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Dual redundancy safety valve (air isolation)

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article image Cross sectional view of Norgren's XSz internally monitored dump valve.

IN any pneumatic system used to operate a machine or robot, the reliable removal of air from the system is essential to the safety of the operator and or public.

It is a requirement to use an emergency dump valve so that the compressed air may be "removed" rendering the equipment safe, such as Norgren’s XSz safety valve.

Functioning of the XSz safety valve

The monitor in this valve uses integral air logic incorporated into the pilot-supply air system. No moving parts exist.

Each main valve poppet is monitored on each cycle by an air chamber and orifice. This design feature eliminates the need for electrical monitoring of the valve and has been assessed as conforming to AS 4024.1 1996 and may be used in applications to Category 4.

For those exporting to Europe, this valve is also Certified by BG as meeting EN 954-1 1997 (Category 4) Safety of Machines - safety related.

Under normal conditions, when the solenoids are energised, the plunger in the pilot valves moves and allows the pilot-supply air to reach the main valve poppets.

This air pressure on top of the poppets forces both main pistons downward, which opens the supply port 1(P) to the operating port 2(A). At the same time, the exhaust port 3(R) is closed.

Air contained in the built-in volume chambers is consumed during actuation. These chambers must refill through their respective control orifices within 300ms to complete a successful cycle.

If any problem occurs (broken poppet, loose solenoid connection, dirt prohibiting a main poppet from moving, etc.) and the poppet does not shift, then the air volume chamber does not recharge in the required time, hence, it is pneumatically "locked-out".

The cross-flow channel is blocked and the air volume chamber is connected to the exhaust. The working port is connected to the exhaust port and no pressure build-up exists. This condition, which inhibits further valve operation, is achieved without any moving parts.

Since no moving parts exist and the air logic monitoring system is integral to the valve, it is impossible to interfere with or override the failure indication system. This provides an added degree of safety above the static pneumatic monitor design or electrically monitored dump valves.

To initiate another valve operation, the solenoids must be de-energised and then re-energised. In other words, users will have to start from the beginning. If a problem still exists, the valve will again experience a malfunction and vent pressure to exhaust.

The safety of the valve with a dynamic pneumatic monitor is inherent in the principle of air logic. However in the event that an electrical signal is required for the press control circuitry (fault location, operations counter etc) a pressure switch interlock can be added. Whether this signal is needed depends upon individual plant and control requirements.

This valve is available in a 3/2 configuration with port sizes 1/4" - 2"or 5/2 configuration with port sizes 1/4" - 1/2". Either valve is available with BSP or NPT connection.

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