June 2008

Never forget the fundamentals of process control

By Fernando Otero

What is the goal of an automatic control system?

From the regulatory controls point of view, a control system maintains important process variables at set point values.

The system should be able to make the necessary compensations when disturbances occur in order to get the controlled process variable back to its set point.

This job happens for multiple process variables in a manufacturing plant. Sure, there are other good ways an automatic control system can excel, as in performing automated sequential tasks, like in batch control. However, the whole idea of a process automation system is to insure the quality and quantity of the products is constant, uniform, and consistent.

Two control valves, one pipe

I have performed hundreds of control performance services in industrial plants. I want to point out some of my singular findings and how they relate to the ultimate goal of the automation system, and more importantly their impact on overall plant performance.

Do not get me wrong, I have seen plenty of excellent, outstanding performances of control systems; however, they call me most of the time to work on those not performing so well.

Maybe the most curious type of finding is a bad control strategy designed and implemented from day one. 

Something as simple, but critical, as finding two control valves on the same pipe separated a few feet from each other. To the question of what the roles of those control valves were, the answer was "one control valve controls temperature, and the other control valve controls pressure."

Of course, the control system did not work, and one of the control loops operated in manual mode.

A couple of other simple examples include generation of a very long dead time in the loop by locating the primary and final control elements very far away from each other, and generation of loop interaction by locating a final control element in the same line of a primary control element of another control loop.

Another interesting finding has to do with the settings and parameters in the configuration of the control strategy. This starts with the tuning parameters of the control loops.

The main thing that has impressed me is finding out at many sites that most, if not all, of the control loops had not been re-tuned since the day when the loops were commissioned.

Simple fundamental steps

Another issue is the use and abuse of the PV-filtering parameter. The first example that comes to my mind is a pressure control loop that had significant derivative action.

Since the pressure process variable is noisy, the derivative action would cause excessive movement of the controller output, and this, in turn, would generate a lot of process variability.

The "solution" was to apply a six-second filter on the PV signal of the controller. The default-dampening factor of the transmitter, which was 2.0 seconds, had never seen modification.

This "worked" very well because the noise was gone; the control valve moved more smoothly although it was still very reactive, but did not stop the presence of nuisance, deviation, and high alarms. The number of unexpected pressure shutdowns was the same.

In reality, by setting such a high PV-filter value, you decide to ignore the actual process behavior and choose not to see on the control system the magnitude of the variability this loop is having. This pressure loop had a time constant of 1.8 seconds and a dead time of 0.9 seconds.

The point here is, in many cases, simple procedures based on the fundamentals of process controls can affect the performance of not only the control system, but also the bottom line of the plant.

In the case of the automatic control system, after investing so much money on plant automation, the last thing I want is a control system that generates variability in my process.

ABOUT THE AUTHOR

Fernando Otero is an ISA Certified Automation Professional (CAP) and a senior member of ISA. He has two chemical engineering degrees. He is a process control consultant with Cornerstone Controls in Indianapolis.  His e-mail is fernando.otero@corner-stonecontrols.com.