I believe that K-25.2 ESFR sprinkler is listed with a 175 psi max. working pressure. With a 50 psi pressure, you are most likely dealing with 30 to 35 ft storage in a 40 ft high building, so the sprinkler would be at a max. elevation of about 39 ft and the highest churn pressure you could ever be allowed would be:
175 psi + 39 ft x 0.433 psi/ft = 191.9 psi. If there are any sprinklers at lower elevations, under obstructions, etc. then even lower max. pressures would be allowed. Therefore you are not allowed to sustain the 211 psi static under any circumstances. My suggestion is to increase the pipe sizing and decrease the pump pressure rating. Also, there is no rule against it in the standard, but I've always felt it to be a poor practice to push towards the 150% of pump flow rating. If the flow demand goes past the 120% +/- range, I recommend going to the next available rating. Larry Keeping From: Sprinklerforum [mailto:[email protected]] On Behalf Of Colin Carew (Cambridge) Sent: April-06-17 4:54 PM To: [email protected] Subject: RE: NFPA 20 4.7.7.2 140 is the pump rating , we sitting at 211 static . I understand the premise , 4.7.7.2 says I am not allowed to do it . but the biggest challenge is that my system design is 12 heads at 50 psi , K25.2 . I am just squeaking under Demand . I am not sure what other option I have . From: Sprinklerforum [mailto:[email protected]] On Behalf Of Prahl, Craig/GVL Sent: April-06-17 4:32 PM To: [email protected]<mailto:[email protected]> Subject: RE: NFPA 20 4.7.7.2 Is the 140 psi total pressure or pump rating? The pressure relief valve comes into play if the combination of supply pressure and pump pressure exceeds the allowable system pressure, which for most cases is 175 psi. So if you had fluctuating supply pressures, during a higher than normal pressure event, the system could relieve and not damage your downstream, 175 psi rated system. Is this a diesel or electric? How much pressure do you need at these heads? Craig L. Prahl Fire Protection Group Lead/SME CH2M 200 Verdae Blvd. Greenville, SC 29607 Direct - 864.920.7540 Fax - 864.920.7129 CH2MHILL Extension 77540 [email protected]<mailto:[email protected]> From: Sprinklerforum [mailto:[email protected]] On Behalf Of Colin Carew (Cambridge) Sent: Thursday, April 06, 2017 4:21 PM To: [email protected]<mailto:[email protected]> Subject: NFPA 20 4.7.7.2 [EXTERNAL] Hi Forum , Could someone shed some light on the requirements for 4.7.7.2 . I have a project where the quoted fire pump comes with a Pressure relief valve and waste cone . City pressure is 58/56/1586 , we are using a 1500gpm pump at 140 psi to get the flow and pressure we need to supply a freezer using LP-46 heads . I find many references to the use of Pressure relief valves being Used but 4.7.7.2 is causing me to scratch my head . Colin
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