Snap Acting Relay

Introduction

This is a 3D model of a Snap Acting Relay.

3D Model Annotations

Snap Acting Relay

Snap acting relays enable designers to build a certain safety factor into a system. Primary air (pilot air, signal air) is used to operate the relay under normal operating conditions, but if the primary air supply should drop below a certain pressure, the relay will switch to a secondary air supply. Switching the air supply may lead to the valve remaining in its current position, retracting, or extending. The action dictated by the secondary air supply is often linked to the desired fail-safe position of the valve.

Signal/Pilot Pressure

Under normal operating conditions, signal air is supplied to the relay through this port. Signal air pressure is maintained at all times in order for the valve to remain in its normal operating position. The air overcomes the spring force on the diaphragm in the upper chamber, which opens a port and allows it to flow into the lower chamber; it then acts upon the diaphragm in the lower chamber.

Exhaust

Upon failure of the primary air supply, the upper plunger presses down on the transfer port beneath it and closes the transfer port. This occurs because the spring pressure acting on the upper side of the diaphragm exceeds that of the air pressure acting on the lower side of the diaphragm. This action causes the lower piston to move downwards, which causes the exhaust port to open. Primary air is then vented through the exhaust port, which leads to the spool valve changing position.

Diaphragm A

A diaphragm provides a large surface area upon which the air pressure can act. The diaphragm spring provides residual pressure to the opposing side of the diaphragm.

Diaphragm B

A diaphragm provides a large surface area upon which the air pressure can act. The diaphragm spring provides residual pressure to the opposing side of the diaphragm. The opposing spring on this relay is below the main spool valve, but the effect is the same as if it was installed directly on the opposite side of the diaphragm (the residual pressure of the spring is transferred to the diaphragm via the spool valve).

Spool Valve

The spool valve will cover or uncover ports B or C if the primary air pressure is below or above a certain pressure.

Upper Chamber

The upper chamber of the valve.

Lower Chamber

The lower chamber of the valve.

Port A

The common flow port is represented by Port A. Port A is not affected by the position of the spool valve and is always connected to the main system.

Port B

Port B is affected by the position of the spool valve and is disconnected from the main system when the spool valve moves to the lower position.

Port C

Port C is affected by the position of the spool valve and is disconnected from the main system when the spool valve moves to the upper position.

Transfer Port

Air is transferred from the upper to lower chamber through the transfer port. The port is bored through the centre line of the piston.

Plunger

The plunger is moved by air or spring pressure. If the air supply fails, the plunger moves down and covers the transfer port.

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