What Air Pressure Matches Hydrostatic Test?

6/24/2020

We are pressure testing a sprinkler system but there is no water available.

How much air pressure should be used to equal a 200 psi hydro test?

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12 Comments

Mike L

6/24/2020 07:20:12 am

40 psi is typically used for air testing.

NFPA-13 permits interim air tests for certain applications (ie, systems subject to freezing, dry-systems, preaction systems). The parameters are specified in Section 25.2.2 of the 2016 edition.

Assuming all stakeholders (AHJ, ownership, insurance carriers, etc) agree that an air test is acceptable.

Is it that a water source is not available at the time of testing? if that is the case, an interim air test is acceptable. The system will need to be hydrostatically tested once water is available.

sean

6/24/2020 07:11:56 pm

it also says to go through with water testing when weather or conditions allow

Dan Wilder

6/24/2020 08:26:25 am

TL:DR - DON'T. Find a garden hose, rent a water truck, but NEVER fill a sprinkler system to 200PSI of air pressure.

PSI is unit of pressure expressed in measure of force over an area (Pounds per square inch in this case). Something exerting 1 pound per square inch is still 1 pound of pressure regardless of what is providing that 1 pound, the difference is the volume needed to exert that pressure (compression).

Lets assume you have a column of water within a 1 cube (1x1x1). The volume of that water (the cube) is ~7.48 gallons (with a gallon weighing ~8.33 pounds). 7.48 x 8.33 = ~62.3 pounds and dividing this by 144 (12"x12" = the base of the cube) gives you ~.433 pounds per square inch...familiar number right? (this is based on gravity providing the compression force in a downward direction)

Air however is much lighter (comes in at ~.0807 pounds per cubic foot) and all things being standard, if you were at sea level, the weight of all the air above you only comes out to ~14.7 PSI (1 atmosphere).

Now this is where I'm going to tell everyone (again) to NEVER use air pressure above the requirements of NFPA. Water is considered uncompressible but air is highly compressible. The compression rate of air per cubic foot to 200PSI is ~13.6x i.e. it takes nearly 14 times the volume of the system to fill with air (within the same space) to bring it to 200 PSI - Boyle's Law equation if you want the longer version. A 500 gallon system (~66.8 Cu Ft) would contain ~909 cubic feet of air under compression so if something were to fail (a cap for instance), it's that release of energy out of a small orifice that becomes the safety issue.

Will the system hold the pressure (200PSI is still 200PSI), possibly, but the safety of your crew and the others on the site alone should far outweigh the need to perform this test with air.

Jonathan Sullivan

6/24/2020 10:14:08 am

Interesting aside, this is recognized by other model codes such as IPC. Air tests are prohibited for PVC piping, and if an air test is performed on drain and waste piping, it can't exceed 5 PSI, where if a water test were performed on the entire system, multi-story stack pressure could be many times greater.

Micheal

6/24/2020 10:17:09 am

I too echo the advice the advice. Considerable damage and even death may result from an air test at the pressures mentioned above.

A leak test at very low pressures may be conducted, under tightly controlled conditions.

6/24/2020 10:25:14 am

Do NOT pump up that wet system with air, for all the reasons listed by Dan Wilder. Even if you were to pump air into that system, it doesn't count-- you would still have to perform the hydrostatic test afterwards. Wait for your APPROVED water source to become available for your system.

Jonathan Joseph

6/24/2020 12:18:38 pm

Have the GC supply the water truck. In most contracts this is an Item that usually gets addressed and typically GC is responsible for supplying water for testing when the tap to a city main, private well or pond is not present.

All systems shall be tested with water. Dry systems should be tested with water and then with air since they are reliant on both pressures and like Dan said the difference of compression is extreme.

Louie Lincoln link

6/24/2020 02:27:46 pm

I could not agree more. When one has a failure with high air pressure it is basically an uncontrolled explosion. This would lead to property damage and possible loss of life. Should chose to do it anyway. Please give advanced notice so that I can arrange to be out of town,

Cliff S

6/25/2020 10:09:52 am

Word of caution on air testing. Depending on the size of your system and the amount of noise on your project, it could be very difficult to find an air leak.

Louie Lincoln link

6/29/2020 07:44:08 am

A little bit of peppermint oil in the air intake works wonders in high noise environments .

Vahe Zohrabian

6/27/2020 12:54:28 pm

Agreed with all points above plus, Viscosity and surface tension of water is greater than that of air, about 89 times, and that viscosity, internal friction of water making it resist the tendency to flow, particularly through a small opening.
So, air can be leaked from pinholes that water would not and excessive presure of air can couse for that pin holes to get bigger and couse real leaks during water test.

Vahe Zohrabian

6/27/2020 01:00:07 pm

Below you will find more information,

https://www.google.com/search?q=viscosity&rlz=1C1CHBF_enUS877US877&oq=viscosity&aqs=chrome..69i57j0l7.1597j0j7&sourceid=chrome&ie=UTF-8

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