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.
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