Marine vol 1 part 1

Should theunit be requested as such, but used on other duties, so that its running hoursbecome more prolonged, the highly stressed conditions in the machine willnecessitate frequent main

MARINE ENGINEERING PRACTICE

L STERLING, C.Eng., M.I.Mar.E.

THE INSTITUTE OF MARINE ENGINEERS

Published by The Institute of Marine Engineers

The Memorial Building

be addressed to The Institute of Marine Engineers

ISBN: 0 900976 06 3

Printed in the UK by

The Chameleon Press Limited, London SW18 4SG

1 PREAMBLE

It is intended to cover the subject in a very broad sense in that installationand selection of compressors will also be covered, since a unit incorrectlyinstalled will always have excessive maintenance requirements It is felt that

a review of the system and installation may readily pinpoint a fault external

to the machine, and the comments on selection may give a guidance that themachine is running in a too highly stressed condition for the duty for which

it is being used Some comments are also made on automation since themajority of marine compressors are now run by automatic systems

1

SELECTION 3

2.1.2 Kinematic Energy

Dynamic compression (usually rotary) imparts kinetic energy to thegases which is converted into pressure by means of a diffuser This group isusually met as fans, blowers; superchargers, condensers, ejectors etc in themarine field and is, therefore, not in the scope of this chapter

2.2 CHOICE OF CORRECT MACHINE

Selection is very important, since a machine which has been incorrectlyselected or has been used on duties other than that for which it was selected,will often cause endless maintenance problems Take for example a recipro-cating air compressor for engine starting Such a machine does not log agreat number of running hours per annum and usually only runs for shortperiods except on one or two occasions, such as when on passage through along, difficult, navigational stretch As such, the machine can, therefore, beselected in its maximum stress condition This normally denotes selecting atits maximum speed and, as such, the unit is perfectly satisfactory Should theunit be requested as such, but used on other duties, so that its running hoursbecome more prolonged, the highly stressed conditions in the machine willnecessitate frequent maintenance and sometimes cause major breakdowns.Even when the duty is for starting air only, the class of vessel has somebearing on its selection, since a starting air compressor on a short voyage,such as a rapid turn round ferry, is in much more constant use and, therefore,must be selected in a lower stressed condition A control air compressor is

at the other end of the scale and is selected for a 24 hour per day continuoususe It is also selected so that the stress conditions are low for the type ofmachine in question Perhaps one of the most abused compressors is theso-called "topping up" compressor which, because of general air usage fromthe receivers and system leakages, often becomes a 24 hour per day categorycompressor, due to its capacity being requested too low, and yet its selection

is often based on a compressor which does little running

The examples have all been given on reciprocating compressors sincethe rotary air compressor is, in the majority of cases, on a 24 hour per dayduty, and mis-selection does not, therefore, often apply

2.3 WHAT CLASS OF COMPRESSOR?

2.3.1 Rotary or Reciprocating

There is no question that above 7 bars the reciprocating machine, withits more positive sealing, is the only correct selection With the reciprocatingmachine, nature's limitation is the temperature caused by compression andthis allows the compression ratio to be up to 7:1 in each following stage, withintercooling between stages Thus a starting air pressure of say 35 bar gaugecan be readily attained with a two stage machine

The rotary machine is limited by gas slip past its seals and a differential

of about 7 bars per stage is about the limit Thus a six stage machine would

be required for 35 bar gauge Comm~rcial considerations, therefore, hold itsusage to single stage (7 bars) applications but multi-staging would also bringtechnical problems in its wake with the shaft sealing

2.3.2 Oil Free and Non-Oil Free Air

Since the air drawn into both types of machine contains atmospheric

4 MARINE ENGINEERING PRACTICE

humidity, the compression ratio will always be sufficient to produce freemoisture and leave the compressed air 100'70 humid The moisture causescorrosion and, therefore, oil in the air, from a non-oil free machine, givesmore protection to the system and, with good machine maintenance, is thecorrect choice for all but "instrument air"

2.3.3. Instrument Air

Too little attention is paid to cleanliness and dryness of instrument air.There are two ways of achieving instrument air:

a) Low Pressure Machines (7-8 bars)

Air leaving a compressor is 100% humid and also has some freemoisture present The free moisture is easily removed by means offilters which must be of the ceramic type to ensure full removal forthis type of duty However, to achieve the necessary dryness anabsorbent type dryer is required to remove the moisture from the airand give the desired dryness factor

b) High Pressure Machines (25-35 bars)

Again the air is 100'70humid and some free moisture is present and,again, a ceramic type filter is necessary to ensure the free moisture isremoved However, in this instance, the absorbent dryer is notrequired because, by using a pressure reducing valve, the dew point

of the air at 7-8 bars will be at an acceptable level, see Fig 2 This

phenomenon needs a little explanation If air is at pressure and 100%humidity, and its pressure is then lowered to half, its volume will bedoubled Allowing the temperature to equalise, the moisture whichwas present now has to occupy twice the volume; the humidity is,therefore, halved This phenomenon can easily be observed in the

SELECTION 5 reverse direction by running a compressor at a low pressure and observing the small amount of moisture that is squeezed out of the air and then increasing the machine pressure and observing the increase in moisture.

c) Oil Removal

Removal of the oil is relatively simple to carry out The usual method being to have a pre-filter followed by a carbon absorber, which removes the oil, and usually followed again by a further after filter

to remove the remains of free moisture.

2.3.4 Air Cooled versus Water Cooled

Technically there is no difference in the two machines apart from the following:

a) Air Cooled Machines

No corrosion problems nor intercooler fouling problems to thing like the degree of water cooled machines (excepting closed circuit fresh water cooling machines) However, in general, they run much hotter and radiate a tremendous amount of heat into the areas where they are installed and, due to this, require special cooling air trunking Because of the hotter running of the machines, carboning problems are also higher.

any-b) Water Cooled Machines

More prone to crankcase condensation in very cold cooling water conditions, but car boning is far less than the air cooled machines and, in general, the noise level is lower, due to the deadening of the water jackets on the machine.

In general, therefore, it is preferable to have water cooled machines, except perhaps in the case of emergency machines where the extra heat input

to the surroundings would be more readily tolerated.

3 INSTALLATION

3.1 SITING

The siting of a marine compressor is not usually critical since the system pipe runs are relatively short and there is, therefore, no great necessity to site the compressor(s) at a mid-system point The greatest care must be taken so that the situation of the compressor is such that it draws in clean atmospheric air, unpolluted by oil, steam or gas leakages Even the picking up of extremely hot air is detrimental in that it reduces the compressor's output and increases the stage temperatures.

Air cooled compressors must be more carefully sited to ensure that sufficient air circulation can occur round the machine and, naturally, all machines must be sited for ease of maintenance On the larger machines it should be ensured that facilities for attaching lifting equipment above the compressor are available.

3.2 COMPRESSOR SEATINGS

All compressors must be installed on an adequately stiff mounting to reduce vibration which is detrimental to compressor life These remarks are aimed at the reciprocating compressor since this requires more sturdy foundations, due to the out of balance forces caused by the inertia masses.

It is important that the loaded area of the compressor is spread and taken by the main structural members of the vessel Fig 3 shows some examples.

3.3 RESILIENT MOUNTINGS

There are many versions of these but only one correct type; this is shown

in Fig. 4, where the mounting is completely resilient Installation of a compressor on such a mounting is not easy and, on the correct installation, the support base of the compressor will normally be constructed so that it can be filled with concrete grout to oppose the inertia forces of the new compressor Fig 5 shows some inertia forces of typical medium size starting air compressors and, as can be seen, it is necessary to put a grouting weight

of 200 kg into the base to satisfactorily oppose the inertia forces and keep the unit correctly mounted on the anti-vibration mounts An important point which is all too often forgotten is that such a mounting necessitates all machine connections being flexible with very low constraining forces on compressor movement.

3.4 AIR SYSTEM

Many people consider that the air pipework system of a compressor is

6

not critical in a design sense This, however, is a complete fallacy Fig 6shows a typical schematic installation which is true for all types of com-pressors and which is described below.

10 MARINE ENGINEERING PRACTICE

3.4.1 Pipeline Non-Return Valve

This valve is put directly on the discharge of a compressor for the reasonsgiven below:

a) Reciprocating Compressor Delivery Side Un/oader (Fig 7)

When the compressor unloads there is only the compressed air in the

INSTALLATION 11

body and coolers of the compressor, together with a very short pipework volume, which unloads to atmosphere This is not only economical, it prevents the air, which has been compressed into the pipeline, from rushing backwards and bringing with it moisture droplets, which sometimes settle on the valves and create malfunctions This inrush of air would occur if the non-return valve was placed any distance along the air-line If the compressor is fitted with automatic unloaders it is essential that automatic cooler drains are also fitted, otherwise the machine is not truly automatic and periodic manual draining of the coolers will be necessary.

b) Reciprocating Compressor Suction Valve Depression (Fig 8)

This should be fitted to the suction valve of each stage if the machine

is to be correctly unloaded To unload, the valve spider pushes against the spring pressure of the valve and puts the valve plate in a mid position so that the compressor inhales and exhales past the same draught A pipeline non-return valve is not strictly necessary with this unloading, since the final delivery valve of the compressor can fulfil this function, and a normal screwdown valve in the pipeline will enable any maintenance to be carried out A pipeline non-return valve is, however, recommended since it operates under much less onerous conditions and, therefore, requires much less attention than

a compressor delivery valve This avoids any accident with the compressed air pushed back on the machine when maintenance is carried out.

c) Rotary Machine

With the positive rotary machine the pipeline non-return valve is still

a necessity, otherwise air will leak past the internal seals when the unit is not in use and, in the worst case, would back rotate the machine The unloading of the machine is usually carried out by a recirculation method and from this point of view the position of the pipeline non-return valve is not critical However, the closer the valve is to the compressor, the less gas is lost on machine shutdown Blow off to the atmosphere can also be used as a control but is not

to be recommended A further method of throttling the intake is also not recommended since the compression ratio of the machine can be raised to such a point that the discharge temperature becomes excessive.

3.4.2 Condensate Pot

A condensate pot should always be so placed that the delivery line from the compressor turns immediately vertically downwards after the non-return valve and into the condensate pot The condensate pot must be fitted with automatic drain trap This unit prevents any flooding of condensate back on

to the compressor, especially when it is under a standby condition, with another machine on the same system running Sometimes the condensate pot has an inbuilt non-return valve and moisture separator.

3.4.3 Moisture Separators

These units must always be fitted in the system and can be of several types but, in general, work more efficiently where the air is at its coolest.

a) Vortex Type

This unit forces the air into a vortex motion so that centrifugal forcesthrow the free moisture clear to where it is trapped and then drained.preferably via an automatic drain

INSTALLATION 13

b) Directional Change Type

This type works by changing the direction of the flow at least once through 180 0 but sometimes more often It also reduces the upward velocity of the air to a very low speed so that moisture droplets are carried to the bottom of the vessel due to the directional change.

c) Coagulating Type

In general these units run most efficiently but require much more attention since they foul up more rapidly There are two types: one a mesh type which coagulates the moisture on its strands and the other

a porous ceramic type unit which coagulates around its pores The latter is more efficient but should usually be preceded by a coarser type of separator.

3.4.4 Safety Device

In any air system a safety device· should be placed between any two points where air can be trapped either between any two valves which can be closed manually, or between a non-return valve and a screwdown valve The cheapest form of protection is a fusible plug, but bursting discs and relief valves can also be used The precaution is simply to prevent the pressure rising sufficiently to burst the section of the system "Locked off" in the event

of a fire occurring Where CO 2 is used for fire extinguishing the discharge from the fusible plug should be piped to the open deck.

3.4.5 Air Receivers

The sizing of the air receivers for starting air usage is well defined and, being relatively large, do not cause any problems Where separate air receivers are used for instrument or general service air, these are often of too small a capacity, causing the compressor to come on and off load too frequently, or

if the unit is on a stop/start operation, to stop and start too frequently This latter feature is very critical On a marine vessel the air receiver should be so sized that a continuous running compressor does not come on and off load for periods under five minutes and for compressors on stop/start the periods should be at least 20 minutes.

The following formula is of sufficient accuracy to establish the correct receiver size:

It goes without saying that all air receivers must be equipped with a safety valve which can handle the full flow of air with not more than a 10% rise in pressure It is also advisable to have the fusible plug, or plugs, of sufficient size to carry out the same function should a fire occur, which might create a problem with the relief valve

The receiver is a very efficient moisture separator and, therefore, its internals should be examined at least every 12 months and re-coated, if necessary It is well worthwhile paying extra for a receiver with a sufficiently large manhole aperture to ensure correct inspection and coating rather than use a receiver with a small handhole/inspection cover.

14 MARINE ENGINEERING PRACTICE

All receivers should be kept in a well drained condition and, in theauthor's opinion, should be fitted with automatic drains and a manualshut-off valve to prevent a low loss of air which can occur from theseautomatic drains when the receiver is isolated as a standby reservoir on avoyage This manual valve will prevent a slow drain unbeknown to crewmembers The arrangement of the inlet and outlet from the receiver is alsoimportant so that advantage of the receiver as a moisture separator is taken.Fig 9 shows how the inlet and outlet should always be widely separated

3.5 COOLING WATER SUPPLY

3.5.1 Direct Salt Water Cooling

The following remarks apply to direct salt water cooled machines, orwhen salt water is supplied to an individual heat exchanger on a compressor

In general, the water pipeline velocities should be kept at a maximum of2·5 metres per second Short sections may, however, have higher velocities.Should more than one compressor be connected to the pipeline, each com-pressor should have its own water cin;ulating pump, to ensure the correctsupply of cooling water, and this is even more important if other units areconnected to the same water mains It is also often anticipated that only one

or the other of two compressors will run at any time, and the pipework sizedfor the flow to only one compressor This is not correct, since there is always

an unanticipated usage where both machines will be running The pipework

INSTALLATION 15should, therefore, be sized for flow to all units as if they would be runningsimultaneously, and only the sections which are directly connected with theunits sized down to the required "single" capacity.

Fig 10 shows a badly designed water system, which, however, isfrequently installed in ships but when fitted with the correct size of pipeworkand water pumps on the compressors, will nullify the system defects If thewater pump on the compressor is of the positive displacement type it acts as

an automatic water shut-off valve, should it be drawing from a pressurizedwater main Fig II shows a very simple air operated water shut-off valve,where the first stage air of the compressor, or final stage air, if single stage,can be connected direct to the cylinder of the valve When the compressor isstopped there is no air pressure present but as the compressor comes on loadthe air pressure increases to a sufficient pressure to force the valve openagainst the return spring and the cooling water pressure When the com-pressor stops the air pressure is lost and the spring closes the valve seat, andsealing is assisted by the water pressure Solenoid valves can be used for thesame action but, in general, are not quite so reliable, and costs are usuallyhigher

3.5.2 Fresh Water Cooling

The velocity of fresh water can usually be permitted up to 3,5 metresper second without causing any pipework problems The fresh water coolingtemperature for a compressor should normally be as low as possible andcertainly preferably below 38°C (100°F) When fresh water cooling is used it

16 MARINE ENGINEERING PRACTICE

is best that each compressor has its own individual heat exchanger since thisgives facility of maintenance much more readily, and also ensures correctcooling for each compressor The fresh water can either be provided by aseparate individual system or taken from the main or auxiliary enginesystems, as shown schematically in Figs 12(a)and 12(b).It is not as economical

to use cooling water from the engine system, for the reasons given below

In the following calculations the b.h.p of the compressor is used sincethe bulk of the heat from the compressor is rejected to the cooling water asheat, as is also the heat from the friction of the pistons and rings, although the

Self-contained Fresh Water Circuit Fig 12(a).

Heat to be removed via heat exchanger = maximum h.p absorbed x10'7 k caI.jmin X maximum fresh water cooling flow rate

Assuming (I) Maximum sea water temperature of 32°C;

(2) Fresh water temperature at cooler outlet not to exceed 40°C

Main or Auxiliary Engine Fresh Water Tap-off Fig 12(b).

In this arrangement the compressor heat exchanger cooling water inlettemperature is about 65°C compared with a maximum of 32°C in the self-

contained fresh water unit This means that a larger cooler is required tocope with the additional heat as up to three times the amount of heat has

to be extracted in this arrangement

3.5.3 Salt Water versus Fresh Water Cooling

The subject of water cooled compressors cannot be left without someremarks on the subject of cooling The corrosion and fouling disadvantages

of seawater cooling on compressed air systems are well known, but the use

of fresh water is quite often not sufficiently studied, particularly in relation

to temperature If temperatures similar to sea water are utilized with freshwater no troubles can be anticipated, and a prolonged life given to a machine,particularly the cast iron components If cooling water is used direct fromthe main auxiliary diesel cooling system at temperatures between 60°C and71DC(l40°F and 160°F) the compressor will be using a coolant at an average

of 27"C (80°F) above normal average seawater temperature This increasedtemperature will result in the intercooler outlet being 32°C (90°F) and thesecond stage outlet approximately 60°C (l50°F) above their normal averagetemperatures The adverse effects of this elevated temperature are numerous,

in particular the heavy carbonizing of the valves Also more maintenanceand heavier wear, due to the poorer lubrication on the cylinder walls, iscaused by the lower oil viscosity A heavier grade of oil is often used to offsetthis problem, but a successful balance is difficult to obtain when the first andsecond stage temperatures are as much as 50°C (120°F) different, due to poorinterstage cooling with the high coolant temperature Also, on start up, thisoil is much too viscous, and "start up" wear is increased Leaving the hotcooling water running through the machine gives little assistance as the sumpremains unheated Multi-grade oils normally mean storing a special com-pressor oil aboard, and these are only readily available with a high detergentquality, which do not tolerate even a very small quantity of condensatewithout emulsifying also their paraffin base usually means a hard valvecarbon

INSTALLATION 19 3.6 STANDBY COMPRESSOR USAGE AND RELIABILITY

The usage of a standby compressor (or any other machine) can radicallyaffect its life, and there are two main schools of thought on standby usage:a) The standby machine is, as its name implies, standby, and as such isonly run when the other unit is non-operational for some reason;

or

b) It is best to share the running period between both units, since a unitwhich has been on standby over a long period of time is prone tohave a failure when its use becomes necessary

Of the two schools of thought it is felt that the latter (b) is the better conditionalthough, in fact, neither condition is ideal The ideal one is explained below.The reliability curve of components and, therefore, machinery has beenlong established as being boat shaped when reliability is plotted against time.Fig 13 shows the shape of the reliability curve From the curve it can be

deduced that new material which is not fully run in is prone to sudden andunpredictable failures, and from the other end of the time scale it can bededuced that as a unit becomes worn, failures again become unpredictableand more frequent The ideal situation is, therefore, to ensure that both unitsare past the running in period and, therefore, lying in the more reliable middleband of the reliability curve, and then put one machine on standby until theother machine has reached the right hand end of the reliability curve, and

is in need of a major overhaul After the major overhaul this unit can then

be fully run in and become the standby machine

There is one more facet which must be borne in mind for the standbymachine, and, ignoring its use when it is brought in to cover maintenance

on the other machine, it must be given a regular short run to ensure that it iskept fully operational A reasonable period is to bring the standby machineoperational for, say, two hours once a week Naturally, if both machines are

in frequent use, it must be ensured that both machines run this minimumperiod

20 MARINE ENGINEERING PRACTICE

3.7 AUTOMATION OF CoMPRESSORS

3.7.1 Methods of Unloading Compressors

There are many methods of unloading compressor cylinders and thefollowing list gives some of the methods used:

a) throttling of the suction;

b) speed variation;

c) depressors to hold suction valve plates on their seats;

d) by-pass, discharge to suction;

e) changes in volumetric clearance of the cylinders;

f) step unloading of cylinders in a multi-cylinder machine

As marine compressors are fairly small, methods (c) and (d) are usually used,and the machines operate in, all on, or all off load condition The depressing

of the suction valve plates gives slightly less power consumption when runningunloaded than the system of by-passing back to suction and thence toatmosphere via the suction silencer However, the latter type can be made inmodular form and is more robust in construction and easier to understand

We must now consider what is involved in the automation of thecompressors, first in remote and secondly in automatic operation of aircompressors, as installed aboard ship This best begins with a review of thefunctions involved in their local manual operation and supervision

3.7.2 Starting Procedure

Before starting the machine the drains must be opened to give anunloaded start and to allow the machine to purge itself of any accumulatedmoisture as soon as it starts to turn After a short interval, the drains may beclosed and the machine put on load

3.7.3 Normal Running

Periodical draining of the accumulated moisture is desirable, and themachine should be checked to ensure that it is getting adequate cooling water,that the stage pressures are correct and, in forced lubricated compressors,that the oil pressure is correct

3.7.4 Stopping the Machine

A short time before stopping the machine, the drains should be opened

to purge the cylinders and coolers of accumulated moisture for the comingidle period

Marine compressors can conveniently be fitted with equipment to carryout all these functions rigorously and continuously The machines thus fittedare almost certainly better run than those started and stopped by humanoperators, who are liable to omit operating the drains and will certainly not

be available for continuous observations of running conditions

3.7.5 Mechanization of Human Functions

Fig 14 shows a typical modular unloader, which will vent the first stageintercooler and the aftercooler to atmosphere via the compressor's inlet filtersilencer, the initial blast of air from coolers helping to keep the filter clean

If the machine runs for a lengthy period recirculation of the air takes place,but this is not detrimental as, due to the reciprocating action of the first stage

INSTALLATION 21 piston, the air demand is fluctuating and hence air continually enters and leaves the filter The coolers remain effective as the heat dissipation is very small compared with normal conditions It should be noted that the unloader is always in the unloaded position unless air is on the diaphragm (normally from the first stage) overcoming the actuating spring thrust.

3.7.6 Automatic Drains

To drain the coolers continuously during running, an automatic device must be fitted to each cooler outlet;

3.7.7 Safety Protection Equipment

It is an easy matter to over-protect a compressor, even allowing for the fact that a failure whilst unsupervised is usually more disastrous than when supervised Commercially, only monitoring of the following is necessary: a) lubricating oil pressure (or oil level if splash fed);

b) the final stage air delivery temperature immediately after the air delivery valve This temperature probe<'1nonitor will take care of the following breakdowns or defects:

i) Water blockage

If the air temperature signal probe is fitted directly after the air delivery valve it will monitor directly the maximum temperature point in the machine and will eliminate any time lag If, however, the probe is fitted so as to measure the air temperature after a cooler and a complete or partial water blockage occurs, the cooler

22 MARINE ENGINEERING PRACfICE

will continue to function sufficiently to delay an air temperaturewarning signal, and a heat seizure or head explosion may occur

If the temperature probe is fitted in the water circuit it will also

be ineffective at no flow as its operation will depend on theconductivity of water

In the case of a fan failure in an air cooled machine, atemperature probe fitted after the cooler will also have a delayfactor in temperate climates, as the cooler has a tropical ratingand is therefore designed to operate under tropical conditions.The correct position for the air temperature warning probe isagain at the delivery valve

Paragraph 5.6.1.e deals with the fusible plug on the aftercooler

ii) Unbalanced load between stages

Can be caused by either broken valves, a blockage of valves bycarbon (insufficient maintenance) or blockage of the air inlet filter

or interstage cooler Broken or partially blocked valves allow airback flow so that the fresh air charge is "diluted" with hot airand its final delivery temperature is raised Partial blockage of theair inlet filter (painting and/or insufficient cleaning) of the inter-stage cooler (m/c running too hot-insufficient cooling water/air

or silted cooler block/heavily painted air cooler) will cause highcompression ratio due to throttling causing reduced inlet pressure

to the cylinder

3.7.8 Automatic Operation

The soundest arrangement is to have the lead starting air compressorrunning continuously whilst manoeuvring, automatically loading andunloading according to demand, but automatically stopping and startingwhilst at sea It is also wise to have the standby machine(s) on automaticcontrol The two different operations will be explained separately, but all that

is necessary to combine them is a changeover switch in the pressure switchelectrical leads

The operation will be explained for a single machine The standbymachine operation will be identical except for pressure switch settings beinglower than the lead machine, so that it only becomes operational on failure

of the lead machine, or on exceptionally heavy air demand Timers can befitted for the standby machine to avoid both lead and standby machinesstarting together should an exceptional air demand take place, but it is anunnecessary feature if the lead and standby pressure switch settings aresufficiently apart Fig 15 shows the system This approach can easily beapplied to single or multiple compressor installations, and the operation is

i) Compressor stationary, machine in the unloaded condition (with

no air pressure the spring keeps the unloaders depressed).ii) Compressor is started, running light, and air bleeds through theorifice to the diaphragm, unloaders cease to function andmachine comes on load

24 MARINE ENGINEERING PRACTICE

iii) Compressor is stopped, air trapped in unloader is released andthe springs depress the unloaders

iii) Air is used and pressure drops to cut-in pressure

iv) Pressure switch de-energises solenoid valve, allowing air through

to diaphragm, unloaders cease to function and machine comes

on load

c) Notes

i) For stop/start operation pressure switch is connected direct toswitchgear as solenoid valve is then de-energised, line is openfrom first stage delivery to unloader

ii) For running unloaded pressure switch is connected direct tosolenoid valve

iii) With pressure switch settings shown, no timing relay is necessaryfor lagging machine as pressure difference gives sufficient delay.However, should this pressure difference be reduced, it isadvisable to have one fitted

The compressor control equipment described is usually works-fitted tothe compressors, except for the pressure switches, selector switches (lead/standby machine and running stop/start operation), control air filters and,

of course, the motor starters Installation aboard can be greatly simplified ifthe loose collection of instruments (not motor starter) are assembled on to asimple panel equipped with a single electrical and pneumatic terminal rail.Mounting the panel is simple and interconnections are at a minimum,whether mounted into a larger panel or in its own console

When planning an installation with fully automatic operations of theair compressor, it is important to consider the electrical running load at sea,and the effect on the generating capacity of an air compressor starting up

at any instant In some installations, the generating capacity budgeted asnormal may not take kindly to unpredicted starting of large compressormotors It is then necessary to allow fully automatic operation of the mainstarting air compressors only when several generators are running

It may be argued that a separate compressor, fully automatic, should beused for topping up the air receiver(s) at sea However, practice has shownthat the size of these compressors is constantly underestimated and also, theaddition of further sources of air demand is not commonplace Unlessthe economies can be shown to have outstanding advantages, the mainstarting air compressors should be used if the generators normally runninghave sufficient capacity The automatics ensure also that the machine starts up

at low torque conditions, keeping electrical surges to a minimum

4 OPERATING INSTRUCTIONS

The handbook of the specific machine will cover this adequately but,unfortunately, it is not always available after the initial years, and thefollowing gives, therefore, the general procedure to be followed First makesure that the unit is isolated

Initial start-This procedure should be carried out after the initial installation,

after any major strip down and when accepting the responsibility of the unitsfor the first time

4 I RECIPROCATING COMPRESSOR

4.1.1 Lubrication Check that all compressor oil levels are correct and, wherepossible, that the oil grades are correct All lubricator lines should be handprimed and the lubricator functions checked Lubricators are met lessfrequently on the more modern machines The prime mover lubricationshould also be checked

4.1.2 Priming Cylinder Walls This should be carried out in conjunctionwith the lubrication as, when the valves are removed for a visual check, asmall quantity of oil can be injected through the valve port, to wet thecylinder walls on non-oil free machines

4.1.3 Unloading All cooler drains should be opened together with anyunloaders Check the unloader lines

4.1.4 Gauge Cocks The oil gauge cock should be open but the air gaugecocks must only be partially cracked to avoid excessive gauge fluctuationswhich can damage the gauges

4.1.5 Air Intake Ascertain that the filter is in a clean condition Heavypainting is not unknown on new equipment

4.1.6 Cooling If water cooled, the necessary cocks should be opened and ifair cooled, the fan drive should be checked

4.1.7 Relief Valves If hand testing levers are fitted, manually lift each valve

to ensure that valves will operate "freely"

4.1.8 System Check that the air system is open to the correct source

25

26 MARINE ENGINEERING PRACTICE

4.1.9 Final Check Bar the machine over several revolutions to ensure thatthe machine is free and operating correctly

4.2 ROTARY LIQUID RING COMPRESSOR

4.2.1 Lubrication Only the bearings (usually grease) need to be attended to 4.2.2 Unloading The water sealing liquid cools and avoids the necessity ofcoolers but the drain on the water separator should be opened along withany unloader

4.2.3 Gauge Cocks Pulsations will not be present but the cocks shouldonly be cracked to clamp down the initial surge

4.2.4 Air Intake Ensure that it is clean

4.2.5 Cooling The reservoir tank should be correctly filled with fresh watersince this is both the cooling and operating media Where cooling coils arefitted, the necessary cocks should be opened

4.2.6 Shaft Seals These should be "outwardly" checked visually

4.2.7 Relief Valves Test with manual lever if fitted

4.2.8 System Ensure correct valves are open

4.2.9 Final Check Turn over machine by hand, for a few revolutions

4.3 ROTARY VANE CoMPRESSOR

4.3.1 Lubrication Check that the correct fillings and oil have been used.4.3.2 Unloading All drains and unloaders should be opened

4.3.3 Gauge Cocks Partially open only

4.3.4 Air Intake It is exceedingly important that this is clean and notrestricted

4.3.5 Cooling Open cocks if water cooled and ensure adequate ventilation

on air cooled machines Watch for oil cooler

4.3.6 Relief Valves Lever vent valve, if fitted

4.3.7 System Ensure correct valves are open

4.3.8 Shaft Seals Visually check external appearance for any unusualappearances, although in general, very little guide can be obtained

4.3.9 Final Check Turn over machine by hand a few revolutions This willensure everything is free except compressor blades which are possibly stuck

at the bottom of the rotor slot