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Why is pressure relief on fire pumps not allowed, except (major exception) below discharge valve?

Question asked by daniel.lewis on Apr 2, 2019
Latest reply on Apr 5, 2019 by sglx

From NFPA 20, why are pressure relief valves not allowed to control high churn pressure of fire pumps (Sec 4.7.7.2)?   And then what is the meaning of the exception A.4.7.7.2 that then allows pressure relief downstream of the discharge isolation valve?

 

Reference attached:

4.7.7 *  Maximum Pressure for Centrifugal Pumps.

4.7.7.1  The net pump shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, shall not exceed the pressure for which the system components are rated.
4.7.7.2 * Pressure relief valves and pressure regulating devices in the fire pump installation shall not be used as a means to meet the requirements of 4.7.7.1.

* A.4.7.7.2   It is not the intent of this subsection to restrict the use of pressure reducing valves downstream of the discharge isolation valve for the purpose of meeting the requirements of 4.7.7.

 

 

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