VACUUM TEST FOR SUBSEA POD 1

[BEMPE16524]

Hi Guys,

Need your help here. I have a subsea POD to operate at maximum depth of 3000mtrs. Because of restriction to use a test chamber for external pressure test, we decided to do vacuum test.

the problem is what should be the internal pressure for the pod?
FYI, the pod will contain no fluid.

 

[MikeHalloran]

Pressure does not add quite like voltage does.
You can't apply a head of minus 3000 meters of seawater to the inside of the pod, because vacuum doesn't work that way. The most pressure you can apply to the outside by evacuating the inside is one atmosphere.

You need to build, buy, or rent the chamber to do the external pressure test, or just don't bother.





Mike Halloran
Pembroke Pines, FL, USA

 

[LittleInch]

I agree with mike, the best you can do is subject the inside of the POD to near vacuum, but this is only equivalent to 10m of water (difference of 1 bar between inside and outside).

So we can only assume that from your point - "..we decided to do vacuum test" that you meant to subject, somehow, the inside of the POD to a vacuum,

Otherwise the pressure difference is the wrong way.

This doesn't look very big, what is the "restriction" on testing??

Can't you build one from some large, thick pipe? Design pressure of 300 barg isn't that high.

What are you testing? The ability of the "POD" to withstand negative pressure or the seals or what?

Is this thing actually subject to hydrostatic pressure or does the POD fill with water?

lots of questions / unknowns

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

 

[BEMPE16524]

Guys,

We have the test chamber actually, but due to other issues, the management insist on vacuum test.
I've spoken with someone who already did this test before. Basically what he did was just maximize the differential pressure between the internal and external.
I saw in his test report, the differential pressure is only 28-29 inHg (maximum value on the gauge). He only tested the seals integrity not the structural basically.

I appreciate all your replies and TQVM.

 

[racookpe1978]

It is the mandatory "Make the management feel good test" while they complain about schedule and cost, right?

 

[3DDave]

Anyone done a stress analysis first to determine how much deflection there will be? I'd look at an FEA that had a slightly pre-distorted geometry, such as ovalization or a dent, to see if the structure is stable.

 

[MikeHalloran]

due to other issues, the management insist on vacuum test.

Actually, when you are using pressure-energized seals, e.g. o-rings, it is possible to have a seal assembly that will withstand several thousand psi, but will leak at a differential pressure of just a few inches of water. But a dp of one atmosphere is usually more than enough to energize the seal and stop the leak, so a vacuum of 29"Hg is not really the best test pressure for that purpose. I'm going to guess that your managers are thinking of other, other issues.



Mike Halloran
Pembroke Pines, FL, USA