Emerson Exchange 365

As a student in high school, I struggled when writing papers. I had a lot to say, but the resulting jumble of thoughts and ideas was poorly presented and confusing to the reader. A ninth grade teacher hammered me for this, and ultimately taught me that no article or paper could be successful if it [...] Read the full text.

If you watch a good operator start up or deal with abnormal conditions, you see him or her put the controller output at a value that puts the process at a good operating point based on past experience. The good operator is patient and confident enough to leave the output at this value long enough [...] Read the full text.

We conclude our review of nonlinearities with an overview of applications and a detailed list of simple solutions to minimize the adverse effects of this everyday problem in nearly all control loops. It is impressive how setting lambda equal to 3x the largest deadtime value helps to solve most nonlinearity problems. Lambda is the closed loop time constant for self-regulating processes and the arrest time for integrating and runaway processes. Read the full text.

I am a risk-averse engineer. My clients do NOT like surprises, and they pay me a decent salary to make sure that their automation projects go in as painlessly as possible. Therefore, I do not apply new technologies unless I know they work and I am content to let others debug the latest software revision [...] Read the full text.

When I was leaving for college, my Dad said, “Make sure you use a good grain analyzer to optimize alcohol batch time and yield.” These words of wisdom would be useful in the years to come at “Purple Passion” parties with tubs of grain alcohol and grape juice, but their greatest value was seen last [...] Read the full text.

Here we use the broader definition of linearity to mean constant dynamics. A linear control loop has a constant total loop dead time, constant primary and secondary time constants, and a constant open loop gain. This perspective reviews the sources of dynamics and causes of nonlinearity. Read the full text.

The power of politics and of human emotion can be mind-boggling and utterly baffling to engineers who are taught throughout their lives to apply sound logical principles and facts to decision-making. I cannot begin to count the number of times when I have found myself desperately trying to apply logic to a situation where absolutely [...] Read the full text.

A change anywhere in a control loop will circle the loop in one loop deadtime. Every part of the control loop in the field and in the control room will be affected by the change within one deadtime. Loop components in the path as the change propagates will see the response before one deadtime but [...] Read the full text.

One of the more heated arguments I had with a client involved his extreme desire to use line size butterfly valves for controlling his process. The entire concept of using control valves for the available pressure drop and for controllability at low flow rates was utterly lost on him. We went round and round until [...] Read the full text.

The speedup of a plant’s response can cause loops to go from a smooth to an oscillatory response. In actual plants, the faster rate of change of a process variable important for product quality such a temperature or composition occurs for various changes in operating conditions. Principal sources of speed up in an actual plant are a smaller mass due to minimization of inventory or at the start of fed-batch operations, an increase in heat transfer coefficient from a cleaner surface or higher fluid flow, and an increase in catalyst activity from fresh catalyst or higher fluid flow.  In virtual plants, an increase in integration step size in first principle models is used for simulation speedup so scenarios take minutes instead of hours to complete. This speedup of simulations is essential for training operators and developing/prototyping control strategies. The bottom line is that the product of two terms can be used to detect and prevent instability in plant speed up. Read the full text.

The reset time can be approximated as a factor of the deadtime, but this dimensionless factor can range from 0.5 to 40 depending upon the type of process. The control literature has focused on self-regulating processes with process time constants in the same range as the deadtime and with little treatment of deadtime dominant processes, [...] Read the full text.

This tip is really an extension of the previous “Never Lie” tip. Every engineer in the history of the world has made mistakes. In fact, the best engineers are the ones who have made LOTS of mistakes—and learned from them. As mentioned previously, pain is instructive, and a painful mistake is a wonderful way to [...] Read the full text.

Since early in my career, I have always been glad to see a diaphragm actuator rather than a piston actuator on a control valve, so I was surprised to read a two-part article in the 1990s by an experienced consultant who maintained that control valves should use piston actuators. The article had excellent information and [...] Read the full text.

Study after study has shown that human perception quickly turns into reality. If a class is told they are troublemakers and slow learners, then over time that is exactly what they will become. However, if that same class is treated as an elite group where expectations are high and good grades are simply a given, [...] Read the full text.

The primary reason why there are so many and so different schools of thought about control algorithms and tuning can be traced back to one parameter in the process response. What PID tuning and what PID structure is pronounced as best and even whether PID control should be used is based on an assumed range of values for this parameter. A simple tool can open the mind to relative merits of different solutions and provide the means to see the whole picture eliminating disagreement. Read the full text.

If there were no unmeasured disturbances, feedback control would not be necessary. Process engineers and operators could home in on the best PID output and just leave it at this value. In fact many process engineers are much more comfortable with setting a stream flow per a process flow diagram than relinquishing to a PID controller that they don’t quite understand. In batch operations, often flows are sequenced based process design knowledge rather than released to a PID loop for fed-batch control. Also algorithms could be designed to focus on providing the best setpoint response and compensating for known disturbances. Read the full text.

For pH control, where the requirements for precision are extraordinary, the reagent flow rates are so low that I was never tempted to use rotary valves other than as expendable on-off valves for pulsing. I once used small solenoid actuated ball valves for pulse duration control because the frequent stroking would wear out any other [...] Read the full text.

Around the middle of my career, I was becoming disenchanted with the reorganization of my plant and decided to pursue another job. I found what appeared to be an excellent opportunity – big raise, generous relocation package, awesome benefits, and a lucrative bonus. It sounded like a “can’t lose” opportunity and I jumped at the [...] Read the full text.

Much of the differences in approaches to controller algorithms and tuning can be traced back to assumptions made about the type and importance of disturbances. Each method has merits based on the disturbance frequency, location, and time lag. Here we gain an understanding of how to reduce process variability from upsets originating from changes in raw materials, production rates, weather, operating conditions, or other loops. Read the full text.

I quickly started to appreciate pipeline and inline control from working on inline pH control where a static mixer (a baffled piece of pipe) was the only source of mixing. Static mixer manufacturers touted radial mixing, but no back mixing, that is, there was only plug flow. No back mixing meant that the components at [...] Read the full text.

Over the course of many years, I have learned to be suspicious of anything that is marketed as “Plug and Play.” I suppose I may be the only person with this problem, but with amazing regularity, I find plug and play devices that do not plug and play.... Read the full text.

A young small company is able to take university graduates and immediately make them productive in industrial applications. The open, positive, and enabling culture fostered by management has recently elevated and accelerated employee knowledge by providing... Read the full text.

I have found over and over again that the simplest and best thing you can do for level, pressure, pH, and temperature control on vessels and columns is to simply increase the reset time by a factor of 10. It is truly amazing how many of these loops are... Read the full text.

The effects of measurement dynamics are similar to the effects of controller dynamics except there are often many more sources of lags and delays and the consequences are generally more severe. Delays are the result of transportation delays and discontinuous... Read the full text.

Over the course of many projects, my project team and I have learned this tip the hard way, and with every project we promise to test new or revised software harder and more thoroughly than the time before. Despite all of our efforts, we STILL find small... Read the full text.