Why Low Pressure at 150% Test for Fire Pump?

1/18/2022

Hello everyone -

I have a problem with two identical fire pumps rated at 145 psi at 2,500 gpm.

When testing for 0% flow and 100% flow the pump gives the correct pressure, but, when testing 150% flow the pressure drops dramatically, it has to show 125 psi and the result is 92 psi.

The pumps has a 10-inch flange suction an as NFPA allows it was installed a 10-inch pipe directly to the pipe, no suction reducer, having 16-ft (5 m) long of distance for one pump and 19'-6" (6 m) for another one, the second one with two 45-degree elbows installed more than 10 times the diameter from pump suction.

The tank is 23-ft (7 m), full of water. The test outlet is installed on the other side of the tank, so the flow does not fall over the suction inlets. The antivortex inlet has a plate of 7.5 inches above the floor.

The pump discharge is 10 inch.

The flow meter is 8-inch diameter with 8 diameters before and more than 5 diameters after.

Any idea why the pressure at 150% is so low?

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14 Comments

Mike L

1/18/2022 07:11:29 am

We would need to get a full description on how the whole arrangement is piped up. Are the pumps piped in parallel or in series?

But, my gut reaction is that your supply piping is too small for the 150% flow. At 150%, the flow rate is 3,750 gpm.

Using Hazen-Williams formula, assuming 8" schedule 40 steel piping, the friction loss at 3,750 gpm is double what it is at 2,500 gpm.

Dan Wilder

1/18/2022 07:21:32 am

Need to get the icosahedron of trouble shooting out....

Are these in series or parallel?

Do these results match the certified pump curve from the factory or previous test results (new vs existing)?

What does the suction pressure drop to?

Are both pumps performing the same or is it only one pump with issues?

Electric or Diesel? Are there any large changes in the volts/amps or RPM's at the testing points? Verify incoming wire sizes & connection points are secure (this typically shows up more on startup but just a check)

Are these results at an actual flow condition (with pitot) or through the flow meter only? Does the setup allow flow through the flow meter to a location that can have a pitot to verify GPM?

Per 20 - 19' Ed - Section 14.2.6.3.4 - As long as you have meet the fire protection demand (and the rating of the pump), you are not required to hit the 150%, but that becomes the new pump curve moving forward for the ITM side.

Factory impeller rating (width/Dia) vs results? Are they consistent.

Just a couple thoughts before the caffeine kicks in....

Anthony

1/18/2022 07:29:27 am

Had to google icosahedron, not upset.

Martin Moffitt

1/18/2022 08:18:35 am

Me Too!

1/18/2022 08:23:13 am

Always nice to keep things light on the "most productive day of the week"...

Never played the actual game but loved the dice!

Glenn Berger

1/18/2022 08:17:51 am

In addition to the above:

Did the Factory Acceptance Test show the correct pressure at 150% of rated flow?

Alex

1/18/2022 02:54:11 pm

Million dollar question, Glenn. What did the factory show?

Paul Pinigis link

1/18/2022 08:22:14 am

We need a bit more information, but my gut reaction is that the suction pipe is too narrow.

Jesse

1/18/2022 08:33:52 am

I'd be curious to know more about the pump arrangement.

Its possible the suction piping is giving you grief. You're trying to move 3,750-GPM and your fiction loss is significantly more than at rated capacity. I'd be curious to see the results of the factory acceptance test as well as the yearly performance test results to see if anything was trending.

You're not far from where you should be. If you can get at 150% flow at 95-psi you're still within the 65% of rated pressure.

If you can't, as long as you still meet the demand and the pump's rated capacity you're still in good shape. This just becomes the new pump curve.

Cliff Schulze

1/18/2022 09:06:10 am

I assume you are getting the 125 psi from the factory curve? Otherwise, per NFPA at 150% of rated flow, you need to get 65% of rated pressure, which is 94.25 psi. The 92 psi you are getting is within the 5% variance allowed.

SCHULMAN

1/18/2022 09:59:43 am

I knew what a 20-sided dice was Dan ,,,
Palladin ... back in the day of course ...

Any chance your supply piping is so poorly manifolded that it is the cause ?

Franck

1/18/2022 10:03:06 am

10 in. suction pipe is fine and this should not be the problem.
The distance to the tank is not an issue, nor apparently the elbows (the elbows does not create a huge friction loss, but could lead to an early ageing of the pump is installed too close to the suction side).

The issue may come from the rotational speed of your engine (if this is a diesel engine). If this is the case, you can check it by resgistering the rotational speed with a tachometer. Normally, the speed should not vary too much between 0 and 150% (less than 10%). But if your engine has issue, it may be that it perorms well up to 100% and then present issues between 100 and 150% when youi ask the engine to "work a bit more".

For diesel fire pumps, as the rotational speed may vary between the shut off point (0% flow) and overload point 150%), a correction to the rotational speed should be performed to compare the curves over the years and to determine possible degradations on the fire pump.
This is not required for electric fire pumps as the rotational speed is constant.

Values should be corrected according to the following formulas:
• Corrected Flow = Flow measured x (speed nominal / speed measured)
• Corrected Pressure = Net Pressure measured x (speed nominal / speed measured)2

If the problem is not related to the rotational speed, you may have other possible issues such as:
- dirt caught in your flowmeter (if you are using a flowmeter for the test), thus giving artificially wrong results
- wearing of the impeller or a small amount of foreign material caught inside the casing or impeller
- Partial plugging at the intake screen
- Partial blockage in the pump suction pipe (from foreign material, including MIC or zebra mussels, or possibly with a valve that is not fully open on the suction side.
- Reduction of the impeller actual size due to cavitation or corrosion with sediments coming with the water from the tank.

I had once a similar issue and it was a plastic bag that was partially blocking the intake of the pump.

Another possible reason could be the change of the impeller, with a proper diameter, but not wide enough, or with less vanes or a greater angle of vanes.

I would recommend you to first open the pump and check the impeller and casing conditions. If no problem found, then the issue is probably an obstruction upstream.

Jack G

1/18/2022 04:11:06 pm

Agree with above.
1. Are you share that psi number for 150% was correct. Per NFPA 20 it only has to be 94 psi.
2. Certified gauges? Are gauges giving incorrect readings.
3. suction and discharges pipes seem fine. Nfpa-20 suggests 15 fps for suction 32 fps for discharge,
4. Did you adjust readings for elbow, vortex plate valve in suction. Could be why you got 92 psi instead of 94. If gauges, we’re accurate.
5. Flow meter, vanes clear? If an old flow meter has it been removed and checked internally.
6. Check the setting of the relief valve. Could be opening too early, or leaking. Or blank for the test, if it is leaking, keeping an eye on things.
7. On that note. What is the churn pressure. Sloppy pump will open the relief. ( 140 % allowed by 20)

Ruben link

2/24/2022 01:46:49 pm

Hello Everyone, thanks for all your recommendations, We are just leaving from the installation, At the end it was an air bubble that made up all the problem.

The flow meter dial was installed looking upwards since is to the waist level, so when the contractor was purging the air from the flow meter it was not correct since the hoses where looking down, and they also didn´t install an air vent as manufacturer recommends

We rotated the flow meters with dial looking down and red hose up, and the miracle happended

Once again, thanks everyone for your recommendations.

Why Low Pressure at 150% Test for Fire Pump?

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