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Critical Point Between Set Pressure & Relieving Pressure


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#1 MFarq

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Posted 28 March 2018 - 07:56 PM

Sorry if this has been posted before, but the search function was not working when I tried.

 

I am sizing a relief valve for a distillation column (unbalanced heat method), and my relief load calculations are based on the relieving conditions (1.1*Pset). This column handles many different components in varying concentrations depending on the batch. One case reaches the critical point nearly halfway between set pressure & relieving pressure, and appears to be the largest load based on calculations at the set pressure.

 

My experience with fluids at or above critical points is very limited, though my understanding is this: Once the critical point is reached, the boiling ceases, and relief load is effectively a result of expansion due to temperature increase as if the column only contained vapor. This expansion results in a much lower relief load than the vapor generation due to boiling based on my preliminary calculations.

 

I have calculated the maximum relief load by varying pressure incrementally between the set pressure and relieving conditions. Should the relief load be based on this maximum vapor generation rate between the setpoint and the relieving conditions as I calculated? If there is any printed guidance on cases like this, please let me know.

 

Thanks!



#2 Bobby Strain

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Posted 28 March 2018 - 11:11 PM   Best Answer

The valve needs only to relieve the required fluid at the maximum allowable relieving pressure. So, you have wasted a lot of time and effort looking at other conditions.

 

Bobby



#3 serra

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Posted 29 March 2018 - 02:08 AM

sizing of PSV is based on specified flow and density (orifice mass flux),

at cheresources you can find several threads comparing results in two phase and dense phase area,
API formulation based on integration of ideal model can give larger errors than other methods,
for a discussion see for example this thread

https://www.cheresou...ng-temperature/



#4 MFarq

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Posted 29 March 2018 - 08:37 AM

The valve needs only to relieve the required fluid at the maximum allowable relieving pressure. So, you have wasted a lot of time and effort looking at other conditions.

 

Bobby

 

This does not seem to be the case for relieving rate estimation for loss of cooling in a distillation column. Per API 521 section 4.4.3.1, "the required relieving rate is determined by a heat and material balance on the system at the relieving pressure".



#5 Bobby Strain

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Posted 29 March 2018 - 08:58 AM

The relieving pressure is the maximum pressure. 1.1 X design pressure, or 1.21 for fire. So there is nothing in the API RP requiring any lower relieving pressure.

 

Bobby



#6 MFarq

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Posted 29 March 2018 - 09:06 AM

The relieving pressure is the maximum pressure. 1.1 X design pressure, or 1.21 for fire. So there is nothing in the API RP requiring any lower relieving pressure.

 

Bobby

 

I misread your original comment. Thanks for the reply.






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