Acetylene, The Principles Of Its Generation And Use

F >> F. H. Leeds and W. J. Atkinson Butterfield >> Acetylene, The Principles Of Its Generation And Use

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This tends to insure the proper action of the bell and decreases the
liability of escaping gas.

(_f_) A space of at least three-quarters of an inch must be allowed
between the sides of the tank and the bell.

(_g_) All water-seals must be so arranged that the water-level may
be readily seen and maintained.

19. WATER-SUPPLY.--(_a_) The supply of water to the generator for
generating purposes must not be taken from the water-seal of any
gasholder constructed on the gasometer principle, unless the feed
mechanism is so arranged that the water-seals provided for in Rules 18,
(_c_), (_d_), and (_e_) may be retained under all
conditions. This provides for the proper level of water in the gasholder.

(_b_) In cases where machines of the carbide-feed type are supplied
with water from city water-mains or house-pipes, the pipe connexion must
discharge into the regularly provided filling trap on the generator and
not through a separate continuous connexion leading into the generating
chamber.

This is to prevent the expulsion of explosive mixtures through the
filling trap in refilling.

20. RELIEFS OR SAFETY BLOW-OFFS.--(_a_) Must in all cases be
provided, and must afford free vent to the outer air for any over-
production of gas, and also afford relief in case of abnormal pressure in
the machine.

Both the above-mentioned vents may be connected, with the same escape-
pipe.

(_b_) Must be of at least 3/4-inch internal diameter and be provided
with suitable means for connecting to the pipe loading outside of the
building.

(_c_) Must be constructed without valves or other mechanical working
parts.

(_d_) Apparatus requiring pressure regulators must be provided with
an additional approved safety blow-off attachment located between the
pressure regulator and the service-pipes and discharging to the outer
air.

This is intended to prevent the possibility of undue pressure in the
service-pipes due to failure of the pressure regulator.

21. PRESSURES.--(_a_) The working pressure at the generator must not
vary more than ten-tenths (1) inch water column under all conditions of
carbide charge and feed, and between the limits of no load and 50 per
cent. overload.

(_b_) Apparatus not requiring pressure regulators must be so
arranged that the gas pressure cannot exceed sixty-tenths (6) inches
water column.

This requires the use of the pressure relief provided for in Rule No. 20
(_a_).

(_c_) Apparatus requiring pressure regulators must be so arranged
that the gas pressure cannot exceed three pounds to the square inch.

The pressure limit of 3 pounds is taken since that is the pressure
corresponding to a water column about 6 feet high, which is about, the
limit in point of convenience for water-sealed reliefs.

22. AIR MIXTURES.--Generators must be so arranged as to contain the
minimum amount of air when first started or recharged, and no device or
attachment facilitating or permitting mixture of air with the gas prior
to consumption, except at the burners, shall be allowed.

Owing to the explosive properties of acetylene mixed with air, machines
must be so designed that such mixtures are impossible.

23. PURIFIERS.--(_a_) Must be constructed of galvanised iron or
steel not less than No. 24 U.S. Standard gauge in thickness.

(_b_) Where installed, purifiers must conform to the general rules
for the construction of other acetylene apparatus and allow the free
passage of gas.

(_c_) Purifiers must contain no carbide for drying purposes.

(_d_) Purifiers must be located inside of gasholders, or, where
necessarily outside, must have no hand-holes which can be opened without
first shutting off the gas-supply.

24. PRESSURE REGULATORS.--(_a_) Must conform to the rules for the
construction of other acetylene apparatus so far as they apply and must
not be subject to sticking or clogging.

(_b_) Must be capable of maintaining a uniform pressure, not varying
more than four-tenths inch water column, at any load within their rating.

(_c_) Must be installed between valves in such a manner as to
facilitate inspection and repairs.

_Class B.--Stationary Apparatus for Central Station Service._

Generators of over 300 lights capacity for central station service are
not required to be automatic in operation. Generators of less than 300
lights capacity must be automatic in operation and must comply in every
respect with the requirements of Class A.

25. GENERAL RULES. GENERATORS.--(_a_) Must be substantially
constructed of iron or steel and be protected against depreciation by an
effective and durable preventive of corrosion.

Galvanising is strongly recommended as a protection against oxidation,
and it may to advantage be reinforced by a thorough coating of asphaltum
or similar material.

(_b_) Must contain no copper or alloy of copper in contact with
acetylene, excepting in valves.

(_c_) Must be so arranged that generation will take place without
overheating; temperatures in excess of 500 deg. F. to be considered
excessive.

(_d_) Must be provided with means for automatic removal of
condensation from gas passages.

(_e_) Must be provided with suitable protection against freezing of
any water contained in the apparatus.

No salt or other corrosive chemical is permissible as a protection
against freezing.

(_f_) Must in general comply with the requirements governing the
construction of apparatus for isolated installations so far as they are
applicable.

(_g_) Must be so arranged as to insure correct procedure in
recharging and cleaning.

(_h_) Generators of the carbide-feed type must be provided with some
form of approved measuring device to enable the attendant to determine
when the maximum allowable quantity of carbide has been fed into the
generating chamber.

In the operation of generators of this type an allowance of at least 1
gallon of clean generating water per pound of carbide should be made, and
the generator should be cleaned after slaking of every full charge. Where
lump carbide is used the lumps may become embedded in the residuum, if
the latter is allowed to accumulate at the bottom of the generating
chamber, causing overheating from slow and restricted generation, and
rendering the mass more liable to form a hard deposit and bring severe
stresses upon the walls of the generator by slow expansion.

26. GENERATING CHAMBERS.--(_a_) Must each be connected with the
gasholder in such a manner that they will, at all times, give open
connexion either to the gasholder or to the blow-off pipe into the outer
air.

(_b_) Must be so arranged as to guard against appreciable escape of
gas to the room at any time during the introduction of the charges.

(_c_) Must be so designed that the residuum will not clog or affect
the operation of the machine and can conveniently be handled and removed.

(_d_) Must be so arranged that during the process of cleaning and
recharging the back-flow of gas from the gasholder or other generating
chambers will be automatically prevented.

27. GASHOLDERS.--(_a_) Must be of sufficient capacity to contain at
least 4 cubic feet of gas per 1/2-foot burner of the rating.
This is to provide for the requisite lighting period without the
necessity of making gas at night, allowance being made for the
enlargement of burners caused by the use of cleaners.

(_b_) Must be provided with suitable guides to direct the movement
of the bell throughout its entire travel.

28. PRESSURE RELIEFS.--Must in all cases be provided, and must be so
arranged as to prevent pressure in excess of 100-tenths (10) inches water
column in the mains.

29. PRESSURES.--Gasholders must be adjusted to maintain a pressure of
approximately 25-tenths (2.5) inches water column in the mains.

CHAPTER V

THE TREATMENT OF ACETYLENE AFTER GENERATION

IMPURITIES IN CALCIUM CARBIDE.--The calcium carbide manufactured at the
present time, even when of the best quality commercially obtainable, is
by no means a chemically pure substance; it contains a large number of
foreign bodies, some of which evolve gas on treatment with water. To a
considerable extent this statement will probably always remain true in
the future; for in order to make absolutely pure carbide it would be
necessary for the manufacturer to obtain and employ perfectly pure lime,
carbon, and electrodes in an electric furnace which did not suffer attack
during the passage of a powerful current, or he would have to devise some
process for simultaneously or subsequently removing from his carbide
those impurities which were derived from his impure raw materials or from
the walls of his furnace--and either of these processes would increase
the cost of the finished article to a degree that could hardly be borne.
Beside the impurities thus inevitably arising from the calcium carbide
decomposed, however, other impurities may be added to acetylene by the
action of a badly designed generator or one working on a wrong system of
construction; and therefore it may be said at once that the crude gas
coming from the generating plant is seldom fit for immediate consumption,
while if it be required for the illumination of occupied rooms, it must
invariably be submitted to a rigorous method of chemical purification.

IMPURITIES OF ACETYLENE.--Combining together what may be termed the
carbide impurities and the generator impurities in crude acetylene, the
foreign bodies are partly gaseous, partly liquid, and partly solid. They
may render the gas dangerous from the point of view of possible
explosions; they, or the products derived from them on combustion, may be
harmful to health if inspired, injurious to the fittings and decorations
of rooms, objectionable at the burner orifices by determining, or
assisting in, the formation of solid growths which distort the flame and
so reduce its illuminating power; they may give trouble in the pipes by
condensing from the state of vapour in bends and dips, or by depositing,
if they are already solid, in angles, &c., and so causing stoppages; or
they may be merely harmful economically by acting as diluents to the
acetylene and, by having little or no illuminating value of themselves,
causing the gas to emit less light than it should per unit of volume
consumed, more particularly, of course, when the acetylene is not burnt
under the mantle. Also, not being acetylene, or isomeric therewith, they
require, even if they are combustible, a different proportion of oxygen
for their perfect combustion; and a good acetylene jet is only calculated
to attract precisely that quantity of air to the flame which a gas having
the constitution C_2H_2 demands. It will be apparent without argument
that a proper system of purification is one that is competent to remove
the carbide impurities from acetylene, so far as that removal is
desirable or necessary; it should not be called upon to extract the
generator impurities, because the proper way of dealing with them is, to
the utmost possible extent, to prevent their formation. The sole
exception to this rule is that of water-vapour, which invariably
accompanies the best acetylene, and must be partially removed as soon as
convenient. Vapour of water almost always accompanies acetylene from the
generator, even when the apparatus does not belong to those systems of
working where liquid water is in excess, this being due to the fact that
in a generator where the carbide is in excess the temperature tends to
rise until part of the water is vapourised and carried out of the
decomposing chamber before it has an opportunity of reacting with the
excess of carbide. The issuing gas is therefore more or less hot, and it
usually comes from the generating chamber saturated with vapour, the
quantity needed so to saturate it rising as the temperature of the gas
increases. Practically speaking, there is little objection to the
presence of water-vapour in acetylene beyond the fear of deposition of
liquid in the pipes, which may accumulate till they are partially or
completely choked, and may even freeze and burst them in very severe
weather. Where the chemical purifiers, too, contain a solid material
which accidentally or intentionally acts as a drier by removing moisture
from the acetylene, it is a waste of such comparatively expensive
material to allow gas to enter the purifier wetter than need be.

EXTRACTION OF MOISTURE.--In all large plants the extraction of the
moisture may take place in two stages. Immediately after the generator,
and before the washer if the generator requires such an apparatus to
follow it, a condenser is placed. Here the gas is made to travel somewhat
slowly through one or more pipes surrounded with cold air or water, or is
made to travel through a space containing pipes in which cold water is
circulating, the precise method of constructing the condenser being
perfectly immaterial so long as the escaping gas has a temperature not
appreciably exceeding that of the atmosphere. So cooled, however, the gas
still contains much water-vapour, for it remains saturated therewith at
the temperature to which it is reduced, and by the inevitable law of
physics a further fall in temperature will be followed by a further
deposition of liquid water from the acetylene. Manifestly, if the
installation is so arranged that the gas can at no part of the service
and on no occasion fall to a lower temperature than that at which it
issues from the condenser, the removal of moisture as effected by such a
condenser will be sufficient for all practical purposes; but at least in
all large plants where a considerable length of main is exposed to the
air, a more complete moisture extractor must be added to the plant, or
water will be deposited in the pipes every cold night in the winter. It
is, however, useless to put a chemical drier, or one more searching in
its action than a water-cooled condenser, at so early a position in the
acetylene plant, because the gas will be subsequently stored in a water-
sealed holder, where it will most probably once again be saturated with
moisture from the seal. When such generators are adopted as require to
have a specific washer placed after them in order to remove the water-
soluble impurities, _e.g._, those in which the gas does not actually
bubble through a considerable quantity of liquid in the generating
chamber itself, it is doubtful whether a separate condenser is altogether
necessary, because, as the water in the washer can easily be kept at the
atmospheric temperature (by means of water circulating in pipes or
otherwise), the gas will be brought to the atmospheric temperature in the
washer, and at that temperature it cannot carry with it more than a
certain fixed proportion of moisture. The notion of partially drying a
gas by causing it to pass through water may appear paradoxical, but a
comprehension of physical laws will show that it is possible, and will
prove efficient in practice, when due attention is given to the facts
that the gas entering the washer is hot, and that it is subsequently to
be stored over water in a holder.

GENERATOR IMPURITIES.--The generator impurities present in the crudest
acetylene consist of oxygen and nitrogen, _i.e._, the main
constituents of air, the various gaseous, liquid, and semi-solid bodies
described in Chapter II., which are produced by the polymerising and
decomposing action of heat upon the carbide, water, and acetylene in the
apparatus, and, whenever the carbide is in excess in the generator, some
lime in the form of a very fine dust. In all types of water-to-carbide
plant, and in some automatic carbide-feed apparatus, the carbide chamber
must be disconnected and opened each time a fresh charge has to be
inserted; and since only about one-third of the space in the container
can be filled with carbide, the remaining two-thirds are left full of
air. It is easy to imagine that the carbide container of a small
generator might be so large, or loaded with so small a quantity of
carbide, or that the apparatus might in other respects be so badly
designed, that the gas evolved might contain a sufficient proportion of
air to render it liable to explode in presence of a naked light, or of a
temperature superior to its inflaming-point. Were a cock, however, which
should have been shut, to be carelessly left open, an escape of gas from,
rather than an introduction of air into, the apparatus would follow,
because the pressure in the generator is above that of the atmosphere. As
is well known, roughly four-fifths by volume of the air consist of
nitrogen, which is non-inflammable and accordingly devoid of danger-
conferring properties; but in all flames the presence of nitrogen is
harmful by absorbing much of the heat liberated, thus lowering the
temperature of that flame, and reducing its illuminating power far more
seriously. On the other hand, a certain quantity of air in acetylene
helps to prevent burner troubles by acting as a mere diluent (albeit an
inferior one to methane or marsh-gas), and therefore it has been proposed
intentionally to add air to the gas before consumption, such a process
being in regular use on the large scale in some places abroad. As Eitner
has shown (Chapter VI.) that in a 3/4-inch pipe acetylene ceases to be
explosive when mixed with less than 47.7 per cent. of air, an amount of,
say, 40 per cent. or less may in theory be safely added to acetylene; but
in practice the amount of air added, if any, would have to be much
smaller, because the upper limit of explosibility of acetylene-air
mixtures is not rigidly fixed, varying from about 50 per cent. of air
when the mixture is in a small vessel, and fired electrically to about 25
per cent. of air in a large vessel approached with a flame. Moreover,
safely to prepare such mixtures, after the proportion of air had been
decided upon, would require the employment of some additional perfectly
trustworthy automatic mechanism to the plant to draw into the apparatus a
quantity of air strictly in accordance with the volume of acetylene made
--a pair of meters geared together, one for the gas, the other for the
air--and this would introduce extra complexity and extra expense. On the
whole the idea cannot be recommended, and the action of the British Home
Office in prohibiting the use of all such mixtures except those
unavoidably produced in otherwise good generators, or in burners of the
ordinary injector type, is perfectly justifiable. The derivation and
effect of the other gaseous and liquid generator impurities in acetylene
were described in Chapter II. Besides these, very hot gas has been found
to contain notable amounts of hydrogen and carbon monoxide, both of which
burn with non-luminous flames. The most plausible explanation of their
origin has been given by Lewes, who suggests that they may be formed by
the action of water-vapour upon very hot carbide or upon carbon separated
therefrom as the result of previous dissociation among the gases present;
the steam and the carbon reacting together at a temperature of 500 deg. C.
or thereabouts in a manner resembling that of the production of water-gas.
The last generator impurity is lime dust, which is calcium oxide or
hydroxide carried forward by the stream of gas in a state of extremely
fine subdivision, and is liable to be produced whenever water acts
rapidly upon an excess of calcium carbide. This lime occasionally appears
in the alternative form of a froth in the pipes leading directly from the
generating chamber; for some types of carbide-to-water apparatus,
decomposing certain kinds of carbide, foam persistently when the liquid
in them becomes saturated with lime, and this foam or froth is remarkably
difficult to break up.

FILTERS.--It has just been stated that the purifying system added to an
acetylene installation should not be called upon to remove these
generator impurities; because their appearance in quantity indicates a
faulty generator, which should be replaced by one of better action. On
the contrary, with the exception of the gases which are permanent at
atmospheric temperature--hydrogen, carbon monoxide, nitrogen, and oxygen--
and which, once produced, must remain in the acetylene (lowering its
illuminating value, but giving no further trouble), extraction of these
generator impurities is quite simple. The dust or froth of lime will be
removed in the washer where the acetylene bubbles through water--the dust
itself can be extracted by merely filtering the gas through cotton-wool,
felt, or the like. The least volatile liquid impurities will be removed
partly in the condenser, partly in the washer, and partly by the
mechanical dry-scrubbing action of the solid purifying material in the
chemical purifier. To some extent the more volatile liquid bodies will be
removed similarly; but a complete extraction of them demands the
employment of some special washing apparatus in which the crude acetylene
is compelled to bubble (in finely divided streams) through a layer of
some non-volatile oil, heavy mineral lubricating oil, &c.; for though
soluble in such oil, the liquid impurities are not soluble in, nor do
they mix with, water; and since they are held in the acetylene as
vapours, a simple passage through water, or through water-cooled pipes,
does not suffice for their recovery. It will be seen that a sufficient
removal of these generator impurities need throw no appreciable extra
labour upon the consumer of acetylene, for he can readily select a type
of generator in which their production is reduced to a minimum; while a
cotton-wool or coke filter for the gas, a water washer, which is always
useful in the plant if only employed as a non-return valve between the
generator and the holder, and the indispensable chemical purifiers, will
take out of the acetylene all the remaining generator impurities which
need, and can, be extracted.

CARBIDE IMPURITIES.--Neglecting very minute amounts of carbon monoxide
and hydrogen (which may perhaps come from cavities in the calcium carbide
itself), as being utterly insignificant from the practical point of view,
the carbide impurities of the gas fall into four main categories: those
containing phosphorus, those containing sulphur, those containing
silicon, and those containing gaseous ammonia. The phosphorus in the gas
comes from calcium phosphide in the calcium carbide, which is attacked by
water, and yields phosphoretted hydrogen (or phosphine, as it will be
termed hereafter). The calcium phosphide, in its turn, is produced in the
electric furnace by the action of the coke upon the phosphorus in
phosphatic lime--all commercially procurable lime and some varieties of
coke (or charcoal) containing phosphates to a larger or smaller extent.
The sulphur in the gas comes from aluminium sulphide in the carbide,
which is produced in the electric furnace by the interaction of
impurities containing aluminium and sulphur (clay-like bodies, &c.)
present in the lime and coke; this aluminium sulphide is attacked by
water and yields sulphuretted hydrogen. Even in the absence of aluminium
compounds, sulphuretted hydrogen may be found in the gases of an
acetylene generator; here it probably arises from calcium sulphide, for
although the latter is not decomposed by water, it gradually changes in
water into calcium sulphydrate, which appears to suffer decomposition.
When it exists in the gas the silicon is derived from certain silicides
in the carbide; but this impurity will be dealt with by itself in a later
paragraph. The ammonia arises from the action of the water upon
magnesium, aluminium, or possibly calcium nitride in the calcium carbide,
which are bodies also produced in the electric furnace or as the carbide
is cooling. In the gas itself the ammonia exists as such; the phosphorus
exists mainly as phosphine, partly as certain organic compounds
containing phosphorus, the exact chemical nature of which has not yet
been fully ascertained; the sulphur exists partly as sulphuretted
hydrogen and partly as organic compounds analogous, in all probability,
to those of phosphorus, among which Caro has found oil of mustard, and
certain bodies that he regards as mercaptans. [Footnote: It will be
convenient to borrow the phrase used in the coal-gas industry, calling
the compounds of phosphorus other than phosphine "phosphorus compounds,"
and the compounds of sulphur other than sulphuretted hydrogen "sulphur
compounds." The "sulphur compounds" of coal-gas, however, consist mainly
of carbon bisulphide, which is certainly not the chief "sulphur compound"
in acetylene, even if present to any appreciable extent.] The precise way
in which these organic bodies are formed from the phosphides and
sulphides of calcium carbide is not thoroughly understood; but the system
of generation employed, and the temperature obtaining in the apparatus,
have much to do with their production; for the proportion of the total
phosphorus and sulphur found in the crude gas which exists as "compounds"
tends to be greater as the generating plant yields a higher temperature.
It should be noted that ammonia and sulphuretted hydrogen have one
property in common which sharply distinguishes them from the sulphur
"compounds," and from all the phosphorus compounds, including phosphine.
Ammonia and sulphuretted hydrogen are both very soluble in water, the
latter more particularly in the lime-water of an active acetylene
generator; while all the other bodies referred to are completely
insoluble. It follows, therefore, that a proper washing of the crude gas
in water should suffice to remove all the ammonia and sulphuretted
hydrogen from the acetylene; and as a matter of fact those generators in
which the gas is evolved in presence of a large excess of water, and in
which it has to bubble through such water, yield an acetylene practically
free from ammonia, and containing nearly all the sulphur which it does
contain in the state of "compounds." It must also be remembered that
chemical processes which are perfectly suited to the extraction of
sulphuretted hydrogen and phosphine are not necessarily adapted for the
removal of the other phosphorus and sulphur compounds.

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