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| Solenoid actuated with forced lifting |
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Valves that are actuated in this way have a combination of direct and
indirect actuation. A mechanical coupling between the solenoid core (pilot
stage) and the piston (or diaphragm) assists the opening movement of the
piston (or diaphragm). This is called forced lifting.
A minimal pressure differential is not necessary with this combined method
of operation, the valve operates from 0 bar. The solenoid is relatively
strong, as it has to open the valve when the pressure differential is not
sufficient without pressure support.
Thus valves that are actuated in this way combine the advantages of direct
actuation (no minimum pressure/flow required) and indirect actuation
(relatively large flows at high pressure).
Description of operation:
Valve closed (Fig. 1)
The solenoid is de-energised and the spindle (piston of pilot valve) connected
to its core closes the vent (pilot seat), which is positioned concentrically on
the piston (or diaphragm). The upstream pressure in P (greater than the
downstream pressure in A) builds up through the two bleed orifices in the
piston (one bleed orifice in the diaphragm) on the top of the piston.
This pressure, multiplied by the area of the top of the piston (or diaphragm),
produces a closing force which isgreater than the opening force exerted on the
piston (or diaphragm). It is forced onto its seat.
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Fig. 1
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Solenoid de-energised
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Valve opens (Fig. 2)
The solenoid is energised. The magnetic force, which is greater than the
closing force (spring and pressure force) exerted on the core, raises it from
the vent. Thus the pressure in the space above the piston (or diaphragm) is
relieved and a balance of pressure comes into operation with the A-side
(valve outlet). This pressure relief continues as less of the fluid can flow
through the bleed orifice than escapes through the vent.
Thus the opening force, resulting from the greater inlet pressure in P, is
predominant. The difference in pressure between the top and bottom side of the
piston raises the piston (or diaphragm) from the valve seat.
This opening operation is therefore identical to that of the valves with
indirect actuation.
Where there is a difference, however, is that the piston (or diaphragm) is
pulled simultaneously into the opening position after a certain core stroke
by means of a screw piece (mechanical coupling).
So no difference in pressure is required to open and keep open the valve.
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Fig. 2
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Solenoid energised
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Valve closes
The solenoid is de-energised and the core closes the vent by spring force
(and by force of pressure where present).
Above the piston (or diaphragm) the same pressure builds up through over the
bleed orifice as on the P-side, and the resultant force forces the piston
(or diaphragm) onto the valve seat.
If there is no or only very slight differential pressure, this closing
operation is brought about by the force of the springs in the core tube and
above the piston only.
The flow direction of the fluid is fixed.
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