Setting for Circulating Relief Valve on Fire Pump?

3/23/2022

Under NFPA 20, Section 4.20.7.1:

The circulating relief valve shall actuate below the opening set point of the pressure relief valve to ensure cooling of the pump during churn operation.

NFPA 20 Handbook:

The pressure setting of the circulation relief valve should be well below the pressure relief valve setting but above the maximum suction pressure.

Based on this, how do I properly specify the circulation relief valve setting that is installed on the pressure relief valve discharge line if the pressure relief valve set point is 175 psi?

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5 Comments

Nick Devine

3/23/2022 08:19:15 am

The circulation relief valve is intended to operate on the start up of the fire pump to keep the casing cool when the pump is running at a no-flow condition (churn).
Therefore, the relief valve should be set so it is closed when exposed to normal supply pressure and open when it is exposed to discharge pressure at churn. There is no exact operating pressure requirement as it just has to satisfy the above conditions.
For example:
If you have a supply pressure of 50 psi and a fire pump with a net shut off pressure of 65 psi, your discharge pressure at churn would be 115 psi. Therefore, a circulation relief valve set to operate at 100 psi, would be closed when exposed to supply pressure (50 psi) and would be open when exposed to discharge pressure at churn (115 psi). This would satisfy the requirements of NFPA 20 by being well below the pressure relief valve setting.

Franck

3/23/2022 08:37:44 am

Perfect explanation by Nick

Having in view the purpose of the device will normally help to better understand the setting.

In the example above, the set up pressure of 100 psi (as well as alsmost any point between 50 and 115 psi) would work perfectly.
To avoid issues, it is preferrable to avoid values too close to the limits.
If too close to 50 psi, it may open and operate all the time, even when the pump is not operating.
If too close to 115 psi, it may not open when the pump is operating at churn.

Having the setting of the PRV is of little help. The argument from NFPA is just to ensure that the PRV is not used as a circulating relief valve.

Jack G

3/23/2022 09:01:56 am

It all depends on design pressure. A lot of Engineers now, design to 175 psi.
I ve found most pump casings are rated for 250 psi. If not, i request it when i buy them. Drilled for 150 psi flange. ( a few times).
You can then play with the settings to suit the situation. 170-180 . 165-175. Etc.

Tim Darling

3/23/2022 09:24:02 am

It is not really practical to try to preset a circulation relief valve at a given pressure. Make sure the circulation relief valve you order is within the pump pressure specs. That is the most common mistake made when it comes to circulation relief valve.
It is easiest to adjust in field on intial pump run. What I do is make sure the circulation relief valve is not open when the pump is off. Start the pump and let it run at churn. Adjust the circulation relief valve until it opens and provides a steady stream of water. When you turn off the pump the circulation relief valve should close. Start the pump again and see if circulation relief valve opens. Turn pump off and it should close. By the time your get though your six pressure drop starts for initial acceptance it will be thoroughly tested. Yes it's really that easy.

Jesse

3/23/2022 10:10:37 am

I started to reply, then got busy. The refreshed my screen and saw that Nick had a most excellent reply. Nothing to add

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Setting for Circulating Relief Valve on Fire Pump?

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