Direct-acting controller

Improper selection of reverse-acting or direct-acting controller... 

The controller action, either direct or reverse acting, which defines the direction of its effect. For a direct-acting controller, when the PV rises above the setpoint (SP), the OP increases, and vice versa. In this case, the static process gain is negative, as illustrated for a level controller in Figure 21.21a. Here, the liquid level is the PV, the flow rate of the effluent stream, Q , is the OP, and the controller action is set to Direct. In contrast, for a reverse-acting controller, when the PV rises above the SP, the OP decreases, and vice versa. In these cases, the static process gain is positive, as illus-... 

As the hydrogen cools, water condenses and accumulates in the column. The level in the bottom of the column can be measured with a d/p cell. The signal from the level transmitter goes to a direct-acting controller with proportional and integral control modes. The fail-closed control valve is in a side branch off the circulating pump dischaige line. 

The reactor installation has two heat removal channels each is able to remove up to 3% of the nominal reactor power. Each channel consists of a cooling condenser connected with a separator and cooled by water, and a condensate drainage pipeline with a direct-acting control valve, which opens if the pressure in the separator exceeds the nominal value. 

But what if the control valve is air-to-close (or fail open) rather than air-to-open Now when the flow rate is too high, the controller output should increase to further close the valve. Here, a direct-acting controller is required. 

The closed-loop system will be stable if this root is negative. Because time constants are always positive (t > 0), the feedback control system will be stable if KcK Kp > -1. This means that as long as the controller has the correct control action (i.e., reverse- or direct-acting, as per Section 8.3), then the system will be stable. For example, if > 0 and Ky > 0, then the controller must be made reverseacting so that Kc > 0. By contrast, if Kp < 0, then a direct-acting controller Kc < 0) is required. 

Using Equation 4.3 for a proportional-only controller (for a direct acting controller, e = CV — SP) and inserting the above values for the set point SP, the bias b, and the controller gain Kc, and rearranging the results gives... 

The S VD analysis provided in Figure 11.6 suggests that there are sensitive regions in the lower part of the column around trays 2 and 3 and in the upper part of the column around trays 11-16. We select trays 2 and 12 to control with the two fresh feeds. Both controllers are direct acting controllers (an increase in temperature increases feed). 

In summary, the two-temperatiu e control structure, which does not use any composition measurement, handles all of these disturbance well except when some of the lightest component C is present in the fresh feed of B. Remember the Fob feed is trying to control the tray 12 temperature, and it is a direct acting controller. Thus, when the light C component enters near the top of the column in the Fob stream, it drops the temperature and the temperature controller decreases Fqb- However, the decrease in Fqb produces an increase in the temperatures in the lower part of the column (remember the T/Fqb gains are positive in the lower part of the column). As a result, the tray 2 temperature increases and the second temperature controller then increases Fqa (see upper left graph. Fig. 11.13). This puts more light material in the column and drives the tray 12 temperature even lower. The result is a system shutdown. 

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