§ 54.15-15 Relief devices for unfired steam boilers, evaporators, and heat exchangers (modifies UG-126). ( a ) An approved safety valve set to relieve at a pressure not exceeding the “maximum allowable working pressure” of the shell must be fitted to all unfired steam boilers and evaporators except for evaporators of the atmospheric type designed for vapor discharge direct to a distiller with no shutoff valve in the discharge line. The distiller connected to atmospheric evaporators must be fitted with a vent to prevent a buildup in pressure. In no case must the vent be less than 1.5 inches in diameter. Evaporators operating between atmospheric pressure and 15 psig, may use a rupture disc as an alternative to the safety valve.
( b ) Safety valves for use on pressure vessels in which steam or pressure is generated must comply with the requirements of § 54.15-10. Rupture discs used in lieu of these safety valves, as provided for in paragraph (a) of this section, must comply with the requirements of § 54.15-13.
( c ) The relieving capacity of evaporator safety valves required by paragraph (a) of this section must be at least equal to the capacity of the orifice fitted in the steam supply to the evaporator. The orifice capacity must be determined in accordance with the formula in paragraph (c)(1) or (2) of this section as appropriate: ( 1 ) Where the set pressure of the evaporator shell safety valve is 58 percent or less than the setting of the safety valve in the steam supply: Formula 1 to § 54.15-15(c)(1) W = 51.45AP
( 2 ) Where the set pressure of the evaporator shell safety valve exceeds 58 percent of the setting of the safety valve on the steam supply: Formula 2 to § 54.15-15(c)(2) W = 105.3A√ P1(P−P1) where: W = The required orifice capacity, in pounds per hour. A = Cross-sectional area of rounded entrance orifice, in square inches. The orifice must be installed near the steam inlet or the coils or tubes and where no orifice is employed the area used in the formula must be that of the inlet connection or manifold. P = Set pressure of steam supply safety valve, in pounds per square inch, absolute. P1 = Set pressure of evaporator shell safety valve, in pounds per square inch, absolute.
( d ) The relieving capacity of safety valves on unfired steam boilers must not be less than the maximum generating capacity of the unfired steam boiler as certified by the manufacturer.
( e ) On new installations and where the orifice size of an existing unfired steam boiler or evaporator is increased, an accumulation test must be made by closing all steam outlet connections except the safety valves for a period of five minutes. When conducting the accumulation test, the water must be at the normal operating level and the steam pressure must be at the normal operating pressure, and while under this test the pressure must not rise more than 6 percent above the safety valve setting.
( f ) A heat exchanger with liquid in the shell and the heating medium in the tubes or coils, must be fitted with a liquid relief valve meeting the requirement of § 54.15-5.
( g ) ( 1 ) A heat exchanger with steam in the shell and liquid in the tubes or coils at a pressure exceeding that in the shell, must have a liquid relief valve fitted to protect the shell against excess pressure.
( 2 ) The discharge capacity of such relief valves must be calculated on the basis of the discharge from one tube using the difference in pressures between that in the shell and that in the tubes and must be not less than that determined by the following formula: Formula 3 to § 54.15-15(g)(2)
Q = 29.81KD 2√ P1−P2 where: Q = Required relief valve discharge capacity, in gallons per minute, based on relief valve set pressure. P1 = Pressure in the tube or coils, in pounds per square inch. P2 = Set pressure of the shell relief valve, in pounds per square inch. D = Internal diameter of the largest tube or coil, in inches. K = Coefficient of discharge = 0.62.
[CGFR 68-82, 33 FR 18828, Dec. 18, 1968, as amended by CGD 72-206R, 38 FR 17226, June 29, 1973]