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Where Does Inlet Start For 3% Rule


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#1 J_Leo

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Posted 04 August 2016 - 08:39 AM

Hello,

 

I have a PV fail open case. The relief valve is on the drum. Per the sketch.

 

My question is the PSV inlet for 3% pressure drop rule should start from PV or from the drum?

 

 

Thank you,

 

Leo

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#2 latexman

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Posted 04 August 2016 - 08:46 AM

In your case, from the drum.



#3 J_Leo

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Posted 04 August 2016 - 09:50 AM

Thank you, Latexman.

 

My original thought was it should be from the Drum. If the PSV is on a line downstream of a fail open control valve, I do see practice to use inlet starting from the control valve.

 

The reason I am asking is I just saw one calculation with inlet starting from PV just as shown in the sketch.

 

I am not sure about this. Because it make the lines to and from the drum extremely larger than that required for separation purpose.

 

Regards,

Leo



#4 latexman

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Posted 04 August 2016 - 11:52 AM

If you have access to API 520 Part II, read section 4 titled INLET PIPING TO PRESSURE RELIEF DEVICES.  Section 4.2.2 probably addresses your query.  My copy is February, 2011, in case there has been a revision.



#5 christopherchoa

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Posted 04 August 2016 - 11:46 PM

Drum is the starting point even the case is control valve failure. Whenever you have a drum in between, the starting point is the drum. The drum acts as a buffer inventory to not let your inlet pressure go down considerably which causes chattering.

 

 

Don't forget to include entrance loss.



#6 J_Leo

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Posted 05 August 2016 - 04:17 PM

Latexman and Christopher,

 

Thank you very much for your confirmation.

 

Regards,

Leo



#7 fallah

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Posted 14 August 2016 - 07:27 AM

 

I have a PV fail open case. The relief valve is on the drum. Per the sketch.

 

My question is the PSV inlet for 3% pressure drop rule should start from PV or from the drum?

 

 

Leo,

 

Why it should be started from PV, while the PSV is protecting the drum? 

 

Actually the starting point is always the first point of PSV inlet line entrance loss hence the outgoing line to the PSV from the drum is the starting point for 3% friction loss rule...






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