Characteristics of Equal Percentage Control Valve

Well you may get a throttle control capability, but it will all depend on a whole variety of other characteristics that you haven't mentioned. Here are a few of the parameters that must be known to attempt any calculation:

• Pressure differential
• Fluid viscosity
• Piping diameter before and after throttle valve
• Actual throttle range of opening

Now I'm certain that there are other pertinent parameters that one should know to answer this. Particularly when %EP exceeds MGH. This will clearly happen when the DCS attempts to put a Z domain pole inside the unit circle. This will certainly make this FUBAR.

>What happens if the control valve is %EP, but we install a linear cam positioner

I assume %EP means equal percentage which I notate as =%.

This is a typical configuration. The valve's trim is =% and since the positioner is linear, then the valve stem responsds in a linear fashion to the control signal. The flow depends on the process conditions, but the trim produces a flow rate with an =% flow profile.

>vice versa

If the valve trim is linear and the positioner is =%, then the valve stem position will not be the same as the control signal (in percent). It will have some relationship similar to the values in the table, (there are other =% curves than that shown in the table; travel of 0-1.0 is 0-100%)

The process response will not be identical because the valve trim is so different, but this combination will mimic the =% valve.

>during normal process if DCS sends signal of say 60% to an equal percentage valve , what will be the local reading? Will it be at 60% opening or different?

I have no idea what the value the local reading shows, but if you're talking about the valve stem position, it will be different than the control signal.

If the =% valve has a linear positioner with a 60% control signal, then the valve stem should be at 60% travel.

But the valve will not be at 60% of maximum flow rate, because 60% travel is only 25% of maximum flow rate for =% trim.

To get 60% of max flow rate from the valve the control system would have to call for something greater than 80% with its control signal.

<...What happens if the control valve is %EP, but we install a linear cam positioner or vice versa? what happens to the process?...>

Well, if the equal percentage characteristic is provided by the valve trim, then absolutely nothing.

<...during normal process if DCS sends signal of say 60% to an equal percentage valve , what will be the local reading?will it be at 60% opening or different?...>

It will be at 60% opening, though the flow through the valve will be determined by a function of the valve trim characteristics, the fluid properties, the upstream and downstream conditions and other characteristics, such as the piping configuration.

Ordinarily, the position of a valve is of secondary interest to what the process is doing and how the control loop is behaving.

Hi bud,

can u share the response for a quick opening valve using a quick open positioner?