Dead-time dominant

Table 8-2 summarizes these rules for minimum-lAE load response for the most common controllers. The process gain and time constant im are obtained from the product of Gt, and Gp in Fig. 8-23. Derivative action is not effective for dead-time-dominant processes. Any secondary lag, sampling interval, or filter time constant should be added to dead time 0. 

Model predictive control (MPC) was developed in the 1970s and 1980s to meet control challenges of refineries. The advantages of MPC are most evident when it is used as a multivariable controller integrated with an optimizer. The greatest MPC benefits are realized in applications with dead-time dominance, interactions, constraints, and the need for optimization. As opposed to a traditional control loop, where the controller responds to a difference (error) between the set point and measurement, the predictive controller uses a vector difference between the future trajectory of the set point and the predicted trajectory of the controlled variable as its input (Figure 2.52). 

The discussion on uncertainty in sampled systems concluded that the sampling interval should not exceed the open-loop response time of the process. For best results with easy processes, the sampling interval should be as short as practicable. But where dead time dominates, the sampling interval is best- keyed to the process response time. Unfortunately, practical considerations take precedence over performance for most applications. 

Nonetheless, the first dead time dominates the control loop. It may be 5 to 30 min in duration, depending principally on the distance of the loop between the distillate valve and the analyzer. (The volume of the accumulator contributes significantly to the response.) The closed loop can then be expected to oscillate at some period between 20 min and 2 hr. 

An important indication of the effect of dead time on a process is the dead time to time constant ratio (Tdt/ )- If this ratio is less than 0.3, then the dead time has little or no effect on the process response. However, if the ratio is greater than 0.3, then the process becomes dead-time-dominated and thus is virtually uncontrollable. 

---