5 replies to this topic

[yincal]

Hi,

 

I have this system:

 

Natural gas from pipeline Oper @ 6200 kPag (with PAHH @ 6700 kPag protection) with a control valve letdown to 5000 kPag. The 5000 kPag side has a PSV set @ 6206 kPag.

 

My question is - is there a control valve wide open scenario here?

 

To consider CV wide open,

    P(in) = 6700 kPag

    P(out) = 6806 kPag (6206 @ 1.1 as per API 521)

    ==> CV wide open case is not applicable.

 

Or should I consider P(out) as 6206 kPag to "force" a CV wide open case just to be conservative?

 

Thank you!

 

Yincal

Edited by yincal, 18 July 2023 - 12:45 PM.

[latexman]

I see no scenario if:

• The pressure is normal (6200 kPag) and the CV fails - single jeopardy.
• The pressure is high (6700 kPag) and the CV does not fail - single jeopardy.

But I'm willing to listen to others.

[snickster]

I would consider designing for high pressure in combination with control valve failure unless I could be totally convinced that the pressure never fluctuates above normal operating except for very very rare instances.  In fact I would look at the source of the pressure and determine what could be the maximum possible pressure and to be safe I would design for that pressure and not depend on the PSHH to warn the operators in enough time to shut down the system.  This may be a conservative design but in this case the cost of a more conservtive design and that of a less conservative is not that much - you still going to have to install a decent sized relief valve in any case.  I would sleep much better at night not having to worry about freak things happening.

[breizh]

Hi,

I will pay attention to the failed mode of the valve, should be closed in case of lack of utilities.

Breizh 

[PRVBoss]

Verify the maximum normal upstream operating pressure (How close to PAHH can we get without tripping alarms and is it reasonable?). Verify the downstream design pressure (is the PSV set appropriately?); we're not just sizing PSVs for the sake of it, we're protecting something with a design pressure / MAWP.  If your upstream pressure is greater than design, but less than allowable accumulation, then any sized RV is adequate. 

[Pavan Deshmukh]

Hi,

 

Your upstream pressure ( PAHH @ 6700 kPag) will cause the overpressure to the downstream system set at 6206 kPag.

 

Because of this you have to do control valve failuer case for protected system of this PSV.

 

If you upstream and downstream system design pressure is same the you can say CV failure is not applicable.

 

 

Thanks!

Edited by Pavan Deshmukh, 27 July 2023 - 01:31 PM.