If the supply is capable of supporting 825 GPM, then we are back to your original question of whether you can pull that through the 400 GPM fire pump. Practical experience says that you MIGHT be able to, but you would be requiring the pump to operate at more than 150% of its rated capacity, which would violate NFPA 20.
Is it feasible to add a bypass around the smaller pump? If so, I think you could make a good argument that you are within code. Then it just comes down to whether or not you have big enough pipe to get you flow to the suction side of the big pump while maintaining positive suction pressure, or if friction loss at those flows will keep you from doing so. Does the site loop have more than one connection to the city main (or wherever the water supply comes from)? If so, that second connection can count as the fire pump bypass, even though you don’t have a traditional bypass in the pump room. -Kyle M at Aero From: Fpdcdesign <[email protected]> Sent: Monday, October 6, 2025 11:22 AM To: Discussion list on issues relating to automatic fire sprinklers <[email protected]> Subject: [Sprinklerforum] Re: [External]Multiple undersized pumps. Kyle, As I mentioned, there are several issues with this site I didn’t mention. One is approximately 1000 ft of 80 year old, severely tuberculated 8” main (estimate C=30) that feeds the site. The first thing is that this needs to be replaced. Once that is done, the 825 is very possible. CW, I have all of that info. I have been working on this since 2021 and it just came back to life after a year away. 825 gpm is the sprinkler demand only. There is a raw water supply that would be used for hose streams. I hesitate to post many specifics of the project here as what is going on could result in litigation. Todd G Williams, PE Fire Protection Design/Consulting Stonington, CT 860-535-2080<tel:860-535-2080> (ofc) 860-554-7054<tel:860-554-7054> (fax) 860-608-4559<tel:860-608-4559> (cell) On Oct 6, 2025 at 11:26 AM, <Kyle.Montgomery<mailto:[email protected]>> wrote: “Neither pump has ever passed its initial test due to the water supply”. So, the water supply can’t even provide 600 GPM to the little pump? If that’s the case, there is no workaround other than adding a water tank. You can commission the pump without reaching 150% of the rated capacity, but you have to at least meet the demand flow and pressure. If you can’t get to 600 GPM, you certainly aren’t getting to 825 GPM. -Kyle M at Aero From: Fpdcdesign <[email protected]<mailto:[email protected]>> Sent: Monday, October 6, 2025 7:24 AM To: Sprinklerforum <[email protected]<mailto:[email protected]>> Subject: [External][Sprinklerforum] Multiple undersized pumps. I am working on a project for a facility with several small and medium sized buildings. Only some of the buildings are sprinklered. This location has a plethora of problems but I only have one that i will bring here. The facility has a very bad water supply and a new building was constructed, complete with fire pump, to meet a demand of 825 gpm. Also on this site is a fire pump house with a 400 gpm at 75 psi pump that takes suction from the main. I was told this pump only feeds a dedicated main to 3 buildings. However, I subsequently found that the pump actually ties into the underground feeding the entire site, including the new building. Now I have a main fed by a pump with a maximum flow of 600 gpm feeding a 750 gpm pump. Is there any way to get this to work? There is no by-pass around the 400 gpm pump. Not sure if that would make a difference. I would appreciate input from those mightier than I. Neither pump has ever passed its initial test due to the water supply. Todd G Williams, PE Fire Protection Design/Consulting Stonington, CT 860-535-2080<tel:860-535-2080> (ofc) 860-554-7054<tel:860-554-7054> (fax) 860-608-4559<tel:860-608-4559> (cell) _________________________________________________________ SprinklerForum mailing list: https://lists.firesprinkler.org/list/sprinklerforum.lists.firesprinkler.org To unsubscribe send an email to [email protected]<mailto:[email protected]>
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