Hydrostatic pressure test increased?

[staedtlerwtc]

I have a hydrostatic pressure test for a HWS/R system, 1-1/4" pipe, approx 40 ft in length.

Initial test pressure was 225psi and after 4 hours the test pressure increase to 280psi. I'm used to test pressure decreasing due to a leak but never seen test pressure increase by 55psi.

I understand that test pressure may increase slightly if the temperature is exteremly hot and humid, but this test was conducted below grade not exposed to any extreme heat or direct sunlight. Air was vented and pressure was given enough time to stablize.

What could be the cause for such an high increase in pressure? Could the short distance of pipe being testing play a factor?

I appreicate the help. Thank you.

 

[LittleInch]

Depending on your start temperature (15 to 30 deg C), the pressure increase per degree C can be between 25 to 50 psi (I know - mixed units). Hence it is very important to let the water stabilise with the ground temperature before pressurisation, 8 hours should do you for such a small pipe or over night.

If even a small section is above ground it can have a big impact on the pressur if exposed to sun. Length is only relevant if the temperature changes all at the same rate. A longer line will generally just even out areas which increase temp with those that decrease, but volume doesn't come into it until you draw off some water.

See also this very comprehensive FAQ.

http://www.eng-tips.com/faqs.cfm?fid=1339

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[GenB]

It was not water expansion but your pipe shrunk. Which is more expansive, pressure increaser genblr

 

[gr2vessels]

The pipe shrunk under the hydrostatic pressure?? Please explain.

 

[chicopee]

Faulty pressure gauge, perhaps?

 

[GenB]

'had a similar issue. in this situation the metal shrinks a little creating the pressure to increase. the water temp may stays the same, the metal changes state.
less space, more pressure.

 

[staedtlerwtc]

I'm not sure I get it. How can a hydrostatic pressure cause the pipe to shrink, w/o temp change? Am I missing something?

 

[tamal1475]

To pressure to increase by 25%, either the volume of the pipe bore must shrink by 25% or the absolute temperarture of water should increase by 25%. I do not think these are possible to be missed during conducting the test. One possible reason could be some internal chemical reaction which produced some gas. Can you repeat the test? Appears to be an interesting mystery to be solved.

 

[LittleInch]

Tamal. Please think again. This is water were taking about here, not gas. If you have a piston full of water and try and compress it 25%, you'll have a lot lot more than 25% preassure increase....

There's no interesting mystery here at all. Even two our three degrees C can easily make up the sort of pressure seen by the test. Staedler, please advise how long you left the pipe full of water before you did the test. Anything less than a couple of hours doesn't give the water time to stabilise with ground temp. Also what was burial depth? Anything less than 300mm or so will react to surface temp (sun etc) if you're somewhere where the sun shines a lot.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way