Hydrptest

[edelvi]

My question is regarding to the following information:
Pipeline Section: D= 40”, WT= 20 mm, L=40 km. High point (end point) = 500 masl./ Test point (DW and lowest point) = 50 masl / Fresh Water filtered to 200 mesh , average water temperature 12°C. Filling operation was interrupted several times taking 3 weeks to do it.
Pressurization commenced from a pressure of 50 bar (static head) with a pressurization rate of 1 bar/ 5 minutes. Pressurization was going on with a pressure reading of 70 bar when suddenly pressure readings were observed between 35 and 75 bar during two seconds (the pressurization stopped), and then the pressure readings oscillated between 70 and 35 bar during 30 minutes until it stabilized at 70 bar. No leaks were found at open locations (two valves and test headers). The section has a normal profile without strong changes. Absolutely sure filling pigs (two) were at the end of the section in the test header…

As we do not find any explanation, some one kindly could tell me the possible(s) cause(s) of this phenomenon.

Thanks in advance

 

[Goahead]

Could the pressure oscillations be due to pressurized air bubbles suddenly collapsing?

http://www.welding-advisers.com/

 

[edelvi]

Just 0.009% of air content was calculated for this secction....

 

[Elvie]

at 0.009% of the pipeline volume you still have a nice air bubble (+/-2.7M3)trapped at the highest point.So this is the explanation.If the hydrotest pressure stay steady for 4-6 hours at least, then your test is positive (provided temperature does not change-but you may record also temperature and then correlate the two trends).

 

[edelvi]

The highest point is the end point. Delta of temperature during the pressurization was 0.03°C (increasing) . As you already know, pressure strongly changed from 70 to 35 (35 bar) and then to 75 (40)Bar. Is there any compatibility whit the only +/- 2.7 pressurized air m3?.

 

[ApC2Kp]

edelvi,
Your 40" pipe x 40km (25 miles) with 70 bar pressure is a rather extreme pressure vessel, but some piping charcteristics can help understand the behavior observed.
First, it is assumed the pipeline is welded carbon steel and buried underground. Also, the 40 km length has turns and changes of direction and elevation - there are peaks and valleys along the run. If filling the pipe was interrupted several times during the three weeks while filling it, then it is likely some air pockets or voids could have been introduced despite the use of two filling pigs.
There are several things that may have occurred after the pressurization was stopped. Sudden changes of pressure by closing (or opening) a valve / pump can result in liquid hammer. A pressure wave from the test header would go into the pipeline. The time for the pressure wave to travel 25 miles to the end and be reflected back to beginning might be compared to any data of the pressure oscillations.
The pressure wave would generate a considerable force at each change in direction. The 40" cross section area would be approximately 1160 sq.inch multiplied by the 35 bar differential pressure spike results in momentary force of 588,700 pounds at each "elbow" or change in direction going down the length of pipe. There may have been some shifting of the pipeline in the ground unless adequate anchor blocks were installed at the changes of direction.
The pressure spikes could have interacted with any air pockets in the line. Air pockets might expand and contract signifcantly with the 35 bar differential. Liquid slugs in valleys between the air pockets at peaks could result in the liquid slugs "sloshing" forward and backward.
Another result of the 70 bar pressure might be simple pipe length expansion. How much would a 40 km length grow due to the pressure? A sudden pressure spike might cause the soil friction to "break" and allow the pipe to move. The movement of pipe would not oscillate, but the liquid could again be "sloshing" to produce the pressure oscillations.

 

[edelvi]

I am very thankful for the instructive answers.