Consult the pump manufacturer (if you don't have the information) and find out what the minimum acceptable NPSH is for that pump. It can vary based on pump design and selection criteria but if the suction pressure falls below the mfgrs. NPSH you can end up with pump cavitation and destroy the pump.
Craig Prahl | Jacobs | Group Lead/SME – Fire Protection | 864.676.5252 | [email protected] | www.jacobs.com 1041 East Butler Road Greenville, South Carolina 29606 -----Original Message----- From: Sprinklerforum <[email protected]> On Behalf Of Steven Jenkins via Sprinklerforum Sent: Friday, December 4, 2020 4:57 PM To: [email protected] Cc: Steven Jenkins <[email protected]> Subject: [EXTERNAL] pump suction presure We have an existing horizontal split case fire pump and ground level water storage tank. The storage in the protected building is changing, and the amount of water needed for fire protection is going up. We are changing the tank to a “break tank”, but even then there is not much water left in the tank after the required supply duration, only a few feet left in the bottom of the tank. The pump is right next to the tank, so not much loss due to friction in the connecting pipe. With the tank full, before the pump is turned on, the gauge on the suction side reads about 15 psi, due to the height of water in the tank. When the pump is running at 150%, this drops to about 7.5 psi. At the end of the required duration I will only have about 1.5 psi of head from the height of water in the tank. So if I lose another 7.5 with the pump on, the gauge would read -6 psi. Is this a problem for compliance with 2019 NFPA 20, or since the pump is already running does this make the negative 6 psi gauge reading OK? 2019 NFPA 20 4.16.3.2 Thanks, J _______________________________________________ Sprinklerforum mailing list [email protected] https://urldefense.com/v3/__http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org__;!!B5cixuoO7ltTeg!Vvw022XTXSIIvtVzOCph3uP9bV0FAoMdghl5xCgOCTO0QdLy_jgO4PGM048eiTVf5A$ ________________________________ NOTICE - This communication may contain confidential and privileged information that is for the sole use of the intended recipient. Any viewing, copying or distribution of, or reliance on this message by unintended recipients is strictly prohibited. If you have received this message in error, please notify us immediately by replying to the message and deleting it from your computer. _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
