how to determine the set pressure of multiple pressure safety valve 3

[phoenixmoca]

There is a case in which the required capacity is provided by more than one PSV.
Someone says the set pressures of multiple PSVs can be the same. But others say it should be staggered in order to prevent chattering.

So which is right?Thank you in advance

 

[danschwind]

I'm sure there are many people with different opinions on this regard.

I've dealt in the past with non-staggered PSVs that had chattering problems, so my personal point of view is that they should be staggered.

I'm not sure of the exact texting of ASME and API, but if I recall correctly they both suggest a staggered configuration albeit not demanding it.

There are a lot of guys here in the forums with way more knowledge and experience than me in this subject, so take their opinions with more weight than mine

Daniel
Rio de Janeiro - Brazil

 

[don1980]

Whether the PSVs needs to be staggered depends on the range of flow rates from the credible scenarios for which they are sized. In other words, what is the overall range of flow rates the relief system is likely to experience? The broader that range, the greater the incentive to stagger the set pressures.

One can't declaratively say that excess relief capacity will result in chattering. There are too many other variables, but it makes sense that excessive capacity is probably a risk factor for chatter. So, I wouldn't say that it's critical to stagger the set pressures, but since it may be beneficial, and there's no down-side risk to doing so, it makes sense to stagger.

 

[phoenixmoca]

Hi, danschwind&don1980,
Thank you for your kind reply. And please find the credible scenarios list and their relieving load respectively.
It is noted that the governing case is overfilling with relieving requirement of 187914kg/hr. So it is recommended that two identical PSVs be in service together. But the current two PSVs are with the same set pressure(equal to the design pressure of the protected equipment) on line at the same time.
In my opinion the two PSVs should be staggered to meet the different relieving requirement of the credible scenarios.

 

[The Obturator]

API-520 States that the set pressures should be staggered for multiple PRV use.

For non-Fire conditions
1st PRV is set at MAWP (100%) and the additional PRV (or PRV's) at 5 % above MAWP (105%)
16 % overpressure is used (105 + 5%) and used for all the installed PRV's (including calculation)

For fire condition, the 5% over rule is also used but overpressure bcomes 21%

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[georgeverghese]

Overfilling? Can you describe how this happens, what the protected equipment is, and what operational controls, safety devices / trip loops have failed to result in this huge relief load? Are your trip loops well maintained?

 

[don1980]

API-520 States that the set pressures should be staggered for multiple PRV use.

API-520 Pt II doesn't say that. It mentions staggering as an option to consider when one has widely varying relief loads. The decision to stagger or not stagger is a decision left to the user based on their specific application. I agree that staggering is a good practice, but it's important for the reader to understand that API 520 doesn't use "should" or "shall".

 

[georgeverghese]

Staggered RVs' would be a good idea if you have operational upset (non fire case) relief loads, and if a single large RV option (or 2 RVs both set at the same pressure) presents acoustic vibration concerns at the relief / flare header. Select a lower flow for the smaller, lower set RV (less than approx 30% of the total calculated load) such that it does not present acoustic vibration concerns when activated.

 

[The Obturator]

don1980 I was not referring to API-520 Part II.

API-520 Part I states it clearly;- (PS. It includes "shall" in the context of referring to ASME VIII)
"
5.4.2.2.2 In accordance with the requirements of the ASME Code, accumulated pressure shall be limited to 116 % of the MAWP in vessels that are protected by multiple pressure-relief devices sized for operating (nonfire) contingencies. The set pressure of the first device shall not exceed the MAWP. The set pressure of the additional device or devices shall not exceed 105 % of the MAWP.
"

For some reason you are referring to API-520 Part II and capacity variations (which in your own extract, refers back to API-520 Part I).

Capacity variation was not the question.



Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[don1980]

Obturator - The OP isn't asking for the set pressure limits and accumulation limits that apply when multiple PSVs are used. He asks for comments on setting multiple valves at the same pressure versus utilizing the permissible option to staggering the set pressures. API 520, parts I and II, are silent on that point. Staggering of the set pressure is indeed permitted, and it's a good idea, but API 520 doesn't say that it should or shouldn't be done. It leaves that decision to the user.

 

[LittleInch]

I think you have a choice - for me if the pressure rise is rapid and doesn't "hover" at the high end ( 90% of set point) then I think you can set them at the same pressure.

If on the other hand the pressure rise is slow and can be close to the set point then staggering the set points is normal / recommended.

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

 

[danschwind]

Obturator,

In my understanding, the wording of API 520 Part I that you kindly pasted here isn't exactly demanding a staggered arrangement. It only says that the second device's set pressure shall not exceed 105% of MAWP, meaning it could range from MAWP to 1.05MAWP, therefore you could have 2 devices at the same set pressure (MAWP). This of course if the first device is at MAWP.

Daniel
Rio de Janeiro - Brazil

 

[The Obturator]

danschwind. Read the last part of the extract again...

"The set pressure of the first device shall not exceed the MAWP. The set pressure of the additional device or devices shall not exceed 105 % of the MAWP."




Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[danschwind]

Obturator,

I'm not a native english speaker, but the highlighted extract reads to me, mathematically, as Set Pressure <= 1.05 MAWP. Therefore, in my understanding, having two PSV's set at 1.0 MAWP would still comply with this rule.

I would agree if it was written "The set pressure of the additional device or devices shall be 105% of the MAWP".

This is the same logic, IMO, applied to the primary device. Although defining the Set Pressure = MAWP is common, I don't think it is prohibited to define Set Pressure < MAWP.

Daniel
Rio de Janeiro - Brazil

 

[The Obturator]

danschwind. I'm sure your English is better than my Portuguese...

If we take MAWP of 100 and 100 as the set pressure,
Any additional device or devices, would be set at 105 MAX.

100 and 105 are staggered set pressures.

Besides my earlier extract, API-520 Pt I has a tabled example which I also attach.

Note we are discussing non fire case. The extracted table contains non fire and fire case.

Re. your last comment. Set pressure can be lower than MAWP. API-520 Pt I addresses this separately to the above.

Hope that helps.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[danschwind]

Obturator,

What I'm trying to say, and maybe failing to convey the information, is that at least from what is written the secondary device set pressure does not need to be set at 1.05 MAWP (or 1.05 of primary device set pressure if not at MAWP).

I do agree with you that they should - as I said in my first reply in this topic. But - at least in my limited understanding - if I for some reason want to set two devices at MAWP (both at 100 in your example) I would not be forbidden by the standard to do so.

Taking the excerpt you highlighted before: "The set pressure of the additional device or devices shall not exceed 105 % of the MAWP" -> 100% MAWP doesn't exceed 105% MAWP.
The image you shared also defines the "Maximum Set Pressure = 105% MAWP" -> 100% MAWP < 105% MAWP so we are under the maximum, therefore OK.

All this is further exacerbated by me having to actually troubleshoot a non-staggered PSV arrangement in a gas processing plant of the country's largest oil operator that - albeit me having a lot of issues with them - don't usually have out-of-standard devices. One of my recommendations was indeed staggering the set pressures.

I don't wish to take this argument further, as I suspect we are maybe talking about the same thing but with different words, but we can also agree to disagree. I agree the PSVs should be staggered, but I disagree that they shall be staggered.



Daniel
Rio de Janeiro - Brazil

 

[The Obturator]

dannschwind. OK. Maybe I'm being to pedantic.

API-RP-520 is a Recommended Practice for end users. It is not mandatory but a good reference for sound practice.

Where required by law or where specified, ASME VIII is mandatory.

ASME VIII UG-134 states that any additional device or devices "may" be set higher, but not more than 105% of MAWP.

In other words, and to answer you, the PRV's can all have the same set pressure. ASME is concerned about the valves opening at minimum 100% MAWP (non fire case only).

I've attached the UG-134 extract for your reference.

I hope that closes the issue.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[phoenixmoca]

Thanks a lot for your kind reply and disucussion.

In my opinion it should be staggered to meet different overpressure scenarios' relief requirements without chattering or cycling.

But it is a little bit difficult for me to find the specific standard in which it is described to ask for staggering.

Anyway thank you again.