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. 

                                                                                                                                                                                                                                                                             

TYPES OF CONTROL SYSTEM

1)LEVEL CONTROL SYSTEM

                                                                   Circuit Diagram

                                                                        Block Diagram
  

CONSTRUCTION

1)The liquid level control sytem is mainly consists of a tank (plant), which is filled with liquid.
2)The liquid sensor(transducer) is placed in a liquid tank, which senses the level of the liquid.
3)That sensor is connected to the transmitter.
4)Again the transmitter is connected to the controller, where the signal is to be controlled and compared with the set point value.
5) The controller is connected to the control valve, which controls the flow of liquid.

WORKING

     If the level of the liquid is increased, that level is sensed by the liquid sensor, where the liquid level is converted into electrical signal. Then the electrical signal is transmitted to the transmitter. Then transmitter passes the signal to the controller. In controller, the signal is compared with the set point value. And if the difference is more, then control valve will be closed by the controller. The liquid level of the tank will be decreased by the outlet valve until the level reaches to the set poin value.

    If the level of the liquid is decreased, that level is sensed by the liquid sensor, where the liquid level is converted into electrical signal. Then the electrical signal is transmitted to the transmitter. Then transmitter passes the signal to the controller. In controller, the signal is compared with the set point value. If the diffrence is less, then controlled valve will be opned by the controller. The liquid level of the tank will be increased until the level of the liquid reaches to the set point value.

2)FLOW CONTROL SYSTEM 

                                                                       Circuit Diagram       

                                                                        Block Diagram                

                                                      Differential pressure transducer

CONSTRUCTION

1) The flow control system is consists of a tank, where the liquid is placed.
2) It is also consists of a pipe, the orific plate is placed in the pipe.
3) That orific plate is connected to the transmitter.
4) The transmitter is then connected to the controller.
5) The controller is connected to the control valve, through which the flow of liquid is to be controlled.

WORKING     

1) If the pressure of the flow is increased, that pressure is sensed by the orific plate. The orific plate will passes that signal to the transmitter. The transmitter will transmits the signal to the controller, where the pressure signal will compared with the set point value. And if the pressure is more than the set point value, the control valve will be automatically closed by the controller. Then the flow will be decreased to the set point value.

2) If the pressure of the flow is decreased, that pressure is sensed by the orific plate. The orific plate will passes that signal to the transmitter. The transmitter will transmits the signal to the controller, where the pressure signal will compared with the set point value. And if the pressure is less than the set point value, the control valve will be opened by the controller. Then the pressure of the flow is increased near the set point value. 

3)PRESSURE CONTROL SYSTEM 

                  
                                                                   Circuit diagram

         Block Diagram 

CONSTRUCTION

1) The pressure control system is mainly consists of a chamber (plant).
2) The pressure sensor is placed in the chamber.
3) The sensor is connected to the transmitter, where the signal is to be processed.
4) The transmitter is connected to the controller.
5) The controller is connected to  the control valve, where the flow of gas is to be controlled.

WORKING 

1) If the pressure in the tank is increased above the set point value, then the present pressure in the tank is sensed by the pressure sensor. The sensor converts the pressure into the electrical signal. Then that electrical signal is processed to the transmitter, where the signal is to be measured. That signal is processed to the controller where the signal is compared with the set point value. Then the control valve will be closed and pressure is decreased to the set point value.

 2)  If the pressure in the tank is decreased , then the present pressure in the tank is sensed by the pressure sensor. The sensor converts the pressure into the electrical signal. Then that electrical signal is processed to the transmitter, where the signal is to be measured. That signal is processed to the controller where the signal is compared with the set point value. Then the control valve will be opened and the pressure will increased near the set point value.


4) TEMPERATURE CONTROL SYSTEM 

                                                                 Circuit Diagram

  
                                                                       Block Diagram 

CONSTRUCTION

1) The temperature control system is mainly consists of chamber (plant).
2) The heater and thermocouple is placed in that chamber.
3) The thermocouple is connected to the amplifier.
4) The amplifier is connected to the controller.
5) And the controller is connected to the relay. If the relay is energised, the heater is swicthed off and if the heater is doesn't energised, the heater is switched on.

WORKING

1) If the temperature is increased, then that present temperature is sensed by the thermocouple. The thermocouple converts the temp into the electrical signal. That electrical signal will processed by the sensor to the amplifier for the amplification purpose. After the amplification the amplifier transmits the signal to the controller, where the signal is measured and compared with the set point value. Then the relay gets energized and the heater will be switched off. The temperature gets decreased and controlled to the set point value.
2)If the temperature is increased,hen that present temperature is sensed by the thermocouple. The thermocouple converts the temp into the electrical signal. That electrical signal will processed by the sensor to the amplifier for the amplification purpose. After the amplification the amplifier transmits the signal to the controller, where the signal is measured and compared with the set point value. Then the relay doesn't gets energized and the temperature will be increased by switching on the heater. The temperature is increased until the temperature reaches to the set point value.  


5) CONDUCTIVITY CONTROL SYSTEM

                                                                     Circuit Diagram

                                                                         Block Diagram

 CONSTRUCTION

1) The conductivity control system is mainly consists of a electrolytic tank.
2) The electrodes(sensor) are placed in the tank, which senses the conductivity.
3) The electrodes are connected to transmitter through the cable wire.
4) The transmitter is connected to the controller, which controls the control valve.
5) The controller is connected to the control valve.

WORKING

1) If the level of the tank is increased above the set point value, that level will be sensed by the electrodes(sensor). Then the conductivity of the liquid is converted into electrical signal. That signal is processed to transmitter for the amplification purpose. That amplified signal is further processed to the controller, where the present signal is measured and compared with the set point value. Then the control valve will be closed and the conductivity level is decreased through the outlet pipe, until the conductivity reaches near the set point value.
2)If the level of the tank is decreased below  the set point value, that level will be sensed by the electrodes(sensor). Then the conductivity of the liquid is converted into electrical signal. That signal is processed to transmitter for the amplification purpose. That amplified signal is further processed to the controller, where the present signal is measured and compared with the set point value. Then the control valve will be opened and more electrolyte solution will be filled in the tank so as to increased the conductivity of the liquid.  

6)PNEUMATIC CONTROL SYSTEM

                                                           Circuit Diagram

CONSTRUCTION

1) The pneumatic control system is main consists of a flapper and nozzle.
2) The orific plate of 0.25 mm diameter is placed in the nozzle.
3) The flapper has two ends, one end is fixed, and another end is movable.
4) There is placed to the control valve in the nozzle.

WORKING

1) If the flapper moves towards the nozzle, then the input is increased. The air supply is given to the nozzle through the orific plate. since, the flapper is moves towards the nozzle, there is a restriction to the air, due to which the output is increased by reaches near the set point value.
2) If the flapper moves away from the nozzle, the input is decreased . The air supply is given to the nozzle through the orific plate. since, the flapper is moves away from the nozzle, there no any restriction to the air, due to which the output is decreased and comes near the set point value. 

Process Control System

Definition
             

The Process control system is defiened as the automatic control system, which Controls the process variable such as temperature, pressure,flow etc.

Advantages 

1) Reaction time is less.
2) The automatic control system can work continiously.
3) Performance of automatic control system is standard.
4)The cost of automatic control system is less.
5) It can work in hazardous area.

Static characteristics

1) Accuracy
2) Repeatability
3) Reproduciability
4)Frequency
5)Stabilty

Dynamic Characteristics

1)Step input
2)Parabolic input
3)Ramp input