Hydrostatic Pressure Testing with entrapped air and temperature

[mech9238]

Hello,

I am new to this forum but have visited it several times.

This might be a basic question but wanted to ask:

My question is for performing hydrostatic testing for B31.9 Piping, Pex Piping, Radiant heating systems, and standpipe hydro tests. All testing would be anywhere between 15psi and 450psi.

1. One question that arises is when performing a hydrostatic pressure test air in the system will lower the pressure (air bubbles in domestic water or trapped air and not vented out).
How would this be possible? How would air in piping change the pressure over-time? Would temperature effect the air and water creating a pressure drop?
Examples: Piping tested at 15psi for 2 hrs
Standpipe testing for several floors (8 stories) with no HVAC
How much would the air effect a pressure test?

2. Radiant heating testing: Testing a radiant heating system with a pump, expansion tank, and electric heater--- pressurized with water and circulated for several hrs, however the outside temeprature and sun would heat the piping. would this cause a pressure drop due to thermal expansion? Would air in the system cause a pressure drop after heating? I would assume pressure would increase or stay the same.

Reading different material online ambient temperature will effect the expansion of the piping material, thus increase the volume of the entire system, lowering the pressure. However, the temperature of the water when increased, say from domestic water temperature to about 120 degrees water will expand. Does the thermal expansion occuring from piping and water temperature create a pressure increase or pressure drop?

Please let me know if you have any answers.

 

[JStephen]

Very small changes in the water volume would change pressure considerably. With time, does more of the trapped air dissolve in the water? That would explain the time element, but I don't know if that's actually a significant effect.

 

[racookpe1978]

You have to do the calc's.

Actual (estimated) maximum volume of water in side the pipe & tanks and plenumns.
estimated volume of air trapped in non-vented spaces, then initial and final temperature of the air in those spaces.
Estimated temperature change: initial pipe wall temperature, initial water temperature, assumed final water and pipe wall temperature.
Estimated length of time the pipe wall is heated by the sun (and location of shade available!)
Estimated outside air temperature (not underground, right?)

Get them into a spreadsheet, then change the pipe internal water volume and entrapped air volumes.

 

[mk3223]

Different from water, air is compressible fluid. So, the air is compressed, which causing the testing pressure dropped.
As the testing fluid temperature raised up for any reason, the trapped air is to be expanded faster which may cause the pressure increased.

 

[chicopee]

You have to vent all dead air pockets first. When that's done, circulate the water with minimal internal pressure for a while to ensure all air entrapped in water can be further vented at the dead air pockets( normally at high points in the piping system) and afterward, if possible I would run the water to freely discharge to a drain line to note if air is still entrapped then shut off the free discharge when satisfied. Temperature of water should be allowed to reach ambient conditions perhaps overnight so that pipe sweating is eliminated then perform you hydrostatic test.
Now here is another thought for food. When I worked at a research department of a Chemical company back in the late 60's and early 70's, we employed small vacuum pumps to de-aerate plastic formulations of trapped air bubbles before the formulations were cured in heating ovens so MAY BE vacuum pumps could be applied to accomplish the same results under partial pressure BUT VERY CAREFULLY as you don't want to collapse any parts of the piping system. Also, review some of the industry standards on performing hydrostatic testing on piping system.

 

[hydtools]

"Does the thermal expansion occuring from piping and water temperature create a pressure increase or pressure drop?"
There will be a pressure increase. The coefficient of expansion for water is nearly 6 times greater than that of steel; 3 times that of PEX.

Ted

 

[mech9238]

Thank You for the responses.

I tried to find industry standards for testing radiant heating panels. I could not find. Do you have any resources?

https://www.achrnews.com/articles/98315-the-truth-about-testing-radiant-panel-heat-systems

The article I read suggests ambient temperature would change the pressure but do not understand how thermal expansion of pipe would decrease pressure.

https://www.scsengineers.com/scs-ad...-pressure-testing-using-incompressible-fluid/

Performed a very quick calculation with online calculators to see if a water filled pipe would increase in volume over the expansion of the pipe. Did not take into account any air or trapped air.
Diamter: 6in
Length: 24IN (2ft)
Volume = 2715.43 cubic inches
Water Start Temp: 50 F, to 120F
Pipe temp: 50F to 120F

Thermal expansion of water = 133cub inches and total of 2848
Thermal expansion of pipe = .1814inches, total of 24.1814
New volume of pipe = 2735.9527

Volume of water increased and should increase the pressure.
I believe I did this correctly, but let me know.

If you could please read the article from SCS as that does not make sense to me.

Thank you for your help, greatly appreciated.

 

[hydtools]

It appears the SCS calculation did not include water volume change with temperature. It was only concerned with pipe change and note the pipe is HDPE.

Ted

 

[mech9238]

I was told my numbers were wrong.
The bulk modulus or coffecient of expansion was wrong.
Not that this anaylsis is needed anymore, but here are the results.
Air will expand as stated.
Air compression and thermal expansion do not effect the above tests for my piping. Only pressure increases
All previous numbers were wrong.

Water Vol:678.58 TempSt:50 Temp End: 120 Cof.Exp:0.000214 Temp Dif.: 70 Volume Change: 10.16
Air ---- Vol:0 ----- TempSt:50 Temp End: 120 Cof.Exp:0.0034 Temp Dif.: 70 Volume Change: 0

Volume Water+Air: 688.749201692763
Volume Piping : 683.714108315001


Start Initial End Bulk Modulus Temp Volume
Water ---- Vol:644.65 TempSt:50 Temp End:120 Cof.Exp:0.000214 Temp Dif:70 Volume Change:9.65
Air5.00% Vol:33.92 TempSt:50 Temp End:120 Cof.Exp:0.0034 Temp Dif: 70 Volume Change:8.07


Volume Water+Air:696.316092023681
Volume Piping:683.714108315001

Start Initial End Bulk Modulus Temp Volume
Water ------Vol:508.93 TempSt50 Temp End:120 Cof.Exp:0.000214 Temp Dif:70 Volume Change:7.62
Air25.00% Vol:169.64 TempSt50 Temp End:120 Cof.Exp:0.0034 Temp Dif:70 Volume Change:40.37

Volume Water+Air:726.583653347357
Volume Piping:683.714108315001

Temp Volume
Water Linear 24in Initial 50 End120 Bulk Modulus0.000108 Temp: 70 Change: 0.18144 24.18144 Polyehtelyne

Volume R 3 D 6 L 24 ----------Volume 678.584013175395
Volume R 3 D 6 L 24.18144 Volume 683.714108315001