Application of override control in olefin projects
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This post was last edited by MTO001 on 2021-10-2 02:07. Tag value: %Normal value, Low limit value, High limit value. LIC2203: %501874; LIC2202: %501195. E1501 is the kettle-type propylene refrigerant heat exchanger, and V1501 is the liquid separation tank. LIC2203 is the main circuit controller, LY2203 is the selector, and LIC2202 is the secondary circuit controller (high liquid level, delayed response). Before explaining the override control, let’s first discuss in detail the issues related to the high limit value and set value of the liquid level in the propylene refrigerant heat exchanger. The reason for setting a high liquid level delay at the liquid level controller of the propylene refrigerant heat exchanger is primarily to ensure the safe operation of the unit, preventing high liquid levels in the heat exchanger from causing gas-liquid entrainment into the propylene compressor’s suction tank and triggering a high liquid level interlock. LIC2202 is a controller for high liquid level with very slow response. Under normal operation, the SP value is 50%, and the alarm threshold of 95% specified by the design team is not set arbitrarily; it is determined based on the height to which the kettle-type heat exchanger covers the U-tube bundle. When the liquid level exceeds 95%, it is very likely for the liquid phase to enter the suction tank.
Professor Wang gave an excellent lecture; thumbs up!
Overshoot control is also known as selective control. It is a soft protection measure taken before interlocking actions occur. Under normal conditions, the selector selects the output from the normal controller to send it to the actuator for normal regulation ； When a parameter reaches the upper/lower limit in an emergency situation, the selector selects the output of the emergency controller to send it to the actuator for emergency intervention, waiting for the production process to return to normal limits ； Both the normal controller and the emergency controller are in automatic mode; there is no mechanism to switch the controllers to manual mode. If they are switched to manual mode, operator intervention is required, and it is no longer considered override automatic control/selective automatic control. The emergency controller is generally chosen to be a pure proportional controller with a high gain; the setpoint for this controller is set at the upper limit of the normal operating parameters ； For super/selective control, it is important to note that at any given moment only one of the multiple controllers outputs a signal to the actuator, while the other control systems operate in an open-loop mode. If these controllers have an integral function, then the issue of integral saturation must be taken into consideration.