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UPSETS
Any one of three events could occur which would require a different flow to
maintain the level in the tank. First, if the position of the output hand valve
were opened slightly, then more flow would leave the tank, causing the level to
fall. This is a change in demand, and to restore balance, the inlet flow valve
has to be opened to supply a greater flow rate. A second type of unbalance
condition is a change in the set point. Maintaining any other level besides
midscale in the tank would cause a different flow out ; this change in demand
would require a different input valve position. The third kind of upset is a
change in the supply. If the pressure output of the pump were to increase, even
though the inlet valve remained in the same position, the increased pressure
would cause a greater flow, which would at first cause the level to begin to
rise. Sensing the increased measurement, the level controller would have to
close the valve on the inlet to hold the level at a constant value. In the same
way, any controller applied to the heat exchanger shown in figure 1 must balance
the supply of heat added by the steam with the heat taken away by the water. The
temperature can only remain constant if the flow of heat in equals the flow of
heat out.
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PROCESS CHARACTERISTICS AND CONTROLLABILITY
The automatic controller uses changes in the position of the final actuator to control the measurement signal, moving the actuator to oppose any change it sees in the measurement signal. The controllability of any process is a function of how well the measurement signal responds to these changes in the controller output; for good control the measurement should begin to respond quickly, but then not change too rapidly. Because of the tremendous number of applications of automatic control, characterizing a process by what it does, or by industry, is an almost hopeless task. However, all processes can be described by the relationship between their inputs and outputs. Figure 4 illustrates the temperature response of the heat exchanger when the control valve is opened by manually increasing the controller output signal.
At first, there is no immediate response at the temperature indication, then the temperature begins to change; it rises steeply at first, and approaches a final, constant level. The process can be characterized by the two elements of its response. The first element is the dead time, or the time before the measurement begins respond , in this example, the dead time arises because the heat in the steam must be conducted to the water before it can affect the temperature, and then to the transmitter before the change can be seen. Dead time is a function of the physical dimensions of a process and such things as belt speeds and mixing rates. Second, the capacity of a process is the material or energy which has to enter or leave the process to change the measurements. It is, for example, the gallons necessary to change level, the BTU's necessary to change temperature, or the standard cubic feet of gas necessary to change pressure. The measure of a capacity is its response to a step input. Specifically, the size of a capacity is measured by its time constant, which is defined as the time necessary to complete 63% of its total response. The time constant is a function of the size of the process and the rate of material or energy transfer. For this example, the larger the tank, and the smaller the flow rate of the steam, the longer the time constant. These numbers can be as short as a few seconds, or as long as several hours. Combined with dead time, they define how long it takes the measurement signal to respond to changes in the valve position. A process will begin to respond quickly, but then not change too rapidly, if its dead time is small and its capacity is large. In short, the larger the time constant of capacity compared to the dead time, the better the controllability of the process. |
