Pressure Relief Valve for Centrifugal Pump as per NFPA-20

What is a Relief Valve (RV): A device that allows liquid diversion to limit excess pressure in a system.

Why is a relief valve for fire pumps required? A Relief Valve is required by the standard to be installed when the diesel engine is turning faster than normal because the pressure is proportional to the square of the pump’s speed.

Where can we install RV: It will be installed in only two cases:

  1. For diesel engine fire pump: Where a total of 121% 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.
  2. For electric variable speed pump or a diesel pressure limiting driver: Where 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

It should be noted that the 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.

RV Installation 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.

( Installation location and types of Relief Valves)

Types of RV: Pressure relief valves shall be either a listed spring-loaded or a pilot-operated diaphragm type.

RV Discharge: The relief valve shall discharge into an open pipe or a cone or funnel secured to the valve outlet. Water discharge from the relief valve shall be readily visible or easily detectable by the pump operator. If a closed-type cone is used, it shall be provided with means for detecting water motion through the cone.

Size of RV: The relief valve size shall be determined by one of the methods specified:

  1. The relief valve shall be permitted to be sized hydraulically to discharge sufficient water to prevent the pump discharge pressure, adjusted for elevation, from exceeding the pressure rating of the system components.
  2. If the relief valve is not sized hydraulically, the relief valve size shall not be less than that given in Section 4.27.
( Sizes of Relief Valves )

Installation details about RV:

  • If the discharge pipe employs more than one elbow, the next larger pipe size shall be used.
  • A shutoff valve shall not be installed in the relief valve supply or discharge piping.
  • Relief valve discharge piping returning water back to the supply source, such as an above-ground storage tank, shall not be combined with the discharge from other relief valves. qRelief valve discharge piping from a single fire pump returning water back to the supply source shall be permitted to be combined with fire pump test piping downstream of any control valve.
  • The discharge piping from multiple relief valves shall not be combined with fire pump test piping. qIsolation of the test header piping from the pressure relief valve discharge shall be possible.
  • where a pressure relief valve has been piped back to suction, a circulation relief valve discharges to the atmosphere shall be provided downstream of the pressure relief valve, and shall actuate below the opening set point of the pressure relief valve to ensure cooling of the pump during churn operation.
  • 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 to prevent the pump from drafting air introduced by the drainpipe discharge.

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References of this article:

  • NFPA-20 (2016 Edition)
  • Google search engine

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