I'm not sure a linear trim makes sense. It suits situations where the upstream pressure and downstream pressure do not change greatly when the valve moves, i.e., the pressure drop across the valve is a relatively small portion of the available pressure drop. When the pressure drop changes appreciably across the valve, an equal-percentage trim normally makes more sense. This would probably have nothing to do with the triangle wave unless the valve was hitting a mechanical limit.
As far as an upset causing the control algorithm to drive the valve limit to limit, there normally will be a gain in every system that can accomplish this. It just gets easier when you can add things like slow control response, e.g., the slow valve stroke. The more lag time you can take out of a system, the harder it is to get it to oscillate. Given that this is a gas system, you never will get rid of the capacitance caused by the compression and expansion of the gas; so, it has some lag built into it. To expect it to never oscillate or expect to get it so that it cannot oscillate is probably unreasonable for all process disturbance cases. Again, this should not cause a triangular oscillation but a sine wave.
Jason L. Noe, process specialist
UOP, Des Plaines, Ill.
Follow a three-step procedure
I assume that the cycle in the loop is not the same frequency as the piston frequency of the compressor; if it is this simple, dampen the pressure transmitter so you don't see the compressor cycle.
I would follow the sequence below and quit at the step that solves the problem:
1. Is the valve "tight?" Play between the positioner and the valve can be the culprit. If you are going to Step 2, make the valve as fast as you can.
2. Install a flow meter in the recycle, cascade the flow set point with a pressure control loop. With proper tuning, the valve characteristics are lost in the flow control loop, which should be tuned fast with PI. Tune the cascade as a slow loop with PID heavy on the derivative. If you use auto tune, apply it only on the first loop; manually tune the second loop.
3. Use multiple PID values in the cascade loop depending upon the deviation from the set point. Close to the set point, the loop is slow; far from the set point, the loop is fast. Another variation on the multiple PID is to select PID values based on rate of change of the input.
Bill Wood, P.E.
Texas Brine Co., Houston, Texas
Check for a sticky valve
It sounds like a sticky valve is causing the triangular results. The valve moves a bit too much and sticks. Then the reverse takes place. Check the valve action and then replace that pneumatic positioner with a new digital one with a greater air volume.
Larry Richardson, technical specialist
UOP, Des Plaines, Ill.
Get back to basics
Does the controller have integral or derivative control and are they being used? Is the valve sized correctly? How much is the valve opening and closing for each cycle? Is valve movement causing a pressure change or is pressure change causing the valve to move? How much must the valve move to see a change in pressure? Essentially, I am asking about the gain of the valve. If the gain is too small or too big, the valve is the wrong size. The slow triangular response in the loop sounds like the PV is ahead of the control valve. If you are using PI rather than derivative control, then adding derivative control may slow or speed up the valve response enough for the process to respond. Other items to check are the calibration of the valve and controller output.
Rick Hildebrand, engineer
Amgen, Thousand Oaks, Calif.