Hi:
I need some help to find relieving capacity based on blocked flow condition.
I have valve CV value, pressure drop but i don't know that what equation should i use to find the
relieving capacity? One more thing that i don't have API book in my office. The person who has this book is on vacation. please help me in this matter.
Thank you very much for your help in advance.

Please submit a sketch for the mentioned system in order to better understanding the problem and giving clear solution.

process101,

There are few control valve handbooks available free i.e. Fisher, Masoneilan, etc. Check out in these collection....Useful Document related to Control valve. With CV correspondent to opening as you specified i.e. 100% open and differential pressure across th control valve, plus the properties of fluid, probably you can get the flow...

I am still not clear with your relieving scenario. You mentioned that it is a blocked flow condition. When you have blocked flow (discharge), then the outlet is blocked. Whatever coming into the system will be relieved via PSV. At this moment, you may consider consider control valve still work correctly. So it is normal flow through the control valve.

However, in the event of control valve failure (failed to full open position), the control valve will be fully open (and consider no credit on the discharge), then whatever maximum flow passing the control valve will relief through the PSV. This is control valve failure case.

Having said few points, i still not very sure where is the control valve (inlet to a drum or on the gas outlet or liquid outlet, etc). So you may need to define you case in detail to get specific and accurate advice.