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Vessel Pressure Calculation - Vapour Breakthrough Scenario

relief valve calculation

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#1 sidewinder

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Posted 10 June 2014 - 06:16 AM

A vapour breakthrough case occurs in which a level control valve fails fully open, and vapour flows into a stripper column. As the vapour flows into the stripper column the pressure in the stripper increases until the relief valve in stripper start to lift, as the relief valve lifts, the flow into the stripper decreases and the flow leaving the relief valve increases until the flow into the stripper and the flow out of the relief valve become equal. I am required to calculate the pressure in the stripper at which these flows into stripper and out of relief valve become equal. How do I approach this problem? Any takers?

 

Thank you 

Matsa



#2 fallah

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Posted 10 June 2014 - 06:26 AM

Matsa,

 

Flow into the stripper and out of relief valve can be equal if a simultaneous blocked outlet case will be occured in the stripper.In fact, if such blocked outlet wouldn't be occured, part of flow into the stripper would go out of the stripper through the relevant overhead line. Please upload a simple sketch of the system and clarify the point about your query...



#3 Nirav

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Posted 10 June 2014 - 06:55 AM

There's very simple approach when you think of relief valve design.

 

(1) You should not consider any "favourable" response from the control systems. It means that the all the streams coming in and out of your system should be considered as operating at its normal position. If I further explain, you should not consider any increase or decrease in flow rate from system except the one which is causing "overpressure".

 

(2) After considering above aspect, take a stream which is causing overpressure due to higher flow than normal. In your case, it's gas break through from a high pressure system upstream of a stripper. I presume that you already know and calculated what is the possible flow if there is gas break through from control valve full opening.

 

Based on both above (1) &(2), Let's say the gas break through flow = "x" kg/hr.

This is maximum vapour flow possible from control valve full opening considering selected Cv. Remember, if there is by-pass to this control valve, you need to consider that by-pass valve is also fully open for finding out total possible gas break through. You can refer API-521 for all these guidelines and recommendations.

 

Hence, PSV needs to be capable of the "x kg/hr" calculated as above to relieve it to flare header. Then only pressure in system will remain below design condition. Hence, the flow into system and out of system are equal which is "x kg/hr". This is the basis PSV sizing.

 

Practically, instrumentation system may respond favourably before PSV starts releasing to flare system. Hence, actual flow might be lower than the value specified as "required relief". But it does not have any significance as far as safety of system or sizing or relief valve is concerned.

 

I hope this helps.

 

Warm regards,






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