Pneumatic Testing of Fuel Gas Conditioning Skid.. 1

[Venkatachalam Ponnusamy]

I have to carry-out a Pneumatic Test by using either air or N2 (Nitrogen) for my 3 identical Fuel Gas Conditioning Skid (FGCS) after assembly of complete vessel & piping's at our shop, this is to verify the integrity of all piping/vessel joints for offshore application.
As per our Customer Requirement the test to be carried out @ Maximum Operating Pressure x 1.1 times (i.e pneumatic test pressure is 52 Kg/cm2).
I have N2 cylinder with the specification of each 7 m3 filled at 155 Kgf/cm2.

The Total volume of each Skid is around 1.5 m3 (including piping & vessel).

Now i have to calculate theoretically for No.of N2 Cylinder required to complete the test at 52 Kgf/cm2. Can i calculate by using Boyle's law for the No.of Cylinders? or any other way..
because last time the no of cylinders consumed at shop vs our theoretical calculations are not matching..

I request experts/experienced persons to assist me how to calculate optimum no.of Cylinders required to complete the test & how effectively we have to carry-out the test at shop for consuming optimum no.of cylinders.

 

[LittleInch]

I think the cylinder size is the amount of gas at atmospheric conditions?

YOu will only be using a max of 2/3 of this volume if you start at 155 bar and end up at 50.

So the simple way is to convert it all to standard conditions and work in rought numbers.

So you need 1.5 x 52 = 78 scm of gas.

The problem now is that each cylinder of nitrogen will gradually use less and less nitrogen before the pressure equalises between the bottle and the test rig.

SO you need to recalcuate for each bottle. Work out voolume put in and end pressure to know how much you can get out of the next bottle

Bottle 1 is good for the first say 7 m3, but the second on only say 6.5, the next 6, the next 5.5 etc....

So a rough figure of say 5.5 per cylinder on average and you're looking at 15 bottles plus or minus.

If you do the next skid you can reduce this by using the last bottle first to delete it of all or most of its nitrogen before starting the new bottles so maybe say a couple of bottles for the enext ones. Or re-use it? You would get 50% of the gas back into the next rig.



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[Venkatachalam Ponnusamy]

@ Littleinch,

Thanks for your response and well noted and will calculate the no of bottles as suggested.

Yes the cylinder size and amount of gas is for the atmospheric condition only.

One more suggestion from our side is, based on the skid volume & test pressure how about connecting 4 or 5 cylinders in a common manifold and start filling the gas to the testing skid with pressure regulator, is that not going to be effective and time consuming process and as well optimum consumption of N2 bottles ?.

Series of N2 bottles connected in common manifold also available in the market.

 

[LittleInch]

It will probably all work out the same, but sure, speed might increase if you use a quad of bottles. But then they will all be at the higher pressure when you stop so maybe less efficient use of gas.

It's the fact that your pressure in the test unit keeps climbing that makes it more complex. Try working it out to see if one bottle at a time or 4 bottles is most efficient use of gas.

But whatever you do you will end up with some bottles with up to 50 bar left in them.

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

 

[r6155]

Venkatachalam Ponnusamy
Can you tell us why the customer's requirement is pneumatic test?

Regards

 

[Venkatachalam Ponnusamy]

@ r6155
The application is at offshore platform for Natural Gas Conditioning, pneumatic test is to check the system integrity & to check leaks if any in the piping joints. since the natural gas also contains H2S content as well so it is hazards and not safe if leaks in offshore-platform.

 

[r6155]

@ Venkatachalam Ponnusamy
Is the water allowable in the system?
If yes, do a hydrostatic pressure test to check the integrity and then leak test with air at low pressure. See ASME V for leak testing.

Regards

 

[LittleInch]

I've seen client requirements before for high pressure gas leak test.

Just make sure you do it outside... Even a small N2 leak at that pressure could result in loss of oxygen in a confined space. Make everyone carry oxygen meters if its inside a building.

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

 

[r6155]

I worked during fabrication of nuclear components and leak testing with helium at low pressure, including vacuum.
Never pneumatic test to check integrity, if water is allowed for the system.
Not all customers make good specs.

Regards

 

[Venkatachalam Ponnusamy]

@ r6155,

Yes hydro test is also done (Pipe spools & vessels separately).

In addition to hydro, we have to carry-out the air or N2 leak test also at Maximum Operating Pressure x 1.1 times.

Many times we have insisted client to accept hydrotest + gas leak test at low pressure (i.e 3.5 Kg/cm2), but our client said no deviation on the leak test is acceptable since this is to make sure the safety at offshore platform as the gas contains H2S as well.
This procedure is mainly because based on their past offshore platform experiences & they consider the safety of personnel with at most care.

@Littleinch,
Noted your points, the test will be carried-out in Open area.

 

[r6155]

@ Venkatachalam Ponnusamy
I had a similar experience and invited the client to witness the pneumatic test and take the same risks as our staff. They did not come.
This was a good opportunity to charge them a very high extra cost. And they paid for it.

Regards

 

[LittleInch]

So have you calculated how many bottles?

Can you reuse the N2 in more than one skid?

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

 

[FacEngrPE]

If all of the vessels and spools were shop proof tested, and only bolted mechanical joints were assembled in the field (no welded joints), then a leak test at 1.1 MAWP is not much different than an operational pressure leak test. This is about the pressure you get when the relief valves are fully lifted.

I would recommend that the pressure increase be stopped along the way so that leak tests can be performed before you get close to 1.1 MAWP. This potentially will reduce test gas consumption as any leaks found will need to be fixed before proceeding. This can also minimize personal exposure at the high pressure end of the test.

Fred

 

[georgeverghese]

If you stop the N2 quad banks at say 65bars, I get 20 bottles - (5 quad banks?)per skid, assuming actual water volume of each N2 cylinder is 45litres.

While there is some possibility of leaks through piping flanges, greater likelihood is through instrumentation compression fittings. For high H2S, think compression fittings should be avoided and all tubing conns should be seal welded.

 

[r6155]

Perhaps this design should have been treated as a lethal service.

Regards

 

[Venkatachalam Ponnusamy]

@ Dear All:-
In Fuel Gas Conditioning Skid, we have done the testing in three different pressures as follows:- Test Pressure is to be done for Maximum Operating Pressure x 1.1 times.
1. Upto PCV - Test Pressure 1 (65 Kg/cm2).
2. After PCV - Test Pressure 2 (52 Kg/cm2).
3. Flare Header (PSV outlet) - Test Pressure 3 ( 15 Kg/cm2).
Overall it consumed around 11-15 N2 bottles approximately per skid. Thanks for the inputs and experiences shared.. definitely it will be helpful for the future..

Regards,
Venkat

 

[LittleInch]

Thanks for letting us know what the real life experience is.

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