Fire Pump Suction Issues
Fire Pump Suction Issues
(OP)
I have a ESFR system that requires 1370 gpm from the pump. The supply is 73 static and 44 residual. Pleanty of water to make this happen. Problem is, the municipality wants to keep the supply line at 20 psi or greater. This only gives us 1320 gpm. They are suggesting we put a valve in to limit our suction. By our calculations our demand would put the suply pressure in the 14 - 15 psi range. Have any of you ran into a similar situation like this and how did you resolve it short of a storage tank?
I just can't find the logic of the 20 psi minimum requirment. 45% just seems to be a high saftey factor
I just can't find the logic of the 20 psi minimum requirment. 45% just seems to be a high saftey factor





RE: Fire Pump Suction Issues
Travis Mack
MFP Design, LLC
www.mfpdesign.com
RE: Fire Pump Suction Issues
I never had this same problem you are discussing but in fact 20 psi is what water companies will allow.
RE: Fire Pump Suction Issues
NFPA 24 piping can go down to 0 psig.. Everything on the potable water side has to stay at 20+ psig. What you need to do is consider a double check BFP in a pit, as close to the city water main as possible. After you go through the BFP, then you can draw that sucker down to 0 psig.
Also I don't know how the supply to the break tank wouldn't also draw down to <20 psig. Once the float valve opens it will draw out whatever can physically flow out. So its likely to pull more than 1320 gpm, since there will be essentially zero back pressure. And if it pulls more than 1320 gpm you are not better off.
For a break tank to work, it would have to be elevated by a height, such that the underground would stay at 20+ psig.
RE: Fire Pump Suction Issues
Consider an old water main that is normally leaking under positive pressure. The worry is drawing in water from the soil. Local to its joints, a fast velocity (high velocity pressure) in the main can reduce the normal pressure acting on the joint to lower than 20 psig. But even if the velocity were 20 ft/sec that would only be 3 psig velocity pressure, or 17 psig normal pressure.
The other argument is that the water mains could collapse if they aren't kept pressurized to 20 psig. The argument against that is, why don't they collapse when they are shutdown for construction work?
I have to think the 20 psig regs were tribal knowledge handed down, probably some guy way back when pulled a round number out of the air, maybe overkill, but that doesn't mean its not in contract.
RE: Fire Pump Suction Issues
what happens to the pump once you get down to 0 to 5 psi range??
and if you install it with a low psi, and over the years water pressure degrades in the city main, then what?? do you have the required stuff to drive the system???
RE: Fire Pump Suction Issues
RE: Fire Pump Suction Issues
RE: Fire Pump Suction Issues