Disturbing Remarks

The Memories of Loops Gone Bad and Processes Gone Wild

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Greg: Liquid pressure disturbances are extremely fast. A volume with a vapor head space is the best way to attenuate these disturbances. Oversized pressure regulators are an insidious source of fast periodic upsets when the field-pressure regulator action and its pressure are not monitored in the control room. Pressure loops with analog or fast digital scan rate controllers and variable-speed drives can catch up with fast nonperiodic pressure upsets. Liquid pressure disturbances turn into flow disturbances across any flow resistance (e.g. valve, nozzle, piping or inline element).

Stan: Finally, measurement noise, drift and resolution are disturbances for all types of loops. Measurement resolution problems are mostly history, but sensor noise—particularly drift—are a continuing source of problems. Disturbances that directly get into the process variable for a control loop are especially disruptive because there’s no filtering of this disturbance by the process-time constant.

Greg: Changes in weather are generally slow, but can be relatively fast and are notorious for affecting the air compressor suction temperature and distillation column reflux temperature. There is nothing like a cold rainstorm on uninsulated lines to perk up control room activity. Even wind and sun can be the source of weight changes in a load cell installation. Wind, sun and shade temperature, humidity and precipitation should be historized and available for trending with the loops.

Stan: Changes in ambient temperature can cause a drift in those sensors affected by sensing line, connection or housing temperature. A severely coated sensor shows up as an attenuated, but much slower and less damped oscillation.

Greg: The story in trend charts is more in the trend of the controller output than its process variable. For stick-slip, the classic saw tooth is evident. For periodic disturbances, the oscillation in the controller output is often larger. The controller output could be hitting output limits or wandering along the upper, almost-flat installed characteristic of a control valve. Load disturbances and measurement drift show up as a shift in the controller output. Sensor drift or weather-induced changes are seen in the controller output. The controller output through the reset and rate action of the controller has important information on the type, persistence and speed of the disturbance.

Stan: I don’t understand why multivariate statistical process control (MSPC), focuses on process variables to the exclusion of controller outputs. I also don’t get  the lack of recognition of the fact that some MSPC data alignment techniques may mask changes in oscillation periods and the duration of key sequences in a batch cycle, which are indicative of measurement and valve problems. Even more important, changes in the time durations of key portions of a batch sequence indicate changes in chemical or biological reaction rates and, consequently, changes in the secret concentration profiles.

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