Process Control Pneumatics:the air relay and the force balance Principle

the air relay and the force balance Principle

Air amplifiers balance input pressure and output pressure. An air relay, on the other hand (illustrated in Figure 7.5), balances input pressure with the force from a range spring. An increasing input signal causes air to pass from the supply to the load, while a decreasing input signal causes air to vent from the load. In the center of the input signal range, there is no net flow to or from the output port.

Process Control Pneumatics-0155

Process Control Pneumatics-0156

An air relay is used to linearize a flapper-nozzle, as shown in Figure 7.6. Here, force from the imbalance in input pressures P1 and P2 is matched exactly by the force from the feedback bellows whose pressure is regulated by the air relay.

Suppose flow in the pipe increases, causing pressure difference P1  P2 to increase. Increased force from the bellows at the top decreases the flapper gap causing pressure at the air relay input to rise. This causes air to pass to the feed- back bellows, which apply a force opposite to that from the signal bellows.

The system balances when the input pressure from the flapper-nozzle to the air relay (point A) is at the center of its range, at which point the air relay neither passes air nor vents the feedback bellows. This corresponds to a fixed flapper-nozzle gap.

Figure 7.6 thus illustrates an example of a feedback system where the pressure in the feedback bellows is adjusted by the air relay to maintain a constant flapper- nozzle gap. The force from the feedback bellows thus matches the force from the input signal bellows, and output pressure is directly proportional to (P1  P2). The output pressure, driven directly from the air relay, can deliver a large air volume.

The arrangement in Figure 7.6 effectively operates with a fixed flapper-nozzle gap. This overcomes the inherent non-linearity of the flapper-nozzle. It is known as the force balance principle and is the basis of most pneumatic process control devices.

Incoming search terms:

Related posts:

Compressed Air Transmission and Treatment:Breathing apparatus standards
Energy and Efficiency:CLOSED LOOP (TWO LEVEL) SYSTEMS
Low pressure and vacuum:Hopper off-loading and Trickle valves
Air flow rate evaluation:Pipeline purging
Control components in a hydraulic system:Flovvrneters.
Conveying characteristics:Component pressure drop relationships
HYDRAULIC PUMPS:Wobble Plate Inline Pumps and Bent Axis Pumps.
Hydraulic pumps:Pump performance
Hydraulic pumps:Pump maintenance.
INTRODUCTION AND OVERVIE:ACCURACY, PRECISION, AND SIGNIFICANT DIGITS
THE FIRST LAW OF THERMODYNAMICS:ENERGY BALANCE FOR STEADY-FLOW SYSTEMS
DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES
GAS MIXTURES AND PSYCHROMETRICS:DEW-POINT TEMPERATURE
INTRODUCTION TO FLUID MECHANICS:THE NO-SLIP CONDITION
HEAT EXCHANGERS:SELECTION OF HEAT EXCHANGERS

Leave a comment

Your email address will not be published. Required fields are marked *