Action and reaction

That’s exactly how the positive action of a controller (regulator) is set

Based on the principle of operation, as pressure increases, the regulator’s output increases; therefore, the opening degree of the air-operated valve should decrease

This is a pressure range control; both valves are FO

Positive and negative action of the throttle valve: 1. Positive and negative action of the actuator (diaphragm head, cylinder): When the signal increases, the push rod moves downward, which is positive action (instrument air enters from above the diaphragm head, with the spring located below); the opposite is negative action (instrument air enters from below the diaphragm head, with the spring located above). 2. Direct and reverse action of the control mechanism (control valve structure): Direct action: When the valve stem moves downward, the valve closes (normal installation). Reaction: When the valve stem moves downward, the valve opens wider (reverse installation). 3. Forward and reverse action of the actuator: The actuation structure and the control structure form four combinations. Actuator, control mechanism: For actuators with positive action, it is a gas-shut valve; for those with reverse action, it is a gas-open valve. In the case of positive action, an increase in signal leads to a decrease in flow rate. The principle behind choosing the positive and negative actions is to ensure that the valve returns to a safe position in the event of a loss of instrument air.

Hey, I won’t go into the theory. As shown in the figure, based on the principle of operation, as pressure increases, the regulator’s output increases; therefore, the opening degree of the air-operated valve should decrease. But shouldn’t the vent valve actually be larger? How to explain it

This post was last edited by ylb913 on 2018-8-4 at 22:12. To be honest, the accompanying images in the original poster’s question are not detailed enough; it’s unclear what is meant by the question regarding the controller being of the direct-acting type and why the relief valve also increases as the controller’s output increases. I’m a process-minded person, not an instrumentation-minded one. With over 20 years of experience in this field, I have always disliked the necessity to include information on positive and negative effects in the early workshop procedures or training materials (we no longer include such details in the procedures used later on). What controller’s output increases or decreases? I only check whether the valves should be opened or closed based on whether the process parameters increase or decrease. The reply on the 5th floor was also based on a video lecture; I thought what was said made sense, so I organized it, but I simply can’t remember it myself – I just copy and paste it. What positive and negative effects? It is known that the actual drying process for opening or closing the air flow is in place ； Those who truly understand the process know when a control valve should open automatically and when it should close automatically. Positive and negative action of the control mechanism (control valve structure): Positive action: When the valve stem moves downward, the valve closes (normal installation). Reaction: When the valve stem moves downward, the valve opens wider (reverse installation). In the workshop where I worked earlier, I did see valves installed in this reverse manner, but after those valves were replaced, I never saw any more instances of reverse installation.

The direct action of a controller means that as the input increases, the output also increases; this is what is referred to as direct action. Your diagram is incomplete. Based on conventional understanding, when the measured value increases, it is necessary to increase the valve opening in order to reduce pressure. For FO valves, the default actuator uses direct action, which means that the signal sent by the controller must decrease in order to increase the valve opening. Therefore, the controller’s output must decrease, which suggests that it is actually reverse action.