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Pressure Relief Valve – Section 4.20

The relief valve is not intended to control pressure when a fire pump has been overdesigned; it is intended to relieve pressure when a diesel engine is turning faster than normal, or when failure of the variable speed controls causes a pump to operate at rated speed.

4.20.1.1* Pressure relief valves shall be used only where specifically permitted by this standard.

A.4.20.1 The pressure is required to be evaluated at 121 percent of the net rated shutoff pressure because the pressure is proportional to the square of the speed that the pump is turned. A diesel engine governor is required to be capable of limiting the maximum engine speed to 110 percent, creating a pressure of 121 percent.

4.20.1.2 Where a diesel engine fire pump is installed and where a total of 121 percent of the net rated shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, exceeds the pressure for which the system components are rated, a pressure relief valve shall be installed.

4.20.1.3 Where an electric variable speed pump or a diesel pressure limiting driver is installed, and the maximum total discharge head adjusted for elevation with the pump operating at shutoff and rated speed exceeds the pressure rating of the system components, a pressure relief valve shall be installed.

4.20.1.3.1 Where an electric variable speed pump or diesel pressure limiting driver is used, the pressure relief valve shall be set to a minimum of 10 psi (0.68 bar) above the set pressure of the variable speed pressure limiting control.”

4.20.2 Size. 4.20.2.2 If the relief valve is not sized hydraulically, the relief valve size shall not be less than that given in Section 4.28. (See also 4.20.7 and A.4.20.7 for conditions that affect size.)

4.20.3 Location. The relief valve shall be located between the pump and the pump discharge check valve and shall be so attached that it can be readily removed for repairs without disturbing the piping.

4.20.4 Type. 4.20.4.1 Pressure relief valves shall be either a listed spring loaded or a pilot-operated diaphragm type.

4.20.8 Discharge to Suction Reservoir. Where the supply of water to the pump is taken from a suction reservoir of limited capacity, the drain pipe shall discharge into the reservoir at a point as far from the pump suction as is necessary to prevent the pump from drafting air introduced by the drain pipe discharge.