Low 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. 

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. 

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. 

A procedure that works is to not close the feedback loop but to feed a fixed control signal into the lag block that corresponds to expected signal from the low selector. For example, if the control valve is designed to be half open at normal conditions, the valve signal will be 50% and the low selector output signal is 50%. So insert a control signal onto the Aspen Dynamics process flow diagram, specify its value to be 50 and make it a fixed variable type. Then open up the all variables view of the lag block and specify the output variable to be an initial variable type. Make an initialization run. The output of the lag block should show 50%. 

Finally, reconnect the source of the input signal to the lag block to be the output of the low selector. At this point the run button at the bottom of the screen will be red, indicating... 

The external reset feedback control structure discussed in Figure 18.3 is inserted in the Aspen Dynamics flowsheet for the flash tank process as shown in Figure 18.8. The feed flow controller has its output signal OPpc sent to a low selector. The other input to the low selector is the output signal OPqrc of the high-level override controller. The override controller is proportional-only, so it does not need anti-reset windup protection. 

Figure 18.9 gives the AllVariables views of the individual blocks in the feed flow controller with external reset feedback. The setpoint of the flow controller SPpc is set by a fixed-variable stream. The low selector inputs are the output signals from the two controllers (OP FC and OP ORC). The level transmitter for the override controller is simulated by using a Multiply block in which the liquid level (with units of ft ) is... 

The next window shows that a Comparator block subtracts the process variable (%) from the setpoint (%). The third window shows that a Multiply block generates the product of the controller gain and the error. The fourth window generates the temperature controller output by adding the output of the previous block to the output of the Lag l block. The next window shows that a Multiply block converts the percent-of-scale signal to a reboiler duty (shown in GJ/h). The final window gives the low selector. Note that the reboiler duties of the two inputs from the two controllers are in GJ/h while the output has units of MMkcal/h. Inputl comes from the normal temperature controller, and it is the lower of the two at design conditions. 

In Fig. 11.26, the compressor flow signal goes to a direct-acting proportional-plus-integral flow controller. The controller output goes to the low selector described above. 

It is important that an output signal from the low selector also serve as external feedback to the flow controller. 

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. 

Traditional Advanced Control (TAC) employs the use of advanced control algorithms combined with regulatory control functions (i.e., lead/lag, ratio, high/low selectors, etc) to implement a control strategy. 

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. 

Consider a systan where a low base level override is to be used to close the steam valve. Its output is compared with that of the normal controller through a low selector. Let us say that we want full override action to occur between zero level (3 psig) and the 25% level (6 psig). Tlien the required proportional gain is ... 

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