- All valves have fully open and fully closed positions. Solenoid valves, such as those used to fill clothes washers, can only be fully closed or fully open, depending on the presence of AC voltage at the solenoid coil. Proportional valves can be fully off or fully on and anywhere in-between. They can be manually adjusted, such as with the above-mentioned domestic faucets, or automated to be opened by motors, pneumatic or hydraulic air pressures, or self-regulating where either upstream or downstream pressure or a flow rate can be dialed in to modulate the valve automatically.
- Most valves consist of a fixed valve seat and a movable valve element in a valve body. The valve body integrates the pipe connections that allow the valve to be installed in an application to control the flow. A valve element such as a plug, ball, poppet, or gate rotates or slides into the valve seat, which is precisely shaped to receive it. When they are pressed together, flow stops. If the valve element is fully withdrawn from the seat, flow is unabated through the valve. Any position between those is a throttling position, and flow through the valve will be a percentage of the fully open flow value.
- There are many methods used to open and close proportional valves. Some manually adjusted valves have a threaded stem and a sealing washer. As the valve stem is twisted with the knob, the sealing washer winds further away from the valve seat, reducing the restriction to flow and thereby increasing it. Ball valves rotate through 90 degrees, blocking or opening a pass-through port. Plug valves lift a tapered plug out of a concentric seat, and flow increases through the opening between them.
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Manually adjusted valves such as home faucets are usually turned several turns. Remotely or automatically modulated proportional valves may employ electric motors with lead screw mechanisms, cams or hydraulics to open and close valves. Industrial valves may use stepper motors or servo motors to drive small valves. Many industrial valves use a linear pneumatic valve motor or convert electronic signals to pneumatic.
Pressure drops in the up or downstream flow from other users can significantly alter the flow through a proportional valve. This is a familiar situation in households wherever two people are trying to take showers at the same time.
Proportional valves that open and close as pressure values pass through a narrow band of pressure change are called proportional safety valves.
