Pump Relief PCV Sizing

[b7031719]

If I wanted to size a regulator that is to be installed in bypass around a pump i.e. back-pressure regulator and I know the desired back-pressure how do I determine the size of the valve based on the pump curve?

 

[LittleInch]

This doesn't sound right.
Back Pressure is normally used to "hold back pressure", i.e. it is the pressure upstream of the valve which is the control point. Doing this on a bypass around a pump won't hold any pressure back. If you want to regulate the pressure coming out of the pump then this isn't a good way to do it.

what are you trying to do? Regulate pressure downstream of the pump? Or somewhere else?

What type of pump ~(positive displacement or centrifugal?)

Is flow always there?

Are the pressures (inlet and outlet constant?)


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[b7031719]

Hi thanks for the reply. This is a pump governor regulator e.g. Fisher 1B-R for regulating system pressure downstream of the pump (positive displacement). The system demand varies quite a bit (fire water system supplying deluge in various zones).

 

[LittleInch]

Aaaah Ok.

Point 1 - If you're messing around with a fire water system you need to follow NFPA20 and the valve (called a dump valve by NFPA) needs to be "listed", i.e. certified by UL. Whether your fisher 1-BR is officially listed by UL I don't know - you would need to check.

Point 2 - A PD pump doesn't have a pump curve per se as the volume is fixed by the speed of the pump so the "curve" is a vertical line at the rated flow of the pump.

So the sizing of the valve in terms of flow is the maximum flow from the pump when there is no forward flow from the pump to the fire water system with differential pressure based on whatever pressure you want to maintain in your system.

Fire water pump systems often need weekly or monthly operation testing for 30 minutes.



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

 

[b7031719]

Hi thanks again for the reply. I got this wrong actually it is a centrifugal pump although I doubt this changes things much?

I do have a copy of NFPA 20 and will take a look at the requirements in there thanks for the reminder.

 

[LittleInch]

Well I'm not sure NFPA allows for these valves on centrifugal pumps.

The problem is that they don't want these valves to malfunction and drain fire water when you need it.

NFPA are fairly brutal with pumps and in my experience don't allow you to circulate water during a no flow test so the pump tends to heat up quite a bit....

see section 4.7.7

Basically don't mess with a Fire water system unless you really know what you're doing otherwise it will come back to bit you. Fire water packages are normally designed and purchased as a single item which is approved and you can't modify it.

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

 

[b7031719]

OK so the weekly test is supposed to be done using some manual dump then opened for testing purposes only?

 

[LittleInch]

NO, they just pump against a closed valve / system.

Like I said NFPA are rather brutal on pumps. You could open up a hydrant somewhere which flows back to the pond, but NFPA 25 8.3.1.2 says " A no flow test shall be performed...

https://www.nfpa.org/codes-and-stan...ds/list-of-codes-and-standards/detail?code=25

NFPA codes are free to view on line.

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

 

[b7031719]

Thanks for your help this is interesting. I will contact the original pump manufacturer to get some more advice.

 

[LittleInch]

Yeh, NFPA codes and standards are literally a law unto themselves and are regarded by many as though they are written on tablets of stone. Don't mess with them - it's not worth it.

There is a dedicated forum on this site in the engineering codes and standards on NFPA.

what's the back ground here? - retrofit? new build? some sort of modification?
and as a electrical engineer ?

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

 

[b7031719]

Yes actually it is a modification that I would like the pump manufacturer to supply I am just interested in the process. There is a closed-loop pressure control system doing this job currently but fitted back in the early 80s and is in need of replacement. I was thinking a regulator would be adequate but perhaps not to latest code from what I have learnt.