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Indirectly controlled solenoid valves normally closed
Servo-controlled 2-way solenoid valves normally closed, Main components: body with main orifice, cover with pilot orifice, diaphragm (or piston) assembly, quill assembly + moving core assembly (kit normally closed), coil.
The normally closed 2-way solenoid valves servo-controlled have an inlet fitting and a usage fitting.
The opening of the main orifice, present in the body, occurs due to an imbalance of pressures between the upper and the lower face of a membrane (or a piston): when the coil is not powered, in the chamber above the membrane there is fluid under pressure, while below the membrane there is pressure only in the area outside the main orifice: therefore the resultant of the forces on the membrane is such as to push the membrane to close the main orifice. When the coil is powered, the movement of the mobile core on which a gasket is mounted causes the opening of an orifice (pilot orifice) and discharges the chamber above the membrane: the imbalance of pressure causes the displacement of the membrane which opens the main port.
In this family of solenoid valves, a minimum pressure difference between the supply fitting and that of use is required to ensure correct operation of the solenoid valve itself. However, an excessive pressure difference between the supply and the use, as on the 2-way solenoid valves normally closed with direct control, causes a
Operation: increase in the force required to open the pilot orifice, so if this pressure difference is greater than the maximum value for which the solenoid valve has been designed, the latter may not open even when the coil is powered.
For the correct operation of the solenoid valve and to avoid the rapid deterioration of the diaphragm, it is desirable that, when starting to close the valve, the flow rate value does not exceed Kv, i.e. the guaranteed flow rate with loss of load through the solenoid valve equal to 1 bar. For this reason, if the supply pressure with the valve open is greater than 1 bar, the use of the free-mouth valve is not recommended, i.e. without a suitable throttle on the discharge port which brings the pressure drop on the solenoid valve back to the value of 1 bar. Furthermore, particular attention must be paid in the design of the hydraulic circuit to the problem of water hammers, which can cause overpressures such as to tear the membrane or damage other parts of the solenoid valve.