I/p Transducer.

Is the PLC doing PID control of the valve?

I'd want to do a step by step troublshooting of, well, everything ;-)

SAFETY NOTE: What I describe here is without regard to any safety issues. If your process involves hazards (like if you're manufacturing an explosive liquid, gas, powder, or a poison, of something flammable, or a biohazard, or whatever) you must understand and comply with appropriate safety procedures and concerns. As I don't know what process you are dealing with, I cannot advise you on these.

You may have done some of this already, but whatever you haven't done.

I forget the exact names for these, but let me call it a loop calibrator--you want to get a loop calibrator (something that, among other things, can output various currents between 4 and 20 ma.). Use that to apply 4 and 20 ma. to the valve (while the 4-20 ma. signal from the PLC is disconnected). Does the valve fully open and close in those circumstances? Good, then the valve is good. (At least, almost certainly.)

Now, look at the output from the PLC (with the loop calibrator which should also be able to read currents between 4 and 20 ma.) (Do this with the PLC disconnected from the valve.) Can you create circumstances in the PLC which generate outputs of 4 and 20 ma.?

(There might be two (or more) ways to do that--there might be a manual means of setting the valve output from the PLC--that would be fine as a first test, but then you want to consider the inputs to the PLC that are driving the PID loop which is driving the valve. Can you change those inputs (using one or more loop calibrators to simulate the signals from those inputs) so that the PLC generates outputs of 4 and 20 ma. (and "everything" in between).

Do you understand PID control? PID control is a way of controlling a process to minimize bad things like oscillation in the system. Hence, even though a fully open (or fully closed) valve might be required for the current input conditions to the PID control loop, the valve will not be instantly driven to the fully open or fully closed position, but moved there gradually (by gradually changing the 4 to 20 ma. output from the PID controller (presumably the PLC) to the valve.

The speed of response can be, and should be, tuned by a process engineer to minimize upsets to the process.

So, from the point of view of troubleshooting, when you are trying to see the valve position as a result of a given set of input conditions (to the PID controller), you may have to wait for the output signal to be driven fully to the 4 or 20 ma. condition.

There should be a process engineer somewhere who has either tuned that loop or is responsible to tune that loop--find him and get him involved.

SAFETY NOTE: What I've described here has been without regard to any safety issues. If your process involves hazards (like you're manufacturing an explosive liquid, gas, powder, or a poison, of something flammable, or a biohazard, or whatever) you must understand and comply with appropriate safety procedures and concerns. As I don't know what process you are dealing with, I cannot advise you on these.

Try calibrating on 0% and 50 % for a start.

Try reversing the action of the PID control loop and report back to the forum what happens.

I agree with rhkramer.

Sounds like you need to fiddle with your gain settings on the controller (proportional action).