When is a Fire Pump Pressure Relief Valve Req'd?

2/7/2024

When is a fire pump pressure relief valve required?

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11 Comments

Dan Wilder

2/7/2024 06:45:32 am

NFPA 20 - 22' (similar in previous editions)
Section 4.20.1
Sized per 4.28

Summary
-Centrifugal Fire Pumps, typically only diesel configurations
-Electric with variable speed or diesel with pressure limiting devices installed
--In both cases - the max total discharge exceeds the working pressure of the downstream components

Pete D

2/8/2024 12:43:14 pm

Just want to tag on one exception that is infrequent, but I have dealt with it. Sometimes for a very tall warehouse we size pumps to exceed sprinkler rated pressure at the discharge flange, but at the sprinkler installation elevation the pressure is at or below the rating. This is done to avoid purchasing high pressure rated sprinklers.
The distinction is in the term "working pressure" which I supposed takes the elevation of the installation into consideration, where rated pressure would be the value listed in the data sheet.

Pete H

2/7/2024 06:49:59 am

For pumps I'd check NFPA 20 (2016 edition) 4.19 and 4.26.5 while keeping 4.7.7.2 into account.

It also may be required on your wet pipe systems as per NFPA 13 (2016 edition) 7.1.2, 7.2.6.5, or 8.16.1.2.5

Ray Hardy link

2/7/2024 08:06:31 am

I thought it was when the pump closed valve pressure exceeded 12 bars?

RayJ link

2/7/2024 08:27:16 am

Along time ago, I was younger and out drinking, when I exceeded 12 bars, I needed an open pressure valve.

Glenn Berger

2/7/2024 08:11:52 am

Dan nailed the NFPA 20 requirements. One thing that I have noticed in some installations is that the additional "relief valves" do not get installed. By this I mean the casing relief valve, etc.

Dan Wilder

2/7/2024 11:11:39 am

So I ran into this on a couple jobs, if the cooling water for the pump (diesel) is pulling water from the discharge side of the impeller it acts like the casing relief.

See 4.13 for Circulation Relief, specifically
4.13.1.8 - The requirements of 4.13.1 shall not apply to engine-driven pumps for which engine cooling water is taken from the pump discharge.

Jack G

2/7/2024 08:18:39 am

Pressure relief valves are not permitted to relieve pressure by dumping large amounts of water.
Design must factor this in before choosing your pump.
A relief valve can be eliminated if 1.21 times the rated pressure plus churn pressure is less than rated system components pressure rating.

Jesse

2/7/2024 09:23:38 am

Dan nailed it, but RayJ is also right...

franck

2/7/2024 11:29:57 am

Indicated above (including references).
But based on 1.21 x pressure at churn + sucrion pressure (from water tank, it can be a few psi additionnal).

As most sprinklers are rated at 175 psi (12 bars), this is the limit. This means that most diesel pumps that are rated at 130+ psi need a PRV.
The 1.21 factor dors not apply for electric as there is no possibility of overspeeding (pressure increases if rotational speed increases).

Note that a PRV should normally not operate under normal conditions, but only unusual event (safety device). This is not a pressure regulating device as outlined by Jack.

KH LIM

7/31/2024 09:07:41 am

Could anyone share how to size the pressure relief valve (hydraulic way) for diesel engine fire pump?

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When is a Fire Pump Pressure Relief Valve Req'd?

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