Pressure setting limitations under NFPA 20 + NFPA 13 sprinkler design 3

[annoynimous]

Dear all,

As we are complying to NFPA standards as close as possible, one of the confusion starts is with NFPA 20 2019 Annex A A.14.2.6 (4):

Fire Pump Settings. The fire pump system, when started by
pressure drop, should be arranged as follows:

(a) The jockey pump stop point should equal the pump
churn pressure plus the minimum static supply
pressure.


Assuming that our pump churn pressure will be 14 Bar, and minimum static supply remains 0, our final jockey pump stop point shall be at 14 bar.

However, when crossing over to NPFA 13 rules, the sprinkler pipeline pressure shall not exceed 175 PSI (12.1 Bar). In this case, how can we formulate a pump pressure setting that will comply both NFPA standards?

I also understand the provision of PRV's to regulate this pressure back to 12.1 Bar, but if the PRV's keep relieving pressure constantly in the system pipeline, wouldn't the jockey pump re-activate constantly?

Thanks all

 

[UFT12]

Such issues may not be answered the best way simply focusing on PRVs etc. They require thorough investigation and may not be able to get answered through a forum.

For example, before talking about PRVs, are you sure you cannot choose a pump below 14 bar? Would it be possible to size your network so that you could get away with it with a lower pump? Depending on system size, this may prove a more economical choice.

If you cannot skip with this pump, you could consider the following

a) a pressure limiting driver controller that keeps the discharge pressure to a fixed predetermined limit (although a PRV may be needed here).

b) use components rated higher than 14 bar (~200 psi), at least for the portion of the system that receives the higher than 12 bar (175 psi) pressures.

There may be more options depending also on what is your water supply. By the way, main PRVs for pumps are typically not intended to deal for the issue you describe and are mainly used for engine over-driven pumps. See NFPA 20 commentaries.

 

[annoynimous]

hi UFT12, the system design pressure is set at 175 PSI. The churn pressure is always higher than system pressure.

But because we are basing on a formula given by NFPA 20 Annex A, whereby the jockey pump cutout setting shall be anything at churn pressure, we feel that this sort of information is causing a paradox situation against the limitations of NFPA 13 on the subject of 175 PSI. Please note the PRV's that I mentioned, are also found at the downstream of check alarm valves under NFPA 13...

for your option b), I am not sure what you meant, if the entire sprinkler wet pipeline is pressurized, I am assuming any drop or rise in pressure will be passed downstream?

 

[cdafd]

Not into pumps,,,,

But are you mixing together requirements, that in away do not need to be mixed,,,



However, when crossing over to NPFA 13 rules, the sprinkler pipeline pressure shall not exceed 175 PSI (12.1 Bar). In this case, how can we formulate a pump pressure setting that will comply both NFPA standards?



You do not give section references, but above I take is the limit pressure on the piping, and if a system exceeds that, you have to go to higher pressure rated pipe.

 

[UFT12]

for your option b), I am not sure what you meant, if the entire sprinkler wet pipeline is pressurized, I am assuming any drop or rise in pressure will be passed downstream?

See specs of the components. Do get listing (certification) they need to be rated for a pressure limit. There a various limits depending on your needs.

 

[LowlySprnkLayoutTech]

This situation was not envisioned (or even allowed) when the appendix information was written. The appendix information provided has not caught up with use of PRVs. Keep in mind the appendix is not the code, just recommendations. In this case your trying to apply a recommendation that does not fit your situation, you need to engineer a solution.

 

[annoynimous]

hi @LowlySprnkLayoutTech , your advice makes the most sense of the situation, we have proceed with our own solution, thanks all!