Tech Tip #14

Proper Control Loop Tuning Saves $

Proper control loop tuning can save your company thousands of dollars annually and provides one of the best returns on maintenance expenditures. Costs can be saved in the following areas :

• Valve Maintenance
• Energy Cost
• Raw Material Cost
• Product Quality

But what is 'proper' loop tuning and how can it save your company money? We will be discussing  the details of 'proper' loop tuning in a future Tech Tip, but for now we will focus on how the lack of proper tuning affects operating cost.

Let's begin with a definition :

Proper loop tuning parameters are the set of tuning parameters that result in the lowest cost of production while maintaining product quality.

Most tuning methods used today, focus on minimizing the absolute error between the setpoint and the controlled process variable. The problem with these methods is that they usually result in controller gains that are too high for proper tuning.

Excessive controller gain results in added production costs through two basic effects. The first effect is increased valve movement. Higher gains result in bigger swings in the controller output resulting in increased valve wear and energy costs associated with valve movement. In an April 2002 Control Magazine article, George Buckbee estimates an average annual per valve savings of $500 in decreased maintenance cost and $380 in energy costs from reduced air consumption.

The second effect of excessive controller gain is added variability in other interacting process variables. Think of your process as the smooth surface of a pond. Every valve movement is like dropping pebbles into the pond at different locations. The resulting ripples move across the pond until they reach the location of another controller that senses the ripple and responds by moving it's control valve, thereby making more ripples. Using excessive controller gain is equivalent to dropping bigger rocks in the pond. With improper tuning, an upset causing small ripples at the far end of the pond can result in waves crashing on the shore at the other end. The cost associated with this added process variability can only be quantified on a case by case basis, but it's general impact can be seen in increases in energy cost, raw material usage and final product variability.

A good example of the second effect is the level control on a surge tank in a process stream. The goal for proper tuning parameters for this loop would be to balance the short term differences in production rates into and out of the tank while minimizing the swings in production rates. For any given vessel and production capacities, there is only one set of tuning parameters that will meet this goal. If you know what these parameters are for each surge tank in your facility, then you are a heck-uva lot smarter than the average bear and you probably don't need to waste your time reading any further. But if you are like most of us, someone tweaked and tuned these loops until they got it relatively close to something the operators couldn't complain about. The difference between the trial and error parameters and the only correct set is dollars down the drain.

Maybe you think your tuning methods have gotten you close enough and that the difference in production costs are therefore relatively small. Before you discount this tip and move on to your next task for the day, consider one last definition :

The relatively small difference between production cost and product price is called profit.