PSV Location 4

[IrwinL]

Hello,

Maybe a dumb question but I can't seem to find a clear answer from the codes (ASME sec. viii/ Xiii / API 520). Any inputs would be much appreciated!

Can a PSV be installed downstream of the protected pressure vessel?

I saw some configurations in a plant where a gas aftercooler (pressure vessel in question) doesnt have a PSV either on it or upstream after the upstream gas compressor. There is a PSV downstream the aftercooler, but I'd think that's more for the protection on the downstream systems, not sure if that will provide sufficient overpressure protection on the aftercooler itself. Most setups I have seen are PSV either connected to the pressure vessel directly or upstream of the vessel.

Thanks in advance.
Kevin

 

[The Obturator]

Your answers are in ASME VIII UG-156 & API-520 Part II.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[shvet]

Your issue relates to terms "overpressure" and "pressure accumulation". You should clearly understand first that accumulation≠overpressure, this point is critical for text below. See API 520-1 for details.

If there is a hydraulic resistance between PSV and protected PV than accumulation will differ moving from pressure source (compressor in the case you have described) to PSV. To compensate this effect a designer should:
- decrease set pressure of PSV, or
- increase overpressure of , or
- decrease accumulation in PV

I am not sure I have described an idea clearly but the core is simple - the further a pressure relieving device is located from a pressure source the higher hydraulic resistance impact is and therefore difference between accumulation and overpressure is. If a PSV is located between a pressure source and a protected PV that there is no flow between PV and PSV and therefore accumulation=overpressure.

So the answer is - Yes. PSV is able to be located downstream protected PV, especially in case a pressure source is located downstream of PV. If a pressure source is located upstream PV then such case requires a hydraulic study which in final overcomplicates the whole system design. It is more safe and robust design to locate PSV between PV and pressure source.

Note that if there is an element between pressure source and PSV then it is risky as velocities inside of hydraulic path during relief are likely much higher that those during operation. This fact creates a risk the internals are able to be detached/damaged and block/interfere relief path. For example valves internals, distributors, baffles, wire turbulators (your case), mist eliminators etc.

Check if internals are designed to withstand the worst relief case plus some safety margin. Also put in operation manual instruction for inspections of those to control deterioration.

 

[georgeverghese]

For cases where PSV is located some distance away from protected equipment, the built up backpressure at max process design relief flow should not exceed 103% of upper design pressure of this protected equipment. If it is higher, then the set pressure of the PSV should be lowered to keep within this limit.

 

[LittleInch]

It may sound obvious but there also needs to be no isolation valve or similar between PV and relief valve.

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

 

[shvet]

... there also needs to be no isolation valve or similar between PV and relief valve.

But there are plenty of installations where such is someway, correct? This restriction is valid but does not work in practice.

 

[The Obturator]

Per ASME VIII UG-156 which I referenced earlier, 'Stop Valves' (ie. isolation valves) are permitted under certain conditions.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[pierreick]

Hi,
To add to the replies above regarding isolation valve/stop valve, consider this document from Leser.

Note: You also may want to implement car seals (lock open) and car seals procedure if you want to install stop valve.

https://www.linkedin.com/pulse/what-does-car-seal-open-closed-mean-tim-hughes

Pierre

 

[LittleInch]

I suppose the issue is that any isolation valve needs to be locked open and not able to be closed whilst you have pressure in the vessel. That's not usual for a vessel main line isolation valve.

Same as any relief valve isolation valve.

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

 

[georgeverghese]

In compression circuits, there is usually a dual plate check valve downstream of the cooler, just before which is the tee off to the antisurge valve. If you have this check valve between the cooler and the PSV, check the Plant Owner company design standards if a check valve is permitted in the path from a protected equipment to its PSV. Lock open full bore isolation valves and phase separation vessel demisters are usually permitted.

 

[IrwinL]

Thanks for the input folks. They are helpful.