NFPA20 Relief Valve discharge piping returning back to water source

[annoynimous]

While designing the main relief valve discharge piping, we decided to connect this pipe to the downstream of the test pipe that is returning to an above ground storage tank.

However, the return pipe going into the storage tank is elevated above the return pipe itself.

If the test pipe were to have water flowing back to the water storage tank but the PRV is not activated, would there be a backflow direction to the PRV discharge piping that affects the PRV system?

the PRV is a CLA-VAL 2050B-4KG1 Angle

Thanks all!

 

[LittleInch]

Yes.

You need to consider both the static pressure on the discharge of the PRV in the set point calculation and the flowing back pressure at full relief flow so you don't over pressure the protected system. Usually max 10% above set point of the PRV.

Is that what you meant?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[annoynimous]

Hi LittleInch, thanks for the response.

I think an illustration on my concerns would be easier to understand as indicated by the red arrows in below image:

Basically I'm not sure if the PRV/MRV of this type can prevent such as above from happening as the source of backflow can come from the downstream of the test pipe (NFPA20: 2019 clause 4.20.6.4)

But we were guessing that the pressure at the downstream of test pipe would never be higher than the inlet of the MRV, hence the valve will never open for backflow to occur.

Would like to get some advice on the functionality of these type of MRV's, thank you.

 

[UFT12]

I can't imaging how the pressure exerted downstream the PRV can be higher than the pressure upstream if you follow the sizes of NFPA 20 (4.28) and given that your test piping is always downstream the PRV (considering also equipment such as check & shut-off valves in the way). Therefore I find your guessing valid. Normally the flows during pump testing are fairly large and therefore the PRV shouldn't be able to trip.

 

[TravisMack]

I think they may be talking about the rare situation where you pipe the relief valve and the flow meter back into the suction piping. If I am understanding correctly.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, a Ferguson Enterprise

http://www.mfpdesign.com