Question

Describe with a sketch pneumatic relay and show how feed back can be achieved when such a relay is used in conjunction with a flapper mechanism.

Answer Fig.

Pneumatic valves and actuators are similar to hydraulic valve and actuators. However air is compressible where oil is frequently assumed to be incompressible so that pneumatic valves need to be pressure locked in position.

The essential difference between the pneumatic and hydraulic system is that hydraulic control valves control the oil flow, flow where as pneumatic control valves control air pressure.

A typical nozzle flapper control valve is used extensively in low pressure control systems.

Air is supplied to the control valve at a constant pressure ‘Ps’ and the restriction generate a pressure reduction. As the flapper is in its null position air exhausts through the gap between flapper and the nozzle so that the pressure 'Pc' remains at a steady value.

How ever as flapper is moved towards the nozzle the gap between the flapper and the nozzle is reduced and the pressure 'Pc' increases. The maximum value of Pc is reached as the nozzle tends to be fully closed and Pc tends to Ps.

As the flapper is moved away from the nozzle the gap between the flapper and the nozzle is increased and Pc is reduced to its minimum value. Clearly the movement of the flapper regulates the control pressure 'Pc' so that this pressure can be used in order to operate an output actuator.

However this simple controller is non linear and is extremely sensitive.

In a nozzle flapper valve a negative feedback generates a control pressure that is proportional to the flapper displacement. This principle is known as proportional control action. As the flapper is moved towards the nozzle the pressure 'Pc' increases rapidly and causes the bellows to expand. The bellows act upon pivoted end of the flapper and increases the gap between the flapper and the nozzle until a stable condition is obtained. In this condition pressure 'Pc' is maintained at higher value than the null pressure and its increase is proportional to the flapper displacement’d’.