Stan: OK, say we can throttle and measure the flow rate to turn down rather than shutting down the plant, can we control the plant? What is happening in the process? For example, the heat transfer coefficient decreases to the 0.65 power with flow.
Greg: Decreases in heat transfer coefficients increase thermal lags, which translate to an increase in loop dead time. Decreases in flow cause an additional increase in loop dead time from transportation delays and increase the process gains for temperature and composition loops.
The increase in dead time and process gain necessitates a decrease in the controller gain. It takes longer for the controller output to work through the dead band and stick-slip of the valve because of the slower rate of change of the controller output from a lower controller gain. The result is an increase in the dead time from the valve and a further decrease in loop performance. This is where a control loop with on-demand or adaptive tuning and gain scheduling is beneficial.
Stan: Are people going to be put to work improving the rangeability of the plant or let go to save short-term costs?
Greg: This is a great time to do some testing of the plant and design of experiments to define how to optimize the plant. If you wait till you need the capacity, it is too late.
Stan: One of the sure signs of a downturn in the economy is the amount of time your manager's door is closed.