Fit and Adjust a Service Unit

1. Introduction

The service unit ensures that the air is conditioned so that it can be used for operating in a system. All the contaminants in the air must be removed and the water must be trapped and drained. This is done by fitting a filter with a water trap.

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The air must be regulated to a safe constant pressure by fitting a pressure reducing valve. It must also be enriched by oil so that it can lubricate the component parts.

The service unit is always located on the supply side of the pneumatic installation.

2. Service Unit

The service unit consists of four separate components. They are:

a) an air filter with a water trap,

b) a lubricator,

c) a pressure gauge, and

d) a pressure reducing valve.

The function of each part will be described.


The detailed representation of the service unit is shown in Fig. 1.


The simplified representation is shown in Fig. 2.


3. Air Filter with Water Trap

This component consists of the following parts (Fig. 3).


a) the connection part,

b) the filter bowl,

c) the deflector cone,

d) the filter element,

e) the baffle, and

f) the drain.

The air enters the filter through the inclined nozzle-type opening. It is given a swirl by the deflector cone (c), (Fig. 3 and 4).


Due to the centrifugal (moving from the centre) force of the air, which is caused by the deflector, the water is deposited on the walls of the filter bowl (b). The water and the larger particles of dirt accumulate at the bottom of the filter. The compressed air continues to flow through the filter element (d) where the remaining contaminations are removed from the air. The filter element becomes blocked by the fine particles of dirt and therefore, should be changed at regular intervals. The maximum water level is indicated by a mark. The water should be drained regularly to prevent it exceeding the maximum level.


The symbol for a filter only is shown in Fig. 5.


The symbol for a filter with a trap is shown in Fig. 6.


4. Lubricator

A lubricator consists basically of the following parts (Fig. 7 on the next page):

a) the housing,

b) the lubricator bowl,

c) the rise tube,

d) the throttling screw,

e) the drip cap,

f) the check ball,

g) the nozzle ring gap, and

h) the oil bore.


The lubricator will only operate when there is a flow of air from port “A” through the nozzle ring gap (g), into chamber “D”, through port “B” and into the system.

If this flow does not occur, the same pressure will exist throughout the device and oil will not be injected into the air system.

When flow occurs a partial vacuum occurs in the oil bore (h) and a reduction in pressure is created in the drip cap chamber “E”. The pressure in chamber “D” is now greater than the pressure in the chamber “E” and therefore the oil is forced up in the rise tube (c). The oil drips into the oil bore (h) and is atomized into the compressed air which flows into the system at port “B”. The amount of oil which is injected into the system can be controlled by adjusting the throttling screw (d). If the throttling screw is screwed in deeper, more oil will be injected into the



The symbol for a lubricator is shown in Fig. 8.


5. Pressure Gauge

The pressure gauge is used to measure the pressure in any part of the system. The symbol for the pressure gauge is shown in Fig. 9.


6. Pressure Reducing Valve

The pressure reducing valve consists of the following parts (Fig. 10):

a) the housing,

b) the adjusting screw,

c) the diaphragm,


d) the valve seat ring,

e) the valve disc,

f) the valve plunger, and

g) springs,

h) springs.


When the adjusting screw (b) is screwed out until the tension in spring (h) is released completely, spring (g) will push the valve disc (e) against the seat and close off the valve.

When adjusting screw (b) is screwed in, the tension in spring (h) will exert a force on the plunger (f) which will then tend to open the valve disc (e). If this force becomes greater than the force exerted by the supply pressure Pel and by the tension in spring (g) the valve disc (e) will be lifted off its seat and air will flow from port “P” to port “A” into the system (Fig. 11).


When the system pressure starts to build up, the force exerted by it onto the diaphragm © will tend to close the valve. If this force becomes greater than the force exerted by the spring (h), the spring (g) will push the valve disc onto its seat and cut off the air supply.

If air is drawn off continuously, the gap between the valve disc and its seat will always remain slightly open.

If for some reason the supply pressure Pe2 rises above the preset pressure, the valve disc (e) will close first. The force acting on the diaphragm © due to the excess pressure will lift the valve seat ring (d) off the plunger (f) seat, thereby allowing the excess air from the system to flow through the escape holes “R” (Fig. 12).



The symbol for a pressure reducing valve is shown in Fig. 13.


7. Adjusting the Service Unit

* Select the components shown in Fig. 14.

* Place them on the trainer and construct the circuit.


* Undo the lock-nut on the pressure reducing valve (Fig. 15).


* Unscrew the adjusting screw until there is no tension on the spring.

* Open the air supply.

* Shift the control valve levers to such a position that the flow of air will move the cylinder outwards.

* Turn in the adjusting screw on the pressure reducing valve until the piston starts to move.

* Let the piston move out to the furthest position.

* While the control valves are kept in this position adjust the pressure reducing valve until a pressure of 300 kPa is registered on the gauge.

* Close the air supply.

* Operate the control valves and release any pressure in the system.

* Disconnect the pipe between the control valves at the second valve.

* Open the air supply to obtain a continuous flow.

* Check the rate at which the oil drips from the rise tube in the drip cap (Fig. 16).


* Turn the throttling screw on the side until a rate of 2 drops/minute is obtained.

NB: Do not add too much oil as this could lead to the fine nozzles in the units being blocked.

It should also be mentioned that some units, and some branches of industry, require oil-free compressed air.


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