|Circulation Relief Valve|
First let's clarify that we are taking about a main pressure relief valve and not a circulation relief valve. A circulation relief valve is generally 3/4-inch in size if less than 3,000 gpm and intended to provide a little fresh water into the pump casing for cooling purposes.
|Pressure Relief Valve|
So how does this 10% maximum permitted over-speed RPM relate to the output pressure? The speed (RPM) of a fire pump and the pressure developed do not increase linearly. That is if you double the RPM you do not double the pressure. When you double the speed you actually get four times the pressure. The formula is written as follows:
|Pump Speed vs. Pressure Affinity Formula|
|Pump Speed vs. Pressure Affinity Formula - Rearranged|
NFPA 20 – 2010 Edition
184.108.40.206 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.
FM Global Standard 3-7 (Fire Protection Pumps – May 2010)FYI - Remember that utilizing a Pressure Relief Valve to reduce discharge pressures is not permitted by NFPA 20 section 220.127.116.11. Designing system to discharge large amounts of water every time it runs is not considered good design practice and code recognizes this fact.
2.3.3 Pressure Relief Valves
18.104.22.168 Provided a main relief valve on all diesel engine-driven fire pumps when 121% of the net rated shutoff (churn) pressure plus the maximum pump static suction pressure exceeds any system component rated pressure.