Must Fire Pump Curve Be 5% Less than the PLD?

4/25/2024

AHJ is requesting the design curve for the fire pump to be 5% lower than the pressure limiting driver (PLD) set pressure to account for the tolerance allowed NFPA 20-2019 Section 6.2.2.2 "When operating below the rated speed in a self-regulating mode, a self-regulating variable speed fire pump unit shall maintain the discharge pressure within 5 percent of the set pressure."

Is this an appropriate application for this requirement in NFPA 20?

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4 Comments

Pete H

4/25/2024 06:48:31 am

Not sure, my reading of it would have it be anything from 0-5% to be "within 5%" rather than just requesting only 5%.

But 5% is still within 5%. So I don't think it's wrong.

Jack G

4/25/2024 08:31:13 am

It doesn’t look like an unreasonable requirement.
The code is asking for a “ safety factor “ that is within 5% of the set pressure built into the pump curve is how I interpret 6.2.2.2.

Mike Russell

4/25/2024 11:25:42 am

I guess there's an argument to be had for applying a 5% Safety to the pump curve IF your system demand places you in a point along the supply curve that is above the set-point of the PLD, but that's not typically where the demands fall... Keep in mind, once your demand pressure at the pump discharge drops below the set-point, the PLD is no longer "regulating" the pump. The pump will simply deliver it's rated capacity.

Colin Lusher

4/25/2024 12:02:05 pm

"When operating below the rated speed in a self-regulating mode, a self-regulating variable speed fire pump unit shall maintain the discharge pressure within 5 percent of the set pressure."

This means the discharge pressure is allowed to be 5% above or 5% below the PLD pressure setting. It's not clear in your statement, but it sounds to me like your AHJ is actually asking for your PLD set pressure to 5% below your desired system set pressure to account for the possibility that your PLD runs 5% over the set pressure....or am I misinterpreting that?

This does make some sense if you're operating very close to your systems maximum pressure. If your system pressure limit is 175 psi, this means you'd set your PLD to 167 psi, so that if your PLD runs at 5% over that you still won't exceed your system max pressure limit. Buuuuut, if you have a diesel pump your pump should have a main relief valve valve which would operate at any pressure above your PLD setting anyway.

In conclusion, while this doesn't seem completely unfounded, it may be unnecessary.

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Must Fire Pump Curve Be 5% Less than the PLD?

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