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Sep
15

Exceptional Opportunities in Process Control – New Sensor and Valve Technologies

I spent the first 7 years of my career in instrument design and construction. After being responsible for the calibration, installation, and commissioning of instruments for a half dozen plants in the 1970s, I became painfully aware that the actual performance of the measurements and control valves was largely unknown. These were the days before the advent of smart instrumentation. We didn’t know the effect of stiction and backlash on valve position or the effect of impulse line, process and ambient conditions on sensors. We didn’t know what was the installed accuracy of measurement or if a valve or measurement had a timely and sensitive response. We shifted set points and just shook our heads when the material and energy balances did not close. Since we were mostly interested in capacity we just pushed on to make more product. Operating efficiency and turndown were not as much an issue, which was fortunate because we didn’t have the spectrum and accuracy of instruments for knowing process performance. The time I spent in the 1980s working on pH, furnace pressure, and compressor surge loops were the ultimate test of sensor and valve sensitivity and speed. My perspective on the importance of the field devices was solidified in the 1990s, when I was part of a corporate wide process control improvement program, most of the opportunities involved tuning loops and adding feedforward control and loops for fed-batch operation. A lot of great ideas went by the “way side” because of missing or imprecise measurements and unresponsive valves.

An important point is that if you don’t have the capability of determining actual capability and benefits of the automation system, projects will seek the lowest cost alternatives. A classic example of capital cost superseding performance was the proliferation of rotary piping valves that were posed as throttling valves by the addition of spool type positioners to modulate a piston actuator, linkages, and stem connections fundamentally designed for on-off service. The leakage specs and price were attractive. Deadband and resolution limit were not considered. Since the valve specification didn’t require the valve actually move in response to the small changes in signal commonly incurred in a control loop and there was no position feedback measurement either locally or remotely, the user did not know the real price paid. Aggravated by noisy flow measurements with poor turndown, increased process variability was attributed to mysterious sources. Without online loop metrics, there was little recognition of the deterioration in loop performance. Since the normal practice of testing whether a valve worked was to make 25% or larger changes in signal, instrument technicians and engineers where unaware that the valve did not respond to the small changes in controller output each scan. Putting a smart positioner on a piping valve with the feedback measurement of actuator shaft rather than ball or disk stem position in a rotary piping valve only added to the confusion. The actuator shaft would move in response to the positioner but the ball or disk did not due to extensive seal friction, ball or disk shaft windup, and backlash in the connection and linkages. It was only after actual tests in the flow labs of control valve manufactures was the true cost of these valve recognized. The publication of the lab test results and the subsequent ISA standards developed on valve step testing, the availability of position feedback as a secondary process variable on digital signals, and the analysis of resolution (e.g. stick-slip) and deadband (e.g. backlash) lead to an increased awareness and hence dramatic improvement in valve dynamics.

Today we have smart transmitters and control valves with a rangeability, resolution, and sensitivity that is an order of magnitude better than the typical fare of the last century. A combination of embedded intelligence and new sensor, transmitter, valve, and positioner technology have resulted in dramatic improvements. Combined with the ability to have additional process variables, diagnostics, and alerts reported to the control room by digital signals and the mobility afforded by wireless communication, we can increase the spectrum and flexibility of the field automation system including finding the optimum locations for process analysis and control. Doors will open for online data analytics, process performance metrics (e.g. energy, quality, and yield) and increased opportunities for basic and advanced control improvements to address the increasing needs of process efficiency, flexibility, and rangeability. My recent Control Talk column “Downturn Turndown” digs into the increased importance of sensor and valve performance with of course a top ten list to cap it off.

Recently it was realized that research and development could greatly benefit from the advanced performance, intelligence, and historization of smart industrial automations systems. The future is best exemplified by the lab optimized industrial distributed control system with industrial pH, dissolved oxygen, pressure, temperature, and mass flow measurements for bench top and pilot plant bioreactors that was pioneered by Broadley-James Corporation. The portability and reduced installation cost of wireless instrumentation increase the already significant advantages of moving advanced industrial automation system capability upstream in the commercialization process.

The foundation of a process automation system is the measurements and final elements. If you don’t get these right not much else matters. Measurements provide the only window into the process and final elements provide the only means of affecting the process. The height of the pyramid consisting of increasingly more advanced layers of process analysis and control depends upon the integrity and breadth of the foundation. The goal of the book I just finished is to create a foundation where the sky is the limit for automation. The book royalties go to the Center for Energy and Environmental Resources at the University of Texas where tests are being conducted on the use of wireless conductivity, flow, pH, pressure, and temperature measurements for carbon dioxide capture research.

The new book titled Essentials of Modern Measurements and Final Elements makes no assumptions other than the reader has some technical background. In Chapter 1 Modern Measurement Fundamentals, special care has been taken to explain technical terms and concepts on the use and performance of measurements in the process industry. There is a special emphasis on the advances in wireless instrumentation and communication. Chapters 2 through 6 focuses on the details needed for the best implementation of specific types of measurements that would be used on automation upgrade and new plant projects today in the process industry. Chapter 7 on Final Element Fundamentals follows an approach similar to Chapter 1 in assuming no industrial experience so the material on control valves, dampers, guide vanes, and variable speed drives is beneficial to students and new employees. Chapter 8 gets into the details on the types of control valves that are used in 95% of the applications in the chemical and petrochemical industry. The book concludes with the latest details on WirelessHART automation systems in Chapter 9. The questions at the end of each chapter are designed to stimulate the thought process involved for a successful application.

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