Relief Valve Size & Flow for Dry System Tripping?

9/23/2025

I have a dry system that is tripping when the county runs their test on the hydrant using tankers and pumpers.

My question is, if I use a 3/4' relief valve, will it be enough?

How much flow (gpm) will it flow, at its maximum flow rate?

Thanks in advance.

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9 Comments

Anthony

9/23/2025 07:39:14 am

The problem is the air pressure on the clapper not the relief valve. the relief valve is located after the system trip valve and only there to prevent water pressures greater than 175psi from reaching the system. You could try a retard chamber to absorb some of the surge pressure. But I think it would be easier to just increase the air pressure to handle the surge in pressure from the hydrant tests.

Louie Lincoln

9/23/2025 08:09:04 am

The air is the max pressure. If you have a answer to my oregonal question please do so.

David Kendrick

9/23/2025 08:30:16 am

If you have the air at the highest pressure the data sheet allows and the additional pressure doesn’t impede your ability to deliver water to the remote areas.
Then find buy/rent a pressure recorder to document your trip conditions.
Fire training that causes you an expense should be reported with documentation to your local AHJ and regulators.
As far as a relief valve I’d suggest contacting your AHJ explaining the situation and insurance provider. Both have an interest in your issue as well as the modifications.

Dan Wilder

9/23/2025 08:38:43 am

The relief valve will not be a viable option...this isn't a slow buildup and release of pressure; this is a water hammer and the differential valve doing its job to open when that balance is tipped.

The solution would be a surge tank to lessen the water hammer issue.

The discharge of the 3/4" valve is dependent on the water supply which is not defined. The MFG would have max flow available through the valve being used, unfortunately, most of those charts will stop just after the rated pressures of the valve. Even with it fully open, it will not lessen that initial hit so the valve will trip regardless.

Jesse

9/23/2025 09:25:52 am

Agree with Dan here. This is a water hammer, so I don't see a relief valve adding any value. A surge tank will oprevent that.

JIM SCHAMERHORN

9/24/2025 11:09:35 am

Dan Wilder is spot on with his analysis and solution. We have parts of the city where mains carry a static of 125 PSI, so water hammer is a real concern when hydrant testing is conducted. Our Board of Public Utilities carefully trains and stresses crews are not to slam the hydrant shut. We have had instances where water hammer on the main has tripped some dry valve systems.
Sadly, I didn't hear Louie thank anyone for their help regarding his problem.
Regards,

Gary Lagnese

9/24/2025 11:19:16 am

Louie never said what his air pressure was. Didn’t really give much meaningful info but requires a detailed solution.

Louie Lincoln

9/24/2025 11:27:02 am

Thank you all for your answers.

Jeff

10/13/2025 09:15:15 am

Watch the video
https://www.youtube.com/watch?v=0Tumj58BEDk

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Relief Valve Size & Flow for Dry System Tripping?

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