He can be reached at liptakbela aol. In process control, there are at least five levels of sophistication that the automation engineer can apply. The advantage of the higher levels is better control, but applying a higher level also requires better understanding of the process. In the analog age, using higher levels of sophistication also involved substantially higher costs, but in the digital age, this consideration has become less significant. To illustrate the five levels, I will use the example of controlling the outlet temperature from a steam-powered liquid heat exchanger.
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Ender, President, Techmation, Inc. Reprinted from Control Engineering September Twenty-five years ago, control systems were built around single loop pneumatic controllers, pneumatic transmitters, and control valves. These were hooked together with pneumatic tubing carrying psi pneumatic signals that implemented control strategies at the speed of sound.
Controllers were either proportional only P or proportional plus integral PI. Derivative cascade, and feedforward were viewed as very advanced control strategies.
Controller tuning was considered more of an art than a science and the subject of active research at many universities. On a tour through a typical plant, one would find as many loops in manual as in auto, or drifting all over because the gain was turned way down to avoid troublesome oscillation thereby also avoiding troublesome control. In a word, control was poor.
Electronic analog controllers promised faster response, the possibility of more precise tuning, and a revolutionary improvement in control accuracy for a slight premium in price usually about double. This triggered a long-lived debate as to which was better, analog or pneumatic instrumentation. The debate lasted nearly a decade and was never settled. In the early s, after DCSs had been available for five or six years, Honeywell, Union Carbide, and others, to quantify the benefits of using DCSs rather than conventional analog instrumentation conducted some studies.
One of the areas identified was better control. Before and after comparisons of performance, when conventional instrumentation was replaced with digital systems, repeatedly revealed better control.
However, data sampling rates to the screen are still slow causing aliasing of true process response. Control valves - Better valve designs, anticavitation, and noise reducing trims are yielding high performance rotary valves with better seals and better packing designs. Valve actuators - Spring and diaphragm valve actuators are still found operating at psi, the most common control media. Excessive hysteresis, stick-slip, valve maintenance, and calibration are still major problems.
Installation, calibration, and maintenance are still problems, though. Smart transmitters - These have only recently become available and their popularity is growing. Some designs contain on-board digital processors to massage the raw data readings, including; a moving window filter technique and a noise suppression deadband. Closed-loop control - The transmitter dynamics change depending on the magnitude of the true signal change.
This causes the feedback controller to introduce a resolution cycle similar to controlling the process with a valve with excessive stick-slip. Not too smart from a process control standpoint. Yet, after all this time, there is no standard for PID implementation in digital controllers. Hence, no two digital controllers on the market work the same. Advanced digital systems - Many do not provide enough functionality for even basic modification of the PID algorithm. For such basic control, as error squared for surge level tank control or PI-D and I-PD selection for modification of the setpoint response to the error, some manufacturers even limit the PID settings that can be entered.
Process applications - There are not enough control engineers available to address all applications properly. States trade deficit. Today, such debate centers around fieldbus standard. And, like the earlier debates, users want to know that whether the new digital fieldbus standards will really help process plants achieve better control.
In testing thousands of individual control loops, in hundreds of operating plants, the following summarizes what Techmation and others have found about how the typical process plant is operating. After presenting these data at a recent seminar, a participant challenged the findings. He was certain that his plant, and most others, could not possibly be running with such a high number of loops in manual. The equipment problems include oversized and undersized valves; excessive hysteresis, resolution, or stick-slip in the valves; and measurement problems.
Recent findings indicate At a recent TAPPI Technical Association of the Pulp and Paper Industry meeting, a representative of a major valve manufacturer presented a paper where they tested 31 valves on a paper machine steam and condensate system.
They found that No checks were made on sizing or installed characteristics. Fifteen percent of the loops have Control engineers at the plant design problems. One reason for this is level will admit that they are so a lack of process control design busy trying to solve real conditions at the time of initial problems they do not have engineering.
All too often, we find control loops added to processes as an time for theory afterthought. Proper design includes a realistic expectation of control over a realistic control rangeability with some stated accuracy requirement. Control rangeability is the ratio of the maximum control setpoint to the minimum control setpoint.
Variability impacts final product uniformity and plant efficiency. It also represents a poor utilization of the capital investment in the process control equipment.
In short, the digital electronic control of modern process plants is typically not much better than was achieved with the pneumatic instrumentation of 25 years ago. Who is responsible? Consultants are typically involved more in the design and construction and have only limited expertise in process control dynamics.
They typically are more involved in installation, configuration, and commissioning. Typical tuning during commissioning is done to detune the system to make it stable at steady state.
This type of tuning is not intended to provide minimum variance control. The Buck Stops Here! Plant organization Many engineers in process control took one or more control theory courses in their undergraduate program. The course was typically presented as an abstract math course by a professor who never worked as a controls engineer in industry.
For control engineers working in the plants, once out of school, the material is quickly forgotten, and they practice process control on an ad-hoc basis without reference to the theory that governs the behavior of dynamic systems. Control engineers at the plant level will admit that they are so busy trying to solve real problems they do not have time for theory. Control engineers in industry, in many cases, see process control as a problem of DCS configuration and plantwide systems integration.
In many cases, the advanced control strategies calculate optimum setpoints and demand continuous changes be made by the regulatory control system which is in manual, detuned for steady state operation, has equipment or design problems, or in some cases works well. The technician have the direct engineers cannot so much as touch a loop let alone tune it. The formal training typically varies from engineers cannot so much as none at all, to a brief introduction to touch a loop let alone tune it Ziegler-Nichols tuning rules.
In practice, all technicians teach themselves to tune by the seat of the pants, sometimes referred to as trial and error or SWAG tuning. This typically involves "tweaking" the settings until the loop "looks" right.
Operators know the process and feel that they can control the process and make product "in spite of" the control system. With experience and time, they learn how to stabilize the process, control upsets, and make fast changes manually to overcome the inherent problems in the control system. They are all for optimization, but resist any testing of the system which is running and making product. The following are some observations by leaders in the field: W. Bialkowski writes; "The main barrier is not technological, but a deep seated lack of understanding by industry management of the true nature of process control and its potential to enhance competitiveness.
This leads to an inability to provide the leadership necessary for bringing about the organization and cultural changes needed to meet the challenge. Management has invested greatly in the most modern control systems and assumed that the investments would lead to better control.
Management has also shown a willingness to dedicate resources for improved control. Products like self-tuning controllers and resources dedicated to trial and error tuning by the technicians have not been proven to improve control, reduce variance, improve profits, and provide real tangible benefits.
Results of lessons learned Techmation has developed tools and training programs to provide a systematic approach to process system analysis based on experience gained during the past 8 years in over 2, plant sites in 16 countries.
Developed as a systematic approach to process system analysis, the Techmation Protuner is a software tool for control system dynamic analysis.
Summary Process plants around the world are feeling competitive pressure with escalating demand for lower cost, higher quality products at reasonable profit margins. Obtaining and optimizing the dynamic performance of control systems can make a measurable difference in plant operation and profits.
Instrument Engineers' Handbook, Vol. 2: Process Control and Optimization, 4th Edition
Aug 31, Covering 10 million American homes with solar roofs would trigger the biggest economic expansion of the decade. Distillation, Part 5: Multiple Products Aug 1, In Part 5 of this series on distillation control and optimization, we find that adding a side-stream component to your process gives an additional degree of freedom, but makes it even more essential to not mismatch the variables. What are the potentials of process control in wind, ocean wave, and geothermal energy systems. How to select control valves, Part 2 Sep 12, When it comes to selecting and sizing control valves and positioners, this article not only helps you pick the right one for the right job, but also includes a valuable valve selection chart you can download!
Béla G. Lipták