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Evaluating Accumulation For Relief Valve Scenario


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#1 sparsha

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Posted 18 June 2015 - 07:59 AM

Hi All

I am having trouble understanding the rationale behind using 10% accumulation for non-fire case and 21% accumulation for fire case for the same PSV.

 

Let me elaborate by providing example.

I have a vessel with two possible credible scenarios 1) Fire 2) Blocked Outlet.

I will size for both the cases with an accumulation 21% and 10% respectively. Both the cases area requirements are pretty close.

 

Let's say fire case turned out to be the governing scenario( slightly larger area). So my PSV will be set to open completely at an upstream pressure of 1.21*set pressure.  But for blocked outlet case, i have used an accumulation of 10%. At 10% accumulation my PSV will not be 100% open and the area available is less than actual discharge area. That available area of the PSV at 10% accumulation could be lower than my required area i arrived by my calculation. Is this possible. Are we adopting the correct methodology by comparing the area requirement estimated with 10% accumulation  against a valve which opens completely at 21% accumulations.

 

Consider the other case. If blocked outlet is governing, then the PSV should open completely at 1.1*set pressure. But we have used 21% accumulation for fire case. As i stated previously the area calculated are pretty close. So should i change the accumulation to 10% and check the area requirement, as the PSV cannot experience 21% accumulation even under fire case???

 

Please clarify



#2 Bobby Strain

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Posted 18 June 2015 - 09:19 AM

I think you should do a bit of research on relief valves. Some statements you make are not true.

 

Bobby



#3 fallah

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Posted 18 June 2015 - 10:27 AM

sparsha,

 

Calculation of required orifice area for every credible scenario of a PSV to determine governing scenario is to be done regardless of that scenario might be the governing case or not. Then, the specified accumulation being applied for each scenario at the stage of governing scenario determination. After this stage the important matter is having preset value of accumulation in relieving conditions of governing scenario not higher than that value; means no matter having lower accumulation value for the scenarios other than governing one in relevant relieving conditions...



#4 ChemEng01

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Posted 18 June 2015 - 06:33 PM

All certified relief valves will reach the fully open position at 10% or less accumulation over the set pressure. So you select your governing case based on the governing relief case with the largest required relief rate. 

 

With a fire case the relief valve will still reach the fully open position at 10% or less accumulation above the set pressure. The allowable 21% for ASME vessels is the allowable accumulation for the vessel. Hence the relief valve size will be less than using 10%. The fire case allowable accumulation is based on 10% allowable accumulation over 110%. 

 

So it doesn't matter. You just go for the governing relief case. Just because your allowable accumulation for the vessel is 21% doesn't mean the valve manufacturer will change the spring settings so the valve requires more pressure accumulation to open. 

 

Kind Regards

ChemEng01


Edited by ChemEng01, 18 June 2015 - 06:35 PM.


#5 sparsha

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Posted 21 June 2015 - 11:38 PM

sparsha,

 

Calculation of required orifice area for every credible scenario of a PSV to determine governing scenario is to be done regardless of that scenario might be the governing case or not. Then, the specified accumulation being applied for each scenario at the stage of governing scenario determination. After this stage the important matter is having preset value of accumulation in relieving conditions of governing scenario not higher than that value; means no matter having lower accumulation value for the scenarios other than governing one in relevant relieving conditions...

Dear fallah

This precisely my confusion. Once we have determined the governing scenario, the allowable accumulation for that scenario should be applicable to all other credible scenarios. So if fire case is not governing but credible scenario, should we change the accumulation to 10% and check whether the selected PSV meets the required relieving rate in fire case??



#6 sparsha

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Posted 21 June 2015 - 11:40 PM

I think you should do a bit of research on relief valves. Some statements you make are not true.

 

Bobby

Dear Bobby

Could you please point out those statements, so that i can clear my misunderstanding.



#7 fallah

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Posted 22 June 2015 - 02:29 AM

 

Dear fallah

 

So if fire case is not governing but credible scenario, should we change the accumulation to 10% and check whether the selected PSV meets the required relieving rate in fire case??

 

 

Dear sparsha,

 

No need to do so, because when a PSV can meet the non-fire governing scenario with 10% accumulation, it will certainly meet the lower required relieving rate of fire case with higher orifice area than required and 21% accumulation...

 

Of course, in rare case (due to close relieving rates of governing case and fire case) it might the actual accumulation in fire case as non-governing case to be between 10% and 21% for which there would be no contradiction to 21% allowed accumulation in fire case...
 



#8 sparsha

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Posted 22 June 2015 - 03:19 AM

 

 

Dear fallah

 

So if fire case is not governing but credible scenario, should we change the accumulation to 10% and check whether the selected PSV meets the required relieving rate in fire case??

 

 

Dear sparsha,

 

No need to do so, because when a PSV can meet the non-fire governing scenario with 10% accumulation, it will certainly meet the lower required relieving rate of fire case with higher orifice area than required and 21% accumulation...

 

Of course, in rare case (due to close relieving rates of governing case and fire case) it might the actual accumulation in fire case as non-governing case to be between 10% and 21% for which there would be no contradiction to 21% allowed accumulation in fire case...
 

 

Dear Fallah

Thanks a lot. My confusion is cleared.  The catch here is the pressure inside the vessel need not restrict to 110% of MAWP for fire case, eventhough the valve is set to open 100% at 110% of MAWP. 

 

For the reverse case of fire being the governing scenario and close relieving rates of fire and blocked outlet. Will the valve be set to open 100% at 121% of MAWP or will it be set to open below 110% MAWP as suggested by ChemEng01.

 

Thanks a lot



#9 fallah

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Posted 22 June 2015 - 12:33 PM

 

For the reverse case of fire being the governing scenario and close relieving rates of fire and blocked outlet. Will the valve be set to open 100% at 121% of MAWP or will it be set to open below 110% MAWP as suggested by ChemEng01.

 

 

Dear sparsha,

 

Generally, in fire case the rated capacity of PSV will be evaluated with 21% overpressure for valve selection. The PSV certified capacity, on the other hand, will be evaluated for 10% overpressure according to ASME Sec. VIII Div.I. means PSV will not be tested at 21% overpressure but at 10% overpressure. Then, if certified capacity covers the required capacity of the fire case as governing case, the PSV will be in full open position at below 10% overpressure, otherwise the PSV fully opens at an overpressure between 10% to 21% of the set pressure...



#10 sparsha

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Posted 25 June 2015 - 07:08 AM

 

 

For the reverse case of fire being the governing scenario and close relieving rates of fire and blocked outlet. Will the valve be set to open 100% at 121% of MAWP or will it be set to open below 110% MAWP as suggested by ChemEng01.

 

 

Dear sparsha,

 

Generally, in fire case the rated capacity of PSV will be evaluated with 21% overpressure for valve selection. The PSV certified capacity, on the other hand, will be evaluated for 10% overpressure according to ASME Sec. VIII Div.I. means PSV will not be tested at 21% overpressure but at 10% overpressure. Then, if certified capacity covers the required capacity of the fire case as governing case, the PSV will be in full open position at below 10% overpressure, otherwise the PSV fully opens at an overpressure between 10% to 21% of the set pressure...

 

Thanks a lot Fallah. Now its clear.



#11 HLO

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Posted 10 August 2015 - 06:50 PM

Hi, related with this topic I´m studying a client's requirement that sets:

 

"- In safety or relief valves that protect vessels subject to ASME VIII, the maximum admissible overpressure in each case can reach the difference between the permitted acumulation and the Pset.

- The previous rule shall apply supposing that the maximum accumulation for a fire case with a only one installed valve shall be 10% (an exception to API 520/521)"

 

As a comment in the second phrase, I understant that is an exception to ASME VIII instead of API, because accumulation is related to ASME.

 

Maybe it's easy, but what is the requirement here?: If the PSV is setted below MAWP, and overpressure (Pset x 1.21 for fire) does not exceeds the accumulated pressure ( = 1.10 x MAWP), I'm complying with the requirement?

 

What could be the rationale and benefits in this?

 

Thanks.



#12 Bobby Strain

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Posted 10 August 2015 - 07:55 PM

All you need to understand is that allowable accumulation relates to the equipment design pressure per ASME code. Overpressure is related to the relieving pressure relative to the PSV set pressure.

 

Bobby



#13 fallah

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Posted 11 August 2015 - 12:08 AM

 

As a comment in the second phrase, I understant that is an exception to ASME VIII instead of API, because accumulation is related to ASME.

 

The ASME and API are in the same track in this regard; then no problem on the statement...

 

Maybe it's easy, but what is the requirement here?: If the PSV is setted below MAWP, and overpressure (Pset x 1.21 for fire) does not exceeds the accumulated pressure ( = 1.10 x MAWP), I'm complying with the requirement?

 

Yes, you are complying with the requirement...

 


Edited by fallah, 11 August 2015 - 12:12 AM.





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