Acetylene, The Principles Of Its Generation And Use
by
F. H. Leeds and W. J. Atkinson Butterfield
Part 3 out of 9
with air. In practice the receptacles should fit so tightly into the
outer vessel and into one another that when loaded to the utmost extent
permissible--space being left for the swelling of the charge and for the
passage of water and gas--but little room should be left for the
retention of air in the chamber.
ACTION OF CARBIDE-TO-WATER GENERATORS.--The methods which may be adopted
to render a generator automatic when carbide is employed as the moving
material are shown at M, N, and P, in Fig. 6; but the precise devices
used in many actual apparatus are so various that it is difficult to
portray them generically. Moreover it is desirable to subdivide automatic
carbide-to-water generators, according to the size of the carbide they
are constructed to take, into two or three classes, which are termed
respectively "large carbide-feed," "small carbide-feed," and "granulated
carbide-feed" apparatus. (The generator represented at L does not really
belong to the present class, being non-automatic and fed by hand; but the
sketch is given for completeness.) M is an automatic carbide-feed
generator having its store of carbide in a hopper carried by the rising-
holder bell. The hopper is narrowed at its mouth, where it is closed by a
conical or mushroom valve _d_ supported on a rod held in suitable
guides. When the bell falls by consumption of gas, it carries the valve
and rod with it; but eventually the button at the base of _c_
strikes the bottom of the generator, or some fixed distributing plate,
and the rod can descend no further. Then, when the bell falls lower, the
mushroom _d_ rises from its seat, and carbide drops from the hopper
into the water. This type of apparatus has the defect characteristic of
A^2, Fig. 1; for the pressure in the service steadily diminishes as the
effective weight of bell plus hopper decreases by consumption of carbide.
But it has also two other defects--(1) that ordinary carbide is too
irregular in shape to fall smoothly through the narrow annular space
between the valve and its seat; (2) that water vapour penetrates into the
hopper, and liberates some gas there, while it attacks the lumps of
carbide at the orifice, producing dust or causing them to stick together,
and thus rendering the action of the feed worse than ever. Most of these
defects can be avoided by using granulated carbide, which is more uniform
in size and shape, or by employing a granulated and "treated" carbide
which has been dipped in some non-aqueous liquid to make it less
susceptible to the action of moisture. Both these plans, however, are
expensive to adopt; first, because of the actual cost of granulating or
"treating" the carbide; secondly, because the carbide deteriorates in
gas-making capacity by its inevitable exposure to air during the
granulating or "treating" process. The defects of irregularity of
pressure and possible waste of gas by evolution in the hopper may be
overcome by disposing the parts somewhat differently; making the holder
an annulus round the hopper, or making it cylindrical with the hopper
inside. In this case the hopper is supported by the main portion of the
apparatus, and does not move with the bell: the rod and valve being given
their motion in some fashion similar to that figured. Apparatus designed
in accordance with the sketch M, or with the modification just described,
are usually referred to under the name of "hopper" generators. On several
occasions trouble has arisen during their employment owing to the jamming
of the valve, a fragment of carbide rather larger than the rest of the
material lodging between the lips of the hopper and the edges of the
mushroom valve. This has been followed by a sudden descent of all the
carbide in the store into the water beneath, and the evolution of gas has
sometimes been too rapid to pass away at the necessary speed into the
holder. The trouble is rendered even more serious should the whole charge
of carbide fall at a time when, by neglect or otherwise, the body of the
generator contains much lime sludge, the decomposition then proceeding
under exceptionally bad circumstances, which lead to the production of an
excessively high temperature. Hopper generators are undoubtedly very
convenient for certain purposes, chiefly, perhaps, for the construction
of table-lamps and other small installations. Experience tends to show
that they may be employed, first, provided they are designed to take
granulated carbide--which in comparison with larger grades is much more
uniform and cylindrical in shape--and secondly, provided the quantity of
carbide in the hopper does not exceed a few pounds. The phenomenon of the
sudden unexpected descent of the carbide, popularly known as "dumping,"
can hardly be avoided with carbide larger in size than the granulated
variety; and since the results of such an accident must increase in
severity with the size of the apparatus, a limit in their capacity is
desirable.
[Illustration: FIG. 6.--TYPICAL METHODS OF DECOMPOSING CARBIDE (CARBIDE
TO WATER).]
When it is required to construct a carbide-feed generator of large size
or one belonging to the large carbide-feed pattern, it is preferable to
arrange the store in a different manner. In N the carbide is held in a
considerable number of small receptacles, two only of which are shown in
the drawing, provided with detachable lids and hinged bottoms kept shut
by suitable catches. At proper intervals of time those catches in
succession are knocked on one side by a pin, and the contents of the
vessel fall into the water. There are several methods available for
operating the pins. The rising-holder bell may be made to actuate a train
of wheels which terminate in a disc revolving horizontally on a vertical
axis somewhere just below the catches; and this wheel may bear an
eccentric pin which hits each catch as it rotates. Alternatively the
carbide boxes may be made to revolve horizontally on a vertical axis by
the movements of the bell communicated through a clutch; and thus each
box in succession may arrive at a certain position where the catch is
knocked aside by a fixed pin. The boxes, again, may revolve vertically on
a horizontal axis somewhat like a water-wheel, each box having its bottom
opened, or, by a different system of construction, being bodily upset,
when it arrives at the bottom of its circular path. In no case, however,
are the carbide receptacles carried by the bell, which is a totally
distinct part of the apparatus; and therefore in comparison with M, the
pressure given by the bell is much more uniform. Nevertheless, if the
system of carbide boxes moves at all, it becomes easier to move by
decrease in weight and consequent diminution in friction as the total
charge is exhausted; and accordingly the bell has less work to do during
the later stages of its operation. For this reason the plan actually
shown at N is preferable, since the work done by the moving pin,
_i.e._, by the descending bell, is always the same. P represents a
carbide-feed effected by a spiral screw or conveyor, which, revolved
periodically by a moving bell, draws carbide out of a hopper of any
desired size and finally drops it into a shoot communicating with a
generating chamber such as that shown in L. Here the work done by the
bell is large, as the friction against the blades of the screw and the
walls of the horizontal tube is heavy; but that amount of work must
always be essentially identical. The carbide-feed may similarly be
effected by means of some other type of conveyor instead of the spiral
screw, such as an endless band, and the friction in these cases may be
somewhat less than with the screw, but the work to be done by the bell
will always remain large, whatever type of conveyor may be adopted. A
further plan for securing a carbide-feed consists in employing some
extraneous driving power to propel a charge of carbide out of a reservoir
into the generator. Sometimes the propulsive effort is obtained from a
train of clockwork, sometimes from a separate supply of water under high
pressure. The clockwork or the water power is used either to drive a
piston travelling through the vessel containing the carbide so that the
proper quantity of material is dropped over the open mouth of a shoot, or
to upset one after another a series of carbide receptacles, or to perform
some analogous operation. In these cases the pin or other device fitted
to the acetylene apparatus itself has nothing to do beyond releasing the
mechanism in question, and therefore the work required from the bell is
but small. The propriety of employing a generator belonging to these
latter types must depend upon local conditions, _e.g._, whether the
owner of the installation has hydraulic power on a small scale (a
constant supply of water under sufficient pressure) at disposal, or
whether he does not object to the extra labour involved in the periodical
winding up of a train of clockwork.
It must be clear that all these carbide-feed arrangements have the defect
in a more or less serious degree of leaving the carbide in the main
storage vessel exposed to the attack of water vapour rising from the
decomposing chamber, for none of the valves or operating mechanism can be
made quite air-tight. Evolution of gas produced in this way does not
matter in the least, because it is easy to return the gas so liberated
into the generator or into the holder; while the extent of the action,
and the consequent production of overheating, will tend to be less than
in generators such as those shown in G and H of Figs. 4 and 5, inasmuch
as the large excess of water in the carbide-feed apparatus prevents the
liquid arriving at a temperature at which it volatilises rapidly. The
main objection to the evolution of gas in the carbide vessel of a
carbide-to-water generator depends on the danger that the smooth working
of the feed-gear may be interfered with by the formation of dust or by
the aggregation of the carbide lumps.
USE OF OIL IN GENERATORS.--Calcium carbide is a material which is only
capable of attack for the purpose of evolving acetylene by a liquid that
is essentially water, or by one that contains some water mixed with it.
Oils and the like, or even such non-aqueous liquids as absolute alcohol,
have no effect upon carbide, except that the former naturally make it
greasy and somewhat more difficult to moisten. This last property has
been found of service in acetylene generation, especially on the small
scale; for if carbide is soaked in, or given a coating of, some oil, fat,
or solid hydrocarbon like petroleum, cocoanut oil, or paraffin wax, the
substance becomes comparatively indifferent towards water vapour or the
moisture present in the air, while it still remains capable of complete,
albeit slow, decomposition by liquid water when completely immersed
therein. The fact that ordinary calcium carbide is attacked so quickly by
water is really a defect of the substance; for it is to this extreme
rapidity of reaction that the troubles of overheating are due. Now, if
the basket in the generator B^1 of Fig. 2, or, indeed, the carbide store
in any of the carbide-to-water apparatus, is filled with a carbide which
has been treated with oil or wax, as long as the water-level stands at
_l'_ and _l"_ or the carbide still remains in the hopper, it is
essentially unattacked by the vapour arising from the liquid; but
directly the basket is submerged, or the lumps fall into the water,
acetylene is produced, and produced more slowly and regularly than
otherwise. Again, oils do not mix with water, but usually float thereon,
and a mass of water covered by a thick film or layer of oil does not
evaporate appreciably. If, now, a certain quantity of oil, say lamp
paraffin or mineral lubricating oil, is poured on to the water in B^1,
Fig. 2, it moves upwards and downwards with the water. When the water
takes the position _l_, the oil is driven upwards away from the
basket of carbide, and acetylene is generated in the ordinary manner; but
when the water falls to _l"_ the oil descends also, rinses off much
of the adhering water from the carbide lumps, covers them with a greasy
film, and almost entirely stops generation till it is in turn washed off
by the next ascent of the water. Similarly, if the carbide in generators
F, G, and H (also K) has been treated with a solid or semi-solid grease,
it is practically unattacked by the stream of warm damp gas, and is only
decomposed when the liquid itself arrives in the basket. For the same
reason treated carbide can be kept for fairly long periods of time, even
in a drum with badly fitting lid, without suffering much deterioration by
the action of atmospheric moisture. The problem of acetylene generation
is accordingly simplified to a considerable degree by the use of such
treated carbide, and the advantage becomes more marked as the plant
decreases in size till a portable apparatus is reached, because the
smaller the installation the more relatively expensive or inconvenient is
a large holder for surplus gas. The one defect of the method is the extra
cost of such treated carbide; and in English conditions ordinary calcium
carbide is too expensive to permit of any additional outlay upon the
acetylene if it is to compete with petroleum or the product of a tiny
coal-gas works. The extra cost of using treated carbide falls upon the
revenue account, and is much more noticeable than that of a large holder,
which is capital expenditure. When fluid oil is employed in a generator
of type B^1, evolution of gas becomes so regular that any holder beyond
the displacement one which the apparatus itself constitutes is actually
unnecessary, though still desirable; but B^1, with or without oil, still
remains a displacement apparatus, and as such gives no constant pressure.
It must be admitted that the presence of oil so far governs the evolution
of gas that the movement of the water, and the consequent variation of
pressure, is rendered very small; still a governor or a rising holder
would be required to give the best result at the burners. One point in
connexion with the use of liquid oil must not be overlooked, viz., the
extra trouble it may give in the disposal of the residues. This matter
will be dealt with more fully in Chapter V.; here it is sufficient to say
that as the oil does not mix with the water but floats on the surface,
care has to be taken that it is not permitted to enter any open stream.
The foregoing remarks about the use of oil manifestly only apply to those
cases where it is used in quantity and where it ultimately becomes mixed
with the sludge or floats on the water in the decomposing chamber. The
employment of a limpid oil, such as paraffin, as an intermediate liquid
into which carbide is introduced on its way to the water in the
decomposing vessel of a hand-fed generator in the manner described on
page 70 is something quite different, because, except for trifling
losses, one charge of oil should last indefinitely.
RISING GASHOLDERS.--Whichever description of holder is employed in an
acetylene apparatus, the gas is always stored over, or in contact with, a
liquid that is essentially water. This introduces three subjects for
consideration: the heavy weight of a large body of liquid, the loss of
gas by dissolution in that liquid, and the protection of that liquid from
frost in the winter. The tanks of rising holders are constructed in two
different ways. In one the tank is a plain cylindrical vessel somewhat
larger in diameter than the bell which floats in it; and since there must
be nearly enough water in the tank to fill the interior of the bell when
the latter assumes its lowest position, the quantity of water is
considerable, its capacity for dissolving acetylene is large, and the
amount of any substance that may have to be added to it to lower its
freezing-point becomes so great as to be scarcely economical. All these
defects, including that of the necessity for very substantial foundations
under the holder to support its enormous weight, may be overcome by
adopting the second method of construction. It is clear that the water in
the centre of the tank is of no use,--all that is needed being a narrow
trough for the bell to work in. Large rising holders are therefore
advantageously built with a tank formed in the shape of an annulus, the
effective breadth of which is not more than 2 or 3 inches, the centre
portion being roofed over so as to prevent escape of gas. The same
principle may be retained with modified details by fitting inside a plain
cylindrical tank a "dummy" or smaller cylinder, closed by a flat or
curved top and fastened water- and air-tight to the bottom of the main
vessel. The construction of annular tanks or the insertion of a "dummy"
may be attended with difficulty if the tank is wholly or partly sunk
below the ground level, owing to the lifting force of water in the
surrounding soil. Where a steel tank is sunk, or a masonry tank is
constructed, regard must be paid, both in the design of the tank and in
the manner of construction, to the level of the underground water in the
neighbourhood, as in certain cases special precautions will be needed to
avoid trouble from the pressure of the water on the outside of the tank
until it is balanced by the pressure of the water with which the tank is
filled. So far as mere dissolution of gas is concerned, the loss may be
reduced by having a circular disc of wood, &c., a little smaller in
diameter than the boll, floating on the water of a plain tank.
EFFECT OF STORAGE IN GASHOLDER ON ACETYLENE.--It is perfectly true, as
has been stated elsewhere, that the gas coming from an acetylene
generator loses some of its illuminating power if it is stored over water
for any great length of time; such loss being given by Nichols as 94 per
cent, in five months, and having been found by one of the authors as 0.63
per cent. per day--figures which stand in fair agreement with one
another. This wastage is not due to any decomposition of the acetylene in
contact with water, but depends on the various solubilities of the
different gases which compose the product obtained from commercial
calcium carbide. Inasmuch as an acetylene evolved in the best generator
contains some foreign ingredients, and inasmuch as an inferior product
contains more (_cf._ Chapter V.), the contents of a holder are never
pure; but as those contents are principally made up of acetylene itself,
that gas stands at a higher partial pressure in the holder than the
impurities. Since acetylene is more soluble in water than any of its
diluents or impurities, sulphuretted hydrogen and ammonia excepted, and
since the solubility of all gases increases as the pressure at which they
are stored rises, the true acetylene in an acetylene holder dissolves in
the water more rapidly and comparatively more copiously than the
impurities; and thus the acetylene tends to disappear and the impurities
to become concentrated within the bell. Simultaneously at the outer part
of the seal, air is dissolved in the water; and by processes of diffusion
the air so dissolved passes through the liquid from the outside to the
inside, where it escapes into the bell, while the dissolved acetylene
similarly passes from the inside to the outside of the seal, and there
mingles with the atmosphere. Thus, the longer a certain volume of
acetylene is stored over water, the more does it become contaminated with
the constituents of the atmosphere and with the impurities originally
present in it; while as the acetylene is much more soluble than its
impurities, more gas escapes from, than enters, the holder by diffusion,
and so the bulk of stored gas gradually diminishes. However, the figures
previously given show that this action is too slow to be noticeable in
practice, for the gas is never stored for more than a few days at a time.
The action cannot be accepted as a valid argument against the employment
of a holder in acetylene plant. Such deterioration and wastage of gas may
be reduced to some extent by the use of a film of some cheap and
indifferent oil floating on the water inside an acetylene holder; the
economy being caused by the lower solubility of acetylene in oils than in
aqueous liquids not saturated with some saline material. Probably almost
any oil would answer equally well, provided it was not volatile at the
temperature of the holder, and that it did not dry or gum on standing,
_e.g._, olive oil or its substitutes; but mineral lubricating oil is
not so satisfactory. It is, however, not necessary to adopt this method
in practice, because the solvent power of the liquid in the seal can be
reduced by adding to it a saline body which simultaneously lowers its
freezing-point and makes the apparatus more trustworthy in winter.
FREEZING OF GASHOLDER SEAL.--The danger attendant upon the congelation of
the seal in an acetylene holder is very real, not so much because of the
fear that the apparatus may be burst, which is hardly to be expected, as
because the bell will be firmly fixed in a certain position by the ice,
and the whole establishment lighted by the gas will be left in darkness.
In these circumstances, hurried and perhaps injudicious attempts may be
made to thaw the seal by putting red-hot bars into it or by lighting
fires under it, or the generator-house may be thoughtlessly entered with
a naked light at a time when the apparatus is possibly in disorder
through the loss of storage-room for the gas it is evolving. Should a
seal ever freeze, it must be thawed only by the application of boiling
water; and the plant-house must be entered, if daylight has passed, in
perfect darkness or with the assistance of an outside lamp whining
through a closed window. [Footnote: By "closed window" is to be
understood one incapable of being opened, fitted with one or two
thicknesses of stout glass well puttied in, and placed in a wall of the
house as far as possible from the door.] There are two ways of preventing
the seal from freezing. In all large installations the generator-house
will be fitted with a warm-water heating apparatus to protect the portion
of the plant where the carbide is decomposed, and if the holder is also
inside the same building it will naturally be safe. If it is outside, one
of the flow-pipes from the warming apparatus should be led into and round
the lowest part of the seal, care being taken to watch for, or to provide
automatic arrangements for making good, loss of water by evaporation. If
the holder is at a distance from the generator-house, or if for any other
reason it cannot easily be brought into the warming circuit, the seal can
be protected in another way; for unlike the water in the generator, the
water in the holder-seal will perform its functions equally well however
much it be reduced in temperature, always providing it is maintained in
the liquid condition. There are numerous substances which dissolve in, or
mix with, water, and yield solutions or liquids that do not solidify
until their temperature falls far below that of the natural freezing-
point. Assuming that those substances in solution do not attack the
acetylene, nor the metal of which the holder is built, and are not too
expensive, choice may be made between them at will. Strictly speaking the
cost of using them is small, because unless the tank is leaky they last
indefinitely, not evaporating with the water as it is vaporised into the
gas or into the air. The water-seal of a holder standing within the
generator-house may eventually become so offensive to the nostrils that
the liquid has to be renewed; but when this happens it is due to the
accumulation in the water of the water-soluble impurities of the crude
acetylene. If, as should be done, the gas is passed through a washer or
condenser containing much water before it enters the holder the
sulphuretted hydrogen and ammonia will be extracted, and the seal will
not acquire an obnoxious odour for a very long time.
Four principal substances have been proposed for lowering the freezing-
point of the water in an acetylene-holder seal; common salt (sodium
chloride), calcium chloride (not chloride of lime), alcohol (methylated
spirit), and glycerin. A 10 per cent. solution of common salt has a
specific gravity of 1.0734, and does not solidify above -6 deg. C. or 21.2 deg.
F.; a 15 per cent. solution has a density of 1.111, and freezes at -10 deg.
C. or 14 deg. F. Common salt, however, is not to be recommended, as its
solutions always corrode iron and steel vessels more or less quickly.
Alcohol, in its English denatured form of methylated spirit, is still
somewhat expensive to use, but it has the advantage of not increasing the
viscosity of the water; so that a frost-proof mixture of alcohol and
water will flow as readily through minute tubes choked with needle-
valves, or through felt and the like, or along wicks, as will plain
water. For this reason, and for the practically identical one that it is
quite free from dirt or insoluble matter, diluted spirit is specially
suitable for the protection of the water in cyclists' acetylene lamps,
[Footnote: As will appear in Chapter XIII., there is usually no holder in
a vehicular acetylene lamp, all the water being employed eventually for
the purpose of decomposing the carbide. This does not affect the present
question. Dilute alcohol does not attack calcium carbide so energetically
as pure water, because it stands midway between pure water and pure
alcohol, which is inert. The attack, however, of the carbide is as
complete as that of pure water, and the slower speed thereof is a
manifest advantage in any holderless apparatus.] where strict economy is
less important than smooth working. For domestic and larger installations
it is not indicated. As between calcium chloride and glycerin there is
little to choose; the former will be somewhat cheaper, but the latter
will not be prohibitively expensive if the high-grade pure glycerins of
the pharmacist are avoided. The following tables show the amount of each
substance which must be dissolved in water to obtain a liquid of definite
solidifying point. The data relating to alcohol were obtained by Pictet,
and those for calcium chloride by Pickering. The latter are materially
different from figures given by other investigators, and perhaps it would
be safer to make due allowance for this difference. In Germany the
Acetylene Association advocates a 17 per cent. solution of calcium
chloride, to which Frank ascribes a specific gravity of 1.134, and a
freezing-point of -8 deg. C. or 17.6 deg. F.
_Freezing-Points of Dilute Alcohol._
_________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Alcohol. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 4.8 | 0.9916 | -2.0 | +28.4 |
| 11.3 | 0.9824 | 5.0 | 23.0 |
| 16.4 | 0.9761 | 7.5 | 18.5 |
| 18.8 | 0.9732 | 9.4 | 15.1 |
| 20.3 | 0.9712 | 10.6 | 12.9 |
| 22.1 | 0.9689 | 12.2 | 10.0 |
| 24.2 | 0.9662 | 14.0 | 6.8 |
| 26.7 | 0.9627 | 16.0 | 3.2 |
| 29.9 | 0.9578 | 18.9 | -2.0 |
|_______________|___________________|__________|__________|
_Freezing-Points of Dilute Glycerin._
_________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Glycerin. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 10 | 1.024 | -1.0 | +30.2 |
| 20 | 1.051 | 2.5 | 27.5 |
| 30 | 1.075 | 6.0 | 21.2 |
| 40 | 1.105 | 17.5 | 0.5 |
| 50 | 1.127 | 31.3 | -24.3 |
|_______________|___________________|__________|__________|
_Freezing-Points of Calcium Chloride Solutions._
_________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| CaCl_2. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 6 | 1.05 | -3.0 | +26.6 |
| 8 | 1.067 | 4.3 | 24.3 |
| 10 | 1.985 | 5.9 | 21.4 |
| 12 | 1.103 | 7.7 | 18.1 |
| 14 | 1.121 | 9.8 | 14.4 |
| 16 | 1.140 | 12.2 | 10.0 |
| 18 | 1.159 | 15.2 | 4.6 |
| 20 | 1.170 | 18.6 | -1.5 |
|_______________|___________________|__________|__________|
Calcium chloride will probably be procured in the solid state, but it can
be purchased as a concentrated solution, being sold under the name of
"calcidum" [Footnote: This proprietary German article is a liquid which
begins to solidify at -42 deg. C. (-43.6 deg. F.), and is completely solid
at -56 deg. C. (-69) deg. F.). Diluted with one-third its volume of water,
it freezes between -20 deg. and -28 deg. C. (-4 deg. and-l8.4 deg. F.). The
makers recommend that it should be mixed with an equal volume of water.
Another material known as "Gefrierschutzfluessigkeit" and made by the
Floersheim chemical works, freezes at -35 deg. C. (-3 deg. F.). Diluted
with one-quarter its volume of water, it solidifies at -18 deg. C.
(-0.4 deg. F.); with equal parts of water it freezes at -12 deg. C.
(10.4 deg. F.). A third product, called "calcidum oxychlorid," has been
found by Caro and Saulmann to be an impure 35 per cent. solution of calcium
chloride. Not one of these is suitable for addition to the water used in
the generating chamber of an acetylene apparatus, the reasons for this
having already been mentioned.] for the protection of gasholder seals.
Glycerin itself resembles a strong solution of calcium chloride in being a
viscid, oily-looking liquid; and both are so much heavier than water that
they will not mix with further quantities unless they are thoroughly
agitated therewith. Either may be poured through water, or have water
floated upon it, without any appreciable admixture taking place; and
therefore in first adding them to the seal great care must be taken that
they are uniformly distributed throughout the liquid. If the whole contents
of the seal cannot conveniently be run into an open vessel in which the
mixing can be performed, the sealing water must be drawn off a little at a
time and a corresponding quantity of the protective reagent added to it.
Care must be taken also that motives of economy do not lead to excessive
dilution of the reagent; the seal must be competent to remain liquid under
the prolonged influence of the most severe frost ever known to occur in the
neighbourhood where the plant is situated. If the holder is placed out of
doors in an exposed spot where heavy rains may fall on the top of the
bell, or where snow may collect there and melt, the water is apt to run
down into the seal, diluting the upper layers until they lose the frost-
resisting power they originally had. This danger may be prevented by
erecting a sloping roof over the bell crown, or by stirring up the seal
and adding more preservative whenever it has been diluted with rain
water. Quite small holders would probably always be placed inside the
generator-house, where their seals may be protected by the same means as
are applied to the generator itself. It need hardly be said that all
remarks about the dangers incidental to the freezing of holder seals and
the methods for obviating them refer equally to every item in the
acetylene plant which contains water or is fitted with a water-sealed
cover; only the water which is actually used for decomposing the calcium
carbide cannot be protected from frost by the addition of calcium
chloride or glycerin--that water must be kept from falling to its natural
freezing-point. From Mauricheau-Beaupre's experiments, referred to on
page 106, it would appear that a further reason for avoiding an addition
of calcium chloride to the water used for decomposing carbide should lie
in the danger of causing a troublesome production of froth within the
generator.
It will be convenient to digress here for the purpose of considering how
the generators of an acetylene apparatus themselves should be protected
from frost; but it may be said at the outset that it is impossible to lay
down any fixed rules applicable to all cases, since local conditions,
such as climate, available resources, dimensions, and exposed or
protected position of the plant-house vary so largely in different
situations. In all important installations every item of the plant,
except the holder, will be collected in one or two rooms of a single
building constructed of brick or other incombustible material. Assuming
that long-continued frost reigns at times in the neighbourhood, the whole
of such a building, with the exception of one apartment used as a carbide
store only, is judiciously fitted with a heating arrangement like those
employed in conservatories or hothouses; a system of pipes in which warm
water is kept circulating being run round the walls of each chamber near
the floor. The boiler, heated with coke, paraffin, or even acetylene,
must naturally be placed in a separate room of the apparatus-house having
no direct (indoor) communication with the rooms containing the
generators, purifiers, &c. Instead of coils of pipe, "radiators" of the
usual commercial patterns may be adopted; but the immediate source of
heat should be steam, or preferably hot water, and not hot air or
combustion products from the stove. In exposed situations, where the
holder is out of doors, one branch of the flow-pipe should enter and
travel round the seal as previously suggested. Most large country
residences are already provided with suitable heating apparatus for
warming the greenhouses, and part of the heat may be capable of diversion
into the acetylene generator-shed if the latter is erected in a
convenient spot. In fact, if any existing hot-water warming appliances
are already at hand, and if they are powerful enough to do a little more
work, it may be well to put the generator-building in such a position
that it can be efficiently supplied with artificial warmth from those
boilers; for any extra length of main necessary to lead the gas into the
residence from a distant generator will cost less on the revenue account
than the fuel required to feed a special heating arrangement. In smaller
installations, especially such as are to be found in mild climates, it
may be possible to render the apparatus-house sufficiently frost-proof
without artificial heat by building it partly underground, fitting it
with a double skylight in place of a window for the entrance of daylight,
and banking up its walls all round with thick layers of earth. The house
must have a door, however, which must open outwards and easily, so that
no obstacle may prevent a hurried exit in emergencies. Such a door can
hardly be made very thick or double without rendering it heavy and
difficult to open; and the single door will be scarcely capable of
protecting the interior if the frost is severe and prolonged.
Ventilators, too, must be provided to allow of the escape of any gas that
may accidentally issue from the plant during recharging, &c.; and some
aperture in the roof will be required for the passage of the vent pipe or
pipes, which, in certain types of apparatus, move upwards and downwards
with the bell of the holder. These openings manifestly afford facilities
for the entry of cold air, so that although this method of protecting
generator-houses has proved efficient in many places, it can only be
considered inferior to the plan of installing a proper heating
arrangement. Occasionally, where local regulations do not forbid, the
entire generator-house may be built as a "lean-to" against some brick
wall which happens to be kept constantly warm, say by having a furnace or
a large kitchen stove on its other side.
In less complicated installations, where there are only two distinct
items in the plant to be protected from frost--generator and holder--or
where generator and holder are combined into one piece of apparatus,
other methods of warming become possible. As the reaction between calcium
carbide and water evolves much heat, the most obvious way of preventing
the plant from freezing is to economise that heat, _i.e._, to retain
as much of it as is necessary within the apparatus. Such a process,
clearly, is only available if the plant is suitable in external form, is
practically self-contained, and comprises no isolated vessels containing
an aqueous liquid. It is indicated, therefore, rather for carbide-to-
water generators, or for water-to-carbide apparatus in which the carbide
chambers are situated inside the main water reservoir--any apparatus, in
fact, where much water is present and where it is all together in one
receptacle. Moreover, the method of heat economy is suited for
application to automatic generators rather than to those belonging to the
opposite system, because automatic apparatus will be generating gas, and
consequently evolving heat, every evening till late at night--just at the
time when frost begins to be severe. A non-automatic generator will
usually be at work only in the mornings, and its store of heat will
accordingly be much more difficult to retain till nightfall. With the
object of storing up the heat evolved in the generator, it must be
covered with some material possessed of the lowest heat-conducting power
possible; and the proper positions for that material in order of
decreasing importance are the top, sides, and bottom of the plant. The
generator may either be covered with a thick layer of straw, carpet,
flannel, or the like, as is done in the protection of exposed water-
pipes; or it may be provided with a jacket filled with some liquid. In
view of the advisability of not having any organic or combustible
material near the generator, the solid substances just mentioned may
preferably be replaced by one of those partially inorganic compositions
sold for "lagging" steam-pipes and engine-cylinders, such as "Fossil
meal." Indeed, the exact nature of the lagging matters comparatively
little, because the active substance in retaining the heat in the
acetylene generator or the steam-pipe is the air entangled in the pores
of the lagging; and therefore the value of any particular material
depends mainly on its exhibiting a high degree of porosity. The idea of
fitting a water jacket round an acetylene generator is not altogether
good, but it may be greatly improved upon by putting into the jacket a
strong solution of some cheap saline body which has the property of
separating from its aqueous solution in the form of crystals containing
water of crystallisation, and of evolving much heat in so separating.
This method of storing much heat in a small space where a fire cannot be
lighted is in common use on some railways, where passengers' foot-warmers
are filled with a strong solution of sodium acetate. When sodium acetate
is dissolved in water it manifestly exists in the liquid state, and it is
presumably present in its anhydrous condition (i.e., not combined with
water of crystallisation). The common crystals are solid, and contain 3
molecules of water of crystallisation--also clearly in the solid state.
Now, the reaction
NaC_2H_3O_2 + 3H_2O = NaC_2H_3O_2.3H_2O
(anhydrous acetate) (crystals)
evolves 4.37 calories (Berthelot), or 1.46 calorie for each molecule of
water; and whereas 1 kilo. of water only evolves 1 large calorie of heat
as its temperature falls 1 deg. C., 18 grammes of water (1 gramme-molecule)
evolve l.46 large calorie when they enter into combination with anhydrous
sodium acetate to assist in forming crystals--and this 1.46 calorie may
either be permitted to warm the mass of crystals, or made to do useful work
by raising the temperature of some adjacent substance. Sodium acetate
crystals dissolve in 3.9 parts by weight of water at 6 deg. C. (43 deg. F.)
or in 2.4 parts at 37 deg. C. (99 deg. F.). If, then, a jacket round an
acetylene apparatus is filled with a warm solution of sodium acetate
crystals in (say) 3 parts by weight of water, the liquid will crystallise
when it reaches some temperature between 99 deg. and 43 deg. F.; but when
the generator comes into action, the heat liberated will change the mass of
crystals into a liquid without raising its sensible temperature to
anything like the extent that would happen were the jacket full of simple
water. Not being particularly warm to the touch, the liquefied product in
the jacket will not lose much heat by radiation, &c., into the
surrounding air; but when the water in the generator falls again (after
evolution of acetylene ceases) the contents of the jacket will also cool,
and finally will begin to crystallise once more, passing a large amount
of low-temperature heat into the water of the generator, and safely
maintaining it for long periods of time at a temperature suitable for the
further evolution of gas. Like the liquid in the seal of an isolated
gasholder, the liquid in such a jacket will last indefinitely; and
therefore the cost of the sodium acetate in negligible.
Another method of keeping warm the water in any part of an acetylene
installation consists in piling round the apparatus a heap of fresh
stable manure, which, as is well known, emits much heat as it rots. Where
horses are kept, such a process may be said to cost nothing. It has the
advantage over methods of lagging or jacketing that the manure can be
thrown over any pipe, water-seal, washing apparatus, &c., even if the
plant is constructed in several separate items. Unfortunately the ammonia
and the volatile organic compounds which are produced during the natural
decomposition of stable manure tend seriously to corrode iron and steel,
and therefore this method of protecting an apparatus from frost should
only be employed temporarily in times of emergency.
CORROSION IN APPARATUS.--All natural water is a solution of oxygen and
may be regarded also as a weak solution of the hypothetical carbonic
acid. It therefore causes iron to rust more or less quickly; and since no
paint is absolutely waterproof, especially if it has been applied to a
surface already coated locally with spots of rust, iron and steel cannot
be perfectly protected by its aid. More particularly at a few inches
above and below the normal level of the water in a holder, therefore, the
metal soon begins to exhibit symptoms of corrosion which may eventually
proceed until the iron is eaten away or becomes porous. One method of
prolonging the life of such apparatus is to give it fresh coats of paint
periodically; but unless the old layers are removed where they have
cracked or blistered, and the rust underneath is entirely scraped off
(which is practically impossible), the new paint films will not last very
long. Another more elegant process for preserving any metal like iron
which is constantly exposed to the attack of a corrosive liquid, and
which is readily applicable to acetylene holders and their tanks, depends
on the principle of galvanic action. When two metals in good electrical
contact are immersed in some liquid that is capable of attacking both,
only that metal will be attacked which is the more electro-positive, or
which (the same thing in other words) is the more readily attacked by the
liquid, evolving the more heat during its dissolution. As long as this
action is proceeding, as long, that is, as some of the more electro-
positive material is present, the less electro-positive material will not
suffer. All that has to be done, therefore, to protect the walls of an
acetylene-holder tank and the sides of its bell is to hang in the seal,
supported by a copper wire fastened to the tank walls by a trustworthy
electrical joint (soldering or riveting it), a plate or rod of some more
electro-positive metal, renewing that plate or rod before it is entirely
eaten away. [Footnote: Contact between the bell and the rod may be
established by means of a flexible metallic wire; or a separate rod might
be used for the bell itself.] If the iron is bare or coated with lead
(paint may be overlooked), the plate may be zinc; if the iron is
galvanised, _i.e._, coated with zinc, the plate may be aluminium or
an alloy of aluminium and zinc. The joint between the copper wire and the
zinc or aluminium plate should naturally be above the water-level. The
foregoing remarks should be read in conjunction with what was said in
Chapter II., about the undesirability of employing a soft solder
containing lead in the construction of an acetylene generator. Here it is
proposed intentionally to set up a galvanic couple to prevent corrosion;
there, with the same object in view, the avoidances of galvanic action is
counselled. The reason for this difference is self-evident; here a
foreign metal is brought into electrical contact with the apparatus in
order that the latter may be made electro-negative; but when a joint is
soldered with lead, the metal of the generator is unintentionally made
electro-positive. Here the plant is protected by the preferential
corrosion of a cheap and renewable rod; in the former case the plant is
encouraged to rust by the unnecessary presence of an improperly selected
metal.
OTHER ITEMS IN GENERATING PLANT.--It has been explained in Chapter II.
that the reaction between calcium carbide and water is very tumultuous in
character, and that it occurs with great rapidity. Clearly, therefore,
the gas comes away from the generator in rushes, passing into the next
item of the plant at great speed for a time, and then ceasing altogether.
The methods necessarily adopted for purifying the crude gas are treated
of in Chapter V.; but it is manifest now that no purifying material can
prove efficient unless the acetylene passes through it at a uniform rate,
and at one which is as slow as other conditions permit. For this reason
the proper position of the holder in an acetylene installation is before
the purifier, and immediately after the condenser or washer which adjoins
the generator. By this method of design the holder is filled up
irregularly, the gas passing into it sometimes at full speed, sometimes
at an imperceptible rate; but if the holder is well balanced and guided
this is a matter of no consequence. Out of the holder, on the other hand,
the gas issues at a rate which is dependent upon the number and capacity
of the burners in operation at any moment; and in ordinary conditions
this rate is so much more uniform during the whole of an evening than the
rate at which the gas is evolved from the carbide, that a purifier placed
after the holder is given a far better opportunity of extracting the
impurities from the acetylene than it would have were it situated before
the holder, as is invariably the case on coal-gas works.
For many reasons, such as capacity for isolation when being recharged or
repaired, it is highly desirable that each item in an acetylene plant
shall be separated, or capable of separation, from its neighbours; and
this observation applies with great force to the holder and the
decomposing vessel of the generator. In all large plants each vessel
should be fitted with a stopcock at its inlet and, if necessary, one at
its outlet, being provided also with a by-pass so that it can be thrown
out of action without interfering with the rest of the installation. In
the best practice the more important vessels, such as the purifiers, will
be in duplicate, so that unpurified gas need not be passed into the
service while a solitary purifier is being charged afresh. In smaller
plants, where less skilled labour will probably be bestowed on the
apparatus, and where hand-worked cocks are likely to be neglected or
misused, some more, automatic arrangement for isolating each item is
desirable. There are two automatic devices which may be employed for the
purposes in view, the non-return valve and the water-seal. The non-return
valve is simply a mushroom or ball valve without handle, lifted off its
seat by gas passing from underneath whenever the pressure of the gas
exceeds the weight of the valve, but falling back on to its seat and
closing the pipe when the pressure decreases or when pressure above is
greater than that below. The apparatus works perfectly with a clean gas
or liquid which is not corrosive; but having regard to the possible
presence of tarry products, lime dust, or sludge, condensed water loaded
with soluble impurities, &c., in the acetylene, a non-return valve is not
the best device to adopt, for both it and the hand-worked cock or screw-
down valve are liable to stick and give trouble. The best arrangement in
all respects, especially between the generator and the holder, is a
water-seal. A water-seal in made by leading the mouth of a pipe
delivering gas under the level of water in a suitable receptacle, so that
the issuing gas has to bubble through the liquid. Gas cannot pass
backwards through the pipe until it has first driven so much liquid
before it that the level in the seal has fallen below the pipe's mouth;
and if the end of the pipe is vertical more pressure than can possibly be
produced in the apparatus is necessary to effect this. Omitting the side
tube _b_, one variety of water-seal is shown at D in Fig. 7 on page
103. The water being at the level _l_, gas enters at _a_ and
bubbles through it, escaping from the apparatus at _c_. It cannot
return from _c_ to _a_ without driving the water out of the
vessel till its level falls from _f_ to _g_; and since the area
of the vessel is much greater than that of the pipe, so great a fall in
the vessel would involve a far greater rise in _a_. It is clear that
such a device, besides acting as a non-return valve, also fulfils two
other useful functions: it serves to collect and retain all the liquid
matter that may be condensed in the pipe _a_ from the spot at which
it was originally level or was given a fall to the seal, as well as that
condensing in _c_ as far as the spot where _c_ dips again; and
it equally acts as a washer to the gas, especially if the orifice
_g_ of the gas-inlet pipe is not left with a plain mouth as
represented in the figure, but terminates in a large number of small
holes, the pipe being then preferably prolonged horizontally, with minute
holes in it so as to distribute the gas throughout the entire vessel.
Such an apparatus requires very little attention. It may with advantage
be provided with the automatic arrangement for setting the water-level
shown at _d_ and _e_. _d_ is a tunnel tube extending
almost to the bottom of the vessel, and _e_ is a curved run-off pipe
of the form shown. The lower part of the upper curve in _e_ is above
the level _f_, being higher than _f_ by a distance equal to
that of the gas pressure in the pipes; and therefore when water is poured
into the funnel it fills the vessel till the internal level reaches
_f_, when the surplus overflows of itself. The operation thus not
only adjusts the quantity of water present to the desired level so that
_a_ cannot become unsealed, but it also renews the liquid when it
has become foul and nearly saturated with dissolved and condensed
impurities from the acetylene. It would be a desirable refinement to give
the bottom of the vessel a slope to the mouth of _e_, or to some
other spot where a large-bore draw-off cock could be fitted for the
purpose of extracting any sludge of lime, &c., that may collect. By
having such a water-seal, or one simpler in construction, between the
generator and the holder, the former may be safely opened at any time for
repairs, inspection, or the insertion of a fresh charge of carbide while
the holder is full of gas, and the delivery of acetylene to the burners
at a specified pressure will not be interrupted. If a cock worked by hand
were employed for the separation of the holder from the generator, and
the attendant were to forget to close it, part or all of the acetylene in
the holder would escape from the generator when it was opened or
disconnected.
Especially when a combined washer and non-return valve follows
immediately after a generator belonging to the shoot type, and the mouth
of the shoot is open to the air in the plant-house, it is highly
desirable that the washer shall be fitted with some arrangement of an
automatic kind for preventing the water level rising much above its
proper position. The liquid in a closed washer tends to rise as the
apparatus remains in use, water vapour being condensed within it and
liquid water, or froth of lime, being mechanically carried forward by the
stream of acetylene coming from the decomposing chamber. In course of
time, therefore, the vertical depth to which the gas-inlet pipe in the
washer is sealed by the liquid increases; and it may well be that
eventually the depth in question, plus the pressure thrown by the holder
bell, may become greater than the pressure which can be set up inside the
generator without danger of gas slipping under the lower edge of the
shoot. Should this state of things arise, the acetylene can no longer
force its way through the washer into the holder bell, but will escape
from the mouth of the shoot; filling the apparatus-house with gas, and
offering every opportunity for an explosion if the attendant disobeys
orders and takes a naked light with him to inspect the plant.
It is indispensable that every acetylene apparatus shall be fitted with a
safety-valve, or more correctly speaking a vent-pipe. The generator must
have a vent-pipe in case the gas-main leading to the holder should become
blocked at any time, and the acetylene which continues to be evolved in
all water-to-carbide apparatus, even after the supply of water has been
cut off be unable to pass away. Theoretically a non-automatic apparatus
does not require a vent-pipe in its generator because all the gas enters
the holder immediately, and is, or should be, unable to return through
the intermediate water seal; practically such a safeguard is absolutely
necessary for the reason given. The holder must have a safety-valve in
case the cutting-off mechanism of the generator fails to act, and more
gas passes into it than it can store. Manifestly the pressure of the gas
in a water-sealed holder or in any generator fitted with a water-sealed
lid cannot rise above that corresponding with the depth of water in the
seal; for immediately the pressure, measured in inches of water, equals
the depth of the sealing liquid, the seal will be blown out, and the gas
will escape. Such an occurrence, however, as the blowing of a seal must
never be possible in any item of an acetylene plant, more especially in
those items that are under cover, for the danger that the issuing gas
might be fired or might produce suffocation would be extremely great.
Typical simple forms of vent-pipe suitable for acetylene apparatus are
shown in Fig. 7. In each case the pipe marked "vent" is the so-called
safety-valve; it is open at its base for the entry of gas, and open at
its top for the escape of the acetylene into the atmosphere, such top
being in all instances carried through the roof of the generator-house
into the open air, and to a spot distant from any windows of that house
or of the residence, where it can prove neither dangerous nor a nuisance
by reason of its odour. At A is represented the vent-pipe of a
displacement vessel, which may either be part of a displacement holder or
of a generator working on the displacement principle. The vent-pipe is
rigidly fixed to the apparatus. If gas is generated within the closed
portion of the holder or passes through it, and if the pressure so set up
remains less than that which is needed to move the water from the level
_l_ to the levels _l'_ and _l"_, the mouth of the pipe is
under water, and acetylene cannot enter it; but immediately such an
amount of gas is collected, or such pressure is produced that the
interior level sinks below _l"_, which is that of the mouth of the
pipe, it becomes unsealed, and the surplus gas freely escapes. There are
two minor points in connexion with this form of vent-pipe often
overlooked. At the moment when the water arrives at _l"_ in the
closed half of the apparatus, its level in the interior of the vent-pipe
stands at _l'_, identical with that in the open hall of the
apparatus (for the mouth of the vent-pipe and the water in the open hall
of the apparatus are alike exposed to the pressure of the atmosphere
only). When the water, then, descends just below _l"_ there is an
amount of water inside the pipe equal in height to the distance between
_l'_ and _l"_; and before the acetylene can escape, it must
either force this water as a compact mass out of the upper mouth of the
vent-pipe (which it is clearly not in a position to do), drive it out of
the upper mouth a little at a time, or bubble through it till the water
is gradually able to run downwards out of the pipe as its lower opening
is more fully unsealed. In practice the acetylene partly bubbles through
this water and partly drives it out of the mouth of the pipe; on some
occasions temporarily yielding irregular pressures at the burners which
cause them to jump, and always producing a gurgling noise in the vent-
pipe which in calculated to alarm the attendant. If the pipe is too small
in diameter, and especially if its lower orifice is cut off perfectly
horizontal and constricted slightly, the water may refuse to escape from
the bottom altogether, and the pipe will fail to perform its allotted
task. It is better therefore to employ a wide tube, and to cut off its
mouth obliquely, or to give its lower extremity the shape of an inverted
funnel. At the half of the central divided drawing marked B (Fig. 7) is
shown a precisely similar vent-pipe affixed to the bell of a rising
holder, which behaves in an identical fashion when by the rising of the
bell its lower end is lifted out of the water in the tank. The features
described above as attendant, upon the act of unsealing of the
displacement-holder vent-pipe occur here also, but to a less degree; for
the water remaining in the pipe at the moment of unsealing is only that
which corresponds with the vertical distance between _l'_ and
_l"_, and in a rising holder this is only a height always equal to
the pressure given by the bell. Nevertheless this form of vent-pipe
produces a gurgling noise, and would be better for a trumpet-shaped
mouth. A special feature of the pipe in B is that unless it is placed
symmetrically about the centre of the bell its weight tends to throw the
bell out of the vertical, and it may have to be supported at its upper
part; conversely, if the pipe is arranged concentrically in the bell, it
may be employed as part of the guiding arrangement of the bell itself.
Manifestly, as the pipe must be long enough to extend through the roof of
the generator-house, its weight materially increases the weight of the
bell, and consequently the gas pressure in the service; this fact is not
objectionable provided due allowance is made for it. So tall a vent-pipe,
however, seriously raises the centre of gravity of the bell and may make
it top-heavy. To work well the centre of gravity of a holder bell should
be as low as possible, any necessary weighting being provided
symmetrically about its circumference and close to its bottom edge. The
whole length of an ascending vent-pipe need not be carried by the rising
bell, because the lower portion, which must be supported by the bell, can
be arranged to slide inside a wider length of pipe which is fixed to the
roof of the generator-house at the point where it passes into the open
air.
[Illustration: FIG. 7.--TYPICAL FORMS OF VENT-PIPES OR SAFETY-VALVES.]
A refinement upon this vent-pipe is represented at C, where it is rigidly
fastened to the tank of the holder, and has its internal aperture always
above the level of the water in the apparatus. Rigidly fixed to the crown
of the bell is a tube of wider diameter, _h_, which is closed at its
upper end. _h_ is always full of gas, and its mouth is normally
beneath the level of the water in the seal; but when the bell rises to
its highest permissible position, the mouth of _h_ comes above the
water, and communication is opened between the holder and the outer
atmosphere. No water enters the vent-pipe from the holder, and therefore
no gurgling or irregular pressure is produced. Another excellent
arrangement of a vent-pipe, suggested by Klinger of Gumpoldskirchen, is
shown at D, a drawing which has already been partly considered as a
washer and water-seal. For the present purpose the main vessel and its
various pipes are so dimensioned that the vertical height _g_ to
_f_ is always appreciably greater than the gas pressure in the
service or in the generator or gasholder to which it is connected. In
these circumstances the gas entering at _a_ depresses the water in
the pipe below the level _f_ to an extent equal to the pressure at
which it enters that pipe--an extent normally less than the distance
_f_ to _g_; and therefore gas never passes into the body of the
vessel, but travels away by the side tube _b_ (which in former
references to this drawing was supposed to be absent). If, however, the
pressure at _a_ exceeds that of the vertical height _f_ to
_g_, gas escapes at _g_ through the water, and is then free to
reach the atmosphere by means of the vent _c_. As before, _d_
serves to charge the apparatus with water, and _e_ to ensure a
proper amount being added. Clearly no liquid can enter the vent-pipe in
this device. Safety-valves such as are added to steam-boilers and the
like, which consist of a weighted lever holding a conical valve down
against its seat, are not required in acetylene apparatus, for the
simpler hydraulic seals discussed above can always be fitted wherever
they may be needed. It should be noticed that these vent-pipes only come
into operation in emergencies, when they are required to act promptly. No
economy is to be effected by making them small in diameter. For obvious
reasons the vent-pipe of a holder should have a diameter equal to that of
the gas-inlet tube, and the vent-pipe of a generator be equal in size to
the gas-leading tube.
FROTHING IN GENERATORS.--A very annoying trouble which crops up every now
and then during the evolution of acetylene consists in the production of
large masses of froth within the generator. In the ordinary way,
decomposition of carbide is accompanied by a species of effervescence,
but the bubbles should break smartly and leave the surface of the liquid
reasonably free from foam. Sometimes, however, the bubbles do not break,
but a persistent "head" of considerable height is formed. Further
production of gas only increases the thickness of the froth until it
rises so high that it is carried forward through the gas-main into the
next item of the plant. The froth disappears gradually in the pipes, but
leaves in them a deposit of lime which sooner or later causes
obstructions by accumulating at the angles and dips; while during its
presence in the main the steady passage of gas to the holder is
interrupted and the burners may even be made to jump. Manifestly the
defect is chiefly, if not always, to be noticed in the working of
carbide-to-water generators. The phenomenon has been examined by
Mauricheau-Beaupre, who finds that frothing is not characteristic of pure
carbide and that it cannot be attributed to any of the impurities
normally present in commercial carbide. If, however, the carbide contains
calcium chloride, frothing is liable to occur. A 0.1 per cent. solution
of calcium chloride appears to yield some foam when carbide is decomposed
in it, and a 1 per cent. solution to foam in a pronounced manner. In the
absence of calcium chloride, the main cause of frothing seems to be the
presence in the generator of new paint or tar. If a generator is taken
into use before the paint in any part of it which becomes moistened by
warm lime-water has had opportunity of drying thoroughly hard, frothing
is certain to occur; and even if the carbide has been stored for only a
short time in a tin or drum which has been freshly painted, a production
of froth will follow when it is decomposed in water. The products of the
polymerisation of acetylene also tend to produce frothing, but not to
such an extent as the turpentine in paint and the lighter constituents of
coal-tar. Carbide stored even temporarily in a newly painted tin froths
on decomposition because it has absorbed among its pores some of the
volatile matter given off by the paint during the process of desiccation.
THE "DRY" PROCESS OF GENERATION.--A process for generating acetylene,
totally different in principle from those hitherto considered, has been
introduced in this country. According to the original patents of G. J.
Atkins, the process consisted in bringing small or powdered carbide into
mechanical contact with some solid material containing water, the water
being either mixed with the solid reagent or attached to it as water of
crystallisation. Such reagents indeed were claimed as crude starch and
the like, the idea being to recover a by-product of pecuniary value. Now
the process seems to be known only in that particular form in which
granulated carbide is treated with crystallised sodium carbonate,
_i.e._, common washing soda. Assuming the carbide employed to be
chemically pure and the reaction between it and the water of
crystallisation contained in ordinary soda crystals to proceed
quantitatively, the production of acetylene by the dry process should be
represented by the following chemical equation:
5CaC_2 + Na_2CO_3.10H_2O = 5C_2H_2 + 5Ca(OH)_2 + Na_2CO_3.
On calculating out the molecular weights, it will be seen that 286 parts
of washing soda should suffice for the decomposition of 320 parts of pure
calcium carbide, or in round numbers 9 parts of soda should decompose 10
parts of carbide. In practice, however, it seems to be found that from 1
to 1.5 parts of soda are needed for every part of carbide.
The apparatus employed is a metal drum supported on a hollow horizontal
spindle, one end of which is closed and carries a winch handle, and the
other end of which serves to withdraw the gas generated in the plant. The
drum is divided into three compartments by means of two vertical
partitions so designed that when rotation proceeds in one particular
direction portions of the two reagents stored in one end compartment pass
into the centre compartment; whereas when rotation proceeds in the
opposite direction, the material in the centre compartment is merely
mixed together, partly by the revolution of the drum, partly with the
assistance of a stationary agitator slung loosely from the central
spindle. The other end compartment contains coke or sawdust or other dry
material through which the gas passes for the removal of lime or other
dust carried in suspension as it issues from the generating compartment.
The gas then passes through perforations into the central spindle, one
end of which is connected by a packed joint with a fixed pipe, which
leads to a seal or washer containing petroleum. Approached from a
theoretical standpoint, it will be seen that this method of generation
entirely sacrifices the advantages otherwise accruing from the use of
liquid water as a means for dissipating the heat of the chemical
reaction, but on the other hand, inasmuch as the substances are both
solid, the reaction presumably occurs more slowly than it would in the
presence of liquid water; and moreover the fact that the water employed
to act upon the carbide is in the solid state and also more or less
combined with the rest of the sodium carbonate molecule, means that, per
unit of weight, the water decomposed must render latent a larger amount
of heat than it would were it liquid. Experiments made by one of the
authors of this book tend to show that the gas evolved from carbide by
the dry process contains rather less phosphorus than it might in other
conditions of generation, and as a fact gas made by the dry process is
ordinarily consumed without previous passage through any chemical
purifying agent. It is obvious, however, that the use of the churn
described above greatly increases the labour attached to the production
of the gas; while it is not clear that the yield per unit weight of
carbide decomposed should be as high as that obtained in wet generation.
The inventor has claimed that his by-product should be valuable and
saleable, apparently partly on the ground that it should contain caustic
soda. Evidence, however, that a reaction between the calcium oxide or
hydroxide and the sodium carbonate takes place in the prevailing
conditions is not yet forthcoming, and the probabilities are that such
decomposition would not occur unless the residue were largely diluted
with water. [Footnote: The oldest process employed for manufacturing
caustic soda consisted in mixing a solution of sodium carbonate with
quick or slaked lime, and it has been well established that the
causticisation of the soda will not proceed when the concentration of the
liquid is greater than that corresponding with a specific gravity of
about 1-10, _i.e._, when the liquid contains more than some 8 to 10
per cent, of sodium hydroxide.] Conversely there are some grounds for
believing that the dry residue is less useful than an ordinary wet
residue for horticultural purposes, and also for the production of
whitewash. From a financial standpoint, the dry process suffers owing to
the expense involved in the purchase of a second raw material, for which
but little compensation can be discovered unless it is proved that the
residue is intrinsically more valuable than common acetylene-lime and can
be sold or used advantageously by the ordinary owner of an installation.
The discarding of the chemical purifier at the present day is a move of
which the advantage may well be overrated.
ARTIFICIAL LIGHTING OF GENERATOR SHEDS.--It has already been argued that
all normal or abnormal operations in connexion with an acetylene
generating plant should be carried out, if possible, by daylight; and it
has been shown that on no account must a naked light ever be taken inside
the house containing such a plant. It will occasionally happen, however,
that the installation must be recharged or inspected after nightfall. In
order to do this in safety, a double window, incapable of being opened,
should be fitted in one wall of the house, as far as possible from the
door, and in such a position that the light may fall on to all the
necessary places. Outside this window may be suspended an ordinary hand-
lantern burning oil or paraffin; or, preferably, round this window may be
built a closed lantern into which some source of artificial light may be
brought. If the acetylene plant has an isolated holder of considerable
size, there is no reason at all why a connexion should not be made with
the service-pipes, and an acetylene flame be used inside this lantern;
but with generators of the automatic variety, an acetylene light is not
so suitable, because of the fear that gas may not be available precisely
at the moment when it is necessary to have light in the shed. It would,
however, be a simple matter to erect an acetylene burner inside the
lantern in such a way that when needed an oil-lamp or candle could be
used instead. Artificial internal light of any kind is best avoided; the
only kind permissible being an electric glow-lamp. If this is employed,
it should be surrounded by a second bulb or gas-tight glass jacket, and
preferably by a wire cage as well; the wires leading to it must be
carefully insulated, and all switches or cut-outs (which may produce a
spark) must be out of doors. The well-known Davy safety or miner's lamp
is not a trustworthy instrument for use with acetylene because of
(_a_) the low igniting-point of acetylene; (_b_) the high
temperature of its flame; and (_c_) the enormous speed at which the
explosive wave travels through a mixture of acetylene and air. For these
reasons the metallic gauze of the Davy lamp is not so efficient a
protector of the flame as it is in cases of coal-gas, methane, &c.
Moreover, in practice, the Davy lamp gives a poor light, and unless in
constant use is liable to be found out of order when required. It should,
however, be added that modern forms of the safety lamp, in which the
light is surrounded by a stout glass chimney and only sufficient gauze is
used for the admission of fresh air and for the escape of the combustion
products, appear quite satisfactory when employed in an atmosphere
containing some free acetylene.
CHAPTER IV
THE SELECTION OF AN ACETYLENE GENERATOR
In Chapter II. an attempt has been made to explain the physical and
chemical phenomena which accompany the interaction of calcium carbide and
water, and to show what features in the reaction are useful and what
inconvenient in the evolution of acetylene on a domestic or larger scale.
Similarly in Chapter III. have been described the various typical devices
which may be employed in the construction of different portions of
acetylene plant, so that the gas may be generated and stored under the
best conditions, whether it is evolved by the automatic or by the non-
automatic system. This having been done, it seemed of doubtful utility to
include in the first edition of this work a long series of illustrations
of such generators as had been placed on the markets by British, French,
German, and American makers. It would have been difficult within
reasonable limits to have reproduced diagrams of all the generators that
had been offered for sale, and absolutely impossible within the limits of
a single hand-book to picture those which had been suggested or patented.
Moreover, some generating apparatus appeared on the market ephemerally;
some was constantly being modified in detail so as to alter parts which
experience or greater knowledge had shown the makers to be in need of
alteration, while other new apparatus was constantly being brought out.
On these and other grounds it did not appear that much good purpose would
have been served by describing the particular apparatus which at that
time would have been offered to prospective purchasers. It seemed best
that the latter should estimate the value and trustworthiness of
apparatus by studying a section of it in the light of the general
principles of construction of a satisfactory generator as enunciated in
the book. While the position thus taken by the authors in 1903 would
still not be incorrect, it has been represented to them that it would
scarcely be inconsistent with it to give brief descriptions of some of
the generators which are now being sold in Great Britain and a few other
countries. Six more years' experience in the design and manufacture of
acetylene plant has enabled the older firms of manufacturers to fix upon
certain standard patterns for their apparatus, and it may confidently be
anticipated that many of these will survive a longer period. Faulty
devices and designs have been weeded out, and there are lessons of the
past as well as theoretical considerations to guide the inventor of a new
type of generator. On those grounds, therefore, an attempt has now been
made to give brief descriptions, with sectional views, of a number of the
generators now on the market in Great Britain. Moreover, as the first
edition of this book found many readers in other countries, in several of
which there is greater scope for the use of acetylene, it has been
decided to describe also a few typical or widely used foreign generators.
All the generators described must stand or fall on their merits, which
cannot be affected by any opinion expressed by the authors. In the
descriptions, which in the first instance have generally been furnished
by the manufacturers of the apparatus, no attempt has therefore been made
to appraise the particular generators, and comparisons and eulogistic
comments have been excluded. The descriptions, however, would
nevertheless have been somewhat out of place in the body of this book;
they have therefore been relegated to a special Appendix. It has, of
course, been impossible to include the generators of all even of the
English manufacturers, and doubtless many trustworthy ones have remained
unnoticed. Many firms also make other types of generators in addition to
those described. It must not be assumed that because a particular make of
generator is not mentioned it is necessarily faulty. The apparatus
described may be regarded as typical or well known, and workable, but it
is not by reason of its inclusion vouched for in any other respect by the
authors. The Appendix is intended, not to bias or modify the judgment of
the would-be purchaser of a generator, but merely to assist him in
ascertaining what generators there are now on the market.
The observations on the selection of a generator which follow, as well as
any references in other chapters to the same matter, have been made
without regard to particular apparatus of which a description may (or may
not) appear in the Appendix. With this premise, it may be stated that the
intending purchaser should regard the mechanism of a generator as shown
in a sectional view or on inspection of the apparatus itself. If the
generator is simple in construction, he should be able to understand its
method of working at a glance, and by referring it to the type
(_vide_ Chapter III.) to which it belongs, be able to appraise its
utility from a chemical and physical aspect from what has already been
said. If the generator is too complicated for ready understanding of its
mode of working, it is not unlikely to prove too complicated to behave
well in practice. Not less important than the mechanism of a generator is
good construction from the mechanical point of view, _i.e._, whether
stout metal has been employed, whether the seams and joints are well
finished, and whether the whole apparatus has been built in the workman-
like fashion which alone can give satisfaction in any kind of plant.
Bearing these points in mind, the intending purchaser may find assistance
in estimating the mechanical value of an apparatus by perusing the
remainder of this chapter, which will be devoted to elaborating at length
the so-called scientific principles underlying the construction of a
satisfactory generator, and to giving information on the mechanical and
practical points involved.
It is perhaps desirable to remark that there is scarcely any feature in
the generation of acetylene from calcium carbide and water--certainly no
important feature--which introduces into practice principles not already
known to chemists and engineers. Once the gas is set free it ranks simply
as an inflammable, moisture-laden, somewhat impure, illuminating and
heat-giving gas, which has to be dried, purified, stored, and led to the
place of combustion; it is in this respect precisely analogous to coal-
gas. Even the actual generation is only an exothermic, or heat-producing,
reaction between a solid and a liquid, in which rise of temperature and
pressure must be prevented as far as possible. Accordingly there is no
fundamental or indispensable portion of an acetylene apparatus which
lends itself to the protection of the patent laws; and even the details
(it may be said truthfully, if somewhat cynically) stand in patentability
in inverse ratio to their simplicity and utility.
During the early part of 1901 a Committee appointed by the British Home
Office, "to advise as to the conditions of safety to which acetylene
generators should conform, and to carry out tests of generators in the
market in order to ascertain how far those conform with such conditions,"
issued a circular to the trade suggesting that apparatus should be sent
them for examination. In response, forty-six British generators were
submitted for trial, and were examined in a fashion which somewhat
exceeded the instructions given to the Committee, who finally reported to
the Explosives Department of the Home Office in a Blue Book, No. Cd. 952,
which can be purchased through any bookseller. This report comprises an
appendix in which most of the apparatus are illustrated, and it includes
the result of the particular test which the Committee decided to apply.
Qualitatively the test was useful, as it was identical in all instances,
and only lacks full utility inasmuch as the trustworthiness of the
automatic mechanism applied to such generators as were intended to work
on the automatic system was not estimated. Naturally, a complete
valuation of the efficiency of automatic mechanism cannot be obtained
from one or even several tests, it demands long-continued watching; but a
general notion of reliability might have been obtained. Quantitatively,
however, the test applied by the Committee is not so free from reproach,
for, from the information given, it would appear to have been less fair
to some makers of apparatus than to others. Nevertheless the report is
valuable, and indicates the general character of the most important
apparatus which were being offered for sale in the United Kingdom in
1900-1901.
It is not possible to give a direct answer to the question as to which is
the best type of acetylene generator. There are no generators made by
responsible firms at the present time which are not safe. Some may be
easier to charge and clean than others; some require more frequent
attention than others; some have moving parts less likely to fail, when
handled carelessly, than others; some have no moving mechanism to fail.
For the illumination of a large institution or district where one man can
be fully occupied in attending to the plant, cleaning, lighting, and
extinguishing the lamps, or where other work can be found for him so as
to leave him an hour or so every day to look after the apparatus, the
hand-fed carbide-to-water generator L (Fig. 6) has many advantages, and
is probably the best of all. In smaller installations choice must be made
first between the automatic and the non-automatic principle--the
advantages most frequently lying with the latter. If a non-automatic
generator is decided upon, the hand carbide-feed or the flooded-
compartment apparatus is almost equally good; and if automatism is
desired, either a flooded-compartment machine or one of the most
trustworthy types of carbide-feed apparatus may be taken. There are
contact apparatus on the markets which appear never to have given
trouble, and those are worthy of attention. Some builders advocate their
own apparatus because the residue is solid and not a cream. If there is
any advantage in this arising from greater ease in cleaning and
recharging the generator and in disposing of the waste, that advantage is
usually neutralised by the fear that the carbide may not have been wholly
decomposed within the apparatus; and whereas any danger arising from
imperfectly spent carbide being thrown into a closed drain may be
prevented by flooding the residue with plenty of water in an open vessel,
imperfect decomposition in the generator means a deficiency in the amount
of gas evolved from a unit weight of solid taken or purchased. In fact,
setting on one side apparatus which belong to a notoriously defective
system and such as are constructed in large sizes on a system that is
only free from overheating, &c., in small sizes; setting aside all
generators which are provided with only one decomposing chamber when they
are of a capacity to require two or more smaller ones that can more
efficiently be cooled with water jackets; and setting aside any form of
plant which on examination is likely to exhibit any of the more serious
objections indicated in this and the previous chapters, there is
comparatively little to choose, from the chemical and physical points of
view, between the different types of generators now on the markets. A
selection may rather be made on mechanical grounds. The generator must be
well able to produce gas as rapidly as it will ever be required during
the longest or coldest evening; it must be so large that several more
brackets or burners can be added to the service after the installation is
complete. It must be so strong that it will bear careless handling and
the frequent rough manipulation of its parts. It must be built of stout
enough material not to rust out in a few years. Each and all of its parts
must be accessible and its exterior visible. Its pipes, both for gas and
sludge, must be of large bore (say 1 inch), and fitted at every dip with
an arrangement for withdrawing into some closed vessel the moisture, &c.,
that may condense. The number of cocks, valves, and moving parts must be
reduced to a minimum; cocks which require to be shut by hand before
recharging must give way to water-seals. It must be simple in all its
parts, and its action intelligible at a glance. It must be easy to
charge--preferably even by the sense of touch in darkness. It must be
easy to clean. The waste lime must be easily removed. It must be so
fitted with vent-pipes that the pressure can never rise above that at
which it is supposed to work. Nevertheless, a generator in which these
vent-pipes are often brought into use is badly constructed and wasteful,
and must be avoided. The water of the holder seal should be distinct from
that used for decomposing the carbide; and those apparatus where the
holder is entirely separated from the generator are preferable to such as
are built all in one, even if water-seals are fitted to prevent return of
gas. Apparatus which is supposed to be automatic should be made perfectly
automatic, the water or the carbide-feed being locked automatically
before the carbide store, the decomposing chamber, or the sludge-cock can
be opened. The generating chamber must always be in communication with
the atmosphere through a water-sealed vent-pipe, the seal of which, if
necessary, the gas can blow at any time. All apparatus should be fitted
with rising holders, the larger the better. Duplicate copies of printed
instructions should be demanded of the maker, one copy being kept in the
generator-house, and the other elsewhere for reference in emergencies.
These instructions must give simple and precise information as to what
should be done in the event of a breakdown as well as in the normal
manipulation of the plant. Technical expressions and descriptions of
parts understood only by the maker must be absent from these rules.
ADDENDUM.
BRITISH AND FOREIGN REGULATIONS FOR THE CONSTRUCTION AND INSTALLATION OF
ACETYLENE GENERATING PLANT
Dealing with the "conditions which a generator should fulfil before it
can be considered as being safe," the HOME OFFICE COMMITTEE of 1901
before mentioned write as follows:
1. The temperature in any part of the generator, when run at the maximum
rate for which it is designed, for a prolonged period, should not exceed
130 deg. C. This may be ascertained by placing short lengths of wire,
drawn from fusible metal, in those parts of the apparatus in which heat
is liable to be generated.
2. The generator should have an efficiency of not less than 90 per cent.,
which, with carbide yielding 5 cubic feet per pound, would imply a yield
of 4.5 cubic feet for each pound of carbide used.
3. The size of the pipes carrying the gas should be proportioned to the
maximum rate of generation, so that undue back pressure from throttling
may not occur.
4. The carbide should be completely decomposed in the apparatus, so that
lime sludge discharged from the generator shall not be capable of
generating more gas.
5. The pressure in any part of the apparatus, on the generator side of
the holder, should not exceed that of 20 inches of water, and on the
service side of same, or where no gasholder is provided, should not
exceed that of 5 inches of water.
6. The apparatus should give no tarry or other heavy condensation
products from the decomposition of the carbide.
7. In the use of a generator regard should be had to the danger of
stoppage of passage of the gas and resulting increase of pressure which
may arise from the freezing of the water. Where freezing may be
anticipated, steps should be taken to prevent it.
8. The apparatus should be so constructed that no lime sludge can gain
access to any pipes intended for the passage of gas or circulation of
water.
9. The use of glass gauges should be avoided as far as possible, and,
where absolutely necessary, they should be effectively protected against
breakage.
10. The air space in a generator before charging should be as small as
possible.
11. The use of copper should be avoided in such parts of the apparatus as
are liable to come in contact with acetylene.
The BRITISH ACETYLENE ASSOCIATION has drawn up the following list of
regulations which, it suggests, shall govern the construction of
generators and the installation of piping and fittings:
1. Generators shall be so constructed that, when used in accordance with
printed instructions, it shall not he possible for any undecomposed
carbide to remain in the sludge removed therefrom.
2. The limit of pressure in any part of the generator shall not exceed
that of 20 inches of water, subject to the exception that if it be shown
to the satisfaction of the Executive of the Acetylene Association that
higher pressures up to 50 inches of water are necessary in certain
generators, and are without danger, the Executive may, with the approval
of the Home Office, grant exemption for such generators, with or without
conditions.
3. The limit of pressure in service-pipes, within the house, shall not
exceed 10 inches of water.
4. Except when used for special industrial purposes, such as oxy-
acetylene welding, factories, lighthouses, portable apparatus containing
not more than four pounds of carbide, and other special conditions as
approved by the Association, the acetylene plant, such as generators,
storage-holders, purifiers, scrubbers, and for washers, shall be in a
suitable and well-ventilated outhouse, in the open, or in a lean-to,
having no direct communication with a dwelling-house. A blow-off pipe or
safety outlet shall be arranged in such a manner as to carry off into the
open air any overmake of gas and to open automatically if pressure be
increased beyond 20 inches water column in the generating chamber or
beyond 10 inches in the gasholder, or beyond the depth of any fluid seal
on the apparatus.
5. Generators shall have sufficient storage capacity to make a serious
blow-off impossible.
6. Generators and apparatus shall be made of sufficiently strong material
and be of good workmanship, and shall not in any part be constructed of
unalloyed copper.
7. It shall not be possible under any conditions, even by wrong
manipulation of cocks, to seal the generating chamber hermetically.
8. It shall not be possible for the lime sludge to choke any of the gas-
pipes in the apparatus, nor water-pipes if such be alternately used as
safety-valves.
9. In the use of a generator, regard shall be had to the danger of
stoppage of passage of the gas, and resulting increase of pressure, which
may arise from the freezing of the water. Where freezing may be
anticipated, steps shall be taken to prevent it.
10. The use of glass gauges shall be avoided as far as possible, and
where absolutely necessary they shall be effectively protected against
breakage.
11. The air space in the generator before charging shall be as small as
possible, _i.e._, the gas in the generating chamber shall not
contain more than 8 per cent. of air half a minute after commencement of
generation. A sample of the contents, drawn from the holder any time
after generation has commenced, shall not contain an explosive mixture,
_i.e._, more than 18 per cent, of air. This shall not apply to the
initial charges of the gasholder, when reasonable precautions are taken.
12. The apparatus shall produce no tarry or other heavy condensation
products from the decomposition of the carbide.
13. The temperature of the gas, immediately on leaving the charge, shall
not exceed 212 deg. F. (100 deg. C.)
14. No generator shall be sold without a card of instructions suitable
for hanging up in some convenient place. Such instructions shall be of
the most detailed nature, and shall not presuppose any expert knowledge
whatever on the part of the operator.
15. Notice to be fixed on Generator House Door, "NO LIGHTS OR SMOKING
ALLOWED."
16. Every generator shall have marked clearly upon the outside a
statement of the maximum number of half cubic foot burners and the charge
of carbide for which it is designed.
17. The Association strongly advise the use of an efficient purifier with
generating plant for indoor lighting.
18. No composition piping shall be used in any part of a permanent
installation.
19. Before being covered in, all pipe-work (main and branches) shall be
tested in the following manner: A special acetylene generator, giving a
pressure of at least 10 inches water column in a gauge fixed on the
furthest point from the generator, shall be connected to the pipe-work.
All points shall be opened until gas reaches them, when they shall be
plugged and the main cock on the permanent generator turned off, but all
intermediate main cocks shall be open in order to test underground main
and all connexions. The gauge must not show a loss after generator has
been turned off for at least two hours.
20. After the fittings (pendants, brackets, &c.) have been fixed and all
burners lighted, the gas shall be turned off at the burners and the whole
installation shall be re-tested, but a pressure of 5 inches shall be
deemed sufficient, which shall not drop lower than to 4-1/2 inches on the
gauge during one hour's test.
21. No repairs to, or alterations in, any part of a generator, purifier,
or other vessel which has contained acetylene shall be commenced, nor,
except for recharging, shall any such part or vessel be cleaned out until
it has been completely filled with water, so as to expel any acetylene or
mixture of acetylene and air which may remain in the vessel, and may
cause a risk of explosion.
_Recommendation_.--It being the general practice to store carbide in
the generator-house, the Association recommend that the carbide shall be
placed on a slightly raised platform above the floor level.
THE BRITISH FIRE OFFICES COMMITTEE in the latest revision, dated July 15,
1907, of its Rules and Regulations _re_ artificial lighting on
insured premises, includes the following stipulations applicable to
acetylene:
Any apparatus, except as below, for generating, purifying, enriching,
compressing or storing gas, must be either in the open or in a building
used for such purposes only, not communicating directly with any building
otherwise occupied.
An acetylene portable apparatus is allowed, provided it holds a charge of
not more than 2 lb. of carbide.
A cylinder containing not more than 20 cubic feet of acetylene compressed
and (or) dissolved in accordance with an Order of Secretary of State
under the Explosives Act, 1875, is allowed.
The use of portable acetylene lamps containing charges of carbide
exceeding the limit of 2 lb. allowed under these Rules (the average
charge being about 18 lb.) is allowed in the open or in buildings in
course of erection.
Liquid acetylene must not be used or stored on the premises.
The pipe, whether flexible or not, connecting an incandescent gas lamp to
the gas-supply must be of metal with metal connexions.
(The reference in these Rules to the storage of carbide has been quoted
in Chapter II. (page 19).)
These rules are liable to revision from time to time.
The GERMAN ACETYLENE VEREIN has drawn up (December 1904) the following
code of rules for the construction, erection, and manipulation of
acetylene apparatus:
I. _Rules for Construction._
1. All apparatus for the generation, purification, and storage of
acetylene must be constructed of sheet or cast iron. Holder tanks may be
built of brick.
2. When bare, galvanised, or lead-coated sheet-iron is used, the sides of
generators, purifiers, condensers, holder tanks, and (if present) washers
and driers must be built with the following gauges as minima:
Holder bells. All other apparatus.
Up to 7 cubic feet capacity 0.75 mm. 1.00 mm.
From 7 to 18 " 1.00 1.25
From 18 to 53 " 1.25 1.50
Above 53 " 1.50 2.00
When not constructed of cast-iron, the bottoms, covers, and "manhole"
lids must be 0.5 mm. thicker in each respective size.
In all circumstances, the thickness of the walls--especially in the case
of apparatus not circular in horizontal section--must be such that
alteration in shape appears impossible, unless deformation is guarded
against in other ways.
Generators must be so constructed that when they are being charged the
carbide cannot fall into the residue which has already been gasified; and
the residues must always be capable of easy, complete, and safe removal.
3. Generators, purifiers, and holders must be welded, riveted or folded
at the seams; soft solder is only permissible as a tightening material.
4. Pipes delivering acetylene, or uniting the apparatus, must be cast- or
wrought-iron. Unions, cocks, and valves must not be made of copper; but
the use of brass and bronze is permitted.
5. When cast-iron is employed, the rules of the German Gas and Water
Engineers are to be followed.
6. In generators where the whole amount of carbide introduced is not
gasified at one time, it must be possible to add fresh water or carbide
in safety, without interfering with the action of the apparatus. In such
generators the size of the gasholder space is to be calculated according
to the quantity of carbide which can be put into the generator. For every
1 kilogramme of carbide the available gasholder space must be: for the
first 50 kilos., 20 litres; for the next 50 kilos., 15 litres; for
amounts above 100 kilos., 10 litres per kilo. [One kilogramme may be
taken as 2.2 lb., and 28 litres as 1 cubic foot.]
The generator must be large enough to supply the full number of normal
(10-litre) burners with gas for 5 hours; the yield of acetylene being
taken at 290 litres per kilo. [4.65 cubic feet per lb.]
The gasholder space of apparatus where carbide is not stored must be at
least 30 litres for every normal (10-litre) flame.
7. The gasholder must be fitted with an appliance for removing any gas
which may be generated (especially when the apparatus is first brought
into action) after the available space is full. This vent must have a
diameter at least equal to the inlet pipe of the holder.
8. Acetylene plant must be provided with purifying apparatus which
contains a proper purifying material in a suitable condition.
9. The dimensions of subsidiary apparatus, such as washers, purifiers,
condensers, pipes, and cocks must correspond with the capacity of the
plant.
10. Purifiers and washers must be constructed of materials capable of
resisting the attack of the substances in them.
11. Every generator must bear a plate giving the name of the maker, or
the seller, and the maximum number of l0-litre lights it is intended to
supply. If all the carbide put into the generator is not gasified at one
time, the plate must also state the maximum weight of carbide in the
charge. The gasholder must also bear a plate recording the maker's or
seller's name, as well as its storage capacity.
12. Rules 1 to 11 do not apply to portable apparatus serving up to two
lights, or to portable apparatus used only out of doors for the lighting
of vehicles or open spaces.
II. _Rules for Erection_
1. Acetylene apparatus must not be erected in or under rooms occupied or
frequented (passages, covered courts, &c.) by human beings. Generators
and holders must only be erected in apartments covered with light roofs,
and separated from occupied rooms, barns, and stables by a fire-proof
wall, or by a distance of 15 feet. Any wall is to be considered fire-
proof which is built of solid brick, without openings, and one side of
which is "quite free." Apparatus may be erected in barns and stables,
provided the space required is partitioned off from the remainder by a
fire-proof wall.
2. The doors of apparatus sheds must open outwards, and must not
communicate directly with rooms where fires and artificial lights are
used.
3. Apparatus for the illumination of showmen's booths, "merry-go-rounds,"
shooting galleries, and the like must be erected outside the tents, and
be inaccessible to the public.
4. Permanent apparatus erected in the open air must be at least 15 feet
from an occupied building.
5. Apparatus sheds must be fitted at their highest points with outlet
ventilators of sufficient size; the ventilators leading straight through
the roof into the open air. They must be so arranged that the escaping
gases and vapours cannot enter rooms or chimneys.
6. The contacts of any electrical warning devices must be outside the
apparatus shed.
7. Acetylene plants must be prevented from freezing by erection in frost-
free rooms, or by the employment of a heating apparatus or other suitable
appliance. The heat must only be that of warm water or steam. Furnaces
for the heating appliance must be outside the rooms containing
generators, their subsidiary apparatus, or holders; and must be separated
from such rooms by fire-proof walls.
8. In one of the walls of the apparatus shed--if possible not that having
a door--a window must be fitted which cannot be opened; and outside that
window an artificial light is to be placed. In the usual way acetylene
lighting may be employed; but a lamp burning paraffin or oil, or a
lantern enclosing a candle, must always be kept ready for use in
emergencies. In all circumstances internal lighting is forbidden.
9. Every acetylene installation must be provided with a main cock, placed
in a conveniently accessible position so that the whole of the service
may be cut off from the plant.
10. The seller of an apparatus must provide his customer with a sectional
drawing, a description of the apparatus, and a set of rules for attending
to it. These are to be supplied in duplicate, and one set is to be kept
hanging up in the apparatus shed.
III. Rules for Working the Apparatus.
1. The apparatus must only be opened by daylight for addition of water.
If the generator is one of those in which the entire charge of carbide is
not gasified at once, addition of fresh carbide must only be made by
daylight.
2. All work required by the plant, or by any portion of it, and all
ordinary attendance needed must be performed by daylight.
3. All water-seals must be carefully kept full.
4. When any part of an acetylene apparatus or a gas-meter freezes,
notwithstanding the precautions specified in II., 7, it must be thawed
only by pouring hot water into or over it; flames, burning fuel, or red-
hot iron bars must not be used.
5. Alterations to any part of an apparatus which involve the operations
of soldering or riveting, &c., _i.e._, in which a fire must be used,
or a spark may be produced by the impact of hammer on metal, must only be
carried out by daylight in the open air after the apparatus has been
taken to pieces. First of all the plant must be freed from gas. This is
to be done by filling every part with water till the liquid overflows,
leaving the water in it for at least five minutes before emptying it
again.
6. The apparatus house must not be used for any other operation, nor
employed for the storage of combustible articles. It must be efficiently
ventilated, and always kept closed. A notice must be put upon the door
that unauthorised persons are not permitted to enter.
7. It in forbidden to enter the house with a burning lantern or lamp, to
strike matches, or to smoke therein.
8. A search for leaks in the pipes must not be made with the aid of a
light.
9. Alterations to the service must not be made while the pipes are under
pressure, but only after the main cock has been shut.
10. If portable apparatus, such as described in I., 12, are connected to
the burners with rubber tube, the tube must be fortified with an internal
or external spiral of wire. The tube must be fastened at both ends to the
cocks with thread, copper wire, or with ring clamps.
11. The preparation, storage, and use of compressed or liquefied
acetylene is forbidden. By compressed acetylene, however, is only to be
understood gas compressed to a pressure exceeding one effective
atmosphere. Acetylene compressed into porous matter, with or without
acetone, is excepted from this prohibition.
12. In the case of plants serving 50 lights or less, not more than 100
kilos. of carbide in closed vessels may be kept in the apparatus house
besides the drum actually in use.
A fresh drum is not to be opened before the previous one has been two-
thirds emptied. Opened drums must be closed with an iron watertight lid
covering the entire top of the vessel.
In the case of apparatus supplying over 500 lights, only one day's
consumption of carbide must be kept in the generator house. In other
respects the store of carbide for such installations is to be treated as
a regular carbide store.
13. Carbide drums must not be opened with the aid of a flame or a red-hot
iron instrument.
14. Acetylene apparatus must only be attended to by trustworthy and
responsible persons.
The rules issued by the AUSTRIAN GOVERNMENT in 1905 for the installation
of acetylene plant and the use of acetylene are divided into general
enactments relating to acetylene, and into special enactments in regard
to the apparatus and installation. The general enactments state that:
1. The preparation and use of liquid acetylene is forbidden.
2. Gaseous acetylene, alone, in admixture, or in solution, must not be
compressed above 2 atmospheres absolute except under special permission.
3. The storage of mixtures of acetylene with air or other gases
containing or evolving free oxygen is forbidden.
4. A description of every private plant about to be installed must be
submitted to the local authorities, who, according to its size and
character, may give permission for it to be installed and brought into
use either forthwith or after special inspection. Important alterations
to existing plant must be similarly notified.
5. The firms and fitters undertaking the installation of acetylene plant
must be licensed.
The special enactments fall under four headings, viz., (_a_)
apparatus; (_b_) plant houses; (_c_) pipes; (_d_)
residues.
In regard to apparatus it is enacted that:
1. The type of apparatus to be employed must be one which has been
approved by one of certain public authorities in the country.
2. A drawing and description of the construction of the apparatus and a
short explanation of the method of working it must be fixed in a
conspicuous position under cover in the apparatus house. The notice must
also contain approved general information as to the properties of calcium
carbide and acetylene, precautions that must be observed to guard against
possible danger, and a statement of how often the purifier will require
to be recharged.
3. The apparatus must be marked with the name of the maker, the year of
its construction, the available capacity of the gasholder, and the
maximum generating capacity per hour.
4. Each constituent of the plant must be proportioned to the maximum
hourly output of gas and in particular the available capacity of the
holder must be 75 per cent. of the latter. The apparatus must not be
driven above its nominal productive capacity.
5. The productive capacity of generators in which the gasholder has to be
opened or the bell removed before recharging, or for the removal of
sludge, must not exceed 50 litres per hour, nor may the charge of carbide
exceed 1 kilo.
6. Generators exceeding 50 litres per hour productive capacity must be
arranged so that they can be freed from air before use.
7. Generators exceeding 1500 litres per hour capacity must be arranged so
that the acetylene, contained in the parts of the apparatus which have to
be opened for recharging or for the removal of sludge, can be removed
before they are opened.
8. Automatic generators of which the decomposing chambers are built
inside the gasholder must not exceed 300 litres per hour productive
capacity.
9. Generators must be arranged so that after-generation cannot produce
objectionable results.
10. The holder of carbide-to-water generators must be large enough to
take all the gas which may be produced by the introduction of one charge
of carbide without undue pressure ensuing.
11. The maximum pressure permissible in any part of the apparatus is 1.1
atmosphere absolute.
12. The temperature in the gas space of a generator must never exceed
80 deg. C.
13. Generating apparatus, &c., must be constructed in a workmanlike
manner of metal capable of resisting rust and distortion, and, where the
metal comes in contact with carbide or acetylene, it must not be one
(copper in particular) which forms an explosive compound with the gas.
Cocks and screw connexions, &c., of brass, bronze, &c., must always be
kept clean. Joints exposed to acetylene under pressure must be made by
riveting or welding except that in apparatus not exceeding 100 litres per
hour productive capacity double bending may be used.
14. Every apparatus must be fitted with a safety-valve or vent-pipe
terminating in a safe place in the open, and of adequate size.
15. Every apparatus must be provided with an efficient purifier so fitted
that it may be isolated from the rest of the plant and with due
consideration of the possible action of the purifying material upon the
metal used.
16. Mercury pressure gauges are prohibited. Liquid gauges, if used must
be double the length normally needed, and with a cock which in automatic
apparatus must be kept shut while it is in action.
17. Proper steps must always be taken to prevent the apparatus freezing.
In the absence of other precautions water-seals and pressure-gauges must
be filled with liquid having a sufficiently low freezing-point and
without action on acetylene or the containing vessel.
18. Signal devices to show the position of the gasholder bell must not be
capable of producing sparks inside the apparatus house.
19. Leaks must not be sought for with an open flame and repairs requiring
the use of a blow-pipe, &c., must only be carried out after the apparatus
has been taken to pieces or freed from gas by flooding.
20. Apparatus must only be attended to by trustworthy and responsible
adults.
21. Portable apparatus holding not more than 1 kilo. of carbide and of
not more than 50 litres per hour productive capacity, and apparatus fixed
and used out of doors are exempt from the foregoing regulations except
Nos. 11 and 12, and the first part of 13.
In regard to (_b_), plant houses, it is enacted that:
1. Rooms containing acetylene apparatus must be of ample size, used for
no other purpose, have water-tight floors, be warmed without fireplaces
or chimneys, be lighted from outside through an air-tight window by an
independent artificial light, have doors opening outwards, efficient
ventilation and a store of sand or like material for fire extinction.
Strangers must be warned away.
2. Apparatus of not more than 300 litres per hour productive capacity may
be erected in basements or annexes of dwelling houses, but if of over 50
litres per hour capacity must not be placed under rooms regularly
frequented. Rooms regularly frequented and those under the same must not
be used.
3. Apparatus of more than 300 litres per hour productive capacity must be
erected in an independent building at least 15 feet distant from other
property, which building, unless it is at least 30 feet distant, must be
of fire-proof material externally.
4. Gasholders exceeding 280 cubic foot in capacity must be in a detached
room or in the open and inaccessible to strangers, and at least 30 feet
from other property and with lightning conductors.
5. In case of fire the main cock must not be shut until it is ascertained
that no one remains in the room served with the gas.
6. All acetylene installations must be known to the local fire brigade.
In regard to (_c_), pipes, it is enacted that:
1. Mains for acetylene must be separated from the generating apparatus by
a cock, and under a five-minute test for pressure must not show a fall of
over eight-tenths inch when the pressure is 13.8 inches, or three times
the working pressure, whichever is greater.
2. The pipes must as a rule be of iron, though lead may be used where
they are uncovered and not exposed to risk of injury. Rubber connexions
may only be used for portable apparatus, and attached to a terminal on
the metal pipes provided with a cock, and be fastened at both ends so
that they will not slip off the nozzles.
In regard to (_d_), residues, it is enacted that special open or
well-ventilated pits must be provided for their reception when the
apparatus exceeds 300 litres per hour productive capacity. With smaller
apparatus they may be discharged into cesspools if sufficiently diluted.
The ITALIAN GOVERNMENT regulations in regard to acetylene plant are
divided into eight sections. The first of these relates to the production
and use of liquid and compressed acetylene. The production and use of
liquid acetylene is prohibited except under the provisions of the laws
relating to explosives. Neat acetylene must not be compressed to more
than l-1/2 atmospheres except that an absolute pressure of 10 atmospheres
is allowed when the gas is dissolved in acetone or otherwise rendered
free from risk. Mixtures of acetylene with air or oxygen are forbidden,
irrespective of the pressure or proportions. Mixtures of acetylene with
hydrocarbons, carbonic oxide, hydrogen and inert gases are permitted
provided the proportion of acetylene does not exceed 50 per cent. nor the
absolute pressure 10 atmospheres.
The second section relates to acetylene installations, which are
classified in four groups, viz., (_a_) fixed or portable apparatus
supplying not more than thirty burners consuming 20 litres per hour;
(_b_) private installations supplying between 30 and 200 such
burners; (_c_) public or works installations supplying between 30
and 200 such burners; (_d_) installations supplying more than 200
such burners.
The installations must comply with the following general conditions:
1. No part of the generator when working at its utmost capacity should
attain a temperature of more than 100 deg. C.
2. The carbide must be completely decomposed in the apparatus so that no
acetylene can be evolved from the residue. The residues must be diluted
with water before being discharged into drains or cesspools, and sludge
storage-pits must be in the open.
3. The apparatus must preclude the escape of lime into the gas and water
connexions.
4. Glass parts must be adequately protected.
5. Rubber connexions between the generator, gasholder, and main are
absolutely prohibited with installations supplying more than 30 burners.
6. Cocks must be provided for cutting off the main and connexions from
the generator and gasholder.
7. Each burner must have an independent tap.
8. Generators of groups (_b_), (_c_), and (_d_) must be
constructed so that no after-generation of acetylene can take place
automatically and that any surplus gas would in any case be carried out
of the generator house by a vent-pipe.
The third section deals with generator houses, which must be well
ventilated and light; must not be used for any other purpose except to
store one day's consumption of carbide, not exceeding 300 kilos.; must be
fire-proof; must have doors opening outwards; and the vent-pipes must
terminate at a safe place in the open. Apparatus of group (_b_) must
not be placed in a dwelling-room and only in an adjoining room if the
gasholder is of less than 600 litres capacity. Apparatus of group
(_c_) must be in an independent building which must be at least 33
feet from occupied premises if the capacity of the gasholder is 6000
litres and upwards. Half this distance suffices for gasholders containing
600 to 6000 litres. These distances may be reduced at the discretion of
the local authorities provided a substantial partition wall at least 1
foot thick is erected. Apparatus of group (_d_) must be at least 50
feet from occupied premises and the gasholder and generator must not be
in the same building.
The fourth section deals with the question of authorisation for the
installation of acetylene plant. Apparatus of group (_a_) may be
installed without obtaining permission from any authorities. In regard to
apparatus of the other groups, permission for installation must be
obtained from local or other authorities.
The fifth section relates to the working of acetylene plant. It makes the
concessionaires and owners of the plant responsible for the manipulation
and supervision of the apparatus, and for the employment of suitable
operators, who must not be less than 18 years of age.
The sixth section relates to the inspection of acetylene plant from time
to time by inspectors appointed by the local or other authorities.
Apparatus of group (_a_) is not subject to these periodical
inspections.
The seventh section details the fees payable for the inspection of
installations and carbide stores, and fixes the penalties for non-
compliance with the regulations.
The eighth section refers to the notification of the position and
description of all carbide works, stores, and acetylene installations to
the local authorities.
The HUNGARIAN GOVERNMENT rules for the construction and examination of
acetylene plant forbid the use of copper and of its alloys; cocks,
however, may be made of a copper alloy. The temperature in the gas space
of a fixed generator must not exceed 50 deg. C., in that of a portable
apparatus 80 deg. C. The maximum effective pressure permissible is 0.15
atmosphere.
The CONSEIL D'HYGIENE DE LA SEINE IN FRANCE allows a maximum pressure of
1.5 metres, i.e., 59 inches, of water column in generators used for the
ordinary purposes of illumination; but apparatus intended to supply gas
to the low-pressure oxy-acetylene blowpipe (see Chapter IX.) may develop
up to 2.5 metres, or 98.5 inches of water pressure, provided copper and
its alloys are entirely excluded from the plant and from the delivery-
pipes.
The NATIONAL BOARD OF FIRE UNDERWRITERS OF THE UNITED STATES OF AMERICA
has issued a set of rules and requirements, of which those relating to
acetylene generators and plant are reproduced below. The underwriters
state that, "To secure the largest measure of safety to life and
property, these rules for the installation of acetylene gas machines must
be observed."
RULES FOR THE INSTALLATION AND USE OF ACETYLENE GAS GENERATORS.
[Footnote: The "gallon" of these rules is, of course, the American
gallon, which is equal to 0.83 English standard gallon.]
The use of liquid acetylene or gas generated therefrom is absolutely
prohibited.
Failure to observe these rules is as liable to endanger life as property.
To secure the largest measure of safety to life and property, the
following rules for the installation of acetylene gas machines must be
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