which employ water as a seal are now required by the insurance rules. The
valve itself (Figure 13) consists of a large cylinder containing water to a
certain depth, which is indicated on the valve body. Two pipes come into
the upper end of this cylinder and lead down into the water, one being
longer than the other. The shorter pipe leads to the escape pipe mentioned
above, while the longer one comes from the generator. The upper end of the
cylinder has an opening to which is attached the pipe leading to the
torches.

[Illustration: Figure 13.--Hydraulic Back-Pressure Valve.
_A_, Acetylene supply line;
_B_, Vent pipe;
_C_, Water filling plug;
_D_, Acetylene service cock;
_E_, Plug to gauge height of water;
_F_, Gas openings under water;
_G_, Return pipe for sealing water;
_H_, Tube to carry gas below water line;
_I_, Tube to carry gas to escape pipe;
_J_, Gas chamber;
_K_, Plug in upper gas chamber;
_L_, High water level;
_M_, Opening through which water returns;
_O_, Bottom clean out casting]

The gas coming from the generator through the longer pipe passes out of the
lower end of the pipe which is under water and bubbles up through the water
to the space in the top of the cylinder. From there the gas goes to the
pipe leading to the torches. The shorter pipe is closed by the depth of
water so that the gas does not escape to the relief pipe. As long as the