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Emergency Depressurization Downstream Piping

emergency depressurization choked flow downstream piping

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#1 ayan_dg

ayan_dg

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Posted 11 January 2017 - 05:51 AM

In our present Project we have a scenario where we have a emergency depressurization system through an RO. During peak flow the upstream pressure is 21 barg. As critical pressure ratio is 0.55 the downstream pressure is 21*0.55 = 11.55 barg. As such choked flow through orifice happens.

 

Now the downstream goes to Flare the design pressure of this header is 3.5 barg. Now how to protect this 3.5 barg header and what will happen to this excess pressure .

 

Back calculation from flare to the downstream of RO predicts only 1.5 barg at this peak flow.



#2 fallah

fallah

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Posted 11 January 2017 - 06:18 AM

In our present Project we have a scenario where we have a emergency depressurization system through an RO. During peak flow the upstream pressure is 21 barg. As critical pressure ratio is 0.55 the downstream pressure is 21*0.55 = 11.55 barg. As such choked flow through orifice happens.

 

Now the downstream goes to Flare the design pressure of this header is 3.5 barg. Now how to protect this 3.5 barg header and what will happen to this excess pressure .

 

Back calculation from flare to the downstream of RO predicts only 1.5 barg at this peak flow.

 

Hi,

 

At first you should size the depressuring line for peak flow rate based on relevant project's line sizing criteria; then size the RO based on upstream pressure=21 barg for peak flow rate in choked flow conditions. RO downstream piping should be supported enough against vibration due to AIV and so on...

 

One more point: critical pressure ratio should be calculated based on absolute pressures not gauge ones...



#3 flarenuf

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Posted 11 January 2017 - 07:04 AM

Hello Ayan

The back pressure on the RO will not actually be 11.55barg, The backpressure will be calculated by the build up of pressure in your flare header system. You may find for example that the bP  is only 3.00 barg , this will still give you critical flow on the initial flow of gas through the RO.
You will need to check the pressure drops through your network using Flarenet (AFSA) or similar.






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