How to Size a Relief Valve for Fire Water Pump Surges
Industrial manufacturing plants are required to provide and maintain fire water pumps to feed water hoses, sprinkler systems and deluge systems. These pumps must be checked weekly for proper operation and adequate flow. Many sprinkler and deluge systems have certain pressure ratings that must be protected against fire water pump surges. Pressure relief valves are generally installed for this protection.
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Instructions
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Determine the maximum allowable working pressure (MAWP) of the sprinkler or deluge system. This is provided by the system's manufacturer. For instance, assume the system has a MAWP of 150 psi (pounds per square inch). The relief valve will be set for 150 psi.
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Determine the fire water pump dead-head pressure (maximum pressure the pump can deliver) and the corresponding capacity in gallons per minute. Assume the dead-head pressure is 175 psi with a capacity of 500 gallons per minute. This would be the pressure surge that could damage a deluge system.
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Apply the API (American Petroleum Institute) sizing formula for liquids. The sizing for a relief valve is based on the minimum interior orifice required to relief the capacity of the pump. The formula is A = Q / (38 x Kd x Kw x Kc x Kv) x √(G/(P1 - P2)), where A is the minimum required orifice area in square inches, Q is the pump flow rate in gallons per minute, "38" is a conversion factor, all "K" variables are sizing coefficients usually equal to 1.0, G is the liquid specific gravity (for water, G = 1.0), P1 is the pump dead-head pressure and P2 is the relief valve set pressure.
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Determine the minimum relief valve orifice size. This is calculated by A = 500 / (38 x 1.0) x √(1.0)/(175-150), which equals 2.6 square inches. Therefore, a relief valve that has a minimum orifice area of 2.6 square inches is required to relieve the pressure surge.
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References
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