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Size Of Inlet Piping To Pressure Relief Valve


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#1 Vegeta

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Posted 10 May 2021 - 06:57 AM

Dear professionals, 

 

API 520 II says the nominal size of inlet piping to the relief valve must be the same as or larger than the nominal size of the relief valve inlet flange connection.  This is a common practice in the design.  

 

We are currently facing a case at which the pressure vessel service is to be changes to a new service.  The new calculation of the relief load shows that the new relief valve is to be one bigger size than the original one (the original is 1.5" G 3" and the new one is 2" H 3").  The issue is that the nozzles on the vessel are 1.5" and it is extremely difficult to revamp these nozzles on the 50-year old vessel.  The calculation shows that the piping expansion from 1.5" to 2" is not bad and acceptable hydraulically.  So, is that enough to waive the above API 520 II requirement? Or is there something overlooked?    

 

Appreciate your thoughts and your usual help....

 

 

Regards, 


Edited by Vegeta, 10 May 2021 - 06:57 AM.


#2 breizh

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Posted 10 May 2021 - 07:51 AM

Hi,

Consider the doc 6-3-1 . You need to stick to code.

Note : You should also consider to get all the calculation notes and tests for this Old vessel .

 

Good luck

Breizh 



#3 Chemitofreak

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Posted 10 May 2021 - 01:47 PM

What do you mean by new service? What kind of modifications are carried out in the existing 50 year old unit? 

 

If there are no modifications, only the HC/Fluid handled by the system is changed and considering that if the inlet line loss at the rated flow are less than 3% of the set pressure, I would say it is good enough considering the plant is old. All these criteria and stuff have originated in the last 10-20 years i.e. after the plant was designed.  

 

Having said that you need approval from your Principal Engineer for accepting this.  


Edited by Chemitofreak, 10 May 2021 - 01:48 PM.


#4 latexman

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Posted 10 May 2021 - 06:56 PM

Can set pressure be increased? What is set pressure and MAWP?

#5 Pilesar

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Posted 10 May 2021 - 07:54 PM

If you have another top nozzle on the vessel, it might be cost effective to use multiple PSVs instead of increasing the nozzle size.



#6 Vegeta

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Posted 23 May 2021 - 03:03 PM

What do you mean by new service? What kind of modifications are carried out in the existing 50 year old unit? 

 

If there are no modifications, only the HC/Fluid handled by the system is changed and considering that if the inlet line loss at the rated flow are less than 3% of the set pressure, I would say it is good enough considering the plant is old. All these criteria and stuff have originated in the last 10-20 years i.e. after the plant was designed.  

 

Having said that you need approval from your Principal Engineer for accepting this.  

 

The old service was liquified Freon from a compressor.  The new service is liquid propane to be vaporized in the shell.  The modification is due to environmental regulations and abandonment of the compressor. 

 

The relieved Freon was discharged to atmosphere with extremely short inlet piping.  Now, the relieved propane is connected to the flare network with relatively long inlet & outlet piping. 

 

We evaluated the inlet piping pressure losses and they were higher than 3% of the set pressure.  So, we adjusted the blowdown to be higher in order to avoid rapid closure (chattering) of the valve during relief event and the vendor is going to do extra evaluation as per the API. 

 

I'll try to approach a principal engineer as you suggested. 

 

 

Thanks... 



#7 Vegeta

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Posted 23 May 2021 - 03:06 PM

Can set pressure be increased? What is set pressure and MAWP?

 

The set pressure is the same as the vessel design pressure.  The old documents says that the MAWP is the same as the design pressure.  So, I don't see increasing the set pressure feasible.  

 

 

Thanks... 



#8 Vegeta

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Posted 23 May 2021 - 03:12 PM

Team, 

 

I just wonder what the reason of the API 520 II clause is "nominal size of inlet piping to the relief valve must be the same as or larger than the nominal size of the relief valve inlet flange connection".  For example, the 3% rule is mainly to avoid the rapid opening and closure (chattering) of the relief valve. 

 

What is the reason of mandating the nominal size of the inlet piping of the relief valve to be as or bigger than the valve inlet flange from physics point of view? 



#9 breizh

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Posted 23 May 2021 - 07:27 PM

HI,

a) MAWP is related to the actual condition , not at the time the vessel was fabricated/purchased .

B) Just follow the code .

Breizh 






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