Control Valve Operational Range

The operational range of a valve depends on several factors. One is the type of valve you have. Look at valve curves for different types of valves to see the differences. You typically want to operate in the most linear part of the curve. This really helps in tuning your control loop. This means that in the linear range, a change in valve position of X will give you a change in flow roughly of Y. When you operate in a very non-linear portion of the valve curve, the change in valve position will not give you a consistent change in flow. The 50% to 75% area is a good area to run in for a few of reasons. First, even a linear valve has some non-linearity at the extreme ends. Second, At the top end (>85-90%) a large change in valve position gives you a small change in flow as well as a very small buffer for control when the valve is almost full open. Third, at the low end, a small change in valve position gives a large change in flow as well as the fact that you are pumping against a large resistance. This resistance will reduce efficiency.

The control valve position is proportional to the process load. The control valve must be sized to handle the maximum load conditions, and will usually be partially open during typical load conditions.

This is not always true. In a pipeline application, for instance, the valve will normally be fully open during normal operation, and will only throttle back to protct against low suction presure, or high discharge pressure or motor overload.

May I ask if this is a general question or can we help with an application. The reason for my question is a characterized seat valve is completed different in flow than butterfly valves and there are numerous designs in between.

Hi there,

A very simple answer to your question without getting too technical is this. You do not want the valve controlling at it's limits. i.e. controlling at 95% as you do not have anywhere to go from there.

When sizing your control valve you will size it so that the normal operating flow required will be passed when the valve is 50% open. This gives you a bigger range of control i.e 50 - 100% or 50 - 0%.

The other thing to bear in mind is that depending on the valve type, from +- 70 - 80% open, the valve will already be passing maximum or very close to maximum flow (especially butterfly valves).

It sounds like whoever sized your valves did a very good job, but bear in mind that it is coincidence that your machine load corresponds to the valve opening percentage. This is not a rule of thumb.

For more information on control valves try to get hold of the Fischer control valve handbook - you can download it for free from the Emmerson web site. It is a very good reference to have and will answer most of your questions on conreol valves.

Regards,

Craig