Fire Pump that Won't Flow 100% in Annual Test?

7/18/2019

Existing building with a vertical in-line fire pump that has not been tested in the last 5-6 years. The fire pump is fed from a water storage tank where the top of the tank is roughly 12-15 feet above the fire pump.

The fire pump is rated 55 psi at 500 gpm. There is about 60 feet of 4-inch main that connects the discharge side of the fire pump to the fire pump test header. This stretch of main also includes at least 6 elbows and a tee. The test header has two 2-1/2" connections.

With a hose monster and 15-feet of hose connected to the pump test header, we are only able to flow about 400 gpm out of the test header. At this flow rate, the suction side fire pump gauge reads -20 psi and the discharge reads 25 psi.

In short we are not getting the 100% flow (of 500 gpm) or the rated pressure (of 55 psi).

I can see some flow being restricted due to the long run from the pump room to the building exterior, but are there other issues I'm not thinking about here that would cause this? Obstruction in the main perhaps? Any tips on actually getting the flow up to 100%? Thanks.

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11 Comments

Mike

7/18/2019 08:05:59 am

What size/TDL is the piping from the tank to the suction side of the pump?..

The tank elevation is giving you about 5 psi of suction pressure which should give you enough NPSH...but if the pipe is too small or there are check valves on the suction side, you won't get the flow needed.

Justin

7/18/2019 10:32:28 am

Mike the suction side pipe is 3" and there is a check valve right after the 90 coming from the wall then a 90 immediately down to the pump level.

Colin Lusher

7/18/2019 10:42:35 am

Justin, you just solved your problem....3" suction is WAY to small for a gravity tank suction supply of a 500 gpm pump. Its even too small for a pressurized suction supply....NFPA 20 requires a minimum 5" suction line for a 500 gpm pump. You'll need to increase the suction size to 5 or 6" in order to be in compliance with NFPA 20, and I guarantee this will solve your issue. I would also remove the check valve....check valves are prohibited within 50-ft of the suction side of a fire pump.

Colin Lusher link

7/18/2019 10:36:41 am

It sounds clear to me that the problem is on the suction side of the pump. At -20psi on the suction side of the pump, the pump would be experiencing some pretty heavy cavitation (you'll hear the sound of the pump change when the suction pressure drops below about -5 psi...popping/crackling sound), which is not an acceptable condition for pump operation.

With a gravity tank, and a 4" suction line, I'm guessing you're loosing whatever gravity head the tank provides with the friction loss in the 4" line. I ran into this exact same problem early in my career, and even with a low gpm pump (250 gpm in my case), we had to increase the suction from the gravity tank from 4" to 6" in order to flow the pump at 150%.

Look into the following: How long is your 4" suction line? Are all the valves on the suction line wide open? What is the water level in the gravity tank? Your solution will likely be the answer to one of these questions. Worst case, you'll need to increase the size of the suction line to the pump if the run of 4" pipe is too long.

7/18/2019 10:50:43 am

Thank you Colin, I will have to let the individual know what we have come up with and what we think would be the solution.. I would like for them to be covered as well as me since this particular pump is covering a middle school..

7/18/2019 10:58:45 am

Good luck Justin.

Yes, they absolutely need to resolve this issue as their pump is essentially useless without fixing this.

And I would definitely mention to them that the installing contractor did not install a code compliant system. This pump would never have passed it's original acceptance testing, so the original installing contractor AND the pump supplier are responsible here for faulty design and workmanship.

Dan W

7/18/2019 10:57:15 am

Does the pump have any 5" or greater pipe on the suction/discharge side?

Visually check the water height in the tank, could be a bad float valve.

Can you see inside the tank for any growth (vortex plate obstruction), color of the discharge water may also show an issue (green or brown)

Exercise any OS&Y's into a verified closed (verify by bleeding off pressure) then open position (pressure should return), the pin to the disc may have sheared off. Flowing water (no pump) may also show a difference in the before/after if the disc is only partially moving.

Can you verify the correct power is getting to the motor (can be done at churn)?

The length isn't helping anything, so if an internal inspection of the impeller & piping yield nothing and the power is correct, then a pipe size increase may be next.

7/18/2019 11:05:04 am

Start with Table 4.27(a) Summary of Centrifugal Fire Pump Data (U.S. Customary) for 500 GPM pump

Min. suction is 5"
Min test header should be 4" but only if 15 ft of supply is used.

3 problems:
1. Test header should be 6"

Per NFPA 20, 4.21.3.4 Pipe Size. The pipe size shall be in accordance with
one of the following two methods:
(1) Where the pipe between the hose valve header and the
connection to the pump discharge pipe is over 15 ft
(4.5 m) in length, the next larger pipe size than that
required by 4.21.3.1.3 shall be used.

2. suction should be 5" min.
3. Suction pressure is too low

4.15.3.2* The requirements of 4.15.3.1 shall not apply where
the supply is a suction tank with its base at or above the same
elevation as the pump, and the gauge pressure at the pump
suction flange shall be permitted to drop to -3 psi (-0.2 bar)

How did this system even pass inspection or testing the first time?

Doug

7/18/2019 12:13:33 pm

I would also check the power supply to the pump and make sure the pump has the power needed and that it is turning in the proper range.

pjd

7/18/2019 12:43:10 pm

Justin, I just wanted to add that when the cavitation occurs that Colin described above, what is happening essentially is that pressure in the suction line has dropped below the vapor pressure of the water, which varies based on its temperature. At 60 F, which is about ground water temperature, the vapor pressure of water is around 0 psi absolute, which translates to around -15 psi gauge. At or below that pressure, the technical description of what occurs is that the water boils at 60 F, and it undergoes a phase change from liquid to vapor (steam). This vapor is sometimes referred to as "low-quality" steam, at least in the power generation world, because there is very low energy in it. The droplets that make up the steam are relatively densely packed, they are larger and have more mass than the droplets in steam at higher pressure and temperature. The steam particles formed by cavitation impact the pump impeller vanes at its tangential velocity at the radius of site of the impact, and cause pitting in the brass material. Repeated for a long enough time, operating a pump in a cavitation condition will destroy the pump. That is why the NPSH required is listed on the manufacturer's pump curve, and it is especially important to pay close attention to it in suction tank and in cascade vertical turbine pump installations.

7/18/2019 03:32:48 pm

@PJD

Thanks for the scientific explanation. Learn something new every day here.

Michael

Fire Pump that Won't Flow 100% in Annual Test?

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