Feedforward control was explained in the previous loop signature articles. One of the examples used was feedforward control of load changes on a heat exchanger when variations occurred in the flow of the process fluid through the exchanger.

In the previous articles in this series, the basic theory behind feedforward control was discussed, and it was also shown how to apply feedforward in practice. In this article, it will be shown how well feedforward can work in practice by giving a couple of examples.

Feedforward control tuning is not nearly as critical as feedback tuning, and fairly simple models are usually fine for the purpose in hand.

Feedforward control is a powerful technique that can dramatically improve control variance in cases where load changes cause big deviations from setpoint and the actual process dynamics are too slow to allow the feedback controller to operate fast enough to catch these disturbances.

The previous series of loop signature articles dealt with the basics of control loop optimisation, and concentrated on troubleshooting and ‘SWAG’ tuning of simple processes. In this new series, consideration will be given to dealing practically with more difficult issues like interactive processes, and with processes with much more complex dynamics.

For this final article I have chosen to relate a problem that existed in a desuperheater temperature control on a boiler in a petrochemical refinery.

This article is a continuation of Loop Signature 30 published in the last issue in this series, exploring reasons for non-linearities which may be encountered in feedback control loops

If nonlinearity occurs it means that if one is to carry on controlling with the same response to changes in load or setpoint, then the tuning of the controller will also need to be adjusted to meet the new conditions.

In my last article I wrote about how cascade control systems can effectively overcome valve problems. This article gives another example of how a temperature control was able to perform well, in spite of really severe valve problems.