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Limiting Cv Of A Control Valve

li limiting cv of a control valv

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#1 chem2013

chem2013

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Posted 14 January 2016 - 12:09 PM

Dear All,

 

I have a pressure control valve with a bypass globe valve venting into LP flare header.

 

Control valve failure open is critical factor for LP flare sizing. Control valve failure open load is calculated at PAHH of upstream vessel.

 

There is a proposal to limit Cv of control valve in order to limit total max flow through both control valve and bypass valve at xx kg/h.

 

What note should we specify correctly to vendor ?

 

Option 1: Limit Cv of control valve to limit  max flow through control valve at xx/2 kg/h. (a division factor 2 is used as failure open considers both control valve and by pass valve).

 

Option 2:same as option 1 with indicating upstream pressure: xx barg corresponding to PAHH and outlet pressure: yy barg.

 

Please advise which option is more appropriate and why ?

 

Regards.

 

 

 

 

 



#2 fallah

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Posted 14 January 2016 - 12:41 PM

chem2013,

 

In my opinion a combination of both options should be specified for control valve vendor; because they should be informed of upstream/downstream pressures and max allowable flow passing through the combination of control valve and its by pass...



#3 S.AHMAD

S.AHMAD

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Posted 27 January 2016 - 01:05 PM

Normally we size control valve by determining the Cv such that the valve is between 70-80% valve opening during normal operations.. If you lower the Cv then the valve shall be more than 80% opening. Is this acceptable? The normal approach is to size the valve within acceptable valve opening and size the flare header accordingly. If you have problem in doing the normal way then let us know more 

 

s.ahmad


Edited by S.AHMAD, 27 January 2016 - 01:24 PM.





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