Leak test at high pressure 2

[sechon302]

We are going to carry out leak test for slug catcher in gas seperation plant. However pressure requirement for leak test is max. operation preassure. Slug catcher is designed to operate at 80 barg to max.120 barg.
In addition media for leak test is designated with nitrogen. I think that this requirement is unreasonable to carry-out.
If anyony has an experience such kind of leak test, pls give advice or alternate methods.
thanks,

 

[1969grad]

What types of leaks are you looking for? Flanges, valve seats, valve stems, etc.?

If it is valves then look at the requirements for the seat leakage test in API 6D. For valve seats, a low pressure leak test may be more severe especially using nitrogen.

I agree that testing at 120 bar is more a strength test than a leak test.

 

[1969grad]

BTW, I am assuming that this is not a new installation.

 

[dcasto]

A leak test is not designed to find the mechanical strenght as that was done during the design, material qualification, welding qualification and NDT. The leak test is to check the mechanical connections of the system. You can select a leak test pressure based risk. Improper flange torque or misalignments will be for at 20 bar. The same is true for threaded connections.

 

[sechon302]

It's slug catcher,and TOTAL specification for leak test said that leak test should be done at the operation pressure.Is there any mechine or compressor to increase such a high pressure,120 bar.?
Or any alternative you have?

 

[TD2K]

Sure, 120 bar is easily within the capabilities of a recip compressor. You might also want to talk to a well servicing company if they are available to see if a nitrogen pumper truck might be an option.

 

[JimCasey]

Generally hydrostatic tests are done with water..an incompressible. 120bar (1740 psi to us on the English system) represents a huge/dangerous potential release of energy if the test specimen fails. Also, a water leak more accurately represents the service for a slug catcher, and a water leak is a lot easier to detect and measure than a Nitrogen leak.

 

[vzeos]

sechon302,

It is not unusual to perform a leak test after performing the hydrostatic test and just before the piping is placed into service. The purpose of the hydro test is to prove the structural integrity of the pipe. The purpose of the leak test is to prove the tightness of the piping assembly including valves and other components. Since the MAOP is 120 bar, this is a possible operating pressure and should be leak tested at that pressure. Nitrogen is a good choice for the leak test since you can achieve your test pressure with bottled N2 and there would be no need for an inert purge after the leak test. I suggest that the pressure be brought up in stages (say 10%, 20%, 30% …. of MAOP) and leak test every joint with soap water at each stage. Assuming that the product is a combustible gas, it would be a good idea to check all joints with a combustible gas indicator after the pipe is put into service.

 

[EdStainless]

note: Do not use soap and water for bubble testing. Purchase fluids specifically made for this use. They contain surfactants for reliable bubble formation and no chlorides.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[TD2K]

If this is a slug catcher, I don't think bottled N2 is going to work. You are going to need a huge number of bottles and you get effectively less and less N2 out of each bottle as the system pressure is raised.

 

[vzeos]

Nitrogen is commercially available in high-pressure cylinders, tubes, or tube trailers depending upon the quantity required. The available pressures range up to 7500 psig. Quick-change connections and valves make it possible to use as many cylinders (tubes, or tube trailers) as necessary to achieve 120 bar in the test section.

 

[sechon302]

Thanks
I would like to add more explanation on this subject.
System volume of slug ccatcher is about 4000 m3 and nitrogen source is limited. Hence I am looking for alternate method instead of high pressure leak test at 120 bars.
Now I am considering that just after finishing the hydrotest, deprassurize teat pressure and pressurize with nitrogen from nitrogen bullet,150 bars, with water which was filled for hydrotest.
If anyone has another idea or comments,pls

 

[Homayun]

I am in no way a specialist on this subject, but after reading all the responses, my judgment says a couple of things:

1. this type of leak-tests is surely not the first ones being carried out. So there must exist somewhere within your company, or TOTAL where they describe how to Leak Test.
2. why not using Air? If N2 is mandatory, then make sure you have got enough available.

 

[grampi1]

An important consideration on equipment that has been in service is what the typical operating pressure has been. If actual operation is substantially below MAWP, then a gas test (or hydro) at MAWP can find corroded sections and cause failure. Have seen people badly hurt when a corroded screwed plug blew out on hydro. (Comes out like a bullet.) A failure of the primary pressure boundary with gas will kill anyone near by. Gas testing at design pressure should require second level engineering manager sign off and involvment of the safety group.

Please be careful

 

[azertyuiop]

I have worked at a natural gas compression facility , where we had to also do a leak test on a glycol slug catcher, but to 160 bar instead of yours 120 bar.

The volume was not 4000m3 like yours, but we managed to use air mixed with a certain amount of nitrogen to increase the pressure. The client requirement was pure nitrogen, but we talked him into submission.

The pressure was brought up to 60 bar with nitrogen, then use of a rental professional compressor (caterpillar) to increase up to 160 bar using air. At the end of the leak test, purged the system again with nitrogen to get the air out of the installation.

BEFORE starting to put up N2/air pressure on your system, do follow those sequencing steps :
. torque all flanges etc to their required couple.
. then tape all to be tested flanges etc on their circumference, so that the space in between two bolted flanges is covered by the flexible tape.
. then (using a fine needle) make a hole in each tape put in between the two flanges.
. mark the needle hole with a red marker to find it rapidly back during your inspection round.
. then increase your slug catcher internal pressure in steps, to 1 - 6 - 15 - 30 - 60 - 100 - 160 bar.
. in between each pressure increase, spray specialised leak testing fluids on all marked needle holes made on the tape, so that you can detect eventual leaks very rapidly. A bubble will form rapidly on the needle hole if there is a leak.
. if you discover a leak, torque check again, and if torque OK and still a leak, then change the gasket or use a better sealing arrangement, since your mechanical design is then proved NOT good.

good luck.

 

[Robinsnest]

It's not unusual to N2 test at this pressure but there are a number of specialist contractors out there who will do this using liquid N2, vapouriser and compressor on one skid, but it is still a lot of N2. Are you some place where it is difficult to get liquid nitrogen skids?