Marine Machinery 7 E Part 7 ppsx

Generators driven from the main propulsion system Generators can variously be driven from the propeller shaft, through a gearbox or by being mounted on the engine itself.. Auxiliary powe

230 Auxiliary power

closing the pilot valve This action again stops excessive movement of thepower piston and fuel rack As the engine speed drops, the flyweights moveback in towards their former position, while oil leaks through the needle valveallowing the receiving compensating piston to return towards its old position.Again, the two movements act on the floating lever without moving the closedpilot valve

UMS operation

A survey conducted recently (1990) suggests that about 40% of ships in theworld fleet operate with periodically unmanned machinery spaces (UMS) Ofthe 60% with watchkeepers, most are older ships, some are passenger vessels,

or other vessel types where watchkeeping is justified for extra security A feware ships with control equipment which is defective for various reasons.UMS machinery spaces have automatic engine change-over in the event of afault developing on the running machine Some have programmed control ofgenerators with automatic starting and stopping of stand-by engines as thedemand for electrical power rises and falls Synchronization, opening andclosing of breakers, is automatic and load sharing is a function of speed sensing

or load sensing governors

The unattended installations require high dependability which demandsintimate knowledge of the machines and strict attention to the maintenanceschedule

Generators driven from the main propulsion system

Generators can variously be driven from the propeller shaft, through a gearbox

or by being mounted on the engine itself Assuming that residual fuel is used inthe main engine, then all electrical power at sea is provided at much lower cost,

in terms of fuel price and auxiliary generator running hours The diesel drivengenerator is needed only while manoeuvring and in port

Shaft driven direct current generators

Direct current generators are not as sensitive to speed variation as arealternating current machines where frequency has to be maintained If a directcurrent generator has an automatic voltage regulator, the output voltage can

be maintained even with a 10 or 15 per cent speed reduction Belt driven orshaft mounted direct current generators with automatic voltage regulatorswere therefore fitted in ships to save space and to reduce the workload Thesemachines could continue in operation with moderate speed reduction butauxiliary diesels were brought into use when manoeuvring

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Alternators driven from the main propulsion system

One answer to the frequency problem with alternators, is to supply directcurrent from a shaft driven direct current generator to a direct current motorand to use this to drive an alternator at constant speed This arrangementpermits moderate main engine speed reduction before a change over toauxiliary generators is necessary Another solution to maintaining alternatingcurrent frequency, relies on the use of a controllable pitch propeller andconstant speed engine, rather than one which has to be directly reversed.Manufacturers of electrical equipment have also developed various types ofelectronic circuits to maintain level frequency through main engine speedchanges,

Mechanical constant speed drive from variable speed engineThe system shown (Figure 7.12) uses speed increasing gears to deliver drivesfrom the main engine system to two parts of the installation One gear traindrives a variable delivery hydraulic pump (shown at the bottom) The otherdrives the planet carrier for the epicyclic gear train A Rotation of the planetcarrier A with the central sunwheel B fixed, causes the annulus C to drive

Figure 7.12 Constant speed shaft generator drive (Vickers type)

232 Auxiliary power

through its output shaft, the gear train for the generator Any steady rotation

of B will affect the generator speed and frequency

When the speed of the main propulsion system is altered, this is sensed by

an electronic device on the generator and the signal is used to control theswashplate for the variable displacement hydraulic pump unit The outputfrom the latter drives the fixed-displacement hydraulic unit which isconnected to the sunwheel E The annulus for this epicyclic gear is fixed sothat rotation of the sunwheel E, drives the planet carrier G and through theshaft, sunwheel B The speed and direction of B is used to maintain the speed

of output shaft D and thus the speed and frequency of the alternating currentgenerator

Exhaust gas boilers

The original exhaust gas boilers or economizers were of simple constructionand produced, from the low powered engines of the time, a very moderateamount of steam As large slow speed engine powers increased, the largerquantity of steam that could be generated from otherwise wasted exhaustenergy, was sufficient finally for provision of the ships entire electrical powerrequirement through a turbo-alternator, plus any necessary heating steam.Slow-speed diesel power development has increased engine efficiency butactually reduced the waste heat available to an exhaust gas boiler Waste heatsystems have become more sophisticated (Figure 7.13) in order to continue tosupply the electrical requirement and to obtain other economies

Auxiliary steam turbines

Auxiliary steam turbines are used in turbo-generator sets and also for cargopump and fan drives Power outputs vary up to about 1.5 MW for generatorsets The single cylinder turbines can be arranged horizontally or vertically.Both condensing and back pressure turbines have been used, being designedfor steam conditions ranging from about 6 bar to about 62 bar at 510°CThe layout for a closed feed system featured in Chapter 1 shows howturbo-generators and turbine driven cargo pumps are incorporated into asteamship system

Turbo-generators are also fitted in many motor ships in conjunction withwaste heat recovery schemes, based on using the exhaust from very large andpowerful slow-speed diesels Diesel engine builders have developed engineswith greater powers in response to the shipowners demand and also incompetition with steam turbines, for propulsion Diesels are now used almostexclusively for modern ships Only for liquefied gas carriers where the gasboil-off can be burned in the boilers, are steam turbines still being installed

is shown in Figure 7.14 Alternatively the turbine may be a back-pressure unit

in which the exhaust is used as a source of low pressure steam for other

234 Auxiliary power

Figure 7.14 Auxiliary condensing turbine (Peter Brotherhood Ltd)

1 Pedestal end bearing 11, Inner steam labyrinth 19 Tachometer generator

2 Oi! pump and governor 12 Oil seal housings 20, Pinion bearing turbine end worm 13 Interstage labyrinth 21 Pinion bearing outer end

3 Pedestal centre bearing packing 22 Blower seal

4 Internal tooth coupling 14 Inner steam labyrinth 23 Gear shaft oil seal

5 Thrust bearing oil seal 15 Outer steam labyrinths 24 Gear shaft location

6 Michell thrust bearing 16 Rotor bearing bearing

7 Rotor bearing 17 Gear half coupling turbine 25 Gear shaft bearing turbine

8 Oil seal labyrinths end end

9 Outer steam labyrinths 18 Gear half coupling pinion

10, Centre steam labyrinth end

services The casings, split horizontally and supporting the rotors in plainjournal bearings are cast mild steel or, for temperatures exceeding 460°C theyare of 0.5% molybdenum steel, with cast or fabricated mild steel for parts notsubject to high temperatures Solid gashed rotors of chrome-molybdenumalloy steel are usual though some may be encountered having rotor spindles ofthis alloy, with shrunk and keyed bucket wheels Blades may be of stainlessiron, stainless steel or monel metal, with shrouded tips, fitted into the rotors in anumber of root forms

Depending on steam conditions and power the turbine will have a two rowvelocity compounded stage followed by a suitable number, probably five ormore, single row pressure compounded stages, each separated by a cast steelnozzle Steam enters the turbine at the free end via a cast steel nozzle box andflows towards the drive end which is connected to the pinion of the reductiongearing by a fine tooth or other flexible coupling designed to accommodate

Interstage leakage, where the rotor shaft passes through the diaphragm, isminimized by labyrinth glands of a suitable non-ferrous alloy such asnickel-bronze Labyrinth packing may also be used for the turbine shaft/casingglands which are steam-packed In some turbines contact seals utilizingspring-loaded carbon segments as the sealing media, are used instead of thelabyrinth gland (Figure 7.15) A typical labyrinth gland arrangement is shown

in Figure 7.16 The low pressure labyrinth is divided into three separate groups

so as to form two pockets The inner pocket serves as an introduction annulusfor the gland sealing steam; this flows inwards into the turbine and someescapes through the centre labyrinth into the outer pocket The supply ofsealing steam is regulated to keep the pressure in the outer pocket just aboveatmospheric Surplus steam in the outer pocket is usually led to a gland steamcondenser The gland at the high pressure end of the turbine is subject to aconsiderable pressure range from sub-atmospheric at low load to considerablyabove atmospheric at full load and is therefore arranged with three pockets.Gland steam is supplied to the centre pocket The innermost pocket is

Figure 7,15 Example of carbon ring shaft seal

Figure 7.16 Typical labyrinth gland arrangement with air sealing system (Peter Brotherhood Ltd)

Auxiliary power 237

connected to a lower pressure stage further down the turbine, enabling theleakage steam to rejoin the main stream and do further work while theoutermost pocket, connected to the gland condenser, prevents excessiveleakage to atmosphere

The labyrinth packings at both ends of the turbine and in the diaphragms areretained by T-heads on the outer peripheries which slot into matching grooves.Each gland segment is held in position by a leaf spring The retaining lips of theT-head prevent inward movement and the arrangement permits temporaryoutward displacement of the segments Rotating of the segments is prevented

by stop plates or pegs fitted at the horizontal joint

Although there is little residual end thrust on the rotor it is necessary toarrange a thrust bearing on the rotor shaft and it is normal to make this integralwith the high-pressure end journal bearing Sometimes the thrust is ofmulti-collar design but is more frequently a Michell-type tilting pad bearing

Governing

Unlike propulsion turbines, generator turbines work at constant speed andmust be governed accordingly Classification Society rules require that theremust be only a 10% momentary and a 6% permanent variation in speed whenfull load is suddenly taken off or put on On an alternating current installation it

is required that the permanent speed variations of machines intended forparallel operation must be equal within a tolerance of ±0.5% In addition tothe constant speed governor an overspeed governor or emergency trip is alsofitted

Speed-governing system

Speed governing systems consist of three main elements:

1 A speed sensing device, usually a centrifugal flyweight type governordriven through worm and bevel gearing from the turbine shaft

2 A linkage system from the governor to the steam and throttle valve; onlarger turbines this is an oil operated relay consisting of a pressure balancedpilot valve controlling a supply of high pressure oil to a power piston

3 A double-beat balanced steam throttle valve which regulates the amount

of steam passing to the turbine nozzles, according to the speed andelectrical load

To ensure stability, that is freedom from wandering or hunting of the speed,the system is designed to give a small decrease in speed with increase in load.The usual amount of this decrease, called the 'speed droop' of the governor, is3% between no load and full load If the full load is suddenly removed, therewill be a momentary speed increase to a value of 7—10% above normal before itreturns to a value of 3% above normal above the full load speed (the droop

Figure 7.17 Schematic arrangement of speed governing system (Peter

Brotherhood Ltd.)

D Fulcrums O Fulcrum S Sleeve

C Adjusting spring P Pilot valve T Port

G Hand-operated wheel Q Port V Spring

K Piston R throttle valve W, Weights

M,N Levers

Auxiliary power 239

Figure 7.18 Overspeed trip gear (Peter Brotherhood Ltd)

1 Cap 3 Emergency valve

2 Emergency valve spring 4 Casing

An unbalanced steel valve 3, located in the pinion shaft extension, is held on

to the valve seat by a helical spring 2, while the speed of pinion shaft remainsbelow tripping speed If the speed increases 10—15% above the turbine ratedspeed, the centrifugal effect on the trip valve, overcomes the spring force andthe valve lifts rapidly from the valve seat This allows lubricating oil, fed to thecentre of the shaft extension through an orifice plate, to escape Oil systempressure drops to zero downstream from the orifice and this causes the lowpressure oil trip to operate and drain oil from the relay cylinder The relaycylinder spring raising the relay piston and closing the throttle valve cuts offthe steam supply to the inlet of the turbine It is vital to maintain the trip gear ingood working order and this can be greatly aided by testing at regular intervals

In addition to an overspeed trip it is customary to fit a low pressure oil trip tosteam turbines and frequently a back pressure trip (Figure 7.19) is fitted

When the turbine is running the valve is held upwards by the spring In thisposition high pressure oil can pass freely across the upper ports of the valve toactuate the governor relay If the back pressure increases to a predeterminedlevel the load on the bellows unit is sufficient to overcome the adjusting springand allow the operating spindle to move downwards and push the ball valveoff its seat In so doing, oil at relay pressure is admitted through the drilledpassages in the trip body to the piston valve, so depressing the valve against itsspring The valve will, simultaneously, cut off and drain the high pressuresupply to the governor relay The throttle valve is consequently closed by the

Back pressure turbines have most of the features of a condensing turbine but

no condenser The most important difference is in the governing Designed towork against back pressures in the range 1—3.5 bar and with much loweravailable heat drops than with a condensing turbine, governing by the simpleopening or closing of the throttle valve is inadequate Instead the governor

Figure 7.20 Installation for back-pressure turbo-generator (Peter

Brotherhood Ltd.)

1, Main high pressure boilers 4 Auxiliary turbine 8 Steam to ship's

2, HP steam to main propulsion 5 Reduction gear services

turbines 6 Generator 9 Evaporator

3, H.P steam to back pressure 7 Auxiliary turbine exhaust 10, De-aerator

turbo-generator 11 Steam/air heater

242 Auxiliary power

(Figure 7.21) sequentially controls the opening of a number of nozzle contrvalves The control system is arranged for a straight line regulatk(load/speed) with a speed droop of about 4% between full load and no loaThe turbine governor also incorporates speed droop control adjustment withrange of approximately 2.5% to 5.5% to enable load sharing between a grot

of generators to be readily adjusted

Vertical steam turbines

Vertical steam turbines are extensively used for cargo, ballast and other drives Like the horizontal machines used for power generation they can 1:condensing or back-pressure units They are, however, invariably single sta£machines having an overhung wheel (Figure 7.22) The steam casing of th

pun-turbine is a simple steel casting bolted to the top of the exhaust casing Tl

nozzles are fitted and seal welded into the underside of the steam casing inLbelt forming a ring which provides an uninterrupted arc of admission Tlexhaust casing is split in the vertical plane, allowing removal of the front hafor rotor inspection without disturbing the steam or exhaust piping.The rotor shaft is bolted to a head flange on the pinion shaft of the singreduction gearing A thrust bearing located below the pinion supports tl:weight of the rotor and absorbs any vertical thrust This bearing is usually <the Michell multi-pad type

:.d

V

Figure 7.21 Back-pressure governor and control system (Peter

Brotherhood Ltd.)

A uxilia ry po wer 243

Figure 7.22 Sectional arrangement of vertical turbine (Peter Brotherhood

Ltd.)

1 Labyrinth gland housing 5 Gear wheel shaft bearing 12, Tachometer generator

2 Pinion and gear shaft 6, Thrust bearing oil seal 13 Gear shaft location bearing 7 Oil thrower gear shaft bearing

3 Spur gear (govnr and oil 8 Oil seal gear shaft 14, Idler shaft bearings pump drives) 9 Overspeed trip unit 15 Pinion thrust bearing

4 Idle gear (govnr and oil 10, Pinion shaft upper bearing 16 Trip oil inlet fitting pump drives) 11 Pinion shaft upper oil seal

244 A uxiliary po wer

Environment, Proceedings of a Joint Conference Corrosion in the Marine

Environment Trans I Mar E series B.

Murrel!, P.W and Barclay, L (1984) Shaft driven generators for marine application,

Trans I Mar E, 96 paper 50.

Pringle, G G (1982) Economic power generation at sea: the constant speed shaft

driven generator, Trans I Mar E, 94, paper 30.

Schneider, P (1984) Production of auxiliary energy by the main engine, Trans I Mar E,

96, paper 51

The propeller shaft

The simplistic view of the main propulsion shaft installation is that the system

is set up with initial straight alignment and remains in that state during thelifetime of the ship, unless affected by accident or wear The reality is that thereare many factors which can affect and alter alignment during building andthroughout the working lifetime of a vessel

Establishing the shaft centre line

Optical (or laser) equipment can be used to establish the centre line of theshafting system, to give a reference for cutting through bulkheads andmachining of the aperture in the stern frame One method employs a telescopewith crosswires, set up on the shaft centre line at the forward end of the doublebottom engine platform with a plain cross wire target on the same axis at theafter end of the engine seating With both in use, the centre of engine room andaft peak bulkheads can be located and marked prior to cutting holes for theshaft The required centre of the aperture in the stern frame boss, can then befound by line of sight, using a crosswire in an adjustable spider Replacement ofthe crosswire by a plug with a centre gives a location for the divider to be usedwhen marking off the boss for boring Importantly, the telescope and crosswiretarget method can also be used to check the accuracy of the boring operation,work on the installation of the stern tube and siting of shaft bearings Somearrangements as for split stem tubes, involve the welding in of the boss and thisoperation can be guided by constant checking with optical equipment

Deviation while building

With the ship under construction still firmly supported, faults causing shaftmisalignment can and do occur The stern tube aperture can be incorrectlymachined due to flexure of the boring bar or human error Any contraction orexpansion of the hull as a result of temperature variation can conspire withchanges caused by welding of the hull to effect change of hull shape Thewelding in place of a fabricated stern tube requires constant checks to ensurealignment is maintained Some stern tube bearing failures have been traced toalignment errors which should have been detected and remedied duringinstallation

246 The propeller shaft

After fitting the stern tube and propeller shaft, the propeller is mounted Theconsiderable weight of the propeller however, causes droop in the tailshaft andpotential edge loading of the stern tube bearing Arching tends to lift theinboard end of the propeller (or tail) shaft so that the next bearing forwardwhether in the stern tube or beyond, would tend to be negatively loaded.Deformation imposed by the propeller mass, remains even after installation ofthe rest of the shaft system

The remedy for edge loading due to propeller shaft droop, is to arrange forthe stern tube bearing to be slope bored or installed with a downward lie Shaftweight is then fully supported along the bearing surface

After the ship has been launched, the immersed section of the heavily framedstern with the propeller mass, being much less buoyant than the full hull furtherforward, flexes downward This emphasizes the droop of the propeller shaftand resulting inherent misalignment Downward flexing of the stem alsodeforms the hull, changing the line of the tank top It was the normal practice toinstall the intermediate shafting after the launch, when the ship assumed itsin-water shape The shafting was installed from the tailshaft to the engine.Optical equipment, as before, could be used to check the position of thepropeller shaft inboard flange and to locate the centres of plumber bearings.Chocks for the shaft bearings are machined to the correct height

Traditionally, the fairing of couplings has been used to align shafts and tocheck the alignment of adjacent shaft sections The fairing of couplingsinvolves the insertion of feelers between a pair of couplings to check that theyare parallel and the use of a straight edge or dial gauge, to ensure that they areconcentric Incorrect alignment can result if it is assumed that shaft sections arerigid; particularly with the heavy shaft sections for engines of high power.Account must be taken of slight droop due to elasticity and overhangingweight at each shaft flange The natural deformation of shaft sections is takeninto account with rational alignment programmes and coupling conditions can

be used to position shaft sections and to check alignment For this procedure,pre-calculation is used to find the gap and sag that should exist betweencouplings, when shaft alignment is correct

Alignment deviation in service

Shaft line is continually changed through the lifetime of a ship, as the hull isdistorted by hog or sag due to different conditions of loading The weight anddistribution of cargo, ballast, fuel and fresh water are all subject to change andthe changes are known from experiment to affect shaft alignment (Incorrectcargo discharge procedures and resulting excessive hull stresses have resulted

in ships actually breaking in two.)

High deck temperature in the tropics or low sea temperatures can causedifferential expansion and hogging of the hull These types of change can altercrankshaft deflection or shaft alignment readings which are taken even a fewhours apart Heavy weather produces cyclic change of hull shape so that thehull of a moderately sized ship can flex by as much as 150 mm There are also

The propeller shaft 247

local factors which alter shaft alignment Thus the forward tilt of a loadedthrust block, and the lift of its after bearing, causes misalignment of the shaftand possible uneven loading of gear teeth A build-up of fluid film pressure inbearings, as the shaft starts to rotate, lifts the shaft bodily Sinkage of individualpiumber blocks could be another problem

Fair curve alignment

The method of fair curve alignment (using a computer programme developed

at the Boston Navy Yard in 1954 and refined by others) accepts the changes ofline endured by the shaft system and seeks a compromise installation to suit thevarying conditions

The initial calculation is to determine the load on each bearing, assuming allbearings to be in a straight line The computer programme then simulates theraising of each bearing through a range and calculates, for each small change,the increase of its own load and alteration in load on each of the other bearings.The process is then repeated with a simulation of the lowering of each bearing

in turn with the computer rinding resultant load changes on the bearing inquestion and the others Influence numbers, in terms of load change for eachheight variation, are calculated by this exercise for all bearings

The data bank of influence numbers enables the effects of changes inalignment from hull flexure and local factors to be found All of the variablesdescribed above can be assessed to determine the best compromise for shaftinstallation

Shaft checks

The intention of good alignment is to ensure that bearings are correctly loadedand that the shaft is not severely stressed Alignment can be checked withconventional methods, employing light and targets, laser or measurementsfrom a taut wire There is, however, a continuity problem because the line ofsight or taut wire cannot extend over the full length of an installed shaft There

is no access to that part of the shaft within the stern tube and access is difficult

in way of the propulsion machinery Results are also uncertain unless the vessel

is in the same condition with regard to loading and hull temperatures as whenthe shaft system was installed or previously checked

The method of jacking (Figure 8.1) to assess correct bearing loads, is used as

a realistic means of ensuring that statically, the shaft installation is satisfactory

In simple terms the load on each bearing can be stated as the total weight of theshaft divided by the number of bearings The figure for designed load isnormally given in a handbook with the usual permitted deviation of plus orminus 50% The permitted variation may be less for some bearings

The procedure involves the use of hydraulic jacks placed on each side of thebearing, to lift the shaft just clear A dial gauge fixed to the bearing indicateslift Hydraulic pressure, exerted by the jacks, registers the load on the bearing

248 The propeller shaft

Figure 8.1 Method for checking bearing load by jacking (R C Dean)

A plot of lift and load made while hydraulically lifting the shaft, shows adistinctive pattern, due to the elasticity of the steel and removal of deformationfrom the bearing

As the hydraulic jack pressures are raised from zero, the concentratedloading initially causes deformation of the shaft Only after the journal sectionhas been bowed up out of shape to some degree, and the bearing materialresumes its relaxed primary shape, does the sagging centre part of the journallift clear and out of contact with the bearing The plot shows that the dialgauges register upward movement as soon as the shaft is pushed out of shape

by increasing hydraulic pressure The curve takes a different shape as the shaftlifts clear

If the jacking is taken too far, then adjacent bearings gradually become

The propeller shaft 249

unloaded and the plot is affected by a change in the elastic system To guardagainst this, dial gauges are fixed on adjacent bearings to ensure that the lift islimited to the bearing that is being checked

Strain gauges

Shaft stress is sometimes monitored in service, by fitting strain gauges on theshaft These register alternating surface stretch and compression as the shaftrotates,

Change of engine position

The conventional midships position for the engines of older vessels, with theexception of tankers, was based on low engine power and strong hullconstruction Shafts were long, but being of moderate diameter, were able toflex with the hull as loading or other conditions changed (and in heavyweather) A loading or ballast condition which changed hull shape and shaftalignment to an unusual degree, sometimes caused higher temperature in somebearings due to uneven load distribution Shaft stress was the hidden factor.The trend towards higher engine powers and the positioning of engines aft,gave rise to large diameter, short length shafts of increased stiffness Excessivevibration and resulting damage in many dry cargo and container vessels is acommon feature as a result Hull detuners intended to reduce vibration havebeen fitted in steering gear compartments but the improvement to many shipsseems to be marginal Hull vibration seems to be less of a problem in ships withone cargo compartment aft of the machinery space

Shaft bearings

The intermediate shafting (Figure 8.2) between the tailshaft and main engine,gearbox or thrustblock may be supported in plain, tilting pad or roller bearings.The two former types usually have individual oil sumps, the oil beingcirculated by a collar and scraper device; roller bearings are grease lubricated.The individual oil sumps usually have cooling water coils or a simple coolingwater chamber fitted Cooling water is provided from a service main connected

to the sea-water circulating system The cooling water passes directly overboard.Usually for plain and tilting pad bearings, only a bottom bearing half isprovided, the top acting purely as a cover The aftermost plumber blockhowever, always has a full bearing This bearing and any bearing in the forwardend of the stern tube, may be subject to negative loading

Plain bearings

Any oil between a static shaft and plain journal bearing in which it rests, tends

to be squeezed out so that there is metal to metal contact At the start of the