Route Diesel Fire Pump Flow Back Into Suction?

5/12/2026

I have a DoW project with specifications that require the diesel fire pump flow meter to discharge both into the pump suction and through the test header.

I don't have experience with this setup; in all my previous projects, the diesel fire pump discharged back to the water tank as well as to the test header.

I understand that NFPA 20 allows this arrangement (see attached figure from the 2025 edition), but I'm curious how it works in practice for two main reasons:

1. If we close the tank valve during testing, the pump's suction pipe will receive considerable pressure. Will this repeated pressure adjustment after each test impact the longevity of the soft packing, particularly regarding increased water leakage?

2. How does a diesel fire pump cool its motor if the heat exchanger water isn't returned to the tank with fresh cool water coming in?

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5 Comments

Anthony

5/12/2026 08:43:07 am

1) At churn once the pump achieves steady state there will be negligible pressure on the suction side as the water wont go anywhere. Negative pressure is created by a fluid leaving and a vacuum effect. Also as the pump spins up there shouldn't be a significant suction pressure first principals would lead me to believe the higher the volumetric discharge the higher the suction pressure.

2) Depends on the exact arrangement, I've seen some cooling water runs just discharge waste hot water to a floor drain.

Dan Wilder

5/12/2026 09:10:36 am

You will not want to close the suction supply for exactly the reason you state in point 2, you will lose cooling water supply. You can test with it closed provided the temp in monitored.

As to the pressure overage with the loop, it has not been an issue on my projects. The packing will increase water leakage with usage and eventually will needs the banjo clips to be tightened, but this is the sacrificial part of the shaft packing and why these get regularly replaced under normal usage. It's when the local maintenance guy smashes them down to stop the drippage under no run conditions that things go sideways.

Rocket

5/12/2026 11:24:25 pm

If I do not shut off the control valve from the water tank to the pump suction, won't that affect metering? Water leaving the flow meter will flow towards two directions , water tank and pump suction..... Which way would water flow towards?

5/13/2026 08:30:46 am

Once it passes through the meter, doesn't matter where it goes, the flow has been read. The pump will pull the water through regardless at the specified GPM and PSI increase.

5/13/2026 09:50:48 am

A proper measurement requires the difference between suction and discharge pressures....
Not sure how the open valve from the flow meter back to the water tank will affect that gauge reading....

Route Diesel Fire Pump Flow Back Into Suction?

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