Common Control Myths – Part 3

Before I leave to enjoy a crawfish boil and Cajun music, here are some more myths.

(6) Loop oscillations can always be decreased by reducing the controller gain – too low of a controller gain can cause nearly sustained slow rolling oscillations in an integrating process (e.g. level or gas pressure) and instability in a runaway process (e.g. exothermic reactor temperature). Decreasing the gain makes the problem worse for these cases. There are also limit cycles that persist regardless of controller gain.

(7) Limit cycles can always be stopped by eliminating valve stick-slip – limit cycles can also be caused by output limits, IO card resolution limits, deadband in integrating processes and cascade loops, and extreme nonlinearities, such as the pH titration curve.

(8) The installed valve resolution and deadband meet catalog specifications – often tests by manufacturers are for hand tight packing at positions remote from the seating friction. Also, temperature and fouling can make the installed performance worse.

(9) The most accurate type of pH sensors are used most often – the most popular sensors are the ones that require the least amount of maintenance, such as references with solid electrolytes, even though these may require more time to equilibrate and have a more variable junction potential. The flowing liquid junction reference for the right materials of construction and electrolyte is generally the most accurate but the least used type of pH electrode in industry because of the need to pressurize and refill the reservoir.

(10) Thermocouples are faster than RTDs – while this would be true for a bare element, nearly all the installations in the chemical industry I have seen use thermowells. The fit, design, and materials of construction of the thermowell have a far greater effect than the sensor on the speed of response of the temperature measurement.