PSV sizing for Control Valve Failure 2

[carlos81]

Hi all,

I find everywhere that in case of a control valve failing to 100% opening the PSV protecting the system downstream should be sized according to the CV_100%.

Shouldn’t the normal operating flow to the downstream system be part of the sizing as well?
I mean;
relief flow = Control valve flow @CV_100% - Normal operating flow

I just wonder if a PSV is sized for the CV_100% then the PSV will be overdesigned and could produce chattering of the valve.

Any thoughts?

Regards,
Carlos81

 

[Chance17]

The normal flow credit is great for normal operation.
But consider equipment startup. Someone forgets to open a downstream manual valve.
Operations places the control valve on normal control.
The control valve will go wide open against a blocked outlet.
The startup contingency is why one should design safety valves for 100% opening and zero flow credit.
It is also noted equipment outlets can be blocked by means other than manual valves such as line plugging plugging or mechanical equipment shutdowns.
The safety valve has to protect equipment 100% of the time, not just during normal operations.

 

[Ifakiz]

I quite agree wit chance.sizing d psv requires u consider all cases planned n unplanned n add a tolerance of +15% in as much as d pipeline has much ratin

 

[carlos81]

Thanks for your responses.

Considering startup scenario blocking the flow downstream the valve is something that definitely could happen.

I'm good to go with the sizing now.

Best regards!