air pressure test vs. hydrostatic pressure

Hi. guys/gals;

Need some help on information.

Just realized that this board does not have a category for exterior Commercial Water service lines.

Was faced with testing an underground water service today of eight inch diamter and 450 feet long. E. J. Precott, a water specialist supplier of pipes and testing for this sort of thing was on site, and the water pressure failed.

Since this was a line being tied in to about 65’ of existing line to a College Building, the final hook-up was made this morning. The previous section of new line was tested by EJP at 5 lbs. air pressure.

The last time I personnally installed a water line of that magnitude, I had been advised to pre-test with air at 50 lbs. which would equal the hydrostic pressure of 200lbs for half an hour.

My question’ What is the equation back up for air pressure over hydrostatic pressure.

I can not find the back up for this equation. I have previously been told that whatever hydrostatic pressure used, air pressure would be divided by four to attain the same result with air.

I figured some of you wise Plumbers out there would know this.

Thanks in advance.

Marcel :slight_smile: :slight_smile: :slight_smile:

Might find what you need here:


Two different tests, I will E-Mail you a standard.

From what I read, in sounds like you might have a air bubble in your 8" line, this is quite common in 8" and larger water lines, flush out the line more to move the bubble or if you air tested that line over 125% of the working pressure you could have moved a thrust block to cause a air leak.


Very useful thinking, and yes, we did have one 45 degree elbow with a thrust block, but was tested at five pounds of air. The other fittings that were used along with the clamp collars and threaded rods and thrust block was re-ecavated and there was no sign of leakage. Theese last fittings were used to tie in to the existing 65’ of existing lines that have been in the ground since 1977.

The only other possibility, is when they blasted for the sewer line a couple of days ago, it is remotely possible that they could have dislodged a bell joint on the parallel run of the water line.

Anything would help at this point because no one seems to know what to do.
The water specialists are coming back Monday with a machine called the (forget the name) that sences leaks in the ground within five feet.

My goal right now is for once and for all to get back-up for my conclusion of air test pressures vs. hydrostatic pressures, maybe then, some one will listen to me as how to test a water line of this magnitude without water. Most all of the time, the Water Districts will not let you open the gate valve box, so as to introduce bacteria and contamidated water in their system.

Thanks for the help.

Marcel :slight_smile: :slight_smile:

Peter S. ;

Thanks for the information, but what I am looking for is not in there that I could find. I did save it under favorites, though, because there are an abundance of good information.



Pressure is pressure Marcell 5 PSIG is 5 PSIG Air or water.

If you can not hydrostat the line, more Air pressure is really not an option as it is not safe on a line that large.

From your post it is not clear to me what is at the end of your new/repaired line where it ties into the existing line? Could there be problem there? Is the existing line leaking? Good luck Marcel

Brian; Thanks alot for your response. I respect your views always, and read most of them.

We are tied in to an existing 6" main that feeds an exisisting building at the College, and terminates at the Sprinkler System valve that is shut and the domestic 2" line is below that valve, where today you can’t do that. But in anycase, we can not eliminate the possibility that the 30 year line in not holding, but the College people are saying if it had been leaking all these years, evidence of it would have surfaced somewhere. Not much of an argueable point.

Three years ago when I was responsible to install my own 6" service, I was told by the same firm that is doing the testing now for my sub-contractor that I had to induce 200 lbs. hydrostatic divided by four which is 50 lbs. of air pressure to simulate the same pressures. I perceived that to be dangerous and put only 30 lbs. The test barely past at 200lbs. hydrostatic.

I am trying to find out who in hell came up with this conclusion of air pressure vs. hydrostatic so people stop looking at me cross-eyed.

I agree that air pressures are dangerous and water pressures you get wet only.

I do not agree with your statement that 5.lbs. or air is equivelant to 5 lbs. of hydrostatic.
I appreciate your thought and your replies are always well received.


Marcel :slight_smile: :slight_smile: :slight_smile:

I am just guessing that you used C900 for the main and used a mechanical joint for the connection to the old line. Marcel this is going to be way to much typing for me , I will call you instead.

Hi Marcel, good luck with the college people.:slight_smile:
If their line is at a pressure higher than your line than probably not leaking there. However if you have tied into their old line with your new, tested line could be a problem.

Pressure is still pressure, and if the gauge you use to read the pressure is in PSI/ATM/In. Mg/Microns/Torr/etc it really will not care what media is used to produce the pressure.

What weighs more a pound/kilo of air or a pound/kilo of water?:wink:

Thanks Brian;

I am tied in to 65 feet of existing line which was under the same test with the new.

You keep saying Pressure is Pressure, but I am still questioning the fact that water in known to be uncompressable and air is.

Finished reading articles from the AWWA and they keep saying not to test with air at all.
I understand it’s dangers with air, but still asking myself what is equal to what.
I would never perceive seeing 200lbs. of air, but can relate to it when it is water.
Amazing , the education one gets when you can not find the right answer.

Thanks a million.

Marcel :slight_smile: :slight_smile: :slight_smile:


I installed hundreds of miles of Ductile Iron, Transite, and PVC throughout the Phoenix area in my life, never was an Air test allowed. Sanitary sewers are air tested @ 8lbs. here, but not water lines.

Ductile Iron pipe if that is what your using will allow air to pass through the pipe joint rubbers, thus an Air test is worthless, unless they have came up with some sort of rubber which will not let air pass since I installed it.

Air in the line during a water pressure test will keep compressing the air, thus allowing it to escape around the rubbers, until most of the air is gone the pipe will not hold 200lbs, which is the test pressure here and I believe throughout most of the country.

Generally if a thrust block moves enough to allow leakage it has already blown-out. Mechanical Joints will slowly leak and are a bear to find sometimes, I have seen mechanical joints leak under a pressure test but not line pressure. I would start looking at all the mechanical fittings first, generally where the leaks are. A fish-mouthed rubber in a pipe will leak like hell, so I doubt that is the problem.

If your testing against an existing valve, sometimes they will allow water to back-feed under pressure, generally leaving the blow-out at the end of the line open and flushing the line working the valve a few times will help seal the valve your testing against, crap (small stones, pebbles) get lodged in gate valve, and sometimes will bend the bottom of the gate valve if you crank down on a small stone, which will never allow the test to pass.

So basically there are so many variables which will not allow the test to pass, you have to eliminate the easy ones first.

Marcell in a previous life I pressure tested a lot of things, with air, nitrogen, argon, water, liquid nitrogen, liquid heilum (for $6.00/hr:( ). Air compresses, A LOT and if anything failed it can be exciting. Water no compression, so when things break no expansion, depending on where you are you might get wet.

Now 200 PSI in a 1/4 inch line no big deal (area of 1/4 inch circle x 200 psi), 200 psi in an eight inch line, really big deal (area of 8" circle x 200 psi):shock:

I have always held the postition that your line leaks. Sometimes I have been right;)

Dale, and Brian, thank you both, and Dale, I believe what you say is true and what I had been advised 4 years ago by a water pipe, sewer specialist, I was had, and that would explain why I can not find an answer to my equation.

Thanks to the both of you.

Marcel :slight_smile: :slight_smile: :smiley:

Guys with all your help, I believe that I just had a Brain Storm that solved my problem.

The 8" water line from the Main and new valve box, a corporation was installed, and where the pressure test and chlorination was introduced. The line proceeded to climb for about 360’ and another corporation was introduced, supposedly to let the Air out.
My theory at this point, is that the line took a dive toward the existing line at 45 degrees and then 22 1/2 degrees down to mate with the old existing 6" line.

Since I was told that the corporation was installed at the high point to dissolve the air pockets, they where wrong. The air is trapped at the low side of the line going to the building and being compressed at that location losing pressure in minutes of the test.

When I go back Monday, I will instruct everyone to fill the system up again and bleed the air through the existing building low point where I am positive the air is trapped. Since the existing gate valves have been pr oven to seat well, all I have to do is release the pressure in the line and flush from a spigot available at the entrance.

Thanks again.

Marcel :smiley: :smiley:

Marcel, you probably won’t get enough volume with a hose spigot to make a difference.

I would either put another Corp. at the high point or a 2 1/2 inch Corp. somewhere toward the end of what your testing so you can get the water to move quickly.

Marcel; I agree with Dale on the lack of volume with a hose spigot. I also thing you are on the right track with the 45 and 22 1/2. I had a 500’ - 8" line that was feeding a flight simulator building (Luke AFB) that would not hold the pressure and it took about a week of filling and pumping up the pressure before it helded for the two hour test.

Dale; I am glad you did all of that typing, right answer.

Thanks Dale, I will try that tomorrow.

Marcel:) :slight_smile:

Well guys, everyone of you was correct and I respect that very much.
Brian; I finally confirmed today that pressure is pressure. Thanks.
I met up today with the pipeline specialists and told him that a few years ago someone from his office had told me to test with air at 200psi hydro divided by four= 50 psi with air. Well luckily my instincts took over at 30 lbs. and stopped.
He said your instincts served you well. He apologized for someone from his office not telling me the rest of the story.

Got a whole reference manual now guys out of it so watch out.

The key word in this whole concept is DANGEROUS if using air.
Finding leaks at low pressures of 30-5- lbs./sq.inch is as effective as a high pressure of 150lbs./sq.inch water test. This is the divided by four formula I was given a long time ago., and was not elaborated on, which could have caused an accident.

Well my line still leaks after digging up the bells in the blast zone and everything looked alright.
My bacteria test failed also on Saturday.
Had a good day. :slight_smile: :slight_smile:

Next step. Instructed the Site Contractor to install a new gate valve at the location of the old existing line and provide a corporation as was suggested by you guys to bleed the air. Now I will have three corporations. One for injection, one for high point air emittance and one at low point for final air emittance in case the high point don’t take it all out.

Again thank you for your help.

P.S. I know a lot of this is way beyond a regular HI duty, but know that some of you have a back ground in this type of work.
I think that everyone learns a little more by all this extra communication.
Also, I run a job like I am inspecting it myself. That is why the Company is well known in the State and my employer backs me up.

Marcel :slight_smile: :slight_smile: :slight_smile:


That should solve the issue Marcel. Probably is a real good chance the valve your testing against is allowing water to backfeed.

You’ll be able to chlorinate the line without sending high concentrations in the wrong direction also.

We were never allowed to chlorinate a line against an existing valve. You had to either tie the line into the existing after the bacteria test passed and the chlorine was blown off or install a new valve at the start of the new line.

I am sticking with the other guys line leaks Marcell.:smiley: