How to Fix Fluctuating Pressure on Pump Suction?

4/14/2023

Hi everyone - thanks for answering my previous questions and for checking out today's as well.

I have three projects all connected to the same central fire pump assembly. It's a huge network of pipe running for miles.

The client is having a problem with the fluctuating pressure (up and down) on the network for some reason that affects the fire pump's suction pressure. It's in series, and connected to each project.

The client refuses to add a break tank and cut the fluctuation.

In your experience, is there any alternative solution available?

Would a pressure relief valve help? What would happen when the pressure is less than design?

Thanks in advance!

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5 Comments

Jesse

4/14/2023 08:15:52 am

With the mention of a break tank, I presume the water source is municipal? How wildly are the pressures at the intake flange fluctuating?

Eric R

4/14/2023 08:40:14 am

If your water supply pressure is dropping below the point where the fire pump discharge is no longer able to provide the required system demands then that is a pretty big problem.

Since a break-tank would require the installation of a new fire pump anyways, perhaps a less expensive option would be to keep the municipal connection but replace the fire pump to one that can provide the system demands even at worse case suction pressure.

This would likely create a situation where at pump churn you exceed the 175psi pressure rating of your systems, so the fire pump will likely need to be paired with a master pressure reducing valve to keep pressures downstream of the pump under control.

You will want to make sure that during times of low residual pressure you aren't creating dangerously low-pressure condition upstream of your pump at whatever your peak system flows are.

I had a similar situation at an industrial plant that we resolved in this way, and using a PRV to control the pressures across the entire site worked wonders, and made the NFPA 25 testing procedures so much easier at the facility.

Franck

4/14/2023 12:03:56 pm

I think you are spreaking about a Pressure Relief Valve (PRV) not a Pressure Reducing Valve.
A Pressure Relief Valve is commonly used on fire protection systels and are quite reliable.
A Pressure Reducing Valve is normally limited to some situations, such as the lower floors of a high rise building with a pump delivering a high pressure to reach the upper levels.
They are less desirable for fire protection systems because of the additional friction losses (not always accounted in hydraulic calculations) and possible maintenance issues on them.

Dan Wilder

4/14/2023 08:50:44 am

Trying to understand the setup...

1 main fire pump (supply from municipal or tank)
1 fire pump for each of the 3 projects attached?
Any large elevation changes?

If the site is setup more like a tree than a loop, try adding check valves immediately after the tees to the downstream systems to help retain pressures?

Maybe the first jockey pump is undersized for the amount of underground leakage for the site as a whole (underground loss volume near equal to the capacity of jockey pump)?

14-22' Ed - 4.27.3 Pressure maintenance pumps shall have rated capacities not less than any normal leakage rate.
Enhanced Content - An exception to this general rule is when older underground systems leak excessively. In such a case, the jockey pump capacity should be increased further, based on the leakage rate of the underground system.

Is there a possibility to close the supply to the whole system - or sections of that system - and monitor pressure loss over a time period to allow some guess of how many gallons are being lost through the UG system? You would need to know the volume of the water and PSI loss to some accuracy (digital gauges and accurate UG routing).

I had a recent project that the jockey pump was sized correctly as to PSI, but the GPM was oversized causing a water hammer when the JP kicked on, saw the PSI increase and shut off, the system would have a PSI wave drop and the JP would kick back on again....this cycled 3-5 times and would trigger the fire pump to come on. We replaced the JP with a lesser GPM pump (same PSI) and the issue went away. First time I (and many others) had seen that issue in a system.

Casey Milhorn

4/14/2023 11:57:07 am

Check out the Zurn low suction control valve, copied the text below for you. If you are having mostly low suction issues, this may solve it. I'm personally leery about restricting flow to a fire protection supply, but it might be the trick you need.

If you are having high pressure issues, then a PRV on the discharge side would take care of that.

Before installing both on the same system (or anything for that matter) I would reach out to the guys at Zurn. They are extremely knowledgeable about their products and pressure control in general.

I would also put some money in there to monitor the suction and discharge pressures during testing (at the common times for peak fluctuations) and also a 24 hour monitor/recorder for a week or two to see how everything is working.

Just my two cents.

ZW215FP
Fire Pump Suction Control Valve
The ZW215FP Fire Pump Suction Control Valve, with grooved connections, is used to control suction pressure for Fire Suppression Systems to prevent fire pumps from over-drawing from the water supply, causing damage to the pump or supply network due to low or negative pressures on the fire pump suction side. The ZW215FP prevents cavitation and potential backflow conditions by maintaining a minimum inlet pressure to a Fire Protection Pump. Its internal and external epoxy coating resist corrosion, and the test cock on the sensing port allows for quick bleeding of the sensing line.

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How to Fix Fluctuating Pressure on Pump Suction?

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