Control Valve

What is Snap acting relay?

What is snap acting relay?

Snap acting relay senses the supply pressure and changes the direction of air flow when supply pressure is lower than the setting pressure.

Snap-Acting Relay changes ports when the primary supply pressure fails below a predetermined set point to switch or lock in secondary air source.

It also provide fail safe position function when supply pressure gets lower especially when there is a damage in the pipeline.

In the case of a supply or pilot pressure failure, the positive action relay with one prevalent ports and two inlet or outlet ports is automatically applied:

  • Switch from primary to auxiliary power supply
  • Lock an actuator in its last position
  • Extend or retract an actuator stem
  • Divert flow or pressure from one device to another

Snap-Acting Relays have an essential pilot that eliminates the additional piping and connections with other lock-up valves needed. The relays are readily piped and assembled, compact and lightweight.

Working of snap acting relay:

The pressure at which the relays are actuated can be adapted from 25 PSI (172 kPa) to 85 PSI (586 kPa) at any stage. Signal or pilot stress in the upper diaphragm overcomes the spring force in the cap and allows air to flow into the lower chamber.

This pressure accumulation forces the spool valve loaded with the spring to open the prevalent port “A” to “B.”

The exhaust port opens when the pressure falls below the preset level and the popular port “A” is moved from “B” to “C” by releasing the spool valve loaded with the spring.

When the stress on the pilot is less than or equal to 20 percent higher than the set point, the spool valve will return to its initial position (“A” to “B”).


Instrumentation Engineer

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