TYPES OF PNEUMATIC RELAYS

 1) NON-BLEED TYPE RELAY

CONSTRUCTION

1) The non-bleed relay is a type of direct acting relay, it consists of  two bellows connected to the force beam

2)It also consists of a rod, and plugs are connected to the both ends of the rod.

3)The spring is connected to plug at the downward side.

4)The air supply is given from the bottom side of the non bleed type of relay.

WORKING

In direct acting relays, the input is directly proportional to the output. So when the input increases,the output also increases. And when the input decreases, the output also decreases.

  When the nozzle back pressure increases, there is a movement of bellows. The bellows move towards the downward direction.Then the air supply is from the bottom side of relay, so there is a restriction to the air supply, because the nozzle back pressure increases. Hence the output also increases.The air bleed stops when equilibrium condition is obtained, no loss of pressurized air at steady state position.

   When the  nozzle back pressure decreases , the bellows starts moving to upward direction. The air supply is given to the spring from the downward direction, hence the spring moves in upward direction. There is no restriction to the air, because the nozzle back pressure decreases. Hence the output also decreases. The air bleed stops when equilibrium condition is obtained, no loss of pressurized air at steady state position.

2) BLEED TYPE OF RELAY

CONSTRUCTION

1) The bleed type of relay is consists of a main diaphragm on which the nozzle back pressure acts.
2)The diaphragm is connected to the metal rod.
3)At the both ends of metal rods the plugs are connected.
4) The plugs are connected to the spring.
5) The air supply is given to the spring from the bottom side of the relay.

WORKING

The bleed type of relay is a type of direct acting relay. In this relay, the output is directly proportional to the input. Means if the input increases,the output also increases. And if the input decreases, the output also decreases.

   When the nozzle back pressure increases, this back pressure acts on the metal diaphragm. The metal diaphragm moves in downward direction because of increase in pressure. As the diaphragm is connected to the rod, the metal rod and plugs are also moves in downward direction. Then the air supply is given to the spring from the bottom side of the relay. But the air is restricted by the nozzle back pressure, therefore spring also moves in downward direction. so the output also increases.

   When the nozzle back pressure is decreases, that back pressure acts on the metal diaphragm. The metal diaphragm moves in up direction because the pressure is decreases. The air supply is given to the spring from the bottom side of the relay. Because of decrease in pressure, the spring moves in up direction. Therefore the plugs and rods are also moves in up direction. so the output gets decreases.

  In all position of valve excepts the position of shut off the air supply, air continues to bleed in atmosphere even after equilibrium  condition is obtained between nozzle back pressure & control pressure.  

REVERSE ACTING RELAY

CONSTRUCTION

1) The reverse acting relay is mainly consists of a metal diaphragm.
2) The diaphragm is connected to the rod.
3) The rod is connected to the ball.
4) The air supply is given to the ball from the downward side of the relay.

WORKING   

In reverse acting relay the output is indirectly proportional to the input. Means if the input increases the output decreases and if the input decreases the output increases.

    When the nozzle back pressure increases above the set point value, the metal diaphragm moves towards the downward side. As the diaphragm is connected to the ball, the ball also moves towards the down side. The air supply is given to the ball from the bottom side of the relay. Therefore the air is restricted by the nozzle back pressure. Due to this action the air moves to the atmosphere. So the output pressure is decreases and controls to near the set point value.

  When the nozzle back pressure is decreases, the metal diaphragm moves towards the upper side. The ball which is connected to the diaphragm is also moves towards the upper side. The air supply is given to the ball from the bottom side of the relay. Because of decreases in pressure the air does not restricted and the output pressure increases.