Requirement of full bore isolation valves at PSV inlet and outlet as well as BDVs 1

[Sawsan311]

Dear All,

I would like to seek your views on the following:

- Requirement for full bore isolation valves at PSV inlet.. ASME SEC VIII recommends them to be full bore by stating'' full area stop valves'' . While API 520 Part II removed this requirement in its latest revision.
- Requirement for FB isolation valves downstream the PSV
- Requirement for full bore isolation valves downstream the restriction orifice which is on emergency blowdown valves.

Thanks

Regards,

 

[pennpiper]

Sawsan311,
Typical, no detail data:
- No Line sizes?
- No Flange Ratings (Class 300, 400, 6oo, 900, 1500)?
- No Commodity?

So my answer is use Gate Valves for both upstream and downstream PSV isolation isolation.

Sometimes its possible to do all the right things and still get bad results

 

[mk3223]

The isolation valves of PSV are to facilitate the maintenance if required for the operation without an interruption. The full bore design of the isolation valve is to reduce the flow restriction as the relief flow passing through the PSV system which is designed to protect the process from over pressurized.
IMO, it's still a good practice to use FB isolation valves in the PSV piping, if needed.

 

[RVAmeche]

The rationale is if you're going to have an isolation valve, you don't want additional pressure drop loss due to that valve. If you don't want an isolation valve on the inlet (ie, you take the equipment offline to do any repairs), that's your prerogative.

I believe the rationale for the downstream is the same; minimize pressure drop.

 

[georgeverghese]

In most cases, you cannot choose what you want to do, regardless of what justification you present - Owner Company standards will almost always take precedence.

 

[quark]

This is new to me. I thought there shouldn't be any isolation provision for PSVs. All I handled, so far, were batch processes and it didn't matter.

 

[dean427]

This is covered in API 520 Part 2 section 8. The norm is to provide block valves that are full bore but the actual verbiage in the code is that the flow area of the isolation valves has to be at least equivalent to the relief device inlet and outlet connection area and definitely greater than the nozzle flow area.

From 8.3.1..."The opening through all pipe and fittings (including stop valves) between a pressure vessel and its PRV shall have at
least the area of the PRV inlet connection. See 5.2...For outlet isolation valves, to help minimize the built-up backpressure, the flow area in the outlet isolation valve should
be equal to or greater than the outlet area of the PRV."

 

[Sawsan311]

thank you all for your views,

Mr.dean427 , API 520 P-II is past versions back to 5th edition, secton 8.3 on inlet and outlet isolation valves clearly dictated the requirements of full bore isolation valves aligning to the requirements of ASME SEC VIII Non-Mandatory Appendix-M '' stating full area stop valves''. During the development of the sixth edition, the above was striked through and the and kept the area of the inlet of the PSV including the stop valves to be equal to the PSV inlet flange. The question is.. does this statement retained in API 520 PII directly leads to concluding the requirement of full bore valves? Some Engineers challenged that the full area requirement can be met by a reduced bore isolation valve as long as the 3% inlet pressure loss criteria is met for sizing the PSV inlet piping.
Additionally, what about the requirement of full bore isolation valves downstream the emergency blowdown valves, this requirement can be associated with the mach number requirement, FIV vibration and the need to minimize the liquid condensation with potential pockets formation...

For your views and comments

Regards,