Control Talk Blog
The composition measurement with by far the greatest sensitivity and rangeability is pH. Consider that pH routinely detects changes at 7 pH to the 9th decimal place and for a 0-14 pH scale covers 14 orders of magnitude of hydrogen ion concentration. New glass and reference designs have dramatically reduced...
Nearly all process inputs are flows. The measurement of flow is important for process analysis, metrics, and modeling, reducing variability of process inputs, enabling feedforward and ratio control, and isolating valve nonlinearities by secondary flow loops. Inline flow meters offer the best accuracy and rangeability.
Temperature is the most important common measurement. Temperature determines product formation rate and yield in reaction and crystallization processes and product composition in separation processes. Temperature is also important for monitoring energy balances and heat transfer coefficients. The principal considerations are the selection of a TC or RTD and the...
Pressure and differential pressure (DP) measurements are important for regulating inventory, flow, and product quality and are critical for ensuring safe operation. Installation errors and the drift of today’s smart pressure and DP transmitters have decreased by an order of magnitude by compensation of temperature, static pressure, and non-ideal sensor effects.
Radar offers an incredibly sensitive measurement of surface level that can be nearly maintenance free. The potential for an accurate inventory measurement is dramatic but with this extreme capability comes some extraordinary application considerations. A checklist is offered of implementation details to help achieve the full capability of this measurement.
Variable speed pumps can save energy when the flow is reduced. However, the turndown of the pump speed may not achievable if there is overheating at low flows, excessive sensitivity at low rpm, and cycling at high destination pressures.
The threshold sensitivity and resolution of measurements (smallest incremental change) has improved dramatically to better than 0.05% with new sensor technology and smart compensation of extraneous effects of operating conditions. The response of the air-actuated control valve has improved, but the smallest incremental change in position varies widely depending upon...
The value of a key PID feature is increasingly becoming apparent. I got a preview of the importance when I found the 1980s vintage DCS required a fix for override control that was inherently addressed in the next generation of DCS by this feature.
The questions submitted by the 10 worldwide participants in ISA Mentor program are refreshing and cause one to stand back and think about how we do things and how can we convey knowledge we take for granted.
We conclude this series with a look at control schemes that can increase process efficiency for distillation control, compressor control, and neutralizer control. These control schemes can inherently reduce process variability and controller tuning difficulty and for compressors increase onstream time.