Ways to Limit Pump Discharge (175 psi) at Churn?

7/25/2022

Are there any good practices or other viable options allowed by NFPA 20 (other than a Pressure Limiting Driver) to limit the fire pump discharge pressure to at churn to 175psi when there is a high static pressure on the water supply?

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14 Comments

Alex

7/25/2022 05:18:46 am

Can you install a PRV immediately down stream?

Dan Wilder

7/25/2022 08:04:47 am

Talk to your pump rep to get the flattest impeller curve available or maybe (if the water supply is strong enough) increase the GPM of the pump to bring your actual demand point onto a lower part of the curve before it hits 100%. Placing a PRV immediately downstream of the pump is the only other option (verify a low flow bypass isn't required) however, make sure the jockey pump is set correctly.

If the system demand is that close, may think about either a single PRV on a lower floor or changing to 300 PSI sprinklers for that level.

Darin

7/25/2022 08:12:26 am

How about an adjustable pressure regulator at the BF device prior to the pump intake

Glenn Berger

7/25/2022 08:14:36 am

Prior to actually designing or installing any deviations to NFPA 20, you need to get the AHJs approval.

Eric

7/25/2022 08:18:22 am

I'm working on a fire pump with this particular issue atm, so good timing on the question! Your two choices are either a PRV after the pump assembly and before the transition to the sprinkler system riser, or a relief valve piped to a safe discharge location.

Both approaches have their pros and cons. For the PRV you need to make sure everything upstream of it is properly rated for the higher pressure. This includes flanges, control valves, check valves, and even the sensing line to the controller. For the relief valve you need to make sure it is sized correctly and has a safe place to discharge a potentially large amount of water, and if you are in a colder climate not create freeze risks at the discharge location.

Generally the relief valve is more of a pain since you will have to deal with it's water discharge every time the pump does it's weekly churn test.

In my case we are actually going with both due to the water pressure being unstable with occasional high static peaks. We will have the PRV set to 170psi leaving the pump room, and a 4" relief valve set to 275psi so it won't open during standard churn static pressures will if the pump churns during one of the pressure spikes we've noticed recently.

Greg

7/25/2022 08:19:17 am

If the answer is not being found within the pump driver, or pump / impeller set up, and the problem is more of a water-hammer issue, I would suggest reading the following blog on topic by Bermad, a manufacturer of pressure reducing valves and pressure regulating bladder tanks.

https://blog.bermad.com/fire-protection/reducing-water-pressure-surge-and-water-hammer-in-fire-protection-systems

David Kendrick

7/25/2022 08:26:40 am

Forgive me, I should look up the current standard.

My experience with this issue is that an upstream device has to fail in the open position.

It was a long time ago and finding a UL listed PRV that failed in the open position wasn't available.

Since then I think Inbal has a PRV that both fails in the open position and is UL listed for fire protection use.

Casey Milhorn

7/25/2022 09:08:44 am

Don't forget if you are adding a PRV before a feed to standpipes, then you must install (2) PRVs (pressure reducing valves) in series with a bypass around them. (Per NFPA 14). When feeding more than 2 hose valves.

As others have said, there are quite a few pumps out there now that have very flat curves, if not completely flat. We had one recently that actually had a 99.6% churn. There are even some multiport pumps on the market in the states now.

Also, make sure you really do need to limit the pressure to 175 psi. Many of your fittings, equipment, and sprinklers are rated to 250 psi plus now. You just have to make sure EVERYTHING is rated for whatever churn pressure you are seeing. Also, make sure you order (or adjust) the pressure relief valves at any floor zone assemblies. Typically those are factory provided at 175 psi. Also be aware that they are not known for being 100% accurate. Sometimes they have a 5 to 10 percent variance.

Great question.

Dave

7/25/2022 11:28:58 am

Be cognizant of restrictions on the use of PRV's on fire pump system piping in NFPA #20 "Maximum Pressure for Centrifugal Pumps" section (4.7.7 in the 2013 through 2019 editions). If you have the NFPA #20 Handbook, it goes into even greater detail.

4.7.7.1 The net pump shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, shall not exceed the pressure for which the system components are rated.

4.7.7.2* Pressure relief valves and pressure-regulating devices in the fire pump installation shall not be used as a means to meet the requirements of 4.7.7.1.

NFPA #20 covers through the discharge isolation valve. Downstream of that is covered by other standards.

Franck

7/25/2022 01:09:17 pm

As indicated by Dave, PRV is mandatory per NFPA 20

Greg

7/26/2022 04:03:31 pm

I'm wondering now if the question involves a supply from a elevated water tower, tank on a high-rise building, or an outside water cistern on a mountaintop ?

Is the fundamental question really, "What is the water supply graph showing theoretically at the pump intake?"

and based on that, is the pump needed ? is it in the right place on the system ?

a very curious problem to have.

Jack G

7/26/2022 05:13:05 pm

Its possible to recirculate the relief valve discharge back into the suction to control the excess pressure.
Remember the following: you must pay attention to the pump design so as not to create major problems.
An improperly designed pump with a 6 inch relief valve, and say relieving 25 psi, would be throwing out the relief drain, approximately 750 gpm. This could impact the pump suction and waste a lot of fire protection water. ( at churn)
Now if there is a fire , the relief valve will close as the system demand increases.

RAYMOND DELLING

7/27/2022 05:45:06 am

To add to the Question.

I have a scenario:
Static/Residual: 50psi/49psi
Elevation of Test and Pump is the same
Electric fire pump churns at 120psi (at factory)

What we have found is that when the pumps are field tested they 'over' perform by 5-6% and ends up churning around 126-127psi and puts the system pressure over 175psi.

What do you do in this scenario?
Install PRV?
Is 1-2psi a big deal?

What if you installed a fire pump in a building adjacent to this property 2 years ago and the flow test was 5psi less, and the fire pump was churning at 125psi?

Where do you draw the line?

Casey Milhorn

7/27/2022 11:48:08 am

In my opinion, no, 1 to 2 psi or even 5 to 10 psi isn't a big deal. We all have systems we've installed right at 175 psi churn and the pump over performs, or the water supply gets better. Many pump suppliers will go ahead and quote a 250 psi flange on the discharge of the pump when approaching 175 psi total churn, with this expectation.

You will lose 5 to 10 psi of pressure in elevation, depending on the elevation of the system at roof, and that is where most of your concern is at (with the sprinkler heads). Most fittings, pipe, and even a decent amount of valves are rated to 250 psi or more, and that's why I say it's not a huge concern. Also many sprinklers are now listed to 250 psi.

Also I put on a different hat on for new install vs an existing condition. New, I would really try and follow the book to a T, planning for future variance in water supply. After the fact, we may not have enough data to condemn and could possibly be outside the scope of NFPA 25 anyway, BUT it's definitely worth a side note if it's evident the system began life at 175 psi or below. You want to cover your tail on it for sure. I think this is one of those times you give the owner the info, maybe a price to remedy it, and they get to make the decision on their building and pocket book.

Ways to Limit Pump Discharge (175 psi) at Churn?

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