Calculating flow through a control valve


Thread Starter

Pat Furina

Each branch consists of a coil in line with a two-way control valve. The total system Pressue Drop is measured and can be controlled via variable speed pumps.
I have Cv values for each coil and control valve (at 100% open). I have a formula to work out the total Cv in each AHU. I also have a formula to give me flow with respect to DP and total Cv.
What I require is a formula that will give me the Cv of a control valve with repect to its position. I believe the control vales and actuators are linear in nature and therefore flow at 50% open is exactly half of that at 100% for a fixed DP. Does the same linear relationship also apply to the Cv?
Cv versus % open position for any valve is not something you can arrive at through a formula. These values are determined empirically by the valve manufacturer. Most manufacturers should have that data available. Contact them for the info. Without their data, you might as well just guess.
If the valve characteristic is linear, this is the case. But is it really? This is not typical. Also consider nonlinearities in the position measurement adjustment. This means, at may be 10% position reading, the engineered valve position is just 0%. The same can happen at 100%. Full opening may already occure at 90% or so.
Is it not possible to compare with a known measurement and thus try out the valve curve?
Klaus Moellmann ProConSol
First of all all control valves are not designed to work as linear. As you are not sure it best to confirm with maker or by looking at the design of the valve. Linear valves are cylinderical. There is another type which is called percentage and some actuator valves are simple valves which allow maximum flow when the lift is one quarter of the dia of seat. Another problem is that outside indication may not accurately represent the exact opening of the valve.

Because most of the actuator work in a control loop of either 4-20mA or 1-5V etc and the measured value is usually from a flowmeter feedback, exact position of the valve does not really matter as long as it is in a position to provide thew required flow.

In my opinion it should be necessary to install some kind of flow measurement.

Hope you find it useful.

Winfried Niehoff - ifm electronic gmbh

Very true is, that you don't need a feedback of the valve position to control the fluid by the valve.
But a lot of ppl want to have a continuous signal - feedback to have the valve monitored. We just have developed such a sensor for linear valves. Just follow this link: