Use 140% Churn Pressure with Fire Pump Curve?

9/27/2019

I have a fire pump rated at 100 psi at 1000 GPM. The curve for the pump is starts at 125 psi and goes to 0 GPM. I know 140% is the 125 PSI. On my hydraulic calculations should I use the 140% pump curve or should I use the 100% Pump curve? Where in NFPA 13/20 can I find this guidance?

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12 Comments

Phil Watkins

9/27/2019 10:29:59 am

I think you will find that you will have one pump curve from the manufacturer which should be produced to meet your system demand.
If you advise the pump manufacturer of your largest system pressure and flow demand you can expect a curve from them to suit that particular demand that falls within the rated capacity of the pump (up to 140%)

This is for fully hydraulically calculated systems of course.

I hope this makes sense!

ioan rincu

9/30/2019 02:13:16 pm

I have never seen a fully hydraulically calculated system.

Cameron

9/27/2019 10:44:05 am

Typically you can develop your curve using three points. Churn, 100% and 150%. It sounds like you may have indicated that you have 125 psi at 0 gpm (churn), 100 psi at 1000 gpm (100%), then you would need to reference the manufactures curve to see what you have at 1500 gpm (150%). That should give you a curve to reference in your design. If you are only being allowed to use up to 140%, then see what your pressure is at 1400 gpm and don't let your demand exceed that point.

Greg Stoner

9/27/2019 10:53:41 am

I believe this is accurate: A fire pump shall develop at least 65% of its rated head at 150% of rated capacity and shall not exceed 140% of it's rated head at zero capacity..
In your example, a fire pump rated for 100 psi @ 1000 GPM shall develop at least 65 psi @ 1500 GPM. Church psi= 140

Anupam Majumdar

9/27/2019 11:01:24 am

The hydraulic calculation should with rated capacity & head. No other consideration like 150% or 65& etc should not considered. after calculation if it founds that same is not fulfilling at 100%, then designer should re-calculated with revised pipe sizes.

Colin

9/27/2019 01:51:11 pm

Concur with Cameron. It sounds like at 0gpm (churn) you have 125psi, which is typical for a pump rated at 100 psi/1000gpm. The 140% pressure at 0gpm is the max allowed by NFPA 20, but most pumps are actually in the 120%-130% of rated pressure at churn, and your pump falls right in the middle at 125% of rated pressure at churn.

For your calculations, follow Cameron's advice: 1st point is 125psi at 0 gpm, 2nd point is 100 psi at 1000 gpm, then for the last point find the 1500 gpm flow and use whatever psi that falls on for your 3rd point, or ask the mfgr what the 150%/1500gpm pressure is

Colin link

9/27/2019 01:53:42 pm

And if you want the worst case scenario, you can just use the 65% rated pressure for your 150% flow as per Greg's comment. This will give you 65psi at 1500 gpm, which is WORST CASE scenario, most pumps perform well above this.

MIGUEL ANGEL D ADARIO link

9/27/2019 02:43:15 pm

It is important to know that the pump running at 150% flow (in this case it would be 1500 GPM), the engine delivers maximum power, therefore higher consumption, heat, vibrations and noise.

9/27/2019 06:49:56 pm

I think the question may need to be changed.

When designing a fire pump you can design up to 140 % of the rated capacity. This refers to the gpm demand.

So you can install a 750 gpm rated fire pump with a sprinkler system demand of 1050 gpm.

The problem you could run into is pump fatigue and inefficiency you would want to consult your pump manufacturer as well.

However you would need to consult with the AHJ and verify they allow you to design to the 140% I’ve seen anywhere from only allowed to design to 100% of rated capacity.

Bobby

9/27/2019 07:03:15 pm

Can you clarify your circumstances; are you designing a system and in the process of specifying the required pump performance or has the pump already been identified and you know its performance ?

Tim

9/27/2019 11:04:20 pm

I have identified my pump and I know it’s performance just wasn’t sure where I could design from. Thank you all for the responses it helped me feel more confident in the way I m using it. I would like to know where I can find that info in N.F.P.A. 13 or 20?

Mike L

9/28/2019 05:33:54 pm

Your hydraulic calcs should be based on the duty point of the pump (in your case, 100 psi at 1000 gpm). The 140% pressure at 0 Flow and 65% pressure at 150% flow are pump design limitations set with NFPA-20 as theoretical limits.

The pressure at 0 flow is known as churn pressure or shut off head. It is limited by NFPA-20 to a maximum of 140% of the pumps rated pressure.

If you look at the manufacturer's pump curve for your pump, the churn pressure (or shut off head) is probably well below the theoretical 140% pressure.

This value is given so that if the pump were to run with a valve closed, the pump's churn pressure and available water supply static pressure added together do not exceed the pressure ratings of the pipes/fittings and blow system apart.

On the other side of the coin, NFPA permits the pumps to be run at 150% with a derated pressure (65% max) so that a smaller/more economical pump can be installed for certain applications.

For sprinkler calculations, refer to NFPA-13. also, your hydraulic calculation program should have a method to factor in pumps.

Hope this helps

Mike L

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