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| Pressure actuated by external fluid |
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This type of valve is actuated by pressure acting on a piston or diaphragm.
The fluid is usually neutral e.g. air, water, hydraulic oil. The pressure
range varies and is specified accordingly in a data table.
"Normally Closed" (N/C) valves of this type remain closed under spring
pressure, when de-activated.
"Normally Open" (N/O) valves of this type remain open under spring pressure,
when de-activated.
When pressure is connected to port "Z" the valves open or close according to type.
A separate 3/2 pilot valve is used to supply pressure to port "Z", it is
mounted either directly on the actuator or remotely.
In some cases a 4/2 or 5/2 pilot valve will be required. The main valve
position can be N/C or N/O. The basic position is determined by connecting
the pilot valve output lines accordingly. (The direction of flow is usually
indicated by an arrow or shown schematically on the body).
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Description of operation:
(Normally closed)
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Opening:
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Closing:
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In the basic position the depressurised chamber of the actuator is open to
atmosphere via the exhaust port of the "pilot valve". When the solenoid of the
"pilot valve" is energised the exhaust port closes and fluid passes through
the pilot valve building pressure in the chamber.
The pressure builds up under the main piston/diaphragm and via the valve
spindle lifts the valve seal off the valve seat and continues until the valve
is fully open.
When the pilot valve is de-energised the supply pressure is shut off and the
air pressure within the chamber vents to atmosphere. The piston/diaphragm now
returns under spring pressure, the valve seal covers the valve seat and the
valve closes and remains closed under spring pressure.
In some versions (closing with the flow) the fluid exerts a pressure on top
of the valve seal that assists the closing process.
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