Fire Pump Issues

[Buildtech2]

During commissioning, we have found that pump suction pressure at 150% of test is -5.0 PSI which is more than acceptable limit as per NFPA. We have also confirmed that static lift is positive as the water level in the firefighting tank is 800 mm higher than pump suction inlet. Moreover, the required NPSH is higher than available NPSH but still the pump suction pressure is exceeding the NFPA requirement at 150% of performance test. I an not able to find a solution for this problem. Appreciate, if anybody can provide some guidance to solve this problem. The pump capacity is 1000 gpm @ 441 ft. wg.

 

[UFT12]

In any case you should not have a situation where NPSH(r) > NPSh(a). It seems that either your static head is not enough, there is a problem with the suction line that exhibits considerable pressure loss or the pump may not be working properly? Just thinking out loud. If you feel there is nothing wrong with the pump and the suction pipe, would it possible to increase the static head (> 800 mm) so that you come to NPSH(a) > NPSH(r)?

 

[LCREP]

Check simple stuff ....

Did you use calibrated gauges on the pump during the test?

Check that all valves are open, you may have a partially closed valve.

Was the underground main flushed prior to the pump test, if so did the flows meet NFPA 24 flow rates for the main size?

When the pump was running did you have any evidence of rocks either discharged from hose nozzles or noise from rocks hitting the pump impeller?

What size is the water main to the pump, was it sized properly based on flow, pressure loss, elevation, elbows, valves as per NFPA 24?

Any back flow devices on the suction side of the pump, did you take the pressure loss from the device into consideration when sizing the underground main?

 

[cdafd]

What are the readings at

Churn


And

100%

 

[TravisMack]

If I understand correctly, you have a pump taking suction from a tank. The tank is about 2.5' above the pump suction.

Assuming the tank is adjacent to the pump, there should only be a gate valve in the suction line. Is this gate valve fully open? If the tank and pump are not adjacent, do you have a properly sized line between pump and tank. Basically, you can't afford to lose any pressure due to friction losses. You should have a minimum 8" suction line, but you may need a 10" due to the length of the suction line.

As others have said, if there was underground piping, was it flushed properly per NFPA 24. A simple garden hose spigot is not flushing for 8" underground - yes, I have seen plumbers try to claim that is acceptable.



Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

http://www.mfpdesign.com

 

[Buildtech2]

Thanks everybody for your valuable inputs. I will get back to you with more background information as per commissioning documents soon.

 

[Buildtech2]

Dear Friends,
I received actual information from site as per final commissioning as follows.

The firefighting pumps are selected for 1000 gpm @ 185 psi. As per test results, the pump curve is complying to NFPA 20 requirements. At 150% of rated capacity, the head is more than 65% and shut-off head is less than 140 %. I have attached revised sketch for further clarification. I have also calculated NPSHA value which is still to be verified. It seems NPSHR value provided by the vendor is incorrect.

The readings at:
100% 195 psi
150% : 155 psi
0% : 219 psi

As per commissioning sheet, it was found that at 150 % of rated capacity, the pump suction pressure is not with in NFPA requirement.
Main pump suction pressure = -4.91 psi
Standby pump suction pressure= - 6.87 psi

Next week, I am engage third party for actual measurement after complying above mentioned checklist.

 

[jimnogood]

Can you check if there is an anti-vortex plate in the tank discharge?