Cascade control valve configurations

Cascade configuration controlling the desuperheater control valve. 

Cascade control is one solution to this problem (see Fig. 8-35). Here the jacket temperature is measured, and an error signal is sent from this point to the coolant control valve this reduces coolant flow, maintaining the heat transfer rate to the reactor at a constant level and rejecting the disturbance. The cascade control configuration will also adjust the setting of the coolant control valve when an error occurs in reactor temperature. The cascade control scheme shown in Fig. 8-35 contains two controllers. The primary controller is the reactor temperature coolant temperature controller. It measures the reactor temperature, compares it to the set point, and computes an output, which is the set point for the coolant flow rate controller. This secondary controller compares the set point to the coolant temperature measurement and adjusts the valve. The principal advantage of cascade control is that the secondary measurement (jacket temperature) is located closer to a potential disturbance in order to improve the closed-loop response. 

Step 9. Optimize economics and improve dynamic controllability. It is noted that all of the control valves have been assigned, but the HCl product composition is still uncontrolled. To correct this, a cascade controller is installed to regulate the HCl composition. This controller can adjust the set-point of either (a) the recycle flow controller, FC-2, or (b) the recycle temperature controller, TC-2. Figure 21.35 shows the first alternative, in which the liquid feed rate to the absorber is adjusted to control the mass transfer of the benzene from the vapor stream. Clearly, quantitative analysis is required to select the most appropriate configuration, as will be discussed next. 

In the high reflux ratio case, the reflux flow rate is set by the level controller. The reflux flow rate is measured and sent to a multiplier whose other input is the reciprocal of the desired reflux ratio (D/R). The output signal from the multiplier goes to the SP of a distillate flow controller, which is on cascade. A common simulation error is to send the output signal of the multiplier directly to a control valve. This is an obvious and serious error, but is one that is often made. These control configurations will be used in examples in later chapters. 

One effective method of keeping the valve gain (GJ perfectly constant is to replace the valve with a linear flow control loop. The limitation of this cascade configuration (in addition to its higher cost) is that if the controlled process is faster than the speed of response of the flow loop, cycling will occur. This is because the slave—in this case, the flow control loop—in any... 

The rate of heat removed from the vessel is varied by manipulating the rate of solvent evaporated. The pressure and, thus, the evaporation rate are manipulated by varying the recycle rate of the stream exhausted from the steam ejector. A control loop that links the recycle valve position directly to the operating temperature would not permit compensation for short cycle variations in the steam supply pressure, permitting rapid swings in pressure and surface-solution temperature that could result in spontaneous nucleation. Therefore, a cascade configuration that uses the temperature measurement in a master loop and a pressure measurement in a slave loop is employed. 

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