Admin Controls - locked open valves - API 521 Section 4.2.2

[Justice100]

Hello all,

When considering use of API 521 Section 4.2.2 – use of admin controls. If I have a high pressure vessel feeding a lower pressure heat exchanger with locked open isolation valves then for the closed outlet case if the upstream vessel has a PAHH set at less than 1.3 x design pressure of downstream HE (ASME VIII) can the relief case be excluded? Or can it only be excluded if the downstream design pressure is 1.3 x upstream vessel design pressure?

 

[Latexman]

If administrative controls are used to eliminate an overpressure scenario as a basis for the pressure relief system design, the user shall evaluate the potential overpressure in the event the administrative control fails, compare it to the equipment corrected hydrotest pressure (not the PAHH alarm), and consider additional risk reduction if the corrected hydrotest pressure can be exceeded.

Fail-safe devices, automatic start-up equipment, and other conventional instrumentation should not be a substitute for properly sized PRDs as protection against single jeopardy overpressure scenarios. There can be circumstances, however, where the use of PRDs is impractical and reliance on instrumented safeguards is needed. Where this is the case, if permitted by local regulations, a PRD might not be required.

Bold and red emphasis is mine.

Good Luck,
Latexman

 

[Justice100]

Thanks for the reply Latexman.

I think you might have misunderstood me regards your first extract, I am talking about the PAHH on the upstream vessel. The analogy would be that if I was doing a gas blow by calculation I would take the upstream pressure as the PAHH (max operating pressure), not the design pressure (Granted some companies I've worked for take the PAHH but others the PSV set point). Similarly, I was thinking I could take this approach for this scenario.

 

[Latexman]

You're welcome. Yes, I could have easily misunderstood; the OP lacks detail. In my defense, I don't consider a closed outlet case (blocked outlet) equal to a gas blow by case (liquid seal lost), but we have no details, so I guess it could be.

In your gas blow by case, is it a single jeopardy overpressure scenario? If not, the second extract is very specific about that, and it may not apply.

Good Luck,
Latexman

 

[georgeverghese]

These clauses are almost never used in most applications, unless the relief scenario being considered is of very low operational probability, which requires, for example, the coincidental failure of 2 or more instrumented protection loops or similar. Use of administrative controls alone to address an overpressure scenario by some means other than an RV requires Operations and Engineering Management written approval and is invoked only when a conventional overpressure protection mechanism is very expensive or impractical.
In almost all cases, the process/mechanical design pressure of the connected side of the HX should be the same or exceed the design pressure of the upstream vessel, to account for all operational relief cases.
The only case I can see for avoiding an RV on the connected side of the HX, given the LO isolation valves, is firecase on the HX connected side. Again, subject to the condition that design pressure of the connected side of the HX is the same as that on the upstream vessel, and firecase of both vessel and HX is addressed at this upstream vessel.