TABLE OF CONTENTS Track down and correct the source of sustained oscillations. A power spectrum analyzer may be required to find the loops with the common period of oscillation. Beware of a slow scan time of the I/O and controller that will cause a slower than actual period and a smaller than actual amplitude from aliasing. For trends or data obtained from data historians, make sure the data highway reporting and the time intervals between data points for historical data are not too slow and the trigger for exception reporting and compression is not set too high. Also, the data must be saved for at least a month to catch different process conditions and modes of operation.
The correction most often involves improving the responses of control valves and the tuning of controllers. Stick-slip, dead band, and time delay from the final element can be eliminated by the use of a variable-speed drive. Wave form recognition, step tests, and an understanding of the relative effects of valve response, controller tuning, noise, resonance, interactions, and nonlinearities can pinpoint the root cause.
The most likely source of a sustained oscillation is a control valve with stick-slip from excessive friction in either the packing or sealing surfaces. This occurs most often in valves with high-temperature or environmental packing, particularly if it has been tightened. The next-greatest source of stick-slip is high-performance valves designed to provide tight shutoff. It can also occur in almost any control valve so severely oversized that it rides the...
