Run Fire Pump & Backup Simultaneously?

2/3/2022

We have a newly-installed electric (duty) fire pump, and a back-up diesel fire pump.

Each pump has a 100% required flow capacity for the system (750 gpm each) for the building.

Is it OK to run the two pumps simultaneously during testing - which would result in a 200% flow?

The diesel pump is only intended to be used during power interruption/worst-case, but not with normal power online.

During testing, we release water in the test line (flow meter) - the pressure went down and triggered the cut-in pressure of both the electric and diesel pump which makes it run simultaneously (electric first, then the diesel a few seconds later).

Does this not have a water hammer effect on the system?

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10 Comments

Glenn Berger

2/3/2022 08:11:17 am

It is not okay to run both 100% pumps concurrently.

1) The suction pipe is not sized for the 200% of flow or worse yet, 150% of both pumps running.

2) The water supply may not be able to handle this flow.

3) The flow test header and meters (or hose outlets) are jnot sized for both pumps running.

4) etc. etc.

Casey Milhorn

2/3/2022 08:23:41 am

Agreed. Not a good idea. Each pump should be disabled during the other test run. Each should have a check valve, but still not a bad idea to shut off main control valve on discharge side of the non-testing pump as well.

Pete H

2/3/2022 08:28:02 am

I'm inclined to agree with Glenn, this does have elements that will potentially lead to issues where your system is not built for this massive 200% flow.

If I were try to make the argument for your case, I'd go with:

NFPA 20 (2016 Ed.)14.2.6.2.3.1 - Where simultaneous operation of multiple pumps is possible or required as part of a system design, the acceptance test shall include a flow test of all pumps operating simultaneously.

-

This states "possible or required". So it's not just parallel pumps in sequence, it's all pumps that can possibly run going simultaneously, backups included.

However, I'd note that the potential for water hammer or cavitation due to the system not being sized to handle 200% could definitely void whether this is possible.

If water hammer is your only concern, an anti surge pump start control valve could potentially mitigate that issue ( https://www.bermad.com/product/anti-surge-pump-start-control-valve/ https://blog.bermad.com/fire-protection/solving-water-hammer-with-innovative-deluge-valve )

But given the issues of suction pipe, water supply, and flow test header... I would not expect water hammer to be the only concern.

Please take with a grain of salt. All of the above is 5 minutes of research and musings without heavy focus.

Nick Devine

2/3/2022 08:30:42 am

For an arrangement like this, I believe each pump is required to be flow tested independently to confirm their individual performance. It may be necessary to isolate the pumps from each other while performing testing by either disconnecting from power or by closing valves. Sometimes easier said than done, depending on impairment procedures.

As for water hammer, it depends how the start/stop pressures are set for the pumps. We generally encourage the pump pressures to be set as follows:
Jockey (stop) = minimum static supply + net churn
Jockey (start) = - 10psi
Fire Pump (start) = - 5psi
Addition fire pump (start) = - 10psi
Setting the pressures in this manner will help to minimize potential water hammer issues.

SCHULMAN

2/3/2022 09:07:59 am

I am not sure why you would simulate a condition that would / should never happen in a properly installed system.

Jesse

2/3/2022 09:34:26 am

I think this falls under the "just because we can do it, doesn't mean we should". No, this isn't a good idea. Each pump should be tested independently.

First of all, the suction side of the pump arrangement would have been sized for one pump @ 150% of rated capacity. These are relatively small pumps at 750-gpm but there may be some concerns with the water supply being able to handle both as well.

Second, the test header would be sized for 750-gpm, not double that.

Several other concerns as well, but you get the idea.

Franck

2/3/2022 10:08:30 am

If the question is : is it a good idea to test both pumps simultaneously, then the answer has been assessed above and the answer is : no.
Unless each pump take suction from a different suction pipe and your test line allows to measure up to 300% of the nominal flow of your pump.
If the question is : did your test create problems ?
The answer is;: probably not.

Normally, as indicated above, it is a good practive to isolate one pump and start the other one. Then, when the test is done, isolate the one you have tested and test the second one. And don't forget to put everytrhing back in service at the end.

Now, come back to what happened.
Because you opened the test line, the pressure dropped quickly and activate both pumps quite simultaneously.
But what was the resulting maximum pressure ? The maximum pressure delievered by one pump.
When several pumps are operating, you add flow at the same pressure, not pressure.
And what was the flow ? The one that you could flow through your test line when it happens.
When it happens, I presume you had just started to open the flow test line, meaning that to flow that could go through you test line was something between 0 and 50% of the nominal flow of one pump.
100% maximum.
In that case, you can't flow more water than the size of your test lines enables you. This means, if both pumps have the same rating, that half of the flow was coming from one pump and half of the flow from the other.

If the flow measured throught your test line was 100% of the nominal flow of one pump, each pump was flowing 50%.
And the residual pressure was higher than the one with one pump operating (it should be, in fact, the pressure obtained at 50% flow).

To have waterhammer, you need a sudden increase of pressure in your system by providing too much flow with a limited "escape" for the water. This could happen if a fire pump starts at a too low pressure, and the pressure raise in a second from a very small pressure to the pressure delivered by the pump at churn (when only one sprinkler is opêrating, for example).

Joshua Freedman

2/3/2022 10:40:54 am

I am just agreeing with what everyone else said. I would be very worried about causing supply pipes to collapse with suction dropping as low as I would imagine it could get with both running.

Robert Hughes

2/3/2022 11:11:48 am

I'm no electrician, but I'd say your transfer switch should not allow the diesel pump to automatically kick on anyway without a power loss, either real or simulated.

Jack G

2/3/2022 04:45:44 pm

I disagree with Pete—- agree with Robert.
If one pump is backup……for main pump failure, and the design calculations are based on pressures available with ONE pump running, of course you cannot test both simultaneous.
I m sure the suction supply to both, is sized based on one pump running also.
Did you underbid the job? Having the test fail due to an inappropriate test is no way to make it up.
You can set the test header supply to feed to one header which will cut down your set up time.

Run Fire Pump & Backup Simultaneously?

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