This is to prevent the expulsion of explosive mixtures through the
filling trap in refilling.
20. RELIEFS OR SAFETY BLOW-OFFS.--(_a_) Must in all cases be
provided, and must afford free vent to the outer air for any over-
production of gas, and also afford relief in case of abnormal pressure in
the machine.
Both the above-mentioned vents may be connected, with the same escape-
pipe.
(_b_) Must be of at least 3/4-inch internal diameter and be provided
with suitable means for connecting to the pipe loading outside of the
building.
(_c_) Must be constructed without valves or other mechanical working
parts.
(_d_) Apparatus requiring pressure regulators must be provided with
an additional approved safety blow-off attachment located between the
pressure regulator and the service-pipes and discharging to the outer
air.
This is intended to prevent the possibility of undue pressure in the
service-pipes due to failure of the pressure regulator.
21. PRESSURES.--(_a_) The working pressure at the generator must not
vary more than ten-tenths (1) inch water column under all conditions of
carbide charge and feed, and between the limits of no load and 50 per
cent.
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