Hydrotest Question

[JohnRAA]

Hey All,

We need to Hydrotest a pipeline that has some large elevation changes. ( approx. 1500 ft)
The pipe line will be in nitrogen when we begin to fill it with water for the Hydrotest.
Our current plan is to maintain a nitrogen backpressure on the filling pigs so the line does not go slack
on the down slope sections. The concern is that if we don't maintain enough back pressure on the pig
we could create gas pockets in the Hydrotest section that could then interfere with the test.

1) Do you think this is a valid concern?

2) If we do get some amount of gas in the Hydrotest section. And we allow
the line to stabilize at around 1000 psi, is there a good chance any gas pocket
will re-dissolve into solution.

Not sure how concerned I should be about a potential gas pocket interfering with the test.

Thanks

 

[bimr]

Yes, it is a valid concern.

Care must be exercised in the hydrostatic test operation to make sure that no gas is introduced into the test section, thus minimizing the possibility of air locks. Air locks are caused by gas accumulating in the downhill leg and water accumulating in the uphill leg. In some instances, high pressures may be required to overcome the air lock. In addition, care should be exercised to be sure the maximum allowable pipeline operating pressure is not exceeded. It may also be necessary to tap the line and vent air at the high points.

Never witnessed any problem with dissolving of gas. However, if you have gas in the pipeline, it is difficult to conduct the hydrostatic test.

Where the pipeline traverses hilly terrain, the elevation gradient must be carefully considered in selecting the pipeline test segments. Different companies have differing philosophies on how to do this. Some limit the amount of elevation difference while others may specify a range of allowable percentages of the SMYS of the pipe. The test gradient should be plotted to be sure the test pressure falls within the specified pressure limits.

 

[BigInch]

you are doing testing, not initial filling. If you try to keep enough backpressure on the pipeline with N2 while filling in mountainous terrain, you just might wind up doing a hydrotest with N2 rather than water. Not a great idea. The maximum elevations of the test section you select should allow uniform filling at a velocity high enough to keep the N2 ahead of the water while preventing any run-away water columns. And if the column does break and run-away, where will it go. It will surely stop when it gets to the end of the test section. Keep the end of the test section close and it will recover. The broken water column will not contain N2, it will have water vapor at near vacuum conditions. As the pressure builds from your fill rate, the vacuum pocket will collapse, without leaving any vapor or gas remaining there.

 

[LittleInch]

in answer to your questions

1) Is it a valid conern - Yes I think it is. I can only assume this is designed as a liquid line which allows for the variance in pressure due to hydrostatic head without over stressing the pipeline. It is a bit difficult to make real judegments without looking at the profile, but you should be keeping control of your pig. Without sufficient back pressure you could generate large velocities and in theory cause issue if you have any sharp bends. When the water column finally catches up with the pig it can generate very high pressure spikes.

If you get runaway, the water will vapourise as it reaches very low pressure. Whether all that air / vapour goes back into solution is questionable and given that the limit on most tests is less than 0.2% gas, it might just push you over the limit. if the low pressure is straight behind the pig you might get N2 bypass as well.

2) This is part of the issue - you don't really want pressure to fall without a reason and gas dissolving back into the water is difficult to calculate.

3) Gas in a hydrostatic test leads to strange results and does not allow calculations to be made to show that any changes in pressure ( up or down) are due to temperature fluctuations.

There are many guides on testing, but I've always found the BS PD 8010 part 1 code to have many useful equations, graphs and notes over several pages which allow you to use them and quote from a published code and not some rand=m internet sources.

good luck and let us know how it goes - you have quite a significant elevation change going on there.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JohnRAA]

Thanks for the answers and insights!!

1) I hadn't considered the possibility of an airlock situation. Another good reason
to avoid creating a pocket of gas in the test section during filling.
Especially since the profile is very jagged
with lots of little peaks on the way up the hill and also on the way down. We have a vent at the very top
but likely very little of nitrogen blow/evolved vapor will make it to the top.

2) I had been imagining that in vacuum situation (the pig runs away) that the pockets would be created by dissolved
gases coming out of solution. Now I can see that it is most likely to be water vapor.
Doesn't change the risks, but does clarify my understanding.

3) Our new testing procedures do require the line to sit and thermally stabilize at a fairly high pressure.( over 1000 psi)
This increases the chance that any gase or vapor may go back into solution

 

[BigInch]

Water vapour will condense, gas will most likely simply reduce its volume as its pressure is increased to your 1000 psig stabilization pressure.