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MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : SPECIFICATION, SELECTION AND

MAINTENANCE OF EQUIPMENT

LESSON 4 : AIR SERVICE EQUIPMENT

APH - 3 - 4

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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published and without a similar condition including this

condition being imposed on the subsequent purchaser.

________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

We have previously discussed the design of compressed-air distribution mains.

In this lesson we deal with the air service components that are inserted in the

distribution system at strategic points, to ensure the quality of the air being

delivered is maintained at an acceptable level. This text deals with the

function and operation of the most commonly found of these air service

components.

________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

On completion of this lesson you should be able to:

• explain the need for the installation in the distribution system of

components to control the quality of air being supplied to pneumatic

equipment

• describe the function and operation of the more commonly-found

items of air service equipment.

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________________________________________________________________________________________

AIR SERVICE COMPONENTS ________________________________________________________________________________________

Air service components are items of equipment whose sole purpose in the

distribution system is to maintain the quality of the compressed air being

provided at each system take-off point.

The duty of these components will usually include the removal of moisture or

other contaminates, pressure control and, sometimes, the addition of oil to the

air supply. As the efficient operation of pneumatic equipment is dependent on

the air supply being maintained at a satisfactory quality, the importance of

these components can not be over-stressed.

MOISTURE SEPARATORS

As discussed in the previous lesson, the inclusion of drain legs in the system,

fitted with some form of automatic draining valve, is essential for the removal of

condensed moisture that has collected at the bottom of the main. However, drain

legs by themselves will not remove the moisture droplets that remain suspended

in the air stream. To remove suspended moisture, it is usual to fit some form of

impingement-type moisture separator in the pipeline. In the separator the

velocity and direction of the air is caused to change quite suddenly. The heavier

particles of moisture in the air stream find it much more difficult to respond to

this sudden change, and are deposited on the surface of the separator baffle-

plates. This moisture is then collected in the bottom of the separator, and

removed from the system by the operation of an automatic drain- trap.

Moisture separators for this type of duty are shown in FIGURES 1 and 2: they

should be installed in the system to cater for the normal direction of flow.

They are available in sizes ranging from 15 mm to 200 mm, and have a

maximum operating pressure somewhere between 10 and 14 bar.

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FIG. 1

FIG. 2

Air inlet Air outlet

Water outlet

Pressure balance pipe if necessary

Separator

Strainer

Air trap

Compressed air main

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COMPRESSED AIR DRAIN TRAPS

Moisture draining is required to remove water from the distribution system,

and so prevent the carry-over of moisture into air tools or pneumatic control

circuits. The damage, caused to the equipment in many instances, can be quite

minor and easily rectified; but the cost of the machine downtime and the losses

in production, due to this malfunction, give a truer indication of the costs of

moisture in a pneumatic system.

Draining water from the system is usually achieved by the use of some form of

automatic drain-trap. This is simply a valve that opens automatically on

detecting the presence of water, and closes after the water has been discharged

from the system.

The most common type of trap is the ball-float trap, as shown in FIGURE 3.

In this type of trap, a ball float is connected to the main valve; water coming

into the trap will cause the float to rise, opening the main valve. As the water

is discharged, the float will follow the lowering level until the main valve is

once again closed. This valve will operate continuously in the presence of

water: as long as large quantities of water are coming into the trap, it will be

seen to be discharging. However, in some cases, because the trap is draining

the plant continuously, so little water is being handled at any one time, that it

tends to be discharged as a mist, giving the impression that the trap is not

operating correctly.

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FIG. 3 Simple Float and Lever Trap

Ball float traps can also suffer from air binding which will prevent the valve

from operating. FIGURE 4 will help to explain this undesirable condition.

Tank A is a drain point connected to the system, and water is to be drained via

the ball float trap (B). When the isolation valve (C) is opened, then the in-rush

of water to the body of the trap will compress the air trapped in the

interconnecting pipe and the trap body. When the pressure of the air in the trap

is equal to the pressure in the drain tank, then water cannot be induced to enter

the trap, and it will never operate.

FIG. 4 Air Binding of Ball-float Trap

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FIG. 5 Effect of Adding Balance Pipe

The problem can be solved by fitting a balance pipe to the installation, as in

FIGURE 5. This arrangement will allow any air, that would previously have

been trapped, to be displaced through the balance pipe into the drain tank.

Air binding will only tend to occur when the amount of water coming into the

trap is large. If the amount coming into the trap is small, then the air being

displaced can usually escape up the sides of the inlet line: then the fitting of a

balance pipe is unnecessary.

Ball-float traps will handle water lightly contaminated with oil, but, with the

build-up of oil/water emulsion, the operation of the trap will become sluggish,

and may cease to operate altogether.

Pressure balance pipe

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List four other pieces of equipment connected with compressed-air production and

storage that require to be drained using an automatic drain trap.

...................................................................................................................................................

________________________________________________________________________________________

Four other pieces of equipment are:

• compressor intercooler • compressor aftercooler

• moisture separator • air receiver.

Aerodynamic type drain traps can operate at much higher pressures (up to

24 bar) than ball float types and can cope with water which is more heavily

contaminated with oil. FIGURE 6 shows the operation process of an

aerodynamic type drain trap.

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(a) Trap discharging liquid that lifts the disc

(b) Compressed air following liquid is trapped in the chamber above the disc

and its pressure holds the disc firmly on valve seat

FIG. 6 Operation of an aerodynamic type drain trap

Control chamber

Disc

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This type of trap uses the operating principle that if you bring a fast-moving

fluid to rest there will be a build-up of pressure. The only moving part

contained in the trap is the disc above the flat face inside the control chamber

or cap.

When water flows up through the central port A, it will push the disc back

towards the cap and flow across the underside of the disc through to the outlet

port B, as shown in FIGURE 6(a). However, if compressed air follows the

water, as shown in FIGURE 6(b), the mixture will expand radially across the

underside of the disc and will travel at a very much higher velocity. This

means that the static pressure under the disc is reduced making use of

Bernoulli’s theorem which states that the sum of the pressure and velocity

energy in a flowing fluid remains constant. Therefore, higher velocity can

result in lowering of pressure. Then, the disc starts to move down towards the

seat; some of the air is trapped in the control chamber behind the disc and in

being trapped comes to rest. According to Bernoulli’s theorem, there will be a

build-up in pressure in the control chamber that holds the disc firmly on the

valve seat.

When pressures higher than 24 bar are required, then it is usual to use an

inverted bucket trap (FIGURE 7). This trap, which can handle system

pressures up to 62 bar, has flanged connections and a cast steel body.

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FIG. 7 Inverted Bucket-type Air Trap

Both the aerodynamic and inverted bucket traps are classified as blast

discharge traps, because they do not open until a specific amount of water is

present, and then close when it has been discharged. This results in an

intermittent operation: extreme care should be taken to avoid oversizing this

type of trap, or using them on compressors of very low output.

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MAINS AIR PRESSURE REDUCTION

On occasion it is necessary to vary the air-main distribution pressure, if a

single unit is being used to supply air to a number of plants or workshops.

Each plant or workshop may have a different pressure requirement: it may be

economically more feasible to produce the air at a high pressure, and distribute

it in that state to each installation. At each site the pressure is then reduced to

that required for the satisfactory operation of the equipment on that plant.

The alternative to this arrangement would be for the plants, requiring air at

higher pressure, to have their own production units, or maybe have a booster

compressor to increase the pressure of the air supplied by the main production

unit. It would be necessary to look at the specific needs of the plant before

deciding the best method to adopt.

Those systems requiring mains pressure reduction require a reducing valve,

somewhat larger than those used to regulate the air pressure to individual

pieces of equipment.

The pressure-reducing installation shown in FIGURE 8 shows a diaphragm

type reducing valve, which is a large capacity unit with very good flow

characteristics. The pressure downstream of this valve is controlled remotely

by varying the pilot control signal supplied by a small pressure regulator.

Decreasing the pilot control pressure will decrease the mains pressure

downstream of the valve. This type of valve will give very close control of

downstream pressure, but should not be used where dead-end service is

required, due to problems associated with leakage across the valve in a closed

condition, causing increased downstream pressure.

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FIG. 8 Diaphragm Pressure Reducing Valve

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FILTER, REGULATOR AND LUBRICATOR UNITS

The purpose of the air distribution system is to deliver the air to the point of

usage in a suitable condition with regard to pressure and flow rate. As

compressed air is used for many different purposes, it is finally conditioned for

its particular use at the take-off point, just prior to its entry into the equipment.

This will involve filtering the air to remove dirt and moisture, final pressure

reduction and, sometimes, the addition of lubricating oil to the air to assist in

operation of the pneumatic equipment. This will normally all take place in an

air service unit consisting of a filter, regulator and lubricator (FRL).

FIGURE 9 shows an illustration of such an air service unit.

FIG. 9 Air Service Unit

105

Filter

Swirl disc

Drain

Pressure regulator Drip rate adjuster

Oil filler

Lubrication unit

bar

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COMPRESSED AIR FILTRATION

The compressed air filter will remove solid materials, such as pipe scale and

dirt caused by corrosion inside the pipe, from the air. The filter should also

remove liquids, such as water and compressor oil, which may have been

carried along with the compressed air. Most filters are designed to remove

some of the large particles of contaminate, including liquid droplets, by

centrifugal action.

On entering the filter unit, the air is caused to swirl in the upper section of the

filter bowl. The heavier particles are thrown to the outside of the bowl by

centrifugal action. They then fall to the quiet zone at the bottom of the bowl,

where they are collected and drained. This will reduce the amount of dirt that

has to be handled by the filter element and, in so doing, will increase its

working life.

With its larger particles now removed, the air supply will pass through the

filter element. Two types of element are in general use. One type of element

used is the surface type, which has a uniform pore size of material throughout

its depth. The useful service life of this filter is a function of the free surface

area of the element. Perforated or mesh screens and sintered metal elements

will fall into this type.

The second type of element used is classed as a depth-type filter and will

consist of a cartridge element whose pore size will decrease in the direction of

air flow. This will not only allow dirt to be collected on the outer surface, but

will also trap substantial quantities in the core of the element. In consequence,

depth type filters will normally have a greater dirt-holding capacity, and a

longer working life than surface types.

Regular draining of the filter bowl is essential to prevent the water/oil mixture

from rising to the level of the filter element, where it could be forced through

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the filter element, and into the pneumatic equipment connected to the system.

Filter bowls are therefore fitted with either a manual or automatic drain.

FIGURE 10 shows an illustration of a filter unit fitted with a manual drain.

FIG. 10 Air Filter with Manual Drain

Dome

Head assembly

Air flow

Clamp ring

Vane baffle

Bowl assembly

Drain cock

Filter element

Baffle

Deflecting baffle

"O" ring seal

Air flow

Retainer

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The 'cut off rate' of the filter is the size of the smallest particle that will be

caught by a clean filter element, and is usually expressed in microns (µm, millionths of a metre). Standard filters are provided with 50 µ m or alternatively 25 µm elements. However, it is inadvisable to use a finer element than is necessary, because this will tend to reduce its effective working life.

Whenever a very high degree of filtration is required, a high-performance

coalescing filter can be used. Unlike standard filters, the air flow will pass

from the inside to the outside of the element. The element of the filter will

initially remove the very fine particles of solid contaminate by direct filtration.

Finely dispersed aerosols of oil and water in the air gradually migrate to the

outside of the element: in so doing they 'coalesce' or grow in size. To assist in

this growth and to prevent re-entrainment, a further layer of material is

provided through which the air has to pass. The larger droplets, formed by the

coalescing of the moisture, can now be easily separated from the air stream by

gravitational forces. The collected droplets will now fall to the bottom of the

bowl where they can be drained either manually or automatically.

Most filters will be fitted with see-through polycarbonate plastic bowls to

allow the action of the unit to be monitored. However, although the material

used for the manufacture of the plastic bowls is extremely strong, it can be

weakened by age or contact with certain chemicals: for that reason, bowl

guards are available to prevent personnel from injury, in the event of a bowl

bursting. Metal bowls are also available.

If a plastic bowl needs to be cleaned, then it is advisable to use water and

washing-up liquid which will not attack the plastic. If a filter bowl loses its

transparency, due to ageing or cleaning, then it is advisable to change the bowl

for one made from metal, or for a plastic one with a bowl guard.

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Given below is a list of materials which will attack the polycarbonate material

from which the bowls are manufactured.

• ethylene glycol

• formic acid (conc.)

• freon (refrigerant & propellant)

• gasoline (high aromatic)

• hydrazine

• hydrochloric acid (conc.)

• lacquer thinner

• methyl alcohol

• methylene chloride

• methylene salicylate

• milk of lime (CaOH)

• nitric acid (conc.)

• nitrobenzene.

What effect on the system, other than moisture carry-over, do you think will be

associated with an undrained filter bowl?

...................................................................................................................................................

________________________________________________________________________________________

As well as contaminating the system, the high fluid level in the bowl will block the filter,

reducing its flow capacity and increasing the pressure drop across the element.

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PRESSURE REGULATING OR REDUCING VALVE

For any kind of pneumatic equipment, there is usually one pressure at which it

will operate most efficiently. At a higher pressure, the equipment will wear

excessively with no significant increase in output. This results in compressed

air wastage and higher operational costs.

Pressure-reducing valves at each take-off point, if correctly adjusted, will

ensure that the pressure of the air being used by the equipment is exactly that

required for its efficient operation.

The valves themselves come in two versions, those with a secondary relief

function and those without. The prime function, however, of both types of

valve is to maintain constant, but adjustable, downstream pressure,

independent of pressure fluctuations upstream of the valve, and flow

fluctuations downstream of the valve. This obviously assumes that the

pressure upstream, at the inlet to the valve, always remains higher than the

adjusted pressure downstream, at the outlet of the valve.

FIGURE 11 shows an illustration of a pressure-reducing valve, of the type

used in an air service unit.

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FIG. 11 Pressure-reducing Valve without Secondary System Relief Failure

The set pressure of the valve is adjusted by the knurled adjuster which

acts via the main spring and diaphragm through the spindle to

hold the valve face off its seat . Air enters the valve at P and passes

over the valve to exit at A; at the same time it acts on the air side of the

diaphragm, and exerts a force against the action of the main spring .7

967

P A

1 2

3 4 5

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When the force exerted on the diaphragm by the air, plus the force exerted by

the valve spring through the spindle , can overcome the force applied

by the main spring then the valve will start to close to limit the pressure

build-up.

As demand for air in the system reduces the pressure acting on the diaphragm,

then the main spring will once again become dominant, and the valve will

open to allow the pressure downstream to rise once again. The valve is

therefore modulating constantly, to maintain a constant downstream pressure

independent of system demands.

Valves with built-in relief functions prevent the downstream side of the valve

from experiencing any excessive pressure. In the construction of the valve, a

relief orifice is machined in the centre of the diaphragm hub. During times of

normal operation this orifice is blocked by the valve stem. If the pressure

downstream rises to an unacceptable level, then the diaphragm will deflect

away from the valve stem due to the force exerted. The orifice will now be

open to vent the air, and lower the downstream pressure. FIGURE 12 shows

an illustration of such a valve. (Note that the valve is a normally open valve,

but is drawn in the closed condition, assuming the downstream pressure has

risen high enough to overcome the force of the control spring.)

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FIG. 12 Pressure-reducing Valve with Relief Function

Adjustment knob

Bonnet

Control spring

Air vent hole

Diaphragm

Inlet

Return spring

Valve nose

Gasket

Valve seat

Automatic pressure release valve

Outlet

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What factors do you think need to be considered for the correct selection of a suitable

pressure-reducing valve?

...................................................................................................................................................

________________________________________________________________________________________

The pressure-reducing valve must have a flow capacity greater than the sum of all the

components downstream of the valve. It should also be capable of withstanding the inlet

pressure, and be capable of being adjusted to maintain the downstream pressure at the

required value.

AIR LUBRICATION

Lubricating oil is supplied to compressed air, to reduce friction and corrosion

in pneumatic components connected to the supply. The oil is added to the air

stream to produce an oil mist, which is then carried into the equipment, to

provide the lubrication and corrosion resistance required.

Two types of aerosol lubricator are available, macro-lubricators and micro-

lubricators. Macro-lubricators produce a relatively coarse mist, and are

particularly useful where a generous supply of oil is required.

Micro-lubricators produce a much finer mist which is capable of being carried

over longer distances, and through more complicated circuits. Both utilise the

pressure difference created by compressed air passing through a venturi tube,

as shown in FIGURE 13.

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FIG. 13 Macro-lubricator

The pressure drop, resulting from the increase in velocity in the throat of the

venturi, has the effect of drawing oil up the dip tube, and depositing it in the air

stream. In the air stream, the oil is atomised and leaves the lubricator as a

mist.

With the 'macro-mist' lubricators, all of the oil conveyed into the air supply is

atomised by the air stream: this means that droplets, both large and small, are

carried over into the outlet pipework, where the large droplets tend to fall out

of the air stream. For this reason, macro-lubricators should only be used on

systems where the components to be lubricated are close to the lubricator.

Micro-lubricators pick up and atomise the oil in the same way as the macro

lubricator; but the oil-loaded air supply is then re-directed against a baffle plate

or some other device, which causes further fragmentation of the oil particles.

This causes the air to change direction, and be diverted back into the bowl to

exit through a large opening. In this manner, all of the larger droplets in excess

of 2 microns fall back into the lubricator reservoir.

FIGURE 14 illustrates the operating principle of a micro-lubricator.

Air inlet Air and oil out

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FIG. 14 Micro-lubricator

Both types of lubricator unit will have an adjustment screw to regulate the

supply of oil into the airstream, and incorporate a clear perspex dome so that

the drip rate can be monitored as adjustment takes place. Drip rates should be

frequently monitored, and adjusted as necessary to prevent over-lubrication

which might cause hazardous situations.

FIGURE 15 shows a chart indicating the amount of oil that should be added to

an air supply for general lubrication of pneumatic equipment.

Air inlet Air and oil out

Oil

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FIG. 15 Suggested Oil-feed Rates

Use the chart in FIGURE 15 to suggest an oil-feed rate for a lubricator unit supplying

oil loaded air at a flow rate of 1200 l min-1 FAD.

...................................................................................................................................................

________________________________________________________________________________________

The chart indicates a suggested drip rate between 2 and 6 drops per minute. In practice this

may initially be set at 4 drops/min: then the operation of the system should be monitored

and adjusted, so that the system is receiving just sufficient oil for efficient operation and no

more.

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16

0 10 20 30 40 50 60 70 80 90 100 110 120

500 1000 2000 3000 3500 Flow – litres free air per minute

Suggested general lubrication range

Possible region of over lubrication

1" 4

3" 8

1" 2

3" 4

D ro

p s

p er

m in

u te

Pipe bore (inches)

Possible region of under lubrication

Flow – cubic feet free air per minute

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Manufacturers of pneumatic equipment and lubricator units suggest that light

mineral oils with a viscosity of about 10 cSt (centi-Stokes) at 40°C are used for

air-line lubrication.

It should be noted that, when compressed air is vented through the exhaust

ports of valves and motors, it carries with it a proportion of the added

lubricating oil. Over recent years there have been reports of this exhaust air,

with its oil mist, being inhaled by personnel working in close proximity to

pneumatic equipment and air tools. For this reason, manufacturers of

pneumatic equipment have been developing a range of valves and actuators

that no longer need any form of air lubrication. Some items of pneumatic

equipment, such as pneumatic logic elements and air jet sensors, will

malfunction if oil-loaded air is used. Certain processes also require that the air

supplied be oil-free, a good example being spray painting.

With this in mind the equipment and the process should be closely investigated

to ascertain the level of lubrication required, and if the exhausting of lubricated

air will have any detrimental effects upon personnel, or other processes in the

vicinity.

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

...................................................................................................................................................

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Make a simple sketch of an in-line moisture separator and describe its

principle of operation.

2. List the reasons for the use of a large air-main air pressure-reducing valve

station.

3. What is a 'coalescing' filter?

4. What is meant by a filter's 'cut-off rate' and in what units is it measured?

5. What are the hazards associated with the use of oil-loaded air supplies?

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

...................................................................................................................................................

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Your sketch should be like that shown in FIGURE 2 on page 3. As the air

and entrained moisture enter the separator, the position of the internal

baffles causes the air to change direction. The heavier particles of

moisture cannot respond to this need to change direction as quickly as the

air and, as a consequence of this, will strike and be held by the baffle

plate. From the baffle plate the collected liquid will fall to the bottom of

the vessel, where it is removed by venting. The air continues on its

tortuous path through the separator, and leaves at the opposite end in a

drier condition.

2. A large pressure-reducing valve would be used in air systems where a

single production unit is being used to feed a number of installations,

workshops or sites, with air at different pressures.

In such situations, one alternative is to produce and distribute the air at

the highest pressure requirement, and use a pressure-reducing station at

the inlet to each user site as required.

3. A 'coalescing' filter is a high-performance filter. It will behave initially as

a conventional filter to remove solid contaminate, but will also cause

entrained moisture particles to coalesce or grow, making their removal

much easier.

4. A filter's 'cut-off rate' refers to the smallest particle that, in theory, will be

caught by a clean filter element. Cut-off rates are measured in microns

(millionths of a metre).

5. Exhausted compressed air containing lubricating oil can be harmful to

personnel if inhaled. It can also have a contaminating effect on certain

factory processes such as spray painting or food production.

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

We have here dealt with the items of equipment included in a compressed-air

distribution system, whose function is to ensure that the air delivered is in a

suitable condition, for use by the pneumatic equipment connected to the

system.

You should now appreciate the need for pressure reduction, and know that air

supplied at too high a pressure can result in compressed air wastage and

increased component wear, without any increase in work output.

The importance of the removal of contaminates, both solid and liquid, has also

been stressed. We have also looked in detail at the use of filter, regulator and

lubricator units, and the part they play in providing a compressed-air supply of

a suitable quality.

Teesside University Open Learning (Engineering)

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setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice