Further discussion on the implementation of override control in DCS
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Override control is widely used in petroleum, chemical, and power applications, with pressure protection being a common type. DCS systems are commonly implemented using PID modules and high/low selection modules. If there is a process loop that combines flow and pressure override control. Under normal operating conditions of the process (i.e., when the pressure is neither too high nor too low), the flow controller outputs a signal to control the opening degree of the regulating valve in order to achieve the desired fluid flow rate. When the pressure is abnormal (too high or too low), the pressure controller outputs a signal that replaces the flow controller in controlling the opening degree of the control valve, thereby ensuring pressure protection. Do you use the high-select or low-select module together with the PID module in the following situations? 1. When it is necessary to prevent the fluid pressure in the pipeline from exceeding a certain value, the low-select module should be used. 2. If it is necessary to ensure that the fluid pressure in the pipeline does not fall below a certain value, a high-select module should be used.
This statement is not comprehensive. The selection of the selector should be determined by whether the controller operating under abnormal conditions outputs a high or low signal; if it’s a high signal, a high-selector should be used, and otherwise a low-selector. This is related to the characteristics of the device, as well as whether the control valve operates in an open or closed mode. In the case of pressure control, whether it involves regulating the feed stream or the vent stream makes a significant difference
When an emergency occurs and the output of the emergency controller decreases, the low selection is chosen; when an emergency occurs and the output of the emergency controller increases, the high selection is chosen.
OP, you can handle the high/low selector like this: 1. It is known that under override protection, the operating direction of the valve is determined by the process requirements. Using your example: Under normal conditions, the process is controlled by the FIC control valve ； When the pressure is abnormal, the PIC controller controls the opening degree of the control valve. So you need to understand that this \"abnormal operating condition\" requires knowing the direction in which the control valve should operate (should it be opened more or closed more?) ). Generally, override protection is intended for only one direction of \"pressure anomaly\"; I assume that when P is too high, the PIC triggers an override to close the valve. 2. Is the control valve FO or FC? 3. Based on the required operating direction of the valve as specified in 1, and taking into account the requirements in 2, the high/low selector is chosen; in any case, it is necessary to meet the process requirement that \"if P is too high, the PIC will close the valve.\" 4. We also need to check whether the PIC output is reversed, because generally we reverse the AO output for the FO valve. 5. The H/L override in a PID diagram indicates, as specified in requirement 1, that for example, if the PIC override requires the valve to be closed and the valve is of the FC type, then it denotes an L-low selector.
The problem is integral saturation in override control; some systems can handle it, while others offer no solution at all, such as Freelance...