Feedforward Techniques – Part 3

In many of the plants I worked in the production capacity had been increased over the years by a series of debottlenecking projects. Unfortunately the surge tanks volumes were not increased probably because of a lack of understanding of dynamics. Consequently, unit operations upstream and downstream of the surge tank had to be decreased because of high and low levels, respectively. Also, abrupt changes in the surge tank’s discharge flow which are unavoidable as these level limits are approached were disruptive to nearly every type of unit operation.

If batch units or continuous units that are going up and down are dumping into a surge tank, you have a tough scenario to achieve both maximum availability of the surge volume and maximum smoothing of the outlet flow by feedback control alone. Notch gain and error squared level controllers can help but are difficult to tune. Also, low controller gains cause slow oscillations from reset action unless the reset time is also increased so that the product of the reset time and controller gain stays above a minimum. The fact that a low PI controller gain for an integrating process, such as level, can cause oscillations is not well recognized. For more details on this source of oscillations see the equation on page 109 of Good Tuning – A Pocket Guide (2nd Edition) and Equation 3-3j on page 81 of New Directions in Bioprocess Modeling and Control published by ISA. These equations are consistent once you consider the maximum integrating process gain is the inverse of the fastest full scale ramp time.

One solution is to add a velocity limited feedforward. For a surge tank level controller that manipulates the tank’s discharge flow, the total flow of all units that are dumping into surge tanks is a feedforward signal to set the discharge flow. If the flow engineering units are consistent and there is cascade control of level to discharge flow the feedforward gain is one. The big question is what is the velocity or rate limiting needed to spread the disturbance from batch and on-off operations over the available surge volume.

A material balance and dimensions of the tank can be used to compute the velocity or rate limit on a first principle basis. The attached file shows the calculation and implementation in a graphic representation of a Function Sequence Table (FST). Furthermore, the calculation offers continuous directional adaptation of the velocity or rate limit. The only adjustment is to set a filter time for the feedforward measurement that is equal to the normal time that the feedforward flow could be zero. For a single batch operation upstream, this time would be the batch cycle time plus the normal time between batches. For more info on this technique see Appendix B – Batch to Continuous Transition in Advanced Control Unleashed published by ISA.