Low signal selector

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. 

The reader can easily determine how a low-signal selector works for the fuel flow controller. It would compare the signals from the steam pressure and the air flow. A flue gas oxygen analyzer should be installed to continuously monitor or even trim the air flow. 

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... 

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. 

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 ... 

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. 

Since the gain 4 rday is to put out 15 psig at the 25 percent level, and its output goes to a low signal selector, it clearly will exercise no control action at a level of 25 percent. On the other hand, if level is dropping, the output of the gain 4 relay will drop below that of other controllers at some value of level above zero. We are never sure, therefore, at what point the low-level override will take over, but we know positively that it will be between the zero and 25 percent levels. 

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. 

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). 

In level control applications, this override controller can be a simple fixed-gain relay which acts like a proportional controller. The gain of the controller shown in Fig. 8.4n is five. It would be zeroed so that as the level transmitter dropped from 20 to 0 percent of full scale, the output of the relay would drop from 100 to 0 percent of scale. This means that under normal conditions when the level is above 20 percent, the output of the relay will be at 100 percent. This will be higher than the signal from the temperature controller, so the low selector... 

Hint Sketch the responses for both positive and negative step changes in the input to a circuit which consists of a first-order lag with unity gain and a low-selector. The input signal goes in parallel to the lag and to the low-selector. The output of the lag goes to the other input of the low-selector. 

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