Height of Fire Pump Pressure Relief into Tank?

9/2/2021

We have a water storage tank and a diesel fire pump on our project.

Is there any requirement for the pressure relief valve discharge pipe, when connected to a supply tank, to be extended below the lowest possible water level?

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5 Comments

Alex

9/2/2021 07:02:45 am

Hi Anonymous,

NFPA 20 (2019) 4.20.3 states that the “relief valve shall be located between the pump and the pump discharge check valve” and that it should be installed so it can easily be removed for repairs. I can’t find anywhere that states that the relief has to be installed/extended below the lowest possible water level.

Thanks,
Alex

Jesse

9/2/2021 11:01:16 am

Agree with Alex here.

Dan Wilder

9/2/2021 08:31:50 am

No requirement but make sure to provide a means of draining the discharge pipe via a velocity check or manual valve if the piping discharge is above the outlet. Back pressure on teh discharge side of the valve can have negative affects on performance.

I would recommend against the setup just due to the potential of not being able to break the water discharge (ever drain a fish tank or a spa?).

John G

9/2/2021 09:06:08 am

Hmmm....most fire pump problems are suction related.
In my opinion one would want to limit the amount of air into the fire pump supply piping for cavitation reasons. By having the relief valve pipe discharge above the waterline, you are introducing air into the tank thru the discharge of the relief valve. ( water chzrning) Depending on the size of the valve, and excess pressure relieved ( q= 153 (p) *.54 : flow equals length of relief valve drain times the pressure relieved to the .54 power. The 153 will change with the actual ength and size of the relief valve. This is a 35 ft length, one 90, one 45, 6 inch relief. Relieving 36 psi in this case, can produce over 900 gpm, which hitting the top of the tank water will introduce a lot of dangerous air into your suction.
I ve tested all flows and discharges, in my installations, developing mathematical equations that actually fit the performance.
I would recomend 1 foot below the waterl evel for every 1 fps of discharge velocity. 10 fps-- 10 ft. Also have a vacuum break or air relief on the line. This is what we did 55 years ago, and was recommended practice, and has probably been " list in the
translation,". My opinion.

Jones Edward link

10/15/2021 01:23:56 am

I would advise against the setup due to the possibility of being unable to break the water discharge.

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Height of Fire Pump Pressure Relief into Tank?

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