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Pressure Relief Valve And Rupture Disk In Parallel


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#1 Alejandro Costa

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Posted 25 August 2016 - 03:06 PM

Hello, My name is Alejandro Costa from Argentina i'm an Electronic Engineer and i'm working on Electrical & Instrumentation Department for a Oil & Gas Industry. 

I have a question for you about the pressure set of Pressure Relief Valve and Rupture Disc. Many document said when i have multiple devices i can set the last device with 105% of my MAWP but i think this condition is when the required capacity is provided with both devices. If i size each device separately for the required capacity. Can i reach 105 % of MAWP?

I hope I made my point, From already thank you very much

 



#2 Bobby Strain

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Posted 25 August 2016 - 07:46 PM

No, you are not clear. Is only one device in service and the other isolated with a block valve? A sketch or P&ID would be most helpful and maybe get some good response for your question.



#3 Alejandro Costa

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Posted 26 August 2016 - 07:42 AM

Thank you Bobby Strain for ur response. 

 

I´ll try to be more specific. 

Vessel MAWP: 100 Bar

Operative Pressure: 76.95 Bar

I have one Pressure Relief Valve and one Rupture Disc. PRV have to evacuate the required capacity and If PRV Fail RD is set above and it can evacuate the required capacity too.

I think both device have to set as much on MAWP. API 520 and ASME said that i can set on 105%MAWP when i have multiple devices but in this case i have to look the system with the devices separately.

 

Thank you!

Have i nice day

 

Attached File  PID.jpg   84.38KB   12 downloads



#4 Bobby Strain

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Posted 26 August 2016 - 09:09 AM

As long as both devices are always in service you can set the rupture disc relief pressure at 105% of the PSV.

 

Bobby



#5 Alejandro Costa

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Posted 26 August 2016 - 10:02 AM

105 % of PSV or MAWP?



#6 Saml

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Posted 26 August 2016 - 11:27 AM

Alejandro,

 

The ASME Code (assuming you are designig to ASME), requires protection based on MAWP.

 

You are allowed to set the pressure lower, as you do in your case, for any reason as long as pressure buildup does not exceeds 110% at full relief rate for single valve or 116% for multiple devices.

 

Supplementary relief devices can be set at up to 105% of MAWP.

 

Since you said you were in Argentina (and I assume you speak spanish), I am attaching some speaking notes I gave to engineering students a few years ago. Hope it helps.

 

This was intended as educational, introductory material. May have errors and is incomplete (ie. there is no mention to PED).  Use with care. It is also focused in the Buenos Aires province. May be different if you are in other jurisdiction.

Attached Files


Edited by Saml, 26 August 2016 - 11:28 AM.


#7 Alejandro Costa

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Posted 29 August 2016 - 08:12 AM

Thank you so much for the information. I have a big doubt about set because API said "A multiple-device installation requires the combined

capacity of two or more pressure relief devices to alleviate a given overpressure contingency" i need to set PSV and RD on different ranges with the same capacity to alleviate a given overpressure contingency. So i think i can't set at 105 % MAWP.
What do u think about it?
Thank you!


#8 Saml

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Posted 29 August 2016 - 03:21 PM

You are entering a gray zone:

 

- For whatever reason, you are considering that the simultaneous occurrence of an over pressure and PSV failure is an unacceptable scenario and you need a burst disk with the same capacity the PSV.  If your PSV maintenance procedures are adequate, this may be considered a "double jeopardy" case. In this case API 521 is clear:

 

"The user may choose to go beyond these practices and assess multiple jeopardy scenarios. Since such assessments are outside the basis for design, the user is not required to meet accumulations allowed by the pressure-design code for these scenarios. Acceptance criteria are the sole responsibility of the user."

 

This means. Is up to you and your client.

 

- If you are thinking that the disk is a spare of the valve then you should consider installing interlocking block valves or sizing the tail pipe for simultaneous discharge. You may get into vibration problems in the more common scenario where you have an over-pressure and the PSV does not fail. See "Estimate Vibration Risk for Relief and Process Piping" from CCPS meeting of 2013.



#9 Alejandro Costa

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Posted 30 August 2016 - 02:56 PM

Case 1:

Operating Pressure : 76,5 Bar

MAWP : 98 Bar

PRV: 84.15 Bar (Full Flow : 3.000.000 Sm3/d)

RD: 98 Bar (Full Flow : 3.000.000 Sm3/d)

In this case the MAWP is equal to RD set.  (100% of MAWP)

PRV is  set in a different range than DISC. 

 

Case 2:

Operating Pressure : 76,5 Bar

MAWP : 94.6 Bar

PRV: 84.15 Bar (Full Flow : 3.000.000 Sm3/d)

RD: 99.33 Bar (Full Flow : 3.000.000 Sm3/d)

In this case the RD set is bigger than MAWP.  (105% of MAWP)

PRV is  set in a different range than DISC.
 
Which case is correct?
I have to assume that two contingencies can happen? 
My interpretation of API is that I have to take as individual devices that alleviate the total capacity of contingency and that can not exceed 100% of the MAWP 
 
Many vessels are set like Case 2. But i think like Saml said i'm in a Gray Zone.

Edited by Alejandro Costa, 30 August 2016 - 06:34 PM.


#10 Saml

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Posted 30 August 2016 - 11:13 PM

The total capacity of the worst case should be achieved with all the valves. Not by each valve.

 

 

Case A: you are designing considering single Jeopardy cases.

 

What you need to calculate are the relief scenarios. Say that your design case is for 6 million Sm3/d. You split your relief capacity between both devices with 3 million each, Set the second one (the disc) at 105% MAWP, an the valve at some value below MAWP. You provide a difference greater than 5% because the rupture disc are less precise than valves regarding the setpoint.

 

So, for now: MAWP 94.6 bar

Disc: 99.33 bar

 

Your disk has a manufacturing range of -5%/0%. This means that it could be marked anywhere between 94.36 and 99.33

That disk has a burst tolerance of +/- 5%. So the minimum burst pressure would be 89.64

Then you set your PRV with some margin below this pressure to be sure that your valve will always open before the disc ruptures.

 

You are complying with the ASME and API codes since you have one device at or below the MAWP and a supplementary one at a pressure not higher than 105% of MAWP

 

In this case you have to calculate your relief capacity at 116% of MAWP (109.7 bar). This is the maximum  permitted accumulation when multiple devices are used

 

You can go the other way, starting from the operating pressure, then the PRV, then the burst margin and manufacturing range and ending with the design MAWP

 

------------------------------

 

Case B: you are considering the simultaneous occurrence of the worst case overpressure event AND the failure of the PRV (as you mentioned in earlier post)

 

Your worst case relief requirement is 3 million. You set a PRV at or below MAWP. And you calculate the capacity of 3 million Sm3/d.

 

You also consider a case where you have the same event where the combined with the failure of the PRV needs a 3 million Sm3/d relief. This is a double jeopardy case and ouside the scope of the code. Acceptance criteria here are the sole responsibility of you or your client. You install a Rupture Disk for 3 million Sm3/d at a pressure decided by you. 105% is a possibility, but not something required by the code.

 

-------------------------

 

If you have existing equipment designed with this configuration, take a look at the scenarios that were deemed credible and their quantification.

 

Also, the numbers you mention does not fit well with the P&ID you showed before, you cannot relieve 3 million cubic meters per day using a 1 x 2 valve. 

 

 

Regards


Edited by Saml, 31 August 2016 - 06:20 AM.





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