Hydrotest or Pneumatic test of pipelines

[Mahmoudyounis]

My dear Engineers:

I just joined this group. I am working on a 56" Gas pipeline (450 km long) in the desert of Saudi Arabia. Water availability for future hydrotest is scarce. Is nitrogen pneumatic test a good alternative and what are the pros and cons.

thanks for your help.

 

[LittleInch]

Have you searched this site? There are many similar posts and articles.

One written by zdas04 on this is here

http://www.engineering.com/Designer...c=212727627.7.1459426648509&__hsfp=1774089731

Other posts include

http://www.eng-tips.com/viewthread.cfm?qid=345053,

http://www.eng-tips.com/viewthread.cfm?qid=358563

In my opinion, which has changed over time, is that for your instance, yes it's probably a good idea, just be careful at the exposed above ground ends. Using Nitrogen will be very expensive so maybe you will need to do different sections one after another and re-use the gas or just use filtered, dried air.

Biggest issue is usually getting approval from either client or government department.

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

 

[nickelkid]

My understanding this is very common is Siberia. I also understand that mishaps are not pretty. Be cognizant of how much energy you are storing. To me, that would be more important than N2 costs. I have been near 2 failures during a pneumatic testing, both a results from improper assembly, and one of them almost caused injury - high probability of a fatality.

 

[zdas04]

Mahmoudyounis,
You seem to be on the right track with regard not wanting to do a hydrostatic test (that test would be 4.6 million barrels [74 thousand m³], a lot to acquire or dispose of, not to mention the difficulty of drying it and the certainty of leaving enough water behind to create a very large MIC corrosion risk).

On the other hand nitrogen is pretty expensive and REALLY high risk. The problem is that it arrives on bulk trucks as liquid which is kind of cold. The vaporization equipment on the truck uses heat to change it back into a gas. When I write a nitrogen procedure, I make a big deal in the procedure that the nitrogen has to be heated to above 70F [21C], and a recording thermometer is required on the outlet pipe for any post-accident investigation. There have been a number of occasions where the nitrogen temperature was not monitored (or specified in the procedure) and the truck operator tried to hurry the process by injecting nitrogen that was below the ductile-to-brittle-transition temperature and the brittle failure of the pipe was spectacular (one happened near me that resulted in one fatality and permanent injury to two workers, the injection temperature was not monitored but the settings on the truck resulted in gas at -25F [-32C] in the post-accident investigation). Too expensive and too risky.

A fine alternative that I've used many times is to get an air pack that is used for wellbore cleanouts. These machines are pretty readily available, capable of very high pressures, and no risk of ductile-to-brittle transition. The argument that putting air into a hydrocarbon pipe is nonsense since you are testing new pipe that is already full of air.

In addition to the ENGINEERING.com article that LittleInch kindly mentioned, I have recently added a Static Test Document to my web page that might help you understand the issues.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[weldstan]

What are your design conditions; 72% of SMYS or 80% of SMYS or less? If you are at 80% of SMYS, the test pressure will be equal to 100%
of the SMYS. If you have a flaw in the weld that could lead to fracture initiation, you will need to assure that a minimum toughness is assured to prevent ductile shear fracture or brittle fracture. The pneumatic test can be done safely because personnel can be removed from the hazard area during maximum pressurization. In 2 of 3 pipeline hydrotests that I witnessed, failure occurred at long seam welds and it was easy to detect the water spout from afar. It will be easier to see the explosion during a pneumatic test. You must assure, as best as possible, that the weld is sound and the material will leak rather than burst at a leak causing discontinuity.

It is doubtful that you would test 450 km of pipe at one time. You could test sections thereof as is done in the western USA where water is also often in short supply.

 

[BigInch]

Don't test 450 km in one go. Break it up to managable sections such that any section does not require more water than is practical to supply. You might look into using sea water with appropriate corrosion inhibitors. And filling from destination to source of the gas.

 

[weldstan]

As BigInch stated sea water with corrosion inhibitors are often used in the Middle East. Clean up with fresh water must be done and this can be done in slugs held between pigs.