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.
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