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Book, page 121 / 445 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
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