Fire Pump Suction Issues

[669Sprink]

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

 

[TravisMack]

I believe that 20 psi comes from AWWA guidelines, or something like that. The water purveyor will likely be unbending on that requirement. If you have 1320 from the city, you could do a relatively small break tank. You only need to augment 50 gpm for 60 minutes. Even going conservative and figure 100 gpm for 60 minutes + city supply with an automatic float valve on the tank, you would only need a 6000 gallon tank.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[NJ1]

I reality the 20 psi rule is a myth that water companies established to prevent piping from being cavitated. That is the reason why I do not perform any pump test below 20 psi.
I never had this same problem you are discussing but in fact 20 psi is what water companies will allow.

 

[pipesnpumps]

One thing you could is separate the NFPA 24 requirements from the AWWA or state environmental requirements.

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.

 

[pipesnpumps]

The 20 psig is said to be required to protect the water supply.

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.

 

[cdafd]

not a pump expert

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???

 

[NJ1]

cdafd nothing really happens to the pump when suction goes bellow 10 psi. The ratio of fire pumps cavitating is 1 out 100. Mains cavitating? Unless you are in a very old city is likely not to happened but the problem is if it does happened you will pay for the repairs. The water company will blame your firm and that is the end of it.

 

[669Sprink]

I guess the most frustrating part is that the city will be adding on to the main in the future and looping it back to another exsisting main. This loop should aliviate the situation but they are not wanting to make an acception here for this, in their own estimation, temporary problem. This is a class one commodity stored on racks. I will probably win the lottery the same day as this system flows 12 heads. I would bet a pay check the first head to go off is one that gets hit with a fork lift.

 

[Oremus]

The difference in gpm you're talking about is like a 2.5 psi difference on a pitot guage (Assuming a 2 1/2" orifice). I would re-flow the hydrants (maybe even two different times of day) to make sure the first test was accurate.