How are the forward and reverse actions of controllers and valves determined in the DCS monitoring data table?
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Xiaobai is seeking advice! As the title suggests, how are the forward and reverse actions of controllers and valves determined in the DCS monitoring data table?
In a control system, the role of each individual unit doesn’t matter much; as long as the system operates with negative feedback, that’s sufficient. Generally, the control state of the valve is determined first based on safety or process requirements, and then the state of the controller is determined
For control valves, it is necessary to determine whether they are air-operated open or air-operated closed valves; only by analyzing the PID diagram can the forward or reverse action of the controller be determined. Or, after installation in DCS configuration, proceed with automatic control debugging to determine the forward and reverse action of the regulator.
What was said on the third floor is absolutely correct; the positioning mechanism is generally set to positive action by default, that is, when the input is high, the output is also high, and this is determined based on the type of valve (air-open or air-close) as well as the requirements of the control loop in the PID system.
The forward/reverse operation of a control valve has nothing to do with whether it is air-operated or not; the forward/reverse operation can be adjusted using a positioner, and by combining this with a controller, a negative feedback loop can be created. Whether it is a mechanical or intelligent locator, the type of feedback can be adjusted
The positive and negative actions in the monitoring data table refer to those of the controller, not those of the positioner; the entire circuit must form a negative feedback loop. So it is related to both the on/off operation of the valve and the positive/negative action of the actuator; generally, the actuator is set to positive action by default, though it can also be set to negative action.
The selection of the controller’s positive/negative action is intended to ensure the formation of a negative feedback loop. First, it is necessary to determine whether the control valve should operate in air-open (close on failure) or air-close (open on failure) mode based on the process safety requirements, and then determine the positive/negative action of the controller.
The positive and negative actions of the controller create negative feedback by being used in conjunction with a locator on-site. The term ‘reaction’ refers to the output of 4–20mA or 20–4mA, and it does not relate to the direction in which the valve moves. The opening and closing mode is determined in conjunction with the fault location; it is related to the direction in which the valve moves at the fault site – they are two different things