The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient, and retired Senior Fellow from Solutia, Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

Paul Pathasema is an advanced process control engineer at the plant of a specialty chemicals company. Paul works with Bart Propst, a Mentor Program resource and former colleague of Greg McMillan before he retired from Solutia.

Paul Pathasema’s Question

What are some opportunities for wireless measurement and control, and what are the considerations in terms of process performance and reliability?

Greg McMillan’s Answer

An obvious opportunity for wireless devices is in particularly remote locations where cable access is unreasonable, and for portable rigs (e.g., platforms for testing, verification, diagnostics, reconditioning, and other temporary services for several locations).

Less obvious opportunities include using wireless portable temperature transmitters to find the best measurement location (e.g., distillation tray indicated by largest symmetrical change in temperature for a change in reflux to feed ratio), to detect and quantify problems (e.g., fouling indicated by an increase in temperature drop for a constant coolant flow), and to verify batch cycle times by prediction of batch end points (e.g., zero reactor conversion indicated by zero temperature drop with no coolant make up flow so that constant coolant flow is totally recycle flow).

There are several issues to be addressed if wireless measurements are used for closed loop control. Foremost is the ability to be forewarned and have batteries replaced without business interruption, well before the approach to the end of battery life.

Second, there must be multiple paths of communication from the device to the automation system. The reliability of communication considering all possible modes of failure and interferences must meet process safety requirements. For this reason, wireless measurement and control is mostly used for noncritical flow, pH, level, and temperature loops where a failure to update does not cause a hazardous operation. While diagnostics including prolonged time from last update can help, operator corrective action may not be timely or exact.

Third, the additional dead time must be recognized, evaluated, and addressed through tuning and special PID features. There is an additional dead time that is half the wireless update rate plus the communication delay from transmitter to control system that can potentially range from two to six seconds, as noted in the Control Talk column “The ins and outs of wireless instrumentation.”

Consequently, loops with an original dead time of less than 60 seconds would be adversely affected in the ability to reject load disturbances. Wireless measurement and control would be particularly unsafe on pressure loops, since the process dead time is less than one second and can be an issue for flow loops in terms of the ability to handle changes in origination and destination pressures.

The use of wireless positioners for feedback control is quite limited because the communication delay is not deterministic. An enhanced PID, described in Appendix E of the ISA 5.9 Technical Report on PID Algorithms and Performance, can increase the robustness of PID tuning for wireless control. If the dead time from the wireless device exceeds the 63% process response time of self-regulating processes, any additional delays in wireless updates or communication do not require PID retuning. For this case, the enhanced PID gain can be as large as the inverse of the maximum open loop self-regulating process gain. The reset time can be as small as four times the original loop dead time.

For integrating processes, the PID gain may be slightly increased, but if the wireless update rate gets slower, the PID gain must be decreased. An enhanced PID can also prevent the PID output from ramping off from integral action during a failure to update, enabling a smooth recovery when updates resume. The enhanced PID uses the external-reset feedback capability described in ISA 5.9 Section 4.