Faulty positioner causes instability
Jan 2000
System Integration & Control Systems Design
Mimic Crafts
In a recent course held in a South African chemical plant, a loop was encountered that cycled continuously. The loop was a pressure control that was acting as the slave (secondary) loop in a cascade temperature control system on an oil heater. Figure 1 is a schematic of the control system. Repeated attempts had been made by the plant over an extended period of time to tune the loop to stop the cycling.
Figure 2 shows the closed loop test 'as found' performed by the class on the pressure loop. It can be seen that the loop cycled continuously with an amplitude of approximately 10% on the process variable (pressure signal). (The different changes in the test are where setpoint changes were made.)
The next test performed was in manual. Equal changes of 5% were made in the output of the controller as can be seen in Figure 3. The response on the process variable is very interesting. It can be seen that on every change the pressure seemed to 'overshoot' and then after some seconds slowly ramped back until it reached stability. A response like this is invariably due to a problem associated with the positioner, and sometimes also because the actuator of the valve has insufficient power to move the valve correctly, and the positioner then pumps too much air into the actuator to try and get the valve to move. Experience has shown that this type of response often causes complete instability in automatic mode.
In this particular case the resident plant 'valve expert' was called and the class went into the field with him, and inspected the valve. It was found that there was a problem with the positioner's feedback cam follower which was not moving freely. This was then adjusted. Figure 4 shows the next open loop test performed after fixing the problem. Note how well the process variable follows the controller output. The loop worked extremely well after tuning it correctly and putting it back in automatic.
This was yet another example of the need to analyse a loop first, before trying to tune it.
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