High selectors

A host of gadgets and software are available to perform a variety of computations and logical operations with control signals. For example, adders, multipliers, dividers, low selectors, high selectors, high limiters, low limiters, and square-root extractors can all be implemented in both analog and computer systems. They are widely used in ratio control, in computed variable control, in feedforward control, and in override control. These will be discussed in the next chapter. 

Figure 8.4fc shows another type of selective control system. The signals from the three temperature transmitters located at various positions along a tubular reactor are fed into a high selector. The highest temperature is sent to the temperature controller whose output manipulates cooling water. Thus, this system controls the peak temperature in the reactor, wherever it is located. 

The control model MultiHLoSelect is installed on the flowsheet, and the signals from the three deadtimes are fed into it. The output of the high selector (labeled HS ) is the PV signal to the peak temperature controller (see Fig. 6.47). 

ALC Level difference controller APC Pressure difference controller ATC Temperature difference controller ATT Temperature difference transmitter < Low Selector > High Selector... 

An interesting aspect of furnace control is the need to be always on the air-rich side, never on the fuel-rich side. If the furnace became filled with uncombusted fuel and then air was added, the resulting rapid combustion could blow the furnace apart. The same concern makes it important that the start-up of a furnace follow a very carefully thought-out procedure. The control system shown in Fig. 7.1 accomplishes this air-rich operation by the use of several selectors and a lag unit. When the temperature controller calls for more fuel, the air wall increase first before the fuel increases because the low selector on the fuel passes the low signal from the lag to the fuel flow controller while the high selector on the air passes the high signal to the air flow controller. The reverse operation occurs when the temperature controller calls for less fuel The fuel flow decreases first and then the air flow- decreases. 

High selector switch, 403, 404, 405 Hold elements, 558, 576-81 Hot spot, 405... 

HSS (see High selector switch) Hydraulics, of distillation trays, 72 Hydrodealkylation of toluene plant control system design, 519-29 description, 516-19... 

Luyben also suggested ratioing the entrainer flow to the column to the feed flow. In addition, a control structure that always maintained an excess of solvent was recommended. A first-order lag, a high selector, and a low selector were used to always have an excess of entrainer. The entrainer leads the feed for an increase in feed and lags the feed for a decrease. 

FIG 6.16. A high selector is used to permit control of the peak reactor temperature. 

FIG 6,X7. The high selector prevents a f ilure of either analyzer from damaging the reactor with excess reagent... 

Two level controllers must be used, one set for high level, the other low. The outputs of the flow controller and the high-level controller go to a high selector its output is compared to that of the low-level controller in a low selector, whose output drives the valve. 

One may also construct a median selector with two high selectors and two low selectors as shown in Figure 9.1. The major application has been for auditing multiple flow measurements to a chemical reactor. If either the high or low measurement deviates too far from the median value, an alarm or interlock is activated. So far we have found no applications to distillation, but it is a technique that is worth keeping in mind. 

A selector is a practical solution for choosing the appropriate signal 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. On instrumentation diagrams, the symbol (>) denotes a high selector and the symbol (<) a low selector. 

Figure 16,13 Control of a reactor hotspot temperature by using a high selector.

The block diagram for the selector control loop used in the slurry example is shown in Fig. 16.16. The selector compares signals P and P2, both of which have the same units (e.g., mA or %). There are two parallel feedback loops. Note that Gy is the transfer function for the final control element, the variable-speed drive pump. A stability analysis of Fig. 16.16 would be rather complicated because the high selector introduces a nonlinear element into the control system. Typically, the second loop (pump flow) will be faster than the first loop (level) and uses PI control (although reset windup protection will be required). Proportional control could be employed in the slower loop (liquid level) because tight level control is not required. 

The two basic building blocks for selector systems are the high and the low selector. The high selector, shown in Figure 6.11, will pass the highest value of the multiple input to the output signal, ignoring all other inputs. 

The objective of autioneering is to protect against the highest temperature sensed by one of many temperature transmitters. In the example shown in Figure 6.15, the control equipment consists of one controller, four transmitters, and one FCE. The highest temperature will be selected by the high selectors and will he used as the measurement for controlling the fuel to the oven. 

If the feed flow is within the safe operating region, then the signal from the multiplier will pass through the high selector, since it is higher than the low limit. It will also pass... 

For unsteady-state operation the control scheme is more complex. A high selector is used to set the set point on the airflow controller, and a low selector is used to control the ratio controller on the fuel side (Figure 7.32) [12]. For an increase in heat demand, the demand signal will be higher than either the fuel or air flow measurements. This will be passed by the high selector (>) to the airflow controller [12]. The increase in air flow will then be transferred through the low selector (<) to the ratio controller on the fuel side [12]. This ensures no excess in fuel, since airflow leads fuel flow for an increase in steam demand. 

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