The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #75, and was written by Greg.
In the 1970s, there was a concerted effort at the chemical company I worked for to provide extra process measurement connections throughout the plants for pressure and temperature measurement. These connections could be used as needed to help identify and solve unforeseen problems. The installation of a wired transmitter on a temporary basis was not practical because of the cost to install conduit and wiring that is safe, reliable, and noise free.
Consequently, the company developed standards for pressure and temperature gauges that had scale ranges and materials of construction to meet most application requirements. Unfortunately, only the operator in the field standing in front of the gauge could see the indication and the resolution was poor. I still have flashbacks of how a kind old instrument engineer spent the remaining years of his career figuring out how to compromise scale ranges.
With modern wireless smart transmitters, we could have obtained this information far more easily and accurately. Smart transmitters enable ranges to be readily configured without much of a difference in accuracy. If a gateway is set up and other wireless transmitters are already being used in the plant, the commissioning of a new transmitter may just be a matter of minutes. Line of sight is usually not required because other wireless devices can be used as necessary to find a way to the gateway. Channel hopping eliminates noise concerns.
Encryption and invitation-only communication eliminate security concerns. The measurements can be readily indicated, historized, trended, and analyzed by operators and engineers. Wireless offers portability, analysis by asset management systems, historization, and computations for metrics, with the accuracy and diagnostics afforded by smart instrumentation.
Pressure is the driving force for material flow, and temperature is the driving force for heat flow. Flow disturbances start out as a pressure change and heat transfer disturbances start out as a flow or temperature change. The fouling of a heat transfer exchanger can show up as a change in the temperature or pressure difference between the tube side inlet and outlet streams.
Heat transfer rates and overall heat transfer coefficients can be computed if the heat transfer area and flow are fixed or measured. Recirculation flows are often relatively constant. When a heat exchanger in a recirculation line is used for vessel temperature control, cascade control system may already provide the necessary measurements. The cooling heat transfer rate is simply proportional to the primary loop temperature minus the secondary loop temperature; heat exchanger process inlet minus outlet temperature.
As I started to develop the use conductivity and pH for CO2 reduction at the University of Texas Pickle Research Campus, I realized there was a whole new opportunity for wireless. pH electrodes were failing and the relationships to CO2 load and solvent concentration needed to be quantified. The best sensor type and location needed to be found. Lab test results had to be historized and analyzed.
Concept: Wireless transmitters can be easily stocked, configured, and installed to explore and demonstrate diagnostics, metrics (Tip #61), and improvements, such as inferential measurements, new sensor technologies, new strategies, and better sensor locations.
Details: Use wireless pressure measurements to track down problems such as field regulator problems, screen and filter plugging, heat transfer surface fouling, and insufficient valve pressure drop as a percent of total system pressure drop. Use wireless pressure transmitters to provide the pressure profile for a piping system and enable the computation of the installed characteristic of a control valve. Use wireless temperature transmitters to provide an inferential measurement of heat transfer rate and overall heat transfer coefficient. Heat transfer rate measurements can enable the optimization of the scheduling of batches for energy conservation and the tradeoff between yield and production rate in batch reactors.
Use wireless temperature transmitters to provide a temperature profile in a utility system for pinch analysis to find the location of the limitations in the utility system for better process efficiency and capacity. Use wireless temperature transmitters on distillation columns to find the tray with the largest and most symmetrical change in temperature for a change in reflux to distillate ratio or steam to distillate ratio. Use wireless acoustic and temperature transmitters to monitor steam traps to detect when traps are stuck open or closed. Traps stuck closed can back up condensate into the heat transfer volumes of process equipment, severely disrupting process temperature control systems.
Traps stuck open will blow through steam, causing a loss of energy and higher condensate system pressure, which can cause other steam traps to shut, backing up condensate into other process equipment. Traps that are alternately stuck open and closed cause confusing and erratic plant operation. Use wireless pH transmitters in the lab in worst-case process samples to determine electrode performance, life expectancy, and warning signs of electrode failure for predictive maintenance. Use different electrode types in the lab sample to find the best sensor technology. Use wireless pH transmitters in the field to find the measurement location with the best mixing, least noise, and least deadtime (Tip #67). Use wireless mass flow computers and annubars to provide flow measurements where needed to develop and demonstrate online diagnostics and metrics. Consider a system of plant spares where wireless transmitters can be loaned to enable the rapid transition from idea to a working prototype in the field.
Watch-outs: Spare process connections may not have been provided in the design of the plant. Batteries should be monitored. The default update rate for wireless pressure measurements may not be fast enough to track down disturbances.
Exceptions: Wireless transmitters that use a lot of electrical power are not available. Wireless transmitters are too slow for surge detection and control, compressor control, polymer pressure control, and furnace pressure control.
Insight: Wireless transmitters eliminate the cost and time hurdles for moving innovations from research, engineering, and operations into action, enabling the rapid demonstration of benefits and inspiring creativity.
Rule of Thumb: Use wireless transmitters to rapidly prototype, demonstrate, analyze, and justify installations of online diagnostics, metrics, and improvements.
About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.
Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.