SECTION 7
The surge controller must have an anti-reset windup option. The surge control valves are closed and the measurement is above set point during normal operation. The surge controller is direct-acting since the surge valves are reverse-acting (air fail open) and the process is direct-acting (suction flow increases if the vent or recycle flow increases). The surge controller output is therefore full-scale. Since the flow is above set point with the surge control valves closed, the reset action of the controller integrates the positive error and saturates the output. When a user shuts down, the surge controller output remains saturated above 100 percent until the measurement is below set point (the operating point to the left of the set point) long enough for reset action to integrate the negative error and decrease the output to 100 percent. This time is so long that the operating point reaches the backup set point for even the slowest conceivable disturbance. The anti-reset windup option limits the contribution of reset to the controller output so that it plus the contribution of proportional action is equal to about 105 percent (the extra 5 percent signal is to ensure that the surge valves are tightly shut for normal operation). As soon as the measurement starts to decrease, the contribution of the proportional action to the controller decreases, the controller output decreases below 105 percent, and the surge valves start to open. If the approach to set point is slow, the increasing reset contribution to the output will cancel out the decreasing proportional contribution so that the output stays at 105 percent. Thus proportional action will open the surge valves before the measurement reaches set point for rapid disturbances. Some digital controllers have their algorithm deliberately designed to prevent the output from decreasing until the measurement is within 1/2 proportional band of the set point for even rapid disturbances. While this design is desirable for override control, it is undesirable for surge control (Ref. 21). If a reversing relay is used in the I/P transducer or valve positioner for the reverse-acting control valves, the surge
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