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. overload.
(b) Apparatus not requiring pressure regulators must be so arranged that the gas pressure cannot exceed sixty-tenths (6) inches water column.
This requires the use of the pressure relief provided for in Rule No. 20 (a).
(c) Apparatus requiring pressure regulators must be so arranged that the gas pressure cannot exceed three pounds to the square inch.
The pressure limit of 3 pounds is taken since that is the pressure corresponding to a water column about 6 feet high, which is about, the limit in point of convenience for water-sealed reliefs.
22. AIR MIXTURES.--Generators must be so arranged as to contain the minimum amount of air when first started or recharged, and no device or attachment facilitating or permitting mixture of air with the gas prior to consumption, except at the burners, shall be allowed.
Owing to the explosive properties of acetylene mixed with air, machines must be so designed that such mixtures are impossible.
23. PURIFIERS.--(a) Must be constructed of galvanised iron or steel not less than No. 24 U.S. Standard gauge in thickness.
(b) Where installed, purifiers must conform to the general rules for the construction of other acetylene apparatus and allow the free passage of gas.