Allowable overpressures for Multiple PSVs with staggered set-pressures 1

[tono3]

Hello,
The staggered set-points of two (2) PSVs protecting the same vessel would mean one PSV is set at 100% of the Maximum Allowable Working Pressure (MAWP) while the second PSV is set at 105% of the MAWP. Consider the MAWP is 100psig for simplicity.
The allowable accumulated pressure for a staggered set-up would be 116% of the MAWP. Meaning that the first PSV will have an overpressure of 16% (116psig - 100 psig) while the second PSV will have an overpressure of 11% (116 psig- 105 psig).

However, in my case, I have an existing single PSV and am planning on adding one additional PSV to protect the same vessel. Both the PSVs will work in a staggered set points so the new PSV will be set at 105% of the existing PSVs set presure.

As such, my questions are:
1) If I keep the existing PSV as is (meaning allowable accumulated pressure is 110% of MAWP instead of 116%) can I use a lower allowable over pressure of the new PSV of around~5% to match the relieving pressure of the existing PSV.
2) Can the allowable overpressure of the existing PSV be changed from 10% to 16% of Pset. If so, what will need to be changed in the PSV to attain it?
3) Can the multiple PSV set-up have different relieving pressures (meaning the new PSV will have a higher relieving pressure than existing )or must they be same?

Please advise,

 

[The Obturator]

To answer you;-
1) Will the additional PSV be capable of relieving at 5% overpressure? (certification of capacity per ASME would be at 10% overpressure). Generally, if you do not exceed 105% of the second valve, you are within code rules. Why do you want to match the set pressure of the first valve? The whole point of staggered set pressures is to prevent chatter etc., from too many valves opening at the same time.
2) Yes - the PSV only reacts to overpressure 10% or 16%. If you are changing set pressure, then only the manufacturer will be able to advise you what needs to changed with the existing PSV (nameplate, possibly spring etc.).Important to note that whatever you change (overpressure and/or set pressure), it is highly recommended to do a new sizing/capacity calculation.
3) In theory, both valves will reach the same relieving pressure.

Note. You have not mentioned any sizes of the subject valves. You may want to consider the advantages of limiting (or increasing) flow capacity by using restricted flow spring operated PSV or full bore pilot operated PSV's.

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

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

 

[tono3]

Thanks so much for your reply!
Please see a few clarifications:
1)The set pressure of the new PSV (P size) will indeed be 105% of the set pressure of the existing PSV (L size). I meant that the relieving pressure (Set pressure + Overpressure) of both valves to be equal. Considering 100 psig Set pressure of existing PSV, its relieving pressure is 100 psig + 10 psig = 110psig, and the relieving pressure of the new PSV will be 105 psig + 5 psig = 110 psig considering 5% overpressure.
2) The set pressure of the existing PSV will remain at 100 psig, I want to confirm that its overpressure can be increased from 10 to 16 psig so that its new relieving pressure becomes 100 psig + 16 psig = 116 psig?
3) As mentioned the existing PSV is currently relieving at 110 psig, if I go with ASME certification of 10% overpressure for the new PSV with set point 105% of existing, its relieving pressure would be 105 psig + 10.5 psig = 115.5 psig?

 

[The Obturator]

You should be specifying the set pressure/overpressure as follows;-

Existing Valve Set @ 100 + 16% overpressure = 116
Additional Valve Set @ 105 + 10% overpressure = 115.5

The relieving pressure is considered equal here (as in API/ASME). See API-520 Part I, Table 6

NB: PSV's do not have 'set' overpressures. They are set to a pre determined 'set pressure' and react to the protected systems overpressure situation.

5 % overpressure can only be used if the valve is actually capable of fully opening and discharging 100% flow at 5% overpressure. You most likely have a valve that meets ASME and has had its capacity already certified at 10%. It may well be capable of fully opening at 5% overpressure, but you need the manufacturer to confirm that for you.

Also this discussion is based on a non-fire case and the fluid being of a compressible nature (gas/vapour/steam).



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

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

 

[tono3]

That makes sense, thanks so much!!

 

[goutam_freelance]

Referring to ASME VIII UG 131 c(1) the capacity certification shall be done at 10% overpressure.

So 10% overpressure needs to be specified for purchasing.

In effect although additional relieving capacity will be available for the first valve at 16% overpressure, this can not be used in design.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[The Obturator]

@goutam_freelance No. You are confusing capacity certification, which is required to be done at 10% overpressure, with operational overpressure.

FYI. UG 131 was transferred from ASME VIII to ASME XIII in 2021. In any case (old 2019) ASME VIII UG 134 allowed setting of 105 % of multiple valve installation. This is also referenced in API - 520 as I stated earlier above.

A Pressure-relief Valve conforming to ASME VIII/XIII must be certified at 10% overpressure nothing else. Why? during the certification test the valve must demonstrate full lift (D/4) at 10% overpressure. If it can do it at 10% then it can at other overpressures above 10% as then flow is linear in calculation.



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

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

 

[goutam_freelance]

@The Obturator,

You are right to a great extent, but please see below:
UG 130 (c)(2)

And UG-125 (c)(1)

So 16% overpressure is acceptable for multiple PSVs as you rightly indicated.

The capacity certification tests can also be done at max 120% of MAWP.

The question is, if the existing PSV is tested @110% overpressure(assumed), will it require another certification test @116% of MAWP?

I feel existing PSV need not be recertified and capacity @110% MAWP can be used as available capacity from PSV1 and the required balance capacity can be taken from PSV2 @110% of set pressure(10% overpressure). This is expected to be conservative as we are disregarding the additional capacity possible from PSV1 for changing from 10% to 16% overpressure.

I am from power plant background where max. overpressure is 6% and I feel uncomfortable with 16% or 20% overpressure.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[The Obturator]

You are again referring to extracts from an out of date ASME VIII document.

Also you're still confusing the issue. Capacity tests at 'up to 120%' are for the relief conditions specifically mentioned in (old) UG-125 (c)(3). You seem to be picking out sections to suit your narrative. What exactly is it that you disagree with the comments I made earlier that are referenced back to ASME XIII and API-520 latest editions? If you don't like what is written is ASME, I suggest you contact the relevant ASME Committee with your reservations.

Please refer to ASME XIII which replaced all of these sections in 2021. If you need to make further comments, please start a new thread as this is now diversifying from the OP's questions.

I suggest you acquaint yourself with ASME XIII (2023), and the recommended operating conditions in user standard API-520 Part I (2020), before commenting again.





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

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

 

[georgeverghese]

In some jurisdictions, relieving pressure (to reach required relief rate) for a multiple PSV setup with staggered settings cannot exceed 110% of MAWP, not 116% of MAWP. So pls check your national codes or Company design guidelines. You cannot pick and choose one relieving pressure for a PSV, and another relieving pressure for the companion staggered setpoint PSV for this case.

 

[MortenA]

note that this will not be legal in EU in my understanding of PED and the PED FAQ's

--- Best regards, Morten Andersen