High signal selector

The control signals from the two pressure controllers go to a high-signal selector switch. When the pressure set by the wide-range system reaches a certain point, then, the narrow-range system takes over control. Control response charts are in Fig. 11.38. The situation is similar to that described for the chlorine system and shown in Fig. 11.20. 

Conversely, on decreasing demand, the controller output is passed by the low signal selector to the SP of fuel flow controller. The fuel flow measurement provides an input to the high signal selector. If the fuel flow controller does not respond then this will override the output sent to the air flow controller. Thus air is not permitted to reduce until the fuel flow reduces. 

The idea for safe control is to have the air lead the fuel on increases in demand and fuel lead the air on decreases in demand. On load increases, the air is increased ahead of the fuel. On load decreases, the fuel is decreased ahead of the air. This is accomplished with high- and low-signal selectors. 

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. 

This has both a flow controller and a pressure controller. The flow controller permits the addition of the compensation and feedforward schemes. The pressure controller provides burner protection. In this case their outputs are compared in a low signal selector. This provides protection against too high a pressure. If the pressure exceeds the maximum, as entered as SP in the pressure controller, the controller output will reduce to close the valve... 

We use the term override for the most part to refer to the use of two or more controllers connected to a control valve through high or low signal selectors. Logic is built in to enable one controller to override (i.e., take over fi om) the other controller or controllers ... 

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. 

If an operator saw this problem developing, he would switch the temperature loop into manual and cut back on the steam flow. The control system in Fig. 8,4fl will perform this "override control automatically. The low selector (LS) sends to the steam valve the lower of the two signals. If the steam valve is air-to-open, the valve will be pinched back by cither high temperature (through the reverse-acting temperature controller) or by low base level (through the low-base-level override controller). 

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). 

Alpha-1-acid glycoproteins (220,221), bovine serum albumin (222-224), pepsin, and cellobiohydrolase I (223) have been used as chiral selectors. In general, enantiomeric separation increases with increased protein concentration however, this results in a sensitivity loss due to high background signal. The use of a partial filling technique for the capillary injection to overcome this limitation has been reported (220,221). 

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