Fire Pump Start/Stopping with Jockey Idle?

9/20/2021

To me, it makes more sense to locate a backflow on the suction side of a pump. However, I have recently run across a facility with the backflow downstream of the pump (discharge side).

The main fire pump controller is connected between the the first check valve and the backflow preventer.

The jockey pump is connected after the backflow preventer.

Here's the problem: when the first check valve fails, the main fire pump continuously starts & stops because the pressure is bleeding off they the relief valve while the jockey pump remains idle.

Is this an acceptable scenario?

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8 Comments

Glenn Berger

9/20/2021 08:35:05 am

1) the installation of the BFP after the fire pump is not that unusual. It is easier to account for the pressure loss with the pump. Locating the BFP before the pump does create potential scenarios for low suction pressure.

2) The sensing lines for the jockey pump and the fire pump MUST be in the same general location. Need to examine the pump discharge check valve to see why pressure is not being maintained.

Jesse

9/20/2021 08:46:25 am

Agree with Glenn. Though its not my favorite, there are reasons sometimes to put the BFP downstream of the pump. Usually these have to do with the increased friction loss the BFP creates when the pump is at 150%.

Ed Glynn link

9/20/2021 10:12:49 am

We have installed backflows in a similar fashion as there are inherent issues with low suction potentially.

It is our experience that both sensing lines for the pump and the jockey pump need to be down stream of the back flow because the shorter piece of pipe in-between the check valve and the back flow tens to hold pressure much longer than down stream of the back flow.

Putting it down stream allows for a more accurate reflection of the pressure down stream.

Alex

9/20/2021 10:34:22 am

Hi Anonymous,

NFPA 20 (2019) Figure A.6.3.1(b) shows the backflow installed on the pump discharge. I typically design this way so I can accurately account for the friction loss.

I agree with Glenn that it is important to have the sensing lines in the same general location. Having them installed within the same vicinity allows for the sensing lines to be more in line with each other. In your case, the jockey pump could then take over rather than starting your pump.

Thanks,
Alex

Mike

9/20/2021 10:36:26 am

Have you tried raising the sensing line pressure start? May be too low.

Franck

9/20/2021 11:28:38 am

Are you speaking of a booster pump ?

In the case of a fire pump taking suction from a tank, you should not have a problem (and normally no need for a backflow preventer in addition to the check valve).
The reason for a check valve is to avoid, if you have several pumps in a parallel arrangement, that the pump that is operating can flow water to the other one that would then turn in the opposite direction. With an electric drive, this may destroy the motor.

In the case of a booster pump, it may make sense to have the backflow preventer (to avoid pollution in the city water supply) upstream the pump. But at a sufficient distance (normally 50 ft) to avoid water flow disturbance at the intake (creating cavitation, turbulences and early aging of the impeller / pump casing)

I do not understand your start and stop sequencing for the main pump as it should start automatically and stop only manually

in all case, if your pump starts because of a defect on the check valve, this would be an indication that you need to repair it and you need to know it.

Franck

9/20/2021 12:47:42 pm

In all cases, as indicated by other peers, it makes sense to have the sensing lines connected on the same side of the check valve / backflow preventer

The pressure reading from the Jockey pump should be the same as for the fire pump(s).

And this would prevent the fire pump to start instead of the Jockey pump

Dale

9/21/2021 12:45:00 pm

This is an existing condition, so this is not exactly applicable, but I was told that it's better to have the BFP in front of the pump, as the BFP may not be rated for the higher pressure created by the Fire Pump. Just some food for thought.

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Fire Pump Start/Stopping with Jockey Idle?

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