1) To do hydro-test of a section, you have to calculate the volume of water required to fill in that test section - pressure vessel or piping or pipeline....etc. ( Say it is X)
2) Amount of water required to raise the pressure from atmospheric pressure to desired test pressure ( say it is Y)
3) When the volume of water used for hydro-test is Less than X + Y, then air pockets are formed. This can be due to geometric shape of pressure Vessel or piping component or Pipeline
4) Care has to be taken that water filling to start from the lowest elevation point, and over flow should be from Highest elevation point, this would ensure that there are NO air pockets
5) In case of piping components, at times one has to have additional drain points and VENT Points.........for reasons stated in Point No.4. These are in addition to any drains or vents shown in the P & ID drawing.
6) In case of Pipelines one has to calculate theoretical water and water that is ACTUALLY pumped into the hydro- tests system (using flow meters between pump and test section)...then plot PV (Pressure & Volume). If the slope is seen as 45 deg C, then it is a case of ideal testing, any deviation indicates that much amount of air pockets inside the test section
7) Depending upon number of Air pockets inside test section, during the pressure holding period, should there be a ambient temperature drop.... the corresponding pressure drop would be very steep.... less air pockets, less pressure drop, more pockets more pressure drop due to ambient temperature variations
8) In a real scenario failed test case (pipeline), where hydro-test pressure was 114 bar, there was ambient pressure drop of 15 deg C (Nov - Dec, winter effect), the test pressure dropped to less than 60 bar showing heavy presence of air pockets...........
9) Having lost touch with calculations, as a thumb rule..... for 1 bar pressure variation for every 4 deg C drop or raise in pipe surface temperature due to ambient temperature variations, (taking this data from my grey cell memory and hence, request others to correct on this actual formula results)
10) One way of removing the air pockets is once you have reached 25% of test pressure, open the Vent Valve (highest elevation) and bring down the pressure to ambient (if in doubt) and again repeat it at 50% & 75% test pressure ..... so that entrapped air gets out during the venting........... time taking process, yet worth every bit of it....................
Sreeram_Srinivas
srinivas.sreeram@gmail.com
srinivas.sreeram@gmail.com
On Sun, Aug 12, 2012 at 9:45 AM, <materials-welding@googlegroups.com> wrote:
Sampath Kumar <skthatham@gmail.com> Aug 11 07:26AM +0400
Dear Experts,
During Hydro Test if air gets locked[Air Lock] [1] what will happen?
[2]What is the reason for air lock?[3]What is the precautions one has to
take to avoid?pls explain.
Thanking you all
With Best Regards
sampath
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