Refinery Vessel PSV Requirements

[JWJones76]

During a prior HaZop of a refinery process unit (a H2SO4 Alky), it was stated (I assume by the facillitator) that even though the PSV on the isobutane surge drum was adequately sized for the controlling fire case, API codes now required dual PSVs, so a second PSV should be added. I can't find anything or anyone to confirm this statement about dual PSVs is true. Does anyone know of a recent change to the API RPs on PSVs for ASME VIII vessels? This is not a ASME I boiler vessel.

 

[TD2K]

I'm not familar with an API code requiring dual PSVs. Ask the facilitator to provide the API number and section.

 

[don1980]

That's not correct. There's no API or ASME requirement for a spare PSV. That's a plant reliability decision for the owner to make.

Not having a spare PSV means there's a risk of having to shutdown the process to fix the PSV.

 

[JWJones76]

Thanks for the replies. This was a recommendation from an older HazOp, so the contract facilitator is long gone. Obviously, the "challenge" to supply the appropriate code or guideline reference should be done in future HazOps when issues like this come up. I think the guy was either confusing the ASME I requirement or thinking this item would eventually come under a level of protection analysis (LOPA) which might require additional levels that could be accomplished via a second PSV. However, if the thought was LOPA, any recommendation should be based upon the actual LOPA process, if really required, and the resulting ways to achieve additional levels of protection. This guy seemed to have "fast-forwarded" to an answer.

 

[pennpiper]

It would help me and maybe a lot of others if someone could explain what the difference between these is and why we need two different activities for what sounds like the same thing?

- HazOp - Hazardous Operations Study -

- LOPA - Level of Protection Analysis -

prognosis: Lead or Lag

 

[JWJones76]

The HazOp is required by OSHA's Process Safety Managment (PSM) regulations. LOPA is a subset of the HazOp procedure when the consequence of some "what if" scenario raised in the HazOp has the "highest" or "most severe" outcome such as loss of life, impact beyond the fence line, major fire, etc. LOPA is intended to be a more quantitative approach to developing HazOp recommendations for these types of scenarios.

 

[don1980]

All -

OSHA doesn't mandate HAZOP. HAZOP is fine, but there are other alternatives too, such as LOPA. OSHA 1910.119 mentions HAZOP and then goes on to say that equivalent alternate methodologies are also acceptable. LOPA is one of those equivalent alternatives.

 

[MortenA]

Maybe its not required - but a spare PSV can be worth the extra cost many times in many circumstances e.g.:

- If the vessels is operating 24/7
- If the vessel is connected to a common flare header

Best regards

Morten

 

[123engr]

JWJones76 (Chemical)

The provision of dual PSVs on vessels depends on company operation team. On the project we completed recently, based on HAZOP recommendation with input from the company's operation team, they requested us to replaced all single PSV with dual PSVs.

My past shall not limit my future....

 

[BenThayer]

under what "challenge" or question did this action item appear?

typically a HAZOP would list situations such as "NO FLOW" and then list the causes of No Flow (e.g., closed outlet valve) and then discuss the consequences of No Flow (e.g., pressure accumulation, high level, etc.) and then list safeguards (e.g., high pressure alarm, high level alarm, PSV, etc.). and finally would ask if the safeguards are adequate.

If the additional PSV was under a "High Level" situation then i would be asking if the PSV is large enough for liquid relief instead of vapor.

i think the rationale of the facilator might be hiding in the "challenge" of where it the action item was listed.

 

[123engr]

The challenge is captured under "NO FLOW" condition during the HAZOP study.

My past shall not limit my future....

 

[TD2K]

But the recommendation states that while the PSV was adequately sized, API now required dual safety valves.

I'm simply not familiar with any such requirement.