A modulating control valve is an automated valve that is used to control the amount of flow in a system or process. They go beyond simple shutoff or isolation (on/off) and actually allow precise control of the flow rate. The actuators on these valves use feedback and control signals to accurately open and close the valve. There are two distinct types of valves that can be used for modulating control: "Rising Stem" and "Quarter-turn."
Both types operate using the same logic and principles. A control signal is sent to the actuator to instruct it on how far to raise or turn the stem. The control signal may be in the form of air pressure, or electrical voltage. For instance a 3-15 psi pneumatic signal will cause the actuator to completely closed the valve at 3 psi or lower, and completey open it at 15 psi or above. A 0-10 VDC electric signal will cause the actuator to completely closed the valve at 0 VDC, and completey open it at 10 VDC or above.
Typically there will be a flow meter downstream from the valve that measures the current flow rate, and sends that information to a control computer. The computer will apply the appropriate logic and generate the signal needed to adjust the valve in either direction until the desired flow rate is met.
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