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Demethanizer Column Relief Valve


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#1 jmit

jmit

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Posted 03 January 2017 - 09:26 PM

Dear All,

I have a query related to Demethanizer Column overhead protection system. We have control valve, blowdown valve (BDV) and relief valve.

Kindly refer the sketch for better understanding.

My question is :

1) Licensor has specified the line sizes for control valve, relief valves and blowdown valve. While doing Preliminary FEED verification, I need to check the basis for all three valves. I can do the blowdown valve calculations but for control valve, I am unsure as what flowrate shall I consider (or how to calculate). There is no project specific guidelines. Random searches provide result as 30% of overhead but I need to understand the basis.

2) For Relief valve sizing (for blocked outlet case), can I take the credit of control valve opening ? I mean initially control valve will open then blowdown valve and if still pressure increases the relief valve will open.

3) If answer of point 2) is NO then can the flowrate from PSV should be equal to BDV + Control Valve flowrate ?

4) Staggered pressure set points are specified on Relief Valves. Also in the relief valve datasheet, I can see the flowrates are different (PSV-A is having less flowrate then PSV-BCD). As per my understanding, we can divide equally the overhead flowrate by 4 (if 4 valves are required, based on sizing). Please clarify my understanding.

Kindly provide the correct path forward. If more information is required, then I can share.

Thank you.

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#2 Bobby Strain

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Posted 03 January 2017 - 11:25 PM

It seems to me that you have insufficient experience and also lack any design guides from your employer. I assume that you are working for an engineering company, executing a feed for your client. You need to go to your supervisor and have a discussion with her.

 

Good luck, and be careful.

Bobby



#3 jmit

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Posted 04 January 2017 - 01:11 AM

Thanks for the reply Bobby.

 

I would definitely check with my supervisor.

 

Still I would like to have an opinion of experienced guys who have a vast knowledge in the subject field.

 

Thanks!



#4 colt16

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Posted 04 January 2017 - 12:34 PM

Dear All,
I have a query related to Demethanizer Column overhead protection system. We have control valve, blowdown valve (BDV) and relief valve.
Kindly refer the sketch for better understanding.
My question is :
1) Licensor has specified the line sizes for control valve, relief valves and blowdown valve. While doing Preliminary FEED verification, I need to check the basis for all three valves. I can do the blowdown valve calculations but for control valve, I am unsure as what flowrate shall I consider (or how to calculate). There is no project specific guidelines. Random searches provide result as 30% of overhead but I need to understand the basis.
You need to check design basis for this. The control valve flowrate could be based on the feed and the required amount for pressure control.

2) For Relief valve sizing (for blocked outlet case), can I take the credit of control valve opening ? I mean initially control valve will open then blowdown valve and if still pressure increases the relief valve will open.
You probably can but as the control valve closing is probably a double jeopardy case. But to be conservative I think it is easier to just use the normal flow as the relief. It depends really. Besides, the blocked outlet might not be governing so whether you do it or not seems immaterial. If it is the governing scenario, I also wouldn't do it because the cost saving for taking credit is not so much. Again this depends on YOUR project.


3) If answer of point 2) is NO then can the flowrate from PSV should be equal to BDV + Control Valve flowrate ?
No, why would you do that? The PSV will be unnecessarily bigger?

Unless you mean PSV = blocked outlet - BDV - control valve but still the answer is no because the blowdown valve will not be activated during blocked outlet. The blowdown valve is a manual operation during emergency with a button consciously pushed by the operator. A blocked outlet can happen without the operator being conscious and that is why the relief valve must be sized for the full flow.


4) Staggered pressure set points are specified on Relief Valves. Also in the relief valve datasheet, I can see the flowrates are different (PSV-A is having less flowrate then PSV-BCD). As per my understanding, we can divide equally the overhead flowrate by 4 (if 4 valves are required, based on sizing). Please clarify my understanding.
Kindly provide the correct path forward. If more information is required, then I can share.
Staggered pressure settings please consult API 521/520. I forgot which of these two it is mentioned.
For the amount of PSVs, without exact details I cannot be sure but check the basis.


Thank you.


Edited by colt16, 04 January 2017 - 09:27 PM.


#5 mirandomka

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Posted 05 January 2017 - 03:19 PM

1) follow your HMB,design basis doc or operation manual for design requirement for your PCV.

Consult your supervisor/ client.

2) No credit will be available, if minimum normal flow through the PCV is 0.

Favourable instrument response is not credited by API. Ie uncertainty of responsivity and reliability.

However, check with your supervisor and client see what is their risk acceptance level.

Hard to believe there is no philosophy or basis for overpressure protection system design.

3) the relief load should not include blowdown. You don't blow down and operate the column at the same time

4) the first valve is R size which is smaller.




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