Fire pump, PRV piped back to suction 3

[DavidCR]

Please see the diesel pump installation in the attached drawing.

Due to site limitations, the suction tank is above and not beside the pump house. The hydrants and sprinklers are pretty far and in a lower location. A pressure relief valve is needed to protect the accessories of the system.

Initially the owner told us that the PRV discharge could be wasted to an open channel, so the design and works where built so. Now the owner says that there are problems with this, and in a near future he will have to pay for the water, so he wants the PRV discharge piped back to the suction tank, it is complicated now to build a pipe back to the tank.

From the design point of view we are reluctant to have a PRV with a 40´head min of backpressure. We don´t want to meet particular conditions for the PRV operation (a new thing for us, so we don´t know if this is going to be a problem).

An option to solve this is to pipe the PRV discharge back to the fire pump suction as per NFPA 20, new paragraph 4.18.7.1 in the 2010 Ed.

“Where a pressure relief valve has been piped back to suction, a circulation relief valve sized in accordance 4.11.1.6 and discharged to atmosphere shall be provided downstream of the pressure relief valve.”

I´m not sure if the NFPA20 mentions this PRV loop in other paragraphs.

On piping back the PRV discharge we have heard some opinions, some say that it is OK. A pump manufacturer tech guy said that it was not recommended, and the pump warranty could be compromised. The pump seller says that if it is in the NFPA 20, it is OK. And we´d like to understand the implications of a looped PRV discharge.
I´m curious, why this point is new on the standard, to pipe the PRV discharge back to suction, in general, seems to be a great solution to save money on the PRV discharge pipe.
So my questions, to you guys are:
1. Do you think the system as described in the drawing is OK?.
2. Does anybody know about the implications, risks of a looped PRV discharge?
3. What is the function of this circulation relief valve?
And of course,
4. any related comment will be highly appreciated.

Thanks a lot in advance.

 

[DavidCR]

Sorry I´m not sure if the file is included well...

http://files.engineering.com/getfile.aspx?folder=f0b41129-6b30-493b-8990-0e46fa4a0891&file=P1.pdf

 

[DavidCR]

Guys. Help. I really appreciate some help on this.

There is a problem when I try to include the link in the post, but the file is in the link

http://files.engineering.com/getfile.aspx?folder=f0b41129-6b30-493b-8990-0e46fa4a0891&file=P1.pdf

 

[TravisMack]

1. Do you think the system as described in the drawing is OK?.

It looks like arrangements I have seen in the past

2. Does anybody know about the implications, risks of a looped PRV discharge?

I have seen it done in the past without any known complications during testing.

3. What is the function of this circulation relief valve?

I was told it was to bring a constant stream of "cool" water into the system. You will get things pretty hot in that circulating loop. Whether or not it is true, I can't be certain, but it does make sense.



Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Pumpsonly]

Technically it is alright to pipe the PRV back to the suction line. But you will not be able to run the pump under such condition for extended period as the recirculated water will get heated up quite quickly. As pointed out by Travis the circulation relieve valve may relieve some water in the loop to ensure a continuous stream of cool make up water is drawn into the loop.

In sizing the PRV, you still need to take into account the back pressure it will encounter at the connection point into the suction piping due to the static height of the suction tank water level.