Control Valve Pressure Survey

[Plaxerous]

Most of the control valve stations in my plant have immediate upstream and downstream bleeds where we can easily take pressure surveys and calculate theoretical flow based on Valve % Open and CV and compare with the measured flow.

However, I have this one control valve that I'd like to take a pressure survey, but the nearest bleeds are at least 5 FT away. Would these pressures still be accurate in calculating theoretical flow? Is there a reason why it's best practice to take pressures immediately upstream and downstream of a control valve?

 

[shvet]

incorrect, flow disturbances just up/downstream of a CV are excluded from Cv calculcation
this is a popular misconception
measuring dP exact up/downstream of a CV (points where bleeds normally are provided) Cv will be irrelevant to one provided by a CV's manufacturer

CV's dP is not the P1-P2 just up/downstream of a CV's body, it does not work this way

 

[TiCl4]

Shvet is correct - Cv for control valves is calculated based on permanent pressure loss. If your taps are too close to outlet of the valve, you will be seeing a pressure drop that includes both permanent and recoverable pressure drop.

Note that this arrangement is different that orifice flow meters, which are often tapped to measure the lowest pressure point at the vena contracta, so their Cv is based on a different tap arrangement. Perhaps whoever set up your tap points confused the two different arrangements?