MARINE ENGINEERING PRACTICE Volume Part SELECTION, INSTALLATION AND MAINTENANCE OF MARINE COMPRESSORS by L STERLING, C.Eng., M.I.Mar.E THE INSTITUTE OF MARINE ENGINEERS Published by The Institute of Marine Engineers The Memorial Building 76 Mark Lane London EC3R 7JN Copyright © 1973 Marine Management (Holdings) Ltd Reprinted 1987 Reprinted 1988 Reprinted 1993 Reprinted 1996 All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form of by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher Enquiries should be addressed to The Institute of Marine Engineers ISBN: 900976 06 Printed in the UK by The Chameleon Press Limited, London SW18 4SG CONTENTS Page Preamble Selection Installation Operating Instructions 25 Maintenance 28 Trouble Shooting 47 Basic Theory of Compressing Air 50 PREAMBLE It is intended to cover the subject in a very broad sense in that installation and selection of compressors will also be covered, since a unit incorrectly installed 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 the machine 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 the majority of marine compressors are now run by automatic systems SELECTION 2.1 GASES ARE COMPRESSED BY Two BASIC METHODS 2.1.1 Compression Pressure is imparted by decreasing the volume using positive displacement machines Fig shows the main groupings SELECTION 2.1.2 Kinematic Energy Dynamic compression (usually rotary) imparts kinetic energy to the gases which is converted into pressure by means of a diffuser This group is usually met as fans, blowers; superchargers, condensers, ejectors etc in the marine 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 incorrectly selected or has been used on duties other than that for which it was selected, will often cause endless maintenance problems Take for example a reciprocating air compressor for engine starting Such a machine does not log a great number of running hours per annum and usually only runs for short periods except on one or two occasions, such as when on passage through a long, difficult, navigational stretch As such, the machine can, therefore, be selected in its maximum stress condition This normally denotes selecting at its maximum speed and, as such, the unit is perfectly satisfactory Should the unit be requested as such, but used on other duties, so that its running hours become more prolonged, the highly stressed conditions in the machine will necessitate frequent maintenance and sometimes cause major breakdowns Even when the duty is for starting air only, the class of vessel has some bearing 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 continuous use It is also selected so that the stress conditions are low for the type of machine in question Perhaps one of the most abused compressors is the so-called "topping up" compressor which, because of general air usage from the receivers and system leakages, often becomes a 24 hour per day category compressor, 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 since the rotary air compressor is, in the majority of cases, on a 24 hour per day duty, 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 bars the reciprocating machine, with its more positive sealing, is the only correct selection With the reciprocating machine, nature's limitation is the temperature caused by compression and this allows the compression ratio to be up to 7:1 in each following stage, with intercooling between stages Thus a starting air pressure of say 35 bar gauge can be readily attained with a two stage machine The rotary machine is limited by gas slip past its seals and a differential of about bars per stage is about the limit Thus a six stage machine would be required for 35 bar gauge Comm~rcial considerations, therefore, hold its usage to single stage (7 bars) applications but multi-staging would also bring technical 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 MARINE ENGINEERING PRACTICE humidity, the compression ratio will always be sufficient to produce free moisture and leave the compressed air 100'70 humid The moisture causes corrosion and, therefore, oil in the air, from a non-oil free machine, gives more protection to the system and, with good machine maintenance, is the correct 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 free moisture present The free moisture is easily removed by means of filters which must be of the ceramic type to ensure full removal for this type of duty However, to achieve the necessary dryness an absorbent type dryer is required to remove the moisture from the air and 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 is removed However, in this instance, the absorbent dryer is not required 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 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 be doubled Allowing the temperature to equalise, the moisture which was present now has to occupy twice the volume; the humidity is, therefore, halved This phenomenon can easily be observed in the SELECTION 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 anything 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 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 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 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 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 not critical in a design sense This, however, is a complete fallacy Fig shows a typical schematic installation which is true for all types of compressors and which is described below 42 MARINE ENGINEERING PRACTICE ii) Although hardness testing on a sprayed journal is more difficult to do, due to the above-mentioned porosity, there is no doubt that usually the surface is slightly harder, unless, of course, the original crankshaft had been hardened Hardened journals, however, are unusual in compressors since this has never been found to be warranted j) Crankshaft End Float and Bending It is important that a crankshaft has an end float to allow expansion and these figures are always quoted in the handbook, but if the figures are not available, a minimum of 0·025 mm (0·001 in) per 25 mm (1 in) of allocation length should be allowed If bearing failures are occurring frequently, misalignment of the crankshaft should be suspected, and since the manufacturer and the classification societies will have ensured that the crankshaft is of sufficient strength, the distance between the balance weight throws should be taken at as large a radius as possible, with the machine in its normal bolted down position, and then with its feet unbolted Any discrepancy between the two sets of readings will show a misalignment caused by incorrect chocking on the mountings 5.6.2 Rotary Compressors a) Liquid Ring Compressor This compressor is often cast as a rotary unit but the action is actually reciprocating Fig 24 shows, schematically, such a compressor and it can readily be seen that when the impeller is rotating the centrifugal force throws the liquid (usually fresh water) trapped in the casing (hence liquid ring) to the outside periphery of the rotating wheel By placing a suction port where the casing begins to deviate to a larger MAINTENANCE 43 diameter than the rotating impeller, the outward movement of the liquid creates a pressure drop and draws in the gases After the casing has reached the maximum distance from the centre of the rotation, ensuring that liquid is still available to the impeller tips, the casing then begins to decrease the distance between the casing wall and the impeller tips, and the liquid is forced inwards against the centrifugal force, thus compressing the gases which have been trapped between the vanes By having a suitable discharge port the gases can then be forced either to the following stage, or into a receiver The schematic diagram shows a single lobe but often the units have two lobes to balance the radial force and also increase the capacity for a given size A further point is that axial ports, in the inlets, are shown, but very frequently a large centre stationary boss enters the inner portion of the impeller and the ports are placed in this portion The action in all cases being similar The sealing between the inlet and outlet ports is a weak point and, in general, these units cannot be used with a maximum differential of more than 3-4 bars per stage The efficiency of the unit is usually low and its main features are the complete lack of contamination, other than the liquid of the liquid ring and its simplicity of construction The maintenance of this unit is very simple, being somewhat similar to a centrifugal pump, but the clearances are much more critical Apart from regular greasing of any external bearings, preventative maintenance is best carried out by periodic examination of the running clearances in the unit, and renewal to the manufacturer's recommendations Cooling of the compressed air is carried out by the controlled injection of liquid, which is obtained from the separator on the outlet of the compressor, where the liquid carried over, and the condensate, is collected It is essential to ensure that this cooling system is free at all times, as otherwise the liquid ring will have insufficient liquid for correct sealing The injection of too much liquid is generally not possible, but if the injector size is changed so that it becomes possible, it may be found that there will be a decrease in gas output b) Rotary Vane Compressors These units are often called sliding vane, and the compressive action on the air is again a reciprocating action Fig 25 shows a typical rotary sliding vane compressor, and it will be seen that the rotor is offset in the casing bore and that the sliding vanes maintain contact with the casing so that there are steadily increasing areas, with connecting ports, in which the air is forced by the pressure differential, i.e a vacuum is created and atmospheric pressure forces in the air This is followed by a section of the casing where very little change in area takes place and this is the sealing area between suction and discharge, followed by a decreasing area with connecting port(s) so that the air is compressed and forced into the delivery chamber Compression cooling on these compressors is by oil injection, which also lubricates the vanes The oil is supplied by the initial filling and then re-separation on the machine discharge, and reinjection back into the compression chamber It is usual, also, for these units to have a water cooled jacket around the machine, but air cooled versions are available Maintenance of a sliding vane compressor is relatively straightforward, it being important to maintain the oil in a good clean filtered condition, and to periodically examine the rotor and the vanes to ensure that the vanes are free to slide, and that no excessive wear has taken place The rotor wear is usually lower than the vanes and the sliding width of the slots should be examined for wear On the vanes themselves the majority of the wear takes place on the outer edges but the faces, which slide in contact with the rotor, should also be examined If the vanes show excessive wear on their outer tips, the bore, in which they run, should also be examined Many of these compressors use plastic composition for their vanes, and these should be examined carefully for age deterioration with heat Routine checks should be kept on the shaft ~eals since these are perhaps the most important maintenance point on the machine During these inspections the usual checks on lubricating oil draining should be carried out, and any change in the machine running noted, e.g if the machine is running less smoothly than usual this probably indicates a major deterioration, necessitating a shut down and stripping MAINTENANCE 45 c) Screw Compressor The so-called screw or helical-lobe rotary compressor is built in both an oil-free and non-oil-free form Fig 26 shows a typical machine The machines are used for up to about bars for single stage oil-free compressors and up to 4,5 bars for an oil cooled compressor The vast majority of machines are fitted with timing gears, and it is usual for the male rotor to have four lobes and the female rotor to have six lobes, although many lobe combinations can be used The reason for the female usually having a greater number of lobes is to give a stronger centre section on the rotor The sealing between the screw flanks is attained by accurate machining of the screw forms The sealing between the screws and the casings, and the screws and the roots, to avoid pick up, is usually attained by this raised sealing band with the necessary recess where applicable The condition of these sealing bands is important, to maintain good volumetric efficiency The screw casing is usually water jacketed, but air cooled versions are available, and the timing gears are separated from the "pumping" area by shaft seals which usually consist of split carbon rings and springs similar to many turbines On the non-oil-free version controlled oil injection takes place on the suction side This reduces the rise in the compression temperature and also gives better sealing on the clearances Two-stage versions of the machines are available Compressor control is usually by means of a by-pass but suction throttling is sometimes used, although not recommended The timing gears run in their separate oil-filled gearboxes, and the compressor 46 MARINE ENGINEERING PRACfICE itself usually runs at high speed, since, although the compression action is positive, the machine has running clearances, and high speed is necessary to maintain a reasonable volumetric efficiency Providing the installation is correct, the major features being no pipe strains on the unit and correct cooling both aimed at minimum casing distortion, the unit should rUn relatively troublefree and with little maintenance With either of the above factors present, rubbing will occur on a casing and the machine efficiencies will drop rapidly One point which must be mentioned is that due to high speed, the noise from such compressors is rather troublesome, unless the initial installation takes care of this definite point TROUBLE SHOOTING Since perfection is never achieved, problems can only be minimized by following the earlier comments If logging of the machine's characteristics has been carried out during trouble-free operation, a cross check on the characteristics, together with Tables III and IV, "Trouble Spotting Charts", will assist in a quick solution of the problem Any measuring instruments should be checked before using In the case of a severe failure the operational factors immediately before failure should be checked, and also the Routine Maintenance Sheets in case maintenance has been neglected 47 TROUBLE SHOOTING 49 7.1 BASIC THEORY OF COMPRESSING AIR AIR A "perfect" gas obeys Boyle's Law (PV = C,) Charles's Law (VjT = C) and the Combination Law (PVjT = C) Air is a mixture of several gases, the two major constituents being nitrogen and oxygen (78% N2, 21 % O2 and I % other gases by volume 76% N2, 23% O2 and I % other gases by weight Percentages are approximate only.) Although not "perfect" gases, the major gases in air, and therefore air, obey very closely the above laws Water vapour is also present in variable quantities but affects the calculations only slightly and can be ignored for the purpose to which this book is aimed In the "comination" law it is usual to have the constant as a gas constant for unit mass, times the mass, thus enabling the different gas constants to be readily used from tables, for a given unit mass, i.e PV = RmT where R is the constant per unit mass (e.g the familiar R = 53'3 ft IbjlbtF for air or 286 NmjkgtK; therefore the "Comination" law or characteristic equation for air is PV = 286 mT, P = Njm2 absolute, V = m3, m = kg, T = OK) 7.2 COMPRESSION It should be noted that temperatures and pressures must be "absolute" units 7.2.1 Isothermal Compression This would represent the least energy input and involves no temperature change, i.e PV = C (see Fig 31), Actual Compression This lies between the above two extremes and the law is PVn = C (n for air varies for each compressor but a good mean is 1·3) 7.2.3 Theoretical Work Done a) PV = C Fig 28 shows the compression curve from volume VI to V2• At the 50 7.2.4 Effect of Multi-stages Apart from the practical consideration of keeping the maximum temperature of compression compatible with thermal distortion and lubricating oils, etc., power saving can-be achieved, as shown in Fig 30 7.2.5 Volumetric Clearance Fig 31 shows the effect of volumetric clearance on the "perfect" pressure/volume diagram This volumetric clearance not only lowers the possible output of a given machine but also increases the cylinder charge temperature slightly since the temperature does not drop to the inlet temperature on expansion BASIC 7.3 MOISTURE THEORY OF COMPRESSING AIR 55 EFFECT The humidity of the air is usually expressed as a percentage and a reasonable average figure is 70 per cent humid Since the air must be 100 per cent humid before it is fully saturated and free moisture appears, a compression ratio of only I ·43 would be possible before moisture fall out would occur, from 70 per cent humid air, if compression was isothermal With increasing air temperature the ability of air to hold water increases; it actually doubles for every 15°C temperature increase Since the temperature increase during compression is considerable, the ability to hold moisture increases and usually the relative humidity of the air actually decreases despite the fact that the air volume is decreasing and humidity increases in direct proportion to the compression ratio if the temperature remained constant On cooling in the aftercooler (and pipelines) the ability of the air to retain the moisture volume drops and free moisture is deposited leaving the compressed air 100 per cent humid 7.4 EFFICIENCIES Several types of efficiencies should be considered having their particular merits in compressors, each 7.4.1 Volumetric Efficiency On the compressors in question this is a ratio of the "free" air compressed and delivered at the compressor discharge divided by the displacement of the compressor "Free" air is the air at equivalent condition to the air at the atmospheric condition of the site A high volumetric efficiency, assuming leakages are minimal, ensures that the delivery temperature, before any coolers, is at a minimum for the particular operating conditions since there is less hot compressed air to heat up the new air charge entering, normally the total package (unit) size is also a minimum 7.4.2 Compression Efficiency This is the ratio of the theoretical work of compression divided by the actual work of compression (indicated power) The theoretical work of compression can be either adiabatic for "Adiabatic Compression Efficiency" or isothermal for "Isothermal Compression Efficiency" Since the air will normally be cooled before usage the "Isothermal Compression Efficiency" is recommended This efficiency has a direct bearing on power consumption-see 7.4.4 7.4.3 Mechanical Efficiency This is the summation of all the power losses from bearings, piston rubbing losses, glands etc., giving the total "rubbing" losses of all the machine's sliding surfaces The ratio of the indicated power divided by the actual absorbed power gives this mechanical efficiency 7.4.4 Overall Compressor This is the product Efficiency Efficiency of the Compression Efficiency and Mechanical 56 MARINE ENGINEERING PRACTICE 7.4.5 Overall Efficiency This is simply the product of the Overall Compressor Efficiency, the Driver Efficiency and the Transmission Efficiency 7.5 POSITIVE DISPLACEMENT-ROTARY COMPRESSORS The theory is applicable to these units although the volumetric change and clearances are not as straightforward to calculate If the compression is injection cooled, whilst it is taking place, such as by the oil in the bladed rotary or water in the liquid ring compressor, the on" factor is only about 1'1 Off design pressure working usually causes a quicker efficiencydrop than with the reciprocating machine ... Institute of Marine Engineers The Memorial Building 76 Mark Lane London EC3R 7JN Copyright © 19 73 Marine Management (Holdings) Ltd Reprinted 19 87 Reprinted 19 88 Reprinted 19 93 Reprinted 19 96 All... the majority of marine compressors are now run by automatic systems SELECTION 2 .1 GASES ARE COMPRESSED BY Two BASIC METHODS 2 .1. 1 Compression Pressure is imparted by decreasing the volume using... 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 that