On Pressure Test for Heat Pipe System

[geotechniqa]

I am doing a water-pressure test on U pipe installed 200 m within the ground ( for heat source pump system).I applied a pressure of 700 KPa . However, the pressure after 9 hours drops to 650 Kpa. The temperature is not less than 1 degree. Does that indicate leakage? Is it acceptable

 

[BigInch]

Sounds like a leak, but could be material expansion. What is the piping material, wall thickness of the pipe and joint connection method? In the meantime I suggest a retest.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************

http://virtualpipeline.spaces.live.com/

 

[metengr]

What is the operating pressure in the pipe system and do you have any connections (flange or otherwise) along this length?

 

[geotechniqa]

Thanks for your reply. The pipe should not be expandable. Is it because of the existance of water pubbles in the pipe.
The upper joints seem to be fine. There is only one joint I beleive. By the way before the installation the pipe was air-tested and the drop was insignificant 10 KPa only.
I can't test the operating pressure in the pipe system because it is into the ground. I just apply a pressure through a pressure gauge on the top end of the pip.

 

[pennpiper]

One lesson here is:
Never back-fill the trench until after the Hydro-test is successfully completed.

 

[geotechniqa]

We did not grout the trench yet

 

[vpl]

Ok, bit of rant: this is why you set acceptance criteria BEFORE you do the test -- so you know what is or is not acceptable.

In my world (nuclear), it would be considered indication of a leak. Whether that leak was acceptable or not would depend on what the acceptance criteria were.

Did the pressure decreased gradually throughout the 9 hour duration, or did it drop off suddenly? If it was a sudden drop-off, something more then likely failed. A slow drop-off might be nothing more than a loose test fitting. Or it could indicate a small leak (which might be acceptable for your final situation. However, you would need to decide how much of a leak you would have under normal conditions and what the environmental and finacial repercussions would be of having such a leak.


Patricia Lougheed

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[geotechniqa]

Now if the water in the pipe has air pubbles, could that cause this drop in the pressure.
Appreciate your answer

 

[BigInch]

It could amplify a normal pressure drop expected with a change in temperature. Air must be out before hydros.
Did you put in warm water, then it got cool after it was in the pipe?

Did you measure the water volume you put in the pipe?
How does that compare with the inside volume of the pipe?
Did you keep a chart of water volume injected vs pressure?
Did you wait for the pressure to stabilize? Did you wait for the temperature to stabilize?
Did you have to inject a small amount of water to bring the pressure up to test pressure again before you started the test? How much water?

I'd suggest you retest, but vent the air first. Refill to test pressure, wait until press & temp stabilize, top up again to test pressure, then restart your test time period.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************

http://virtualpipeline.spaces.live.com/

 

[geotechniqa]

How can you gurantee that there are no water pubbles. But even if there is water space in the pipe. It could be very small and it will not be that remarkable to lead to this dramatic drop.And by the way
Imagine that you have a closed container with two liquids:
Water and Gas (say air) and then you apply pressure at the Top of the container through a piston and waited say: 1/2 h until the pressure stabilizes. After then the pressure should remain constant if the container remains closed and assuming the temperature doesnot change.So the existance of the air inside the container should not change the stabilized pressure
Please correct me If I am wrong.


The pressure stabilizes
The Temperature is OK
Water is inserted at normal temperature

 

[BigInch]

That's right. If there is air inside, when subject to temperature change, it is very hard to ever stabilize enough in a reasonable time to start the test period. Besides, that's the only reason you have not to declare a leak now.

If you measured the volume of water going in, you could subtract that from pipe volume and know how much air is still in there.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************

http://virtualpipeline.spaces.live.com/

 

[geotechniqa]

I repeat my question In other words
Can we get a stabilized pressure with the existance of air inside the pipe?
By the way The water is already inside ( too late to measure) and suppose I do not want to reflush and full it again

If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************

http://virtualpipeline.spaces.live.com/

 

[ApC2Kp]

geotechniqa,

There are some high pressure hydrotest methods that first fill the system with water, then use a compressed air or nitrogen bottle (3000 psig + regulator) to let small amount of air/nitrogen into piping for pressure. The test duration is usually shorter - 30 minutes to 2 hours.

Most waters will absorb some air, nitrogen, CO2, or vapors depending on pressure, temperature, water purity. Think of how carbonated beverages get saturated with CO2. Steam boilers with condensate return have deaerators to remove the oxygen and CO2 in condensate waters that would cause corrosion. So yes, the air bubbles could be absorbed into the hydrotest water to result in decrease of pressure.

You do not need to be concerned about corrosion for your hydrotest. Many underground heat source pipe systems also are HDPE plastic pipe systems, so that there is no corrosion on inside or outside of pipe to worry about.

The HDPE plastic pipe will swell (increase in diameter) during long term moderate pressure. Most plastic pipes are rated at only 10 percent of tensile strength because of the long term swelling - plastic cold flow that occurs. So if your hydrotest pressure is more than a few percent of the rated burst pressure of the plastic pipe, then you could very well see a drop in system pressure from the swelling of the plastic pipe.

If your system hydrotest has an initial drop in pressure, but then is stable for several hours, you might be able to conclude that no significant leak exists in the system. You might get a short spool of the HDPE pipe, fabricate it with closed ends as a hydrotest sample, and pressurize it to observe the swelling and pressure decrease over several hours of time to gauge how much pressure decay occurs - even without any observable leakage contribution.

 

[BigInch]

That's exactly why I asked about the pipe material.


"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************

http://virtualpipeline.spaces.live.com/