Getting Loopy with Control Loops

ANSWERS: You have an original idea for identifying at a glance the control loops that need to be fixed in your plant. I suggest that concentrating on tuning should not be your first step. First, look at the process to see if the design itself is correct and if the loop components are defective. You will probably find that 50 out f your 79 loops either have defective sensors or faulty valves (in one nuclear plant, 18% of the motor-operated valves were defective). You also will find that some tasks simply cannot be handled by a single feedback controller, because they require feedforward, de-coupling or model-based multivariable control.

To answer your question on tuning, I have asked some of my colleagues to fill in a summary table with some typical tuning valves. These are below. You will note that even these experts disagree on some aspects, such as the use of derivative on pH control (which I found very valuable on batch neutralization applications), or the use of positioners on flow loops, which in my view are not recommended. If you want to become knowledgeable on the subject of tuning, read the 200 or so pages on that topic in my handbook, the Instrument Engineers Handbook.

You also should consider the correct selection of control valve characteristics when reviewing your control loops. Liquid pressure and temperature control usually requires equal percentage valves, while for flow, level and gas pressure control we use linear valve characteristics if the ratio of maximum to minimum valve drop is under 2.0; above that, we use percent.

As to positional vs. velocity algorithms, you should keep in mind that a velocity algorithm has no internal reference, it gives an oscillatory response to large inputs, and a change in the sampling rate affects the loop again. These should not be used on noise processes such as flow.

When using positional algorithms, make sure they are not susceptible to reset windup and are capable of bumpless transfer. If the supplier provides these features by activating the integral term only when the output is connected, it will impair the testing capability of the loop.

Lots of luck to you in your new job. I like your attitude, because not even a single controller should be in manual in any plant. When a controller is in manual, somebody is not doing his or her job.

Béla Lipták

I DON'T HAVE any problem filling out the table. In fact, there is a very similar one in the 4th Ed. of my book Process Control Systems, (McGraw-Hill, 1996) p.99. Attached is my filled table.

Greg Shinskey