Determine Air Pressure for Sprinkler System?

2/24/2022

I understand the limits for what is allowed, but how do you actually determine what air pressure your sprinkler systems needs to be set at?

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9 Comments

Alex

2/24/2022 06:18:02 am

Hi,

I assume you are talking about a dry pipe system? If so:

Higher air pressure in the system means a larger volume of air. In turn, this means a greater time difference between sprinkler activation to when enough air has been released for the valve to reach its trip point. This valve trip point varies per model/manufacture.

Due to the varying trip points, air pressure is recommended by the manufacture as a ratio. This ratio should be compared to your static water pressure to determine the minimum required pressure within your system.

Thanks,
Alex

Dan Wilder

2/24/2022 08:05:45 am

By the specific type of valve you are installing for the system type needed. In the past it was a rule of thumb ratio of the clapper size (air) vs the incoming supply (water) but most (if not all) the MFG's have charts specific to their valve.

Dry Valves - There are a couple different versions on the market for standard valves using anywhere from 10-70PSI of air (using the static water pressure) and some with low pressure versions 10-25 PSI (again, depending on static water pressure)

Preaction Valves - Standard & low pressure versions using the same approach but with the ability to trip via electronic signal vs pressure loss.

There are also accelerators & exhausters to help with trip times and even some freezer versions for cold storage with some very quick trip times in high hazard applications.

Anthony

2/24/2022 08:08:24 am

Like Alex said Its an equipment supplier question. Each dry valve will have a different set of requirements. Make sure you have a properly sized air compressor too. That can be manually calculated or work with the compressor supplier to figure out the fill times and sizing information.

Derek

2/24/2022 08:19:00 am

Agree with all of the above. The low-diff ratio valves actually use water to hold back the piston that trips the valve. In years past, a general rule of thumb was 5:1; that is, 40psi of air would retain 200psi of water, 30psi of air would hold back 150psi of water, etc.

Glenn Berger

2/24/2022 08:22:58 am

Agree based upon the discussion for a dry-pipe sprinkler system. For a supervised pre-action sprinkler system, a different air pressure level must be met.

Jesse

2/24/2022 08:30:29 am

I read and then re-read the question several times before I understood it was regarding a dry system. Then I saw a squirrel so by the time I got back to it, everyone else very thoroughly answered.

What they all said....

schulman

2/24/2022 08:32:39 am

i've never had a problem getting an engineer at my favorite Fire Sprinkler Compressor Manufacturer to check my design for me

Emil Ohler

2/24/2022 09:26:10 am

NFPA does have a default requirement:
8.2.6.7.1
The system air pressure shall be maintained in accordance with the instruction sheet furnished with the dry pipe valve, or shall be 20 psi (1.4 bar) in excess of the calculated trip pressure of the dry pipe valve, based on the highest normal water pressure of the system supply.
So, as everyone else said, follow the instruction sheet, or add 20psi to the trip pressure

Robert Hughes

2/24/2022 10:22:36 am

Knowing (at installation) that you want water to reach the inspector's test opening at a certain amount of time, you set the cut-off pressure (of the air maintenance device) at the maximum point (above the recommended dry valve differential ratio) wherein you'll meet that time constraint upon operation of the ITV (with system pressure at its cut-off maximum). Set the cut-in pressure within the most comfortable margins between the dry valve trip point and air maintenance cut-off point.

Once installed NFPA 25 takes over, wherein one should watch for scale build-up and obstruction slow-downs by archiving and comparing subsequent test result readings.

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Determine Air Pressure for Sprinkler System?

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