hydrostatic testing in gravity lines

[mgrez]

Before I tested a gravity line I took out the air off the pipe and then proceed to test. Once I reach the 150 psi (using like 50 gallons of water)I stoped my pump and the pressure went down very fast until reach static pressure.
I thought that we had a leak but we decide to perform a pneumatic test in the pipe. We used 80 psi to test the line and it held this pressure for 5 hours.

There are a couple of items to also consider:

1. At the bottom of the line there is a valve and 2ft before the valve there is a hydrant.
2. There are 2 air release at the top of the gravity line but none at the bottom.
3. The line was filled initially with the gravity flow, I mean just opening the upper valve.
4. My testing point was at the top of the line and not at the bottom.

I filled up my pipe very slowly after the pneumatic test, and proceed to re-test the line. I used only 5 gallon to reach the 150 psi but the pressure still drop very, very slowly.

Is there any posibility that air could be still trap in this line and that's the reason why my pressure is still droping?

If the pneumatic test doesn't show prescence of a leak, why could be the reason that the hydrostatic test is showing the drop in pressure?

 

[JStephen]

Water is pretty much incompressible. Air is not. So when you pressurize the line with water, and just a little bit leaks out, the pressure drops a lot. When you pressurize it with air, and a little bit leaks out, the rest just expands and the pressure drops very little. That explains the difference- doesn't help you find the leak, though.

If air is trapped in the line, and there are no leaks, pressure won't drop unless the air (or water) is cooling off- so that probably isn't what's going on there.

 

[rconner]

I noticed you said that your "testing point was at the top of the line and not at the bottom", and you also say, "the pressure went down very fast until reach static pressure". The combination of these statements/conditions is some confusing to me. Is the pressure gauge/indicating device also at the "testing point", and what happened when the pressure dropped fast to whatever this "static pressure" level is?
As far as an 80 psi air test, this is not a normal acceptance test for many pipelines with signficant pressure conditions (most test procedures ask on the other hand that the air be removed), and I would add to what has already been said that if there is an air water mix in a pipeline the already complex air testing conditions could also be some further complicated by temperature variations/air going into (or out of) solution etc.

 

[mgrez]

My testing equipment has the gage attached to it. It is just pump with a valve then a manometer and the line.

In this gravity line the flow at the bottom is arround 58psi. when I talked about static pressure I mean that increasing the pressure inside of the pipe to 150psi then slowly went down until it stabilize at 58 psi.
Maybe what I forgot to tell in my first message was that this gravit line has a difference in elevation of approx. 25ft.

Now, I already fix my problem, and pass the leackage test.
By the way JStpehen, that was a great memory refresher!
the problem was that it was a leak, the test was correct it was a big leak in the line. but I hope that it could help you guys what happend to me because it could happend to other people too.
We install a drainable hydrant. While we test with the hydrant close I was not able to held any pressure at all, but when I open the hydrant the water start putting pressure at the caps of the hydrant and I was able to get to 150psi. In theory when you open or close the hydrant there is a mechanism that close the drain, the thing that in my case the hydrant was defective and I was losing all my pressure at the drain of the hydrant.