Selective and Override Control

Selective and Override Control When there are more controlled variables than manipulated variables, a common solution to this problem is to use a selector to choose the appropriate process variable from among a number of available measurements. Selectors can be based on multiple measurement points, multiple final control elements, or multiple controllers, as discussed below. Selectors are used to improve the control system performance as well as to protect equipment from unsafe operating conditions. 

One type of selector device chooses as its output signal the highest (or lowest) of two or more input signals. This approach is often referred to as auctioneering. On instrumentation diagrams, the symbol 

8 38 Block diagram of the Smith predictor. (Source Seborg et al., Process Dynamics and Control, 2d ed., Wiley, New York, 2004.) 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of antireset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain on the trays, while the upper limit is determined by the onset of flooding. Overrides are also used in forced-draft combustion control systems to prevent an imbalance between airflow and fuel flow, which could result in unsafe operating conditions. 

Other types of selective systems employ multiple final control elements or multiple controllers. In some applications, several manipulated variables are used to control a single process variable (also called split-range control). Typical examples include the adjustment of both inflow and outflow from a chemical reactor to control reactor pressure or the use of both acid and base to control pH in wastewater treatment. In this approach, the selector chooses among several controller outputs which final control element should be adjusted. 

---

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

The control valve is a variable orifice device in which the size of the orifice is adjusted to control a process variable. Consequently, the manufacturer, type, or even the size of a control valve has no effect on the energy dissipated in the control of a selected stream once the process pressure, line size, and pumps have been selected. This energy-independence of the control valve assures that continuous throttling of the flow stream is required to control a process variable. In those cases where a valve is used for shut-off or override control (not a continuous throttling device), energy savings can be realized by selecting a valve with a minimum pressure loss in the full-open position. 

These devices select either the higher or the lower of two input signals. As many input signals as desired may be accommodated by arranging selectors in series. Multiple-input low-signal selectors are now available from vendors. In practice one or more selectors are inserted between the output of a normal controller and its final control element, usually a valve. The outputs of the override controllers are also connected to these selectors. As process constraints are approached, one of the override controllers will take over or override the normal controller and drive the final control element in the proper direction— either to force the process away from a constraint, or to hold it a safe distance from a constraint. 

Fig. 17. Examples of selective control strategy (a) reactor hot spot (b) level override (c) prioritized and (d) constraint controls, where...

The reactions with (25,35,45,55)-5-(tm-butyldimethylsilyloxy)-3-(4-methoxyphenylmethoxy)-2,4-dimethylheptanal (15) are particularly informative reagent (5)-3 is incapable of overriding the intrinsic diastereofacial preference of 15, and the normal Felkin product 17 is obtained with >95% selectivity. In contrast, reagent-controlled mismatched double diastereoselectivity is evident in the reaction with (5)-4 that provides 16 as the major component of a 73 22 5 mixture. The minor product 18 apparently derives from a reaction with the contaminating (/ )-4, since (5)-4 that was used is not enantiomerically pure. 

Thus, override and selective controls are widely used to handle safety problems and constraint problems. High and low limits on controller outputs, as illustrated in Fig. 8.4c, are also widely used to limit the amount of change permitted. 

---