Solenoid valves are electrically operated devices used to control flow. They are used for the remote on/off or directional control of liquids, gases and steam. They do not regulate flow. Solenoid valves consist of two main elements: 1.) An electrical coil in the solenoid, and 2.) A valve body or pressure vessel. The solenoid is the electromagnetic unit that powers (acts to open or close) the valve. The valve is the pressure containing unit that acts to shut off or open media flow.
When the solenoid is energized by an electrical signal, current flow results in the build up of a magnetic field. The field attracts a moveable plunger in the valve. Physical movement of the plunger opens or closes a valve orifice which gives the valve on/off or directional control of media. In general, solenoid valves are constructed to be: 1.) Normally-Open, or 2.) Normally-Closed. Both designations refer to action of the valve on flow when the solenoid is not energized. There would be, for example, no media flow through a normally closed valve until the solenoid is energized.
The most common types of solenoid actuated valves are: 1.) Direct-Acting, and 2.) Pilot-Operated. In a direct-acting valve, the plunger is in direct contact with the body main orifice, and opens or closes the orifice. In a pilot-operated valve, the main orifice is not directly controlled by the plunger, but by a diaphragm, piston or spool. Pilot operated valves contain both a pilot and a bleed orifice.
All solenoid valves are individually rated. Pilot-operated solenoid valves may also have an additional specification, Minimum Operating Pressure Differential (MOP). This is the minimum system pressure differential required to operate the valve and maintain it in the open position. MOP applies only to pilot-operated solenoid valves where system pressure is used to lift the diaphragm off the seat (normally-closed) when the solenoid is energized. Direct-acting or hung-diaphragm valves do not require a minimum operating pressure.
There will be a pressure differential ΔP before the solenoid of a normally-closed valve is energized. Just after flow begins moving through the valve, the pressure differential may decrease. When sizing any normally-closed, normally-open, or universal solenoid valve, pressure differential before and after flow begins must be considered.
Solenoid valves are also rated for Maximum Fluid (media) Temperature due to temperature limitations of the various disc or diaphragm materials used in their construction. Response Time, the time necessary for a fully open valve to fully close, or the time necessary for a fully closed valve to fully open, is affected by several factors including: electrical service, media, valve size, system pressure, pressure drop, and operating mode.
The following general response times (nominal) apply for air service using alternating current (AC).
Viscous liquids have very little effect on response time on small direct-acting valves. However, on all other valves, viscous liquids may increase response time by 50 to 100 percent. DC operated solenoid valves will generally increase response time (relative to AC operated solenoids) by as much as 50 percent. Where response time is critical, consult your authorized local Fluid Control Division distributor.
Two-way solenoid valves have one inlet and one outlet connection with one main orifice and flow path. A normally closed valve is closed when the solenoid is de-energized, open when the solenoid is energized. A normally open valve is open when the solenoid is de-energized, closed when the solenoid is energized. Consideration should be given to the desired fail-safe condition of the valve when selecting the type of operation.
When the solenoid receives an electrical signal, a magnetic field is formed which attracts the plunger. The plunger lifts off the main orifice allowing flow through the valve.
When the solenoid is de-energized, it releases its hold on the plunger. The plunger drops and covers the main orifice.