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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The distributing formulae also assume that the pipe is virtually straight;
bends and angles introduce disturbing influences. If the bend is sharp,
or if there is a right-angle, an allowance should be made if it is
desired to put in pipes of the smallest permissible dimensions. In the
case of the most usual sizes of pipes employed for acetylene mains or
services, it will suffice to reckon that each round or square elbow is
equivalent in the resistance it offers to the flow of gas to a length of
5 feet of pipe of the same diameter. Hence if 5 feet is added to the
actual length of pipe to be laid for every bond or elbow which will occur
in it, and the figure so obtained is taken as the value of _l_ in
formulae (i), (ii), or (iii), the values then found for Q, _d_, or
_h_ will be trustworthy for all practical purposes.

It may now be useful to give an example of the manner of using the
foregoing formulae when the tables of sizes of pipes are not available.
Let it be supposed that an institution is being equipped for acetylene
lighting; that 50 burners consuming 0.70 cubic foot, and 50 consuming
1.00 cubic foot of acetylene per hour may be required in use
simultaneously; that a pressure of at least 2-1/2 inches is required at
all the burners; that for sufficient reasons it is considered undesirable
to use a higher distributing pressure than 4 inches at the gasholder,
outlet of the purifiers, or initial governor (whichever comes last in the
train of apparatus); that the gasholder is located 100 feet from the main
building of the institution, and that the trunk supply-pipe through the
latter must be 250 feet in length, and the supplies to the burners,
either singly or in groups, be taken from this trunk pipe through short
lengths of tubing of ample size. What should be the diameter of the trunk
pipe, in which it will be assumed that ten bonds or elbows are necessary?

In the first instance, it is convenient to suppose that the trunk pipe
may be of uniform diameter throughout. Then the value of _l_ will be
100 (from gasholder to main building) + 250 (within the building) + 50
(equivalent of 10 elbows) = 400. The maximum value of Q will be (50 x
0.7) + (50 x 1.0) = 85; and the value of _h_ will be 1 - 2.5 - 1.5.
Then using formula (i), we have:

d = 0.045122((85^2 x 400)/1.5)^(1/5) = 0.045122(1,926,667)^(1/5)

= 0.045122 x 18.0713 = 0.8154.

The formula, therefore, shows that the pipe should have an internal
diameter of not less than 0.8154 inch, and consequently 1 inch (the next
size above 0.8154 inch) barrel should be used. If the initial pressure
(i.e., at outlet of purifiers) could be conveniently increased from 4 to
4.8 inches, 3/4 inch barrel could be employed for the service-pipe. But
if connexions for burners were made immediately the pipe entered the
building, these burners would then be supplied at a pressure of 4.2
inches, while those on the extremity of the pipe would, when all burners
were in use, be supplied at a pressure of only 2.5 inches. Such a great
difference of pressure is not permissible at the several burners, as no
type of burner retains its proper efficiency over more than a very
limited range of pressure. It is highly desirable in the case of the
ordinary Naphey type of burner that all the burners in a house should be
supplied at pressures which do not differ by more than half an inch;
hence the pipes should, wherever practicable, be of such a size that they
will pass the maximum quantity of gas required for all the burners which
will ever be in use simultaneously, when the pressure at the first burner
connected to the pipe after it enters the house is not more than half an
inch above the pressure at the burner furthermost removed from the first
one, all the burner-taps being turned on at the time the pressures are
observed. If the acetylene generating plant is not many yards from the
building to be supplied, it is a safe rule to calculate the size of pipes
required on the basis of a fall of pressure of only half an inch from the
outlet of the purifiers or initial governor to the farthermost burner.
The extra cost of the larger size of pipe which the application of this
rule may entail will be very slight in all ordinary house installations.

VELOCITY OF FLOW IN PIPES.--For various purposes, it is often desirable
to know the mean speed at which acetylene, or any other gas, is passing
through a pipe. If the diameter of the pipe is _d_ inches, its
cross-sectional area is _d^2_ x 0.7854 square inches; and since
there are 1728 cubic inches in 1 cubic foot, that quantity of gas will
occupy in a pipe whose diameter is _d_ inches a length of

1728/(_d^2_ x 0.7854) linear inches or 183/_d^2_^ linear feet.

If the gas is in motion, and the pipe is delivering Q cubic feet per
hour, since there are 3600 seconds of time in one hour, the mean speed of
the gas becomes

183/_d^2_ x Q/3600 = Q/(19 x 7_d^2_) linear feet per second.

This value is interesting in several ways. For instance, taking a rough
average of Le Chatelier's results, the highest speed at which the
explosive wave proceeds in a mixture of acetylene and air is 7 metres or
22 feet per second. Now, even if a pipe is filled with an acetylene-air
mixture of utmost explosibility, an explosion cannot travel backwards
from B to A in that pipe, if the gas is moving from A to B at a speed of
over 22 feet per second. Hence it may be said that no explosion can occur
in a pipe provided

Q/(19.7_d^2_) = 22 or more;

_i.e._, Q/_d^2_=433.4

In plain language, if the number of cubic feet passing through the pipe
per hour divided by the square of the diameter of the pipe is at least
433.4, no explosion can take place within that pipe, even if the gas is
highly explosive and a light is applied to its exit.

In Chapter VI. are given the explosive limits of acetylene-air mixtures
as influenced by the diameter of the tube containing them. If we
possessed a similar table showing the speed of the explosive wave in
mixtures of known composition, the foregoing formulae would enable us to
calculate the minimum speed which would insure absence of explosibility
in a supply-pipe of any given diameter throughout its length, or at its
narrowest part. It would not, however, be possible simply by increasing
the forward speed of an explosive mixture of acetylene and air to a point
exceeding that of its explosion velocity to prevent all danger of firing
back in an atmospheric burner tube. A much higher pressure than is
usually employed in gas-burners, other than blowpipes, would be needed to
confer a sufficient degree of velocity upon the gas, a pressure which
would probably fracture any incandescent mantle placed in the flame.

SERVICE-PIPES AND MAINS.--The pipes used for the distribution of
acetylene must be sound in themselves, and their joints perfectly tight.
Higher pressures generally prevail in acetylene service-pipes within a
house than in coal-gas service-pipes, while slight leaks are more
offensive and entail a greater waste of resources. Therefore it is
uneconomical, as well as otherwise objectionable, to employ service-pipes
or fittings for acetylene which are in the least degree unsound.
Unfortunately ordinary gas-barrel is none too sound, nor well-threaded,
and the taps and joints of ordinary gas-fittings are commonly leaky.
Hence something better should invariably be used for acetylene. What is
known as "water" barrel, which is one gauge heavier than gas-barrel of
the same size, may be adopted for the service-pipes, but it is better to
incur a slight extra initial expense and to use "steam" barrel, which is
of still heavier gauge and is sounder than either gas or water-pipe. All
elbows, tees, &c., should be of the same quality. The fitters' work in
making the joints should be done with the utmost care, and the sloppy
work often passed in the case of coal-gas services must on no account be
allowed. It is no exaggeration to say that the success of an acetylene
installation, from the consumer's point of view, will largely, if not
principally, depend on the tightness of the pipes in his house. The
statement has been made that the "paint" used by gas-fitters,
_i.e._, the mixture of red and white lead ground in "linseed" oil,
is not suitable for employment with acetylene, and it has been proposed
to adopt a similar material in which the vehicle is castor-oil. No good
reason has been given for the preference for castor-oil, and the troubles
which have arisen after using ordinary paint may be explained partly on
the very probable assumption that the oil was not genuine linseed, and so
did not dry, and partly on the fact that almost entire reliance was
placed on the paint for keeping the joint sound. Joints for acetylene,
like those for steam and high-pressure water, must be made tight by using
well-threaded fittings, so as to secure metallic contact between pipe and
socket, &c.; the paint or spun-yarn is only an additional safeguard. In
making a faced joint, washers of (say, 7 lb) lead, or coils of lead-wire
arc extremely convenient and quite trustworthy; the packing can be used
repeatedly.

LEAKAGE.--Broadly speaking, it may be said that the commercial success of
any village acetylene-supply--if not that of all large installations--
depends upon the leakage being kept within moderate limits. It follows
from what was stated in Chapter VI. about the diffusion of acetylene,
that from pipes of equal porosity acetylene and coal-gas will escape at
equal rates when the effective pressure in the pipe containing acetylene
is double that in the pipe containing coal-gas. The loss of coal-gas by
leakage is seldom less than 5 per cent. of the volume passed into the
main at the works; and provided a village main delivering acetylene is
not unduly long in proportion to the consumption of gas--or, in other
words, provided the district through which an acetylene distributing main
passes is not too sparsely populated--the loss of acetylene should not
exceed the same figure. Caro holds that the loss of gas by leakage from a
village installation should be quoted in absolute figures and not as a
percentage of the total make as indicated by the works meter, because
that total make varies so largely at different periods of the year, while
the factors which determine the magnitude of the leakage are always
identical; and therefore whereas the actual loss of gas remains the same,
it is represented to be more serious in the summer than in the winter.
Such argument is perfectly sound, but the method of returning leakage as
a percentage of the make has been employed in the coal-gas industry for
many years, and as it does not appear to have led to any misunderstanding
or inconvenience, there is no particular reason for departing from the
usual practice in the case of acetylene where the conditions as to
uniform leakage and irregular make are strictly analogous.

Caro has stated that a loss of 15 to 20 litres per kilometre per hour
(_i.e._, of 0.85 to 1.14 cubic feet per mile per hour) from an
acetylene distributing main is good practice; but it should be noted that
much lower figures have been obtained when conditions are favourable and
when due attention has been devoted to the fitters' work. In one of the
German village acetylene installations where the matter has been
carefully investigated (Doese, near Cuxhaven), leakage originally occurred
at the rate of 7.3 litres per kilometre per hour in a main 8.5
kilometres, or 5.3 miles, long and 4 to 2 inches in diameter; but it was
reduced to 5.2 litres, and then to 3.12 litres by tightening the plugs of
the street lantern and other gas cocks. In British units, these figures
are 0.415, 0.295, and 0.177 cubic foot per mile per hour. By calculation,
the volume of acetylene generated in this village would appear to have
been about 23,000 cubic feet per mile of main per year, and therefore it
may be said that the proportion of gas lost was reduced by attending to
the cocks from 15.7 per cent, to 11.3 per cent, and then to 6.8 per cent.
At another village where the main was 2.5 kilometres long, tests
extending over two months, when the public lamps were not in use, showed
the leakage to be 4.4 litres per kilometre per hour, _i.e._, 1.25
cubic foot per mile per hour, when the annual make was roughly 46,000
cubic feet per mile of main. Here, the loss, calculated from the direct
readings of the works motor, was 4.65 per cent.

When all the fittings, burners excepted, have been connected, the whole
system of pipes must be tested by putting it under a gas (or air)
pressure of 9 or 12 inches of water, and observing on an attached
pressure gauge whether any fall in pressure occurs within fifteen minutes
after the main inlet tap has been shut. The pressure required for this
purpose can be obtained by temporarily weighting the holder, or by the
employment of a pump. If the gauge shows a fall of pressure of one
quarter of an inch or more in these circumstances, the pipes must be
examined until the leak is located. In the presence of a meter, the
installation can conveniently be tested for soundness by throwing into
it, through the meter, a pressure of 12 inches or so of water from the
weighted holder, then leaving the inlet cock open, and observing whether
the index hand on the lowest dial remains perfectly stationary for a
quarter of an hour--movement of the linger again indicating a leak. The
search for leaks must never be made with a light; if the pipes are full
of air this is useless, if full of gas, criminal in its stupidity. While
the whole installation is still under a pressure of 12 inches thrown from
the loaded holder, whether it contains air or gas, first all the likely
spots (joints, &c.), then the entire length of pipe is carefully brushed
over with strong soapy water, which will produce a conspicuous "soap-
bubble" wherever the smallest flaw occurs. The tightness of a system of
pipes put under pressure from a loaded holder cannot be ascertained
safely by observing the height of the bell, and noting if it falls on
standing. Even if there is no issue of gas from the holder, the position
of the bell will alter with every variation in temperature of the stored
gas or surrounding air, and with every movement of the barometer, rising
as the temperature rises and as the barometer falls, and _vice
versa_, while, unless the water in the seal is saturated with
whatever gas the holder contains, the bell will steadily drop a little an
part of its contents are lost by dissolution in the liquid.

PIPES AND FITTINGS.--As a general rule it is unadvisable to use lead or
composition pipe for permanent acetylene connexions. If exposed, it is
liable to be damaged, and perhaps penetrated by a blow, and if set in the
wall and covered with paper or panel it is liable to be pierced if nails
or tacks should at any time be driven into the wall. There is also an
increased risk in case of fire, owing to its ready fusibility. If used at
all--and it has obvious advantages--lead or composition piping should be
laid on the surface of the walls, &c., and protected from blows, &c., by
a light wooden casing, outwardly resembling the wooden coverings for
electric lighting wires. It has been a common practice, in laying the
underground mains required for supplying the villages which are lighted
by means of acetylene from a central works in different parts of France,
to employ lead pipes. The plan is economical, but in view of the danger
that the main might be flattened by the weight of heavy traction-engines
passing over the roads, or that it might settle into local dips from the
same cause or from the action of subterranean water, in which dips water
would be constantly condensing in cold weather, the use of lead for this
purpose cannot be recommended. Steam-barrel would be preferable to cast
pipe, because permanently sound joints are easier to make in the former,
and because it is not so brittle.

The fittings used for acetylene must have perfectly sound joints and
taps, for the same reasons that the service-pipes must be quite sound.
Common gas-fittings will not do, the joints, taps, ball-sockets, &c., are
not accurately enough ground to prevent leakage. They may in many cases
be improved by regrinding, but often the plug and barrel are so shallow
that it is almost impossible to ensure soundness. It is therefore better
to procure fittings having good taps and joints in the first instance;
the barrels should be long, fairly wide, and there should be no sensible
"play" between plug and barrel when adjusted so that the plug turns
easily when lightly lubricated. Fittings are now being specially made for
acetylene, which is a step in the right direction, because, in addition
to superior taps and joints being essential, smaller bore piping and
smaller through-ways to the taps than are required for coal-gas serve for
acetylene. It is perhaps advisable to add that wherever a rigid bracket
or fitting will answer as well as a jointed one, the latter should on no
account be used; also water-slide pendants should never be employed, as
they are fruitful of accidents, and their apparent advantages are for the
most part illusory. Ball-sockets also should be avoided if possible; if
it is absolutely necessary to have a fitting with a ball-socket, the
latter should have a sleeve made of a short length of sound rubber-tubing
of a size to give a close fit, slipped over so as to join the ball
portion to the socket portion. This sleeve should be inspected once a
quarter at least, and renewed immediately it shows signs of cracking.
Generally speaking all the fittings used should be characterised by
structural simplicity; any ornamental or decorative effects desired may
be secured by proper design without sacrifice of the simplicity which
should always mark the essential and operative parts of the fitting.
Flexible connexions between the fixed service-pipe and a semi-portable or
temporary burner may at times be required. If the connexion is for
permanent use, it must not be of rubber, but of the metallic flexible
tubing which is now commonly employed for such connexions in the case of
coal-gas. There should be a tap between the service-pipe and the flexible
connexion, and this tap should be turned off whenever the burner is out
of use, so that the connexion is not at other times under the pressure
which is maintained in the service-pipes. Unless the connexion is very
short--say 2 feet or less--there should also be a tap at the burner.
These flexible connexions, though serviceable in the case of table-lamps,
&c., of which the position may have to be altered, are undesirable, as
they increase the risk attendant on gas (whether acetylene or other
illuminating gas) lighting, and should, if possible, be avoided. Flexible
connexions may also be required for temporary use, such as for conveying
acetylene to an optical lantern, and if only occasionally called for, the
cost of the metallic flexible tubing will usually preclude its use. It
will generally be found, however, that the whole connexion in such a case
can be of composition or lead gas-piping, connected up at its two ends by
a few inches of flexible rubber tubing. It should be carried along the
walls or over the heads of people who may use the room, rather than
across the floor, or at a low level, and the acetylene should be turned
on to it only when actually required for use, and turned off at the fixed
service-pipe as soon as no longer required. Quite narrow composition
tubing, say 1/4-inch, will carry all the acetylene required for two or
three burners. The cost of a composition temporary connexion will usually
be less than one of even common rubber tubing, and it will be safer. The
composition tubing must not, of course, be sharply bent, but carried by
easy curves to the desired point, and it should be carefully rolled in a
roll of not less than 18 inches diameter when removed. If these
precautions are observed it may be used very many times.

Acetylene service-pipes should, wherever possible, be laid with a fall,
which may be very slight, towards a small closed vessel adjoining the
gasholder or purifier, in order that any water deposited from the gas
owing to condensation of aqueous vapour may run out of the pipe into that
apparatus. Where it is impossible to secure an uninterrupted fall in that
direction, there should be inserted in the service-pipe, at the lowest
point of each dip it makes, a short length of pipe turned downwards and
terminating in a plug or sound tap. Water condensing in this section of
the service-pipe will then run down and collect in this drainage-pipe,
from which it can be withdrawn at intervals by opening the plug or tap
for a moment. The condensed water is thus removed from the service-pipe,
and does not obstruct its through-way. Similar drainage devices may be
used at the lowest points of all dips in mains, though there are special
seal-pots which take the place of the cock or plug used to seal the end
of the drainage-pipe. Such seal-pots or "syphons" are commonly used on
ordinary gas-distributing systems, and might be applied in the case of
large acetylene installations, as they offer facilities for removing the
condensed water from time to time in a convenient and expeditious manner.

EXPULSION OF AIR FROM MAINS.--After a service-pipe system has been proved
to be sound, it is necessary to expel the air from it before acetylene
can be admitted to it with a view to consumption. Unless the system is a
very large one, the expulsion of air is most conveniently effected by
forcing from the gasholder preliminary batches of acetylene through the
pipes, while lights are kept away from the vicinity. This precaution is
necessary because, while the acetylene is displacing the air in the
pipes, they will for some time contain a mixture of air and acetylene in
proportions which fall within the explosive limits of such a mixture. If
the escaping acetylene caught fire from any adjacent light under these
conditions, a most disastrous explosion would ensue and extend through
all the ramifications of the system of pipes. Therefore the first step
when a new system of pipes has to be cleared of air is to see that there
are no lights in or about the house--either fires, lamps, cigars or
pipes, candles or other flames. Obviously this work must be done in the
daytime and finished before nightfall. Burners are removed from two or
more brackets at the farthest points in the system from the gasholder,
and flexible connexions are temporarily attached to them, and led through
a window or door into the open air well clear of the house. One of the
brackets selected should as a rule be the lowest point supplied in the
house. The gasholder having been previously filled with acetylene, the
tap or taps on the pipe leading to the house are turned on, and the
acetylene is passed under slight pressure into the system of pipes, and
escapes through the aforesaid brackets, of which the taps have been
turned on, into the open. The taps of all other brackets are kept closed.
The gas should be allowed to flow thus through the pipes until about five
times the maximum quantity which all the burners on the system would
consume in an hour has escaped from the open brackets. The taps on these
brackets are then closed, and the burners replaced. Flexible tubing is
then connected in place of the burners to all the other brackets in the
house, and acetylene is similarly allowed to escape into the open air
from each for a quarter of an hour. All taps are then closed, and the
burners replaced; all windows in the house are left open wide for half an
hour to allow of the dissipation of any acetylene which may have
accumulated in any part of it, and then, while full pressure from the
gasholder is maintained, a tap is turned on and the gas lighted. If it
burns with a good, fully luminous flame it may be concluded that the
system of pipes is virtually free from air, and the installation may be
used forthwith as required. If, however, the flame is very feebly
luminous, or if the escaping gas does not light, lights must be
extinguished, and the pipes again blown through with acetylene into the
open air. The burner must invariably be in position when a light is
applied, because, in the event of the pipes still containing an explosive
mixture, ignition would not be communicated through the small orifices of
the burner to the mixture in the pipes, and the application of the light
would not entail any danger of an explosion.

Gasfitters familiar with coal-gas should remember, when putting a system
of acetylene pipes into use for the first time, that the range over which
mixtures of acetylene and air are explosive is wider than that over which
mixtures of coal-gas and air are explosive, and that greater care is
therefore necessary in getting the pipes and rooms free from a dangerous
mixture.

The mains for very large installations of acetylene--_e.g._, for
lighting a small town--may advisedly be freed from air by some other plan
than simple expulsion of the air by acetylene, both from the point of
view of economy and of safety. If the chimney gases from a neighbouring
furnace are found on examination to contain not more than about 8 per
cent of oxygen, they may be drawn into the gasholder and forced through
the pipes before acetylene is admitted to them. The high proportion of
carbon dioxide and the low proportion of oxygen in chimney gases makes a
mixture of acetylene and chimney gases non-explosive in any proportions,
and hence if the air is first wholly or to a large extent expelled from a
pipe, main, or apparatus, by means of chimney gases, acetylene may be
admitted, and a much shorter time allowed for the expulsion by it of the
contents of the pipe, before a light is applied at the burners, &c. This
plan, however, will usually only be adopted in the case of very large
pipes, &c.; but on a smaller scale the air may be swept out of a
distributing system by bringing it into connexion with a cylinder of
compressed or liquefied carbon dioxide, the pressure in which will drive
the gas to any spot where an outlet is provided. As these cylinders of
"carbonic acid" are in common employment for preparing aerated waters and
for "lifting" beer, &c., they are easy to hire and use.

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