Feedforward Techniques – Part 2

Maybe I have just been unlucky or maybe the plants I worked in were as stressed as the typical project schedule these days but often for column temperature, boiler level control, and neutralizer pH, feedforward control didn’t live up to expectations. The problem was abusive unmeasured disturbances. The feedforward signal is often flow, which doesn’t tell the whole story. If there were only flow disturbances, life sure would be simple.

Consider a distillation column with a feedforward of feed flow corrected by a tray temperature controller output that manipulates steam flow. For an increase in feed flow, the feedforward initiates an increase in steam. Seems great but what if the tray temperature is rising because of a change in feed concentration Adding steam adds to the rate of rise of temperature toward a possible product spec limit plus wasting steam.

Consider a boiler drum with a feedforward of steam flow corrected by a drum level controller that manipulates feed water flow. For an increase in steam flow, the feedforward initiates an increase in feed water flow. Technically sound but what if the drop in drum pressure causes a swell from the expansion of bubbles that is headed for the high drum level trip point set to prevent liquid carry over into the steam header. Adding hot feed water adds to rate of rise of level and the possibility of boiler shutdown.

Consider a neutralizer with a feedforward of acid waste flow corrected by a pH controller that manipulates a basic reagent flow. For an increase in waste flow, the feedforward initiates an increase in base flow. Seems smart but what if the pH is rising because of a decrease in acid concentration in the feed. Adding base adds to the rate of rise of pH toward a possible environmental limit plus wasting reagent.

A smart technique would preemptively correct the feedforward signal by subtracting a signal that is the rate of rise of the filtered rate limited controlled variable multiplied by an adjustable factor. Here, the correction for a positive rate of change only occurs when the controlled variable is above the set point plus some noise band. The correction for a negative rate of change only occurs when the controlled variable is below the set point minus some noise band. In each case, the feed forward is corrected to help deal with an unmeasured upset. If the controlled variable is near the set point, the controlled variable is lined out, or the unmeasured upset is driving the controlled variable back to set point, the feedforward correction is zero. The adjustable factor like the feed forward gain can be initialized based on first principles (e.g. material, component, and energy balances). Note that the above scenario is for a reverse controller and a direct feed forward action.

Concentration and temperature measurements of the feed may help make the feedforward signal calculation inherently smarter and reduce the number and size of unmeasured disturbances. But, there can be extenuating circumstances. For example, cold feed water would cause bubbles to collapse and the inverse response that might counteract steam flow induced shrink or swell. Also, a pH feedforward based on a pH measurement of the incoming waste may do more harm than good because the electrode error and failure rate in low or high pH streams is larger and error in reagent demand greater because the titration curve is flatter. Testing is always a must before putting even the best idea online.