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MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : SPECIFICATION, SELECTION AND

MAINTENANCE OF EQUIPMENT

LESSON 5 : MAINTENANCE AND SAFETY REQUIREMENTS OF

PNEUMATIC EQUIPMENT

APH - 3 - 5

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

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

All plant equipment requires maintenance in order to give better service and

efficient operation. The degree and type of maintenance required is dependent

upon many factors, which include:

• the type of equipment to be maintained

• the importance of the equipment to the running of the plant

• the kind of duty undertaken

• manufacturer's specific recommendations.

These are only a few of the considerations. In this lesson we will look at the

general maintenance requirements of air compressors, associated equipment

and air distribution systems.

________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

On completion of this lesson you should be able to:

• explain the advantages to be gained from maintaining a system log

• appreciate the maintenance requirements of the component parts of a

compressed air production plant

• understand the basic maintenance requirements for components used

in a distribution system.

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________________________________________________________________________________________

COMPRESSOR MAINTENANCE ________________________________________________________________________________________

Before starting any maintenance work on the compressor unit, it is essential to

be fully aware of the specific needs of the machine to be maintained. This

information will be available in the machine manufacturer's installation and

maintenance handbook which should be studied in detail and fully understood

before initiating a planned preventative maintenance programme for the

machine. It is, however, of great importance that other information in addition

to that provided by the maker is collected and continually updated.

It is essential that the engineer in charge of the compressor plant should

oversee the recording of such things as air pressures and temperatures, oil

pressures and temperatures, oil consumption and levels of vibration during

normal running. These records should be started when the machine is first

installed and properly run in, and they should be constantly updated so that a

history record for the machine is established. This history will assist in the

spotting of reductions in machine performance and reliability and allow the

maintenance team to initiate corrective action before the fault can develop into

a major problem.

How frequently do you think such readings should be taken?

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

________________________________________________________________________________________

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Usually it should be on a daily or shift basis for pressure and temperature readings.

Recording of vibration levels or oil sampling should be done less frequently, for example,

weekly or monthly intervals dependant upon compressor usage and local conditions.

The charts shown in FIGURES 1 and 2 are examples of maintenance schedules

for both reciprocating piston and rotary machines. They give only general

information which, though it may be useful in establishing a maintenance

programme for a machine of unknown origin and history, should not be used to

replace the manufacturer's specific maintenance schedule for the machine if it

is available.

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FIG. 1 Typical Maintenance Schedule for Reciprocating Compressor

Check oil and air pressures and gauge cock settings

Check setting of mech. lubricator

Clean air inlet filter

Clean, inspect valves

Lift water jacket and air relief valve

Drain and renew crankcase oil

Change fusible plugs

Clean coolers

Clean oil filter, check oil levels in crankcase and crankcase and mech. lub.

Check water pump

Check water level in radiator

Check water pump gland for leakage. Tighten if necessary

Check operation of signal devices

Check tension of all drive belts

Unloaders check operation

Grease brgs. on Electrical motor etc.

Running-in period

10 hrs

20 hrs

50 hrs

Every 2

wks or 50 hrs

Every mth or

150 hrs

Every 3

mths or

250 hrs

Every 6

mths or

500 hrs

Every yr or

1000 hrs

Every 2

yrs or

2000 hrs

Every 4

yrs or

4000 hrs

During normal serviceFrequency

Task

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FIG. 2 Typical Maintenance Schedule for Rotary Compressor

Frequency

Task

Check air pressures and gauge cock settings

Check oil injection

Clean air inlet filter

Lift air and water relief valve

Drain and renew oil

Change fusible plugs

Clean oil separators

Check oil leads

Check water pump

Check water pump gland

Check signal device operation

Check unloader operation

Grease brgs. on Elec. motor, etc.

Check blades or rotor sealing

Check shaft seals

Renew all bearings

Check bplgs., gears, etc.,

Check NRV

Running-in period

5 hrs

20 hrs

Every 2

wks or 50 hrs

Every 3

mths or

250 hrs

Every 6

mths or

500 hrs

Every 1 yr or

1000 hrs

Every 2

yrs or

2000 hrs

Every 4

yrs or

4000 hrs

Every 8

yrs or

8000 hrs

During normal service

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ROTARY VANE COMPRESSORS

The maintenance requirements of rotary vane compressors are relatively

straightforward. They will consist mainly of keeping the lubricating oil,

necessary for the efficient operation of the machine, suitably clean and at the

correct operational temperature.

The rotor will also require periodic examination, to ensure that the vanes are

free to slide and that no excessive wear has taken place. If the vanes show

signs of excessive wear on their outer tips, then the bore in which they run

should be examined.

Many compressors of this type use plastic composite materials for the vanes:

these should be examined closely for signs of deterioration. Routine checks

should also be made on the condition of the shaft seals, whose integrity is of

prime importance for the efficient operation of the machine.

ROTARY SCREW COMPRESSORS

The maintenance requirements of the rotary screw design compressor are

similar to the vane type with regard to maintaining oil in a suitable condition

and carrying out regular inspection of shaft seals.

Providing the installation of the machine is correct, to ensure the absence of

any pipe strains from the unit and satisfactory cooling, both aimed at

minimising casing distortion, the unit should run relatively trouble-free with

little maintenance. If casing distortion is present, this will result in increased

internal wear and the efficiency of the machine will drop rapidly.

FIGURES 3 and 4 show trouble-shooting charts for reciprocating and rotary

design compressors. These can be used to highlight possible causes of

common compressor problems.

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FIG. 3 Trouble-Shooting Chart for Reciprocating Compressors

Air flow to fan blocked

Bearing worn

Condensate carry-over

Crankshaft end play-high

Discharge pressure high

Excessive oil

Foundation poor

Fuses blown

Gaskets leaking

Hz high

Intake filter fouled

Intercooler not drained

Location humid

Motor overload relay tripped

Motor small

NRU defective

Piston ring problem

Piston (cylinder) worn or damaged

Piston/head clearance small

Poor lubrication

Poor piston drain holes

Restricted discharge line

Rotation wrong

System leakage high

Unloader system wrong

Valves fouled

Valves not seated correctly

Valves worn or broken

Ventilation poor

Voltage low

Water quantity too low

Wrong oil

Possible causes

Faults

A p p a re

n t

lo w

o u tp

u t

C o m

p re

ss o r

fa il

s to

s ta

C o m

p re

ss o r

fa il

s to

u n lo

a d

C o m

p re

ss o r

n o is

C ra

n kc

a se

e m

u ls

if ic

a ti

o n

C ro

ss h

ea d m

/c r

o d /p

a ck

in g w

ea r

E xc

es si

ve c

a rb

o n

E xc

es si

ve c

o m

p re

ss o r

vi b ra

ti o n

F re

q u en

t st

a rt

in g

H ig

h a

ir d

is ch

a rg

e te

m p er

a tu

In te

rc o o le

r p re

ss u re

h ig

In te

rc o o le

r p re

ss u re

l o w

O il

p a ss

ov er

O ve

rh ea

ti n g

P ri

m e

m ov

er o

ve rl

o a d in

P is

to n /r

in g s/

b o re

w ea

Notes : (1) Runs unloaded for prolonged periods (2) Detergent type oil being used

(1)

(2)

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FIG. 4 Trouble-Shooting Chart for Rotary Compressors

Air flow to fan blocked

Bearing worn

Discharge pressure high

End Float (rotor)

Excessive oil

Foundation poor

Fuses blown

Gaskets leaking

Hz high

Intake filter fouled

Intercooler not drained

Motor overload relay tripped

Motor small

NRV defective

Poor lubrication

Restricted discharge line

Seals leaking

Unloader system wrong

Vane/bore wear

Vanes stuck

Ventilation poor

Voltage low

Water quantity too low

Wrong oil

Possible causes

Faults

A p p a re

n t

lo w

o u tp

u t

C o m

p re

ss o r

fa il

s to

s ta

C o m

p re

ss o r

fa il

s to

u n lo

a d

C o m

p re

ss o r

n o is

E xc

es si

ve c

a rb

o n

E xc

es si

ve c

o m

p re

ss o r

vi b ra

ti o n

F re

q u en

t st

a rt

in g

H ig

h a

ir d

is ch

a rg

e te

m p er

a tu

In te

rc o o o le

r p re

ss u re

h ig

In te

rc o o le

r p re

ss u re

l o w

O il

p a ss

ov er

O ve

rh ea

ti n g

P ri

m e

m ov

er o

ve rl

o a d in

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Using the fault-finding charts provided, list the possible causes of a low intercooler

pressure on a rotary vane compressor installation.

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

________________________________________________________________________________________

The possible causes listed on the chart are:

• fouled intake filter

• defective unloader system

• vanes stuck.

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________________________________________________________________________________________

MAINTENANCE AND SAFETY COMPRESSOR ANCILLARY EQUIPMENT ________________________________________________________________________________________

Before starting any maintenance procedure on the installation it is essential

that all power supplies to the machine are completely isolated, and that the

machine is also isolated from the compressed-air system. If the machine is a

water-cooled model then it will also be necessary to isolate the cooling water

supply.

The areas that are most frequently checked are those that could cause

catastrophic failure of the machine or a massive reduction in compressor

performance if neglected.

For example, oil level: the oil level in the machine sump should be checked

every day, before starting the machine if possible. If the level has fallen then it

should be filled to the correct level with the correct grade of lubricant. In some

machines, however, the oil level may actually rise due to the accumulation of

condensed water. If this is the case, this water should be drained from the

sump via the drain cock, and the machine then topped up to the correct level

using the correct grade of lubricant.

Periodically, it will become necessary to change the oil in the machine. This is

due to contamination and degradation of the oil, which occurs during operation

of the machine. This will have the effect of reducing the oil's ability to provide

the lubricity required; also the amount of suspended solids carried by the oil

can have a serious effect on the effective working life of the compressor's

internal parts, i.e. bearings, journals, vanes, cam-rings and so on.

The rate at which the oil degrades or becomes contaminated, will vary from

machine to machine and depend upon local factors. It is therefore very

difficult to predict exactly when the oil will require changing: in their

recommendations machine manufacturers will always err on the side caution;

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but situations could occur when following the maker's recommended change

intervals could lead to premature failure of equipment. It is far better to

initiate a programme of oil sampling and testing, to try to establish exactly

when the oil becomes unsuitable, and to change the oil then.

Oil companies do provide an oil analysis service which is available to their

customers. Sampling kits are also available to the engineer with which he can

carry out simple tests to measure the degree of contamination due to solids and

water. These tests are usually quite simple and, if the correct sampling

procedures are followed, are quite accurate. This will provide the maintenance

engineer with a clearer picture, regarding the state of the lubricant in use, and

should result in oil changes taking place at the optimum times.

What are the local factors that you think can affect the working life of the compressor

lubricant?

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

________________________________________________________________________________________

Local factors which affect the working life of the lubricants are ambient temperature,

efficiency of the compressor's cooling system, amount of moisture being drawn into the

machine, the degree and efficiency of the oil filtration unit, all of which will affect the rate

of degradation of the oil.

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CONDENSATE DRAINS

The condensate drains are responsible for the removal of the precipitated

moisture: they will require regular attention if the consequences of a build-up

in moisture in the system are to be avoided. The drains likely to require

regular attention are those fitted between the stages of compression and on the

final separator. Automatic drains should be monitored for regular and correct

operation. Manual drains should be operated at frequent intervals, as indicated

by manufacturer's literature, or more frequently if necessary.

PRESSURE GAUGES

As part of an operation monitoring programme, the habit of recording air-

pressure gauge readings will prove to be invaluable in assessing machine

condition. If it is also possible, as part of the routine maintenance procedure,

to time how long it takes the compressor to reach full operating pressure

without demand from the factory, this information can be used to give an early

indication of a reduction in compressor performance, and once again allow

appropriate action to be taken.

In multi-stage machines readings taken of the various stage pressures will also

give advanced warning of problems developing, such as faulty valve operation.

The gauges themselves, however, should be regularly checked for accuracy, to

ensure that information being recorded is a true reflection of what is actually

happening in the system.

What do you think are the consequences of allowing condensate to build up in the

intercooler and moisture separator?

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

________________________________________________________________________________________

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The consequences of allowing such a build-up in the intercooler would be the carry-over of

moisture into the next stage of compression resulting in damage to the compressor's internal

parts. If condensate is allowed to build up in the moisture separator, it will eventually be

carried over into the system receiver and, if not removed from there, will find its way into

the distribution system, increasing component failure rates and machinery downtime.

TEMPERATURE READINGS

Although it is not always possible on all machines to record operating

temperatures, they can be invaluable as an indicator of impending problems.

Cooling water, air and oil temperatures should all be monitored: a significant

rise in the temperature of any of these elements, not due to an increase in

ambient conditions, should be investigated immediately.

All of the previous functions should be monitored daily and logged as a matter

of course. The taking of the readings and maintenance of oil level need not be

a maintenance engineer's job, but may well be part of the process or production

staff responsibility. However, the analysis of the recorded data is the

responsibility of the maintenance team: this information should be scanned

daily for nonconformance with base-line data.

Component parts of the compressed-air production unit that will require the

most frequent attention are the following.

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SUCTION FILTER

Suction filters, if correctly selected and in good condition, will impose a

minimal restriction on the flow of air into the machine. With most types it is

necessary at frequent intervals to change the filter element, or clean it in

accordance with manufacturer's instructions.

A choked filter causes a vacuum in the compressor inlet, which reduces the

machine output and affects the working temperatures. In extreme cases it has

been known for the choked element to be actually drawn into the compressor

resulting in a major breakdown.

The life of a filter depends upon the amount of dirt in the surrounding

atmosphere: this can only be determined by actual experience. A gauge, fitted

after the filter and measuring suction pressure, will act as a guide to the

increase of air restriction due to filter choking, and provide an indication when

to change or clean the filter.

COMPRESSOR VALVES

All modern piston compressors use automatic valves which operate on a low

pressure differential. FIGURE 5 shows an exploded view of a Hoerbiger type

which is by far the most common type used.

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FIG. 5 Hoerbiger Automatic Valve

It is usual for the suction valve to have a lower spring pressure than the

delivery valve. It is therefore very important that, whenever the valves are

stripped for maintenance or cleansing, none of the springs are interchanged

and, if new springs or valve plates are drawn from spares, they are installed in

the correct valve.

Castle nut

Split pinNut washer

Buffer plate

Valve plate

Guide washer

Spring plates

Damper plate

Centre bolt

Locating pin

Valve seat

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Valve failures occur due to wear and fatigue experienced during compressor

operation. If the frequency of failure increases this could be an indication of

incorrect operation, for example, overloading the machine leading to

overheating.

Failures will also occur due to the presence of particulate contamination which

can either be solid or liquid. If the particles are solid they are quite readily

detected, as they will usually affect other running parts.

If the contamination is in liquid form, this will usually be as a result of water

being drawn into the compressor, or, in the case of non oil-free machines, is

caused by excessive oil coming past worn pistons or rings.

Valves will also suffer from carbon deposits from the oil, causing valve wear

and resulting in breakage. This will tend to affect the delivery valves far more

than the suction valves, and be far more prevalent in oil-loaded machines than

oil-free designs.

Valves should be removed, dismantled and inspected at frequent intervals,

usually as recommended by the machine manufacturer. Damaged and worn

parts should be replaced and the valves thoroughly cleaned. Valve seats should

be lightly lapped on a smooth flat surface, to ensure good sealing. The valves

should then be re-assembled and checked for correct operation by pushing the

valve open, usually with a suitable small piece of wood.

The process of stripping and cleaning the compressor valves can take quite a

long time, especially if the machine is a multi-stage double-acting design.

This can mean that the compressor is out of action for a long period of time.

When this amount of machine downtime cannot be tolerated, it is usual to

purchase a complete set of valves as spares: this will allow one set of valves to

be refurbished, whilst the other set is being used in the compressor.

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Why do you think delivery valves are subject to a greater degree of carbon deposition

than suction valves?

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

________________________________________________________________________________________

The reason for delivery valves having a larger build-up of deposited carbon than suction

valves is that the air passing through a delivery valve will be much hotter than the air

passing through a suction valve. If this air is contaminated with lubricating oil, then carbon

will be deposited on both sides of the valve. However, on the suction valve, carbon will

only build up on its cylinder side.

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INTERCOOLERS

Intercoolers, as discussed in earlier text, can be either water-cooled or of the

fan-assisted air-cooled (air-blast) design. Usually they will provide very few

problems, but it must be borne in mind that they are classed as pressure vessels

and as such are subject to legislation with regard to their operation,

maintenance, inspection and testing.

The current legislation, 'Pressure Equipment Regulations 2002', came into

force on 30th May 2002.

The regulations require users of pressure systems to:

• establish the safe operating limits of the plant

• have a suitable written scheme drawn up or certified by a competent

person, for the examination at regular intervals of most pressure vessels,

all safety devices, and any pipework which is potentially dangerous. The

user may seek the advice of any competent person when deciding what

vessels and parts of the pipework need to be included in the scheme

• arrange to have examinations carried out by a competent person at the

intervals set down by the scheme

• provide adequate operating instructions, to ensure the plant is operated

within its safe operating limits, and emergency instructions

• ensure that the plant is properly maintained

• keep adequate records of the most recent examinations and any

manufacturer's records supplied with the new plant.

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Note that these are the basic requirements of the Regulations; further

information can be obtained from your local Health and Safety Executive

office.

As stated previously, intercoolers should provide few maintenance problems; it

is, however, extremely important for the efficient operation of the machine,

that all heat transfer surfaces are kept clean. On the water-cooled design, this

will entail preventing the water-side of the tubes from becoming fouled and, on

the air blast cooler, preventing the build-up of dirt and foreign matter from

restricting the air flow over the finned tube bank.

On the air side of the intercooler, if the machine is correctly cooled and there

have been no excessive temperatures due to valve failures, the insides of the

tubes will be relatively clean with only a smear of oil present. If excessive

temperatures have been experienced, this can lead to heavy build-up of carbon

at the entrance to the intercooler, which can sometimes interfere with the

operation of the intercooler safety relief-valve. It is therefore of prime

importance that the cause of this overheating be identified and removed.

Condensate draining from the intercooler is also very important. This is

usually achieved using some form of automatic drain trap, which should be

regularly inspected for correct and efficient operation. If a manual valve is

used to remove the condensate, then the valve should be operated at intervals

that will prevent the build-up of excessive amounts of condensate in the

intercooler.

What do you think would be the consequences of a heavy carbon deposit interfering

with the operation of the intercooler relief valve?

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

________________________________________________________________________________________

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If the carbon deposit built up around the valve seat, it could have the effect of holding the

valve closed, delaying its lift pressure, and causing the intercooler to be overpressured. If,

however, the valve did operate at its preset pressure, the build-up of carbon around the seat

could prevent the valve from closing properly, resulting in constant leakage from the

system.

AFTERCOOLER

The previous information given for the intercooler is in the main applicable to

the aftercooler also. Once again the device is classed as a pressure vessel,

fitted with a safety relief valve, and subject to the current legislation governing

the operation of such equipment.

However, since the pressures dealt with are higher and normally the inlet

temperatures are also higher, the cleanliness of this cooler is more critical from

a safety point of view. It should be noted, however, that this cooler has no

bearing on the compressor performance, since its action takes place after all

the work has been done on the air. Its sole function is to bring the delivery

temperature of the air down to a reasonable level and, in so doing, allow the

further removal of condensate from the air.

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AIR RECEIVER

A compressor plant is normally provided with one or more air receivers, which

are sized to suit the compressor capacity, the compressor capacity control

method, and the expected variation of air consumption. The function of the

receiver is to store the compressed air, increase the cooling, collect residual

condensate, and equalise pressure variations in the pipe network.

Air receivers are subject to legislation regarding the operation of pressure

systems. The following equipment is deemed necessary for the safe operation

of the unit, and therefore must be fitted and maintained to operate with

maximum efficiency:

• safety valve

• gauge test cock

• designation and inspection plates

• pressure gauge

• drain valve or cock.

Normal running maintenance checks for air receivers, other than the statutory

checks covered by legislation, should consist of:

• regular functional checks on all safety controls.

• regular visual inspection, looking for early signs of trouble such as leaks,

discolouration of the vessel, or unusual sounds emitted during operation.

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With reference to current legislation what records need to be kept regarding the

pressure vessels included in the pressure system?

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

________________________________________________________________________________________

For new plant, current legislation will require records to be kept, which detail the scheme to

indicate which equipment is to be examined, and at what intervals these examinations need

to take place. The latest examination reports need to be kept, along with any test and

examination reports from the manufacturer or supplier of the equipment.

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________________________________________________________________________________________

LEAKAGE IN COMPRESSED AIR SYSTEMS ________________________________________________________________________________________

Most compressed-air systems have some degree of leakage: it is not normally

financially viable to maintain a 100% leak-free system. It is common practice

to tolerate small amounts of leakage in all but the smallest systems; but as

leakage approaches 10% of the total consumption, remedial action should be

taken. Obviously if a lower rate of leakage can be maintained, the system will

be more efficient.

Unfortunately many systems run with a higher percentage of leakage: it would

appear there are two main reasons for the acceptance of this:

• the leakage is a clean process, that is to say the leaking substance (air) is

not toxic or liquid and therefore is not easily noticed

• the attitude "it's only air" often prevails. If the true cost of the escaping

energy were fully understood, remedial action would soon be undertaken.

Consider the fact that often the largest electric motors on a plant are those

being used to drive compressors. A typical medium-sized manufacturing plant

would run 3 compressors, each driven by a 100 kW motor. If the plant has a

10% leakage rate, this means that at any time the power consumed to maintain

those leaks is:

Many plants are run continuously 7 days a week adding up to a staggering

7 24 30 5040× × = kW hr

3 100 10 30× × =% kW

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This calculation assumes all compressors are on-load together all the time.

This may not be the case, but it illustrates the fact that compressed-air leakage

is very expensive and should be kept to an absolute minimum.

If a distribution system has a 10% leakage rate, what effect will this have on the

velocity of the air, and hence the pressure drop in the system, and why do you think

this is so?

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

________________________________________________________________________________________

The velocity of the air will increase and so will the pressure drop. The reason for this is that

the compressor must produce 10% more air than would otherwise be required which means

the flowrate (Q) within the system also increases by 10%.

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LEAKAGE RATE TESTING

It should now be apparent that some means of monitoring the rate of leakage

within a distribution system is of extreme importance. The best way to do this

is by including, as part of a preventative maintenance programme, regular

leakage rate testing. The results of these tests are used to detect any trend

towards increased leakage, or to assess the degree of improvement obtained by

remedial action. From this financial benefits may be calculated.

METHOD OF TESTING

When the plant is idle and all cooling jets etc. are turned off the system is

pressurised: thereafter any air consumed will be leakage.

The test procedure is as follows:

• a figure for the compressor delivery is obtained: Q (FAD)

• the compressor is timed between "cut in" and "cut out" values: t1 • the compressor is timed between "cut out" and "cut in" values: t2.

The cut-in and cut-out values are typically 7 to 8 bar up and 8 to 7 bar back

down: these pressure values are not important to the calculation.

The amount of leakage can be obtained using the following calculation:

leakage working time delivery

total time

leak

= ×

aage = × +( )

t Q

t t 1

1 2

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FIGURE 6 illustrates the timing procedure between cut-in and cut-out values.

FIG. 6 Timing for Leakage Test

SPECIMEN TEST

The following values were obtained during a leakage rate test on a distribution

system. From them calculate the rate of leakage and express it as a percentage

of total air consumption.

Compressor capacity Q = 10 m3 min–1 FAD

t Q

t t

3 2

9 7

= × +

mins

leakage rate 22

3 2 10 3 2 9 7

2 48

( ) = ×

+( )

. . .

.leakage rate m mi3 nn–1

t2t1

Cut out 8 bar

Cut in 7 bar

time

Down

U p

Pressure

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If we now consider this as a percentage of the total consumption.

This would be an unacceptable value for leakage: remedial action would have

to be taken as quickly as possible.

The problems associated with locating the sources of leakage can be fairly

simple or extremely complex, depending upon the type of plant concerned.

Outline potential sources of leakage and suggest methods which may be employed in

identifying their location.

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

________________________________________________________________________________________

2 48 100 10

24 8 . %

. % ×

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Potential sources for leakage include:

• pipe joints and fittings

• valves

• flexible connections

• water traps

• machine components

• leaking cylinder seals.

The obvious methods, such as looking and listening for leaks, may be employed. Also, the

testing of joints with soapy solution is very effective. A good practice is to encourage plant

operators to be diligent and report leakage as it occurs.

The following values were obtained during a leakage-rate test on a distribution system.

Compressor delivery:

Using these values calculate the rate of leakage (free air) and express it as a percentage

of the total consumption.

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

________________________________________________________________________________________

Q t t

= 15 m min FAD = 2.1 mins = 5.7 mins

3 –1

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Collection and recording of information from leakage-rate tests is best

undertaken with the use of some form of documentation. Data collection

sheets outlining the procedure are a good method of ensuring that the correct

information is recorded: the calculation can be performed on the sheet and

other compressor information may be included.

A typical data collection sheet is shown in TABLE 1, which may be used as a

model for the design of sheets related to specific equipment.

leakage rate

lea

= × +( ) =

× +( )

t Q

t t 1

1 2

2 1 15 2 1 5 7

. . .

kkage rate m min

As a percentage

3 –1= 4 038.

Leakage of the to

4 038 100 15

26 9

. % . %

. %

× =

= ttal consumption

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TABLE 1 Specimen compressor and leakage-rate data collection sheet

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Compressor 3

Number

Flow-rate 10 m3 min–1

(Q) FAD

Date 2.8.06

Charge Time 2.1

t1 mins

Discharge Time 7.6

t2 mins

Cut-in 7 bar

Pressure p1 Cut-out

8.1 bar Pressure p2 Ambient

18°C Temp.

Intercooler 28°C

Temp.

Aftercooler 24°C

Temp.

Oil temp. 50°C

2.16 m3 min–1

FAD

Observations/ Compressor Comments area untidy

Signed PW

Leakage rate

Q t

t t

× +( )

1 2

________________________________________________________________________________________

MAINTENANCE OF THE PIPEWORK SYSTEM ________________________________________________________________________________________

The pipework system has very few requirements for maintenance within a

typical plant. Some of the factors to consider are outlined below.

• Maintaining the exterior of the system by painting, giving particular

attention to sections exposed to the elements. Sections which pass

through areas of plant which may have corrosive fumes must also be

given particular attention.

• Inspection of the pipework supports to ensure that no sagging occurs.

• Checking joints fittings and flanges for corrosion and leakage.

• Sections of pipework, which are suspected of internal corrosion and

partial blocking, may be opened up and inspected during plant shutdown

periods.

• Attention should be given to water drain legs to ensure they are not

blocked or excessively corroded.

MOISTURE SEPARATORS

Maintenance requirements for these are normally limited to cleaning out and

inspection on an annual basis, or in accordance with manufacturer's

recommendations. It is important that the outlet from separators to the drain

trap is kept clear as it is a potential blockage point.

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WATER TRAPS

All types of traps should be checked for air leaks at the water outlet: if any air

leakage is detected, it is a sign that the trap is damaged, and a repair should be

undertaken.

With ball-float traps fitted with a blowdown valve, any water found during a

test blowdown would indicate an inoperative trap, and it should be

investigated. Typical problems are punctured floats, sticking mechanisms due

to high water/oil emulsion content, and damaged valve seats.

It is easier to observe how blast-type traps operate and ascertain whether they

operate correctly or not.

Good practice is to carry spare traps of each type, and to instigate a replace-

and-repair programme. Where suspect traps are identified, remove and replace

with either a new or re-conditioned unit, and overhaul the suspect trap.

AERODYNAMIC TRAPS

These traps normally have filters installed prior to the air and water entering

the disc and seat area. It is essential that the cleaning and replacement of these

filters is included in any maintenance programme. Blocking of the filter will

prevent water from being discharged, causing it to be carried further into the

system. The traps themselves are very robust, the only wearing parts being the

disc face and its seat.

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SAFETY VALVES

These valves are installed to prevent over-pressurisation of the system. They

must comply with current legislation with regard to their being registered,

tested and maintained at the prescribed intervals.

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________________________________________________________________________________________

PRESSURE SYSTEM REGULATORS ________________________________________________________________________________________

The Pressure Equipment Regulations cover systems as a whole, including

pressure vessels, pipework, protective devices and other pressure-containing

components.

The Regulations are designed to guarantee the integrity of all pressure systems

and the components used in them. It is essential that anyone setting up a

compressed-air maintenance programme should obtain a copy of the

Regulations for guidance.

It should be noted that there are other regulations, pertaining to the use and

maintenance of compressed air systems:

COSHH REGULATIONS

The Control of Substances Hazardous to Health Regulations demand that

employers identify and assess the risk to employees from a wide range of

substances, and then limit the exposure to them, by protective clothing and

respiratory equipment, if all other avenues are closed.

NOISE AT WORK

The Noise at Work Regulations recognize that permanent damage to hearing

can result from exposure to excessive noise levels, including those from

exhausting compressed air. Employers are required to monitor noise exposure

and to reduce noise below set levels.

The consequences of failing to acknowledge and comply with these

regulations may be heavy fines and imprisonment for up to two years.

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Outline two dangers to personnel in the vicinity of exhausting compressed air.

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

________________________________________________________________________________________

• The danger of high noise levels if it is unsilenced.

• The danger associated with breathing air which may be contaminated with oil or other

impurities.

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________________________________________________________________________________________

AIR SERVICE OR FILTER, REGULATOR, LUBRICATOR UNITS (FRLs) ________________________________________________________________________________________

This is the final line of defence a system has, but the units are often neglected.

Inclusion within the maintenance programme is essential. It is important that

manufacturer's information regarding maintenance of specific units is studied

wherever possible, but in its absence the following general guide-lines are

given.

The degree of frequency of maintenance will be dictated by the condition of

the air being supplied to the unit, and the duty to which it is subjected.

Obviously a unit, which is supplied with air heavily contaminated with

particles and water, will require the filter changing and the water trap cleaning

at more regular intervals, than one being supplied with relatively clean air. It

can be seen therefore that only local knowledge will dictate precise intervals

between services.

Typical routine maintenance should include the following:

• The Water Trap: if this is of the manual type it may require draining

several times a day, depending upon the air condition. Therefore it is

strongly recommended that auto-drain units are installed.

• The Auto Drain: requires visual inspection, checking for air leaks at the

water outlet, and ensuring that the discharge valve operates to

automatically drain the unit. Occasional dismantling and cleaning of the

bowl and float assembly will be required, to prevent sediment and

particles interfering with the operation of the float assembly and valve.

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• The Filter: must be checked regularly to ensure blocking does not occur.

Some filter types can be cleaned in suitable cleaning agents, and reverse-

blown-through with compressed air; others require replacement every

time. Manufacturer's recommendations should be observed. The

consequences of failing to maintain the filter are high pressure losses

downstream and reduced flow-rates.

• Regulators: do not normally pose many problems: the commonest fault

is puncturing of the diaphragm. This can be spotted by leakage from the

top of the unit, and the inability to regulate the downstream pressure.

Spare diaphragms should always be kept in stock: most manufacturers

supply complete service kits, which contain all necessary seals to

completely overhaul the valve.

• Lubricators: require a regular visual inspection to check on the oil level

and the drip rate under full-flow conditions. Other than this the only

other requirement is for cleaning of the bowl assembly when required,

and the topping up of the oil level. It is important when topping up oil

levels that the unit is isolated, and the air vented before removal of the

top-up plug. The correct type and grade of oil should always be used.

Note that polycarbonate filter and lubricator bowls should only be washed out

with a detergent solution; never use solvents.

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________________________________________________________________________________________

MAINTENANCE PLANNING ________________________________________________________________________________________

It is recommended that the maintenance of the compressed-air system and its

components is incorporated within the existing plant maintenance scheme.

There are many schemes in use, some using computer programmes, others

using simple card and chart methods. Owing to the variety of schemes being

used, it is impossible to give precise information regarding the maintenance

planning for any distribution system. However, we have included in

FIGURE 7 a schedule which illustrates some of the maintenance work which

should be undertaken. The frequency of this work will be dictated by many

factors: local knowledge and past experience within the plant concerned will

have a lot of bearing, as will the manufacturer's specific requirements.

There are special statutory requirements governing the correct operation and

maintenance of pressure systems: some are outlined in this lesson. These

requirements must be understood and satisfied.

FIGURE 7 opposite shows an example of a distribution system maintenance

plan.

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FIG. 7 Distribution System Maintenance Plan

Using these guide-lines, current legislation, manufacturer's recommendations

and local knowledge, you should be able to produce a basic maintenance plan

for a compressed-air distribution system.

Leakage rate test

Inspect system, visually check supports

Paint pipework

Inspect & test water traps

Test safety valve

Overhaul safety valve

Inspect & test auto-drains

Check filter elements

Top up lubricators

Complete service on FRLs

Clean out in-line separators

Hours Months 10 20 30 50 100 1 3 6 12 18 24 36 48 60

As per current legislation

SERVICE PERIOD

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. What factors would control the frequency of lubricating oil changes in a

rotary-vane compressor?

2. What are the effects on compressor operation associated with a choked

suction filter?

3. Explain the procedure to follow when undertaking a leakage-rate test on a

distribution system.

4. Give two reasons why a ball-float trap may fail to discharge moisture.

5. List three important considerations with regard to the maintenance of a

lubricator unit with a polycarbonate bowl.

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

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

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. The frequency of oil changes will be dependant upon a number of factors.

One factor will be the maker's recommendations, another factor will be

the effect that local conditions have on the lubricating oil. Regular

sampling and testing of the oil should very quickly dictate the correct

frequency of oil change.

2. The effects on compressor operation associated with a choked suction filter

are:

• apparent low output

• excessive carbon deposits

• high air-discharge temperature

• inter-cooler pressure low

• overheating.

3. The test is undertaken when the plant is idle and no air other than leakage

is being consumed. The compressor capacity (Q) in FAD is obtained.

The compressor is timed between cut-in and cut-out values (t1) and then

between cut-in and cut-out values (t2).

These values are used in the calculation for leakage

The value is normally expressed as a percentage of the total consumption.

leakage rate = × +( )

Q t

t t 1

1 2

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4. (i) The float may be punctured.

(ii) The mechanism may be jammed.

(iii) Air locked.

5. (i) The unit must be isolated and discharged bef or e any wor k

commences.

(ii) The bowl must only be cleaned with detergent, never solvents.

(iii) The drip rate must not be excessive.

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

In this lesson we have dealt with the maintenance requirements of a

compressed-air production plant and distribution system, and have stressed the

importance of keeping good maintenance records and carrying out regular

checks, in accordance with manufacturer's instructions.

The text also highlights the need to work safely and the importance of regular

leakage-rate testing.

The importance of complying with current legislation has been pointed out,

and reference sources are given where applicable.

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/NOR <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> /SVE <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> >> >> setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice