【Daily Question】20150815
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2 points for participation, 5-9 points for analysis that is accurate.
Direct action of a control valve means that when the signal pressure increases, the valve stem moves downward; the reverse is true for reverse action.
Signal pressure: is it a pneumatic signal or an electrical signal?
During ventilation, downward movement of the valve stem indicates the forward direction, while upward movement indicates the reverse direction.
Direct action of a control valve means that when the signal increases, the valve stem moves downward; the reverse is true for reverse action.
Positive action of a control valve means that as the signal value increases, the valve stem moves downward; the reverse is true for negative action.
Actuator + valve body: The forward/reverse action can be used to determine the forward/reverse action of the control valve!
A positive-action control valve has its valve stem move downward as the signal increases, while a reverse-action control valve has its valve stem move upward as the signal increases.
Positive action: A signal of 4mA–20mA is applied to the control valve; when 4mA is applied, the control valve closes. As the mA value increases, the opening degree of the control valve increases as well, and when the input value reaches 20mA, the control valve is fully open. Reaction: A signal of 4mA–20mA is sent to the control valve; when 4mA is applied, the control valve is fully open. As the mA value increases, the opening degree of the control valve gradually decreases, and when the input value reaches 20mA, the valve closes completely.
Control valves come in pneumatic, electro-hydraulic, electromagnetic, and electric types. The valve core can be of a shut-off type with cutting function, a ball valve with cutting function, or a cage type that only provides regulation. A positive action occurs when the internal diameter of the valve increases as the signal increases; conversely, a negative action occurs when the internal diameter of the valve decreases as the signal increases.
The air-open and air-close types of control valves are determined by the combination of the forward and reverse actions of the actuator, as well as the forward and reverse installation of the valve body components. When ventilation occurs, a downward movement of the valve stem indicates positive action, while the opposite case represents negative action. When the valve stem moves downward, a reduction in the flow area between the valve core and the valve seat is characteristic of a positive installation; otherwise, it is considered a negative installation. Actuator Valve body components Control valve Forward Forward Air-operated shut Forward Reverse Air-operated open Reverse Forward Air-operated open Reverse Reverse Air-operated shut