Marine Machinery 7 E Part 7 ppsx

230 Auxiliary power closing the pilot valve. This action again stops excessive movement of the power piston and fuel rack. As the engine speed drops, the flyweights move back in towards their former position, while oil leaks through the needle valve allowing the receiving compensating piston to return towards its old position. Again, the two movements act on the floating lever without moving the closed pilot valve. UMS operation A survey conducted recently (1990) suggests that about 40% of ships in the world fleet operate with periodically unmanned machinery spaces (UMS). Of the 60% with watchkeepers, most are older ships, some are passenger vessels, or other vessel types where watchkeeping is justified for extra security. A few are ships with control equipment which is defective for various reasons. UMS machinery spaces have automatic engine change-over in the event of a fault developing on the running machine. Some have programmed control of generators with automatic starting and stopping of stand-by engines as the demand for electrical power rises and falls. Synchronization, opening and closing of breakers, is automatic and load sharing is a function of speed sensing or load sensing governors. The unattended installations require high dependability which demands intimate knowledge of the machines and strict attention to the maintenance schedule. 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 in the 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 driven generator is needed only while manoeuvring and in port. Shaft driven direct current generators Direct current generators are not as sensitive to speed variation as are alternating current machines where frequency has to be maintained. If a direct current generator has an automatic voltage regulator, the output voltage can be maintained even with a 10 or 15 per cent speed reduction. Belt driven or shaft mounted direct current generators with automatic voltage regulators were therefore fitted in ships to save space and to reduce the workload. These machines could continue in operation with moderate speed reduction but auxiliary diesels were brought into use when manoeuvring. Auxiliary power 231 Alternators driven from the main propulsion system One answer to the frequency problem with alternators, is to supply direct current from a shaft driven direct current generator to a direct current motor and to use this to drive an alternator at constant speed. This arrangement permits moderate main engine speed reduction before a change over to auxiliary generators is necessary. Another solution to maintaining alternating current frequency, relies on the use of a controllable pitch propeller and constant speed engine, rather than one which has to be directly reversed. Manufacturers of electrical equipment have also developed various types of electronic circuits to maintain level frequency through main engine speed changes, Mechanical constant speed drive from variable speed engine The system shown (Figure 7.12) uses speed increasing gears to deliver drives from the main engine system to two parts of the installation. One gear train drives a variable delivery hydraulic pump (shown at the bottom). The other drives the planet carrier for the epicyclic gear train A. Rotation of the planet carrier 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 the swashplate for the variable displacement hydraulic pump unit. The output from the latter drives the fixed-displacement hydraulic unit which is connected to the sunwheel E. The annulus for this epicyclic gear is fixed so that rotation of the sunwheel E, drives the planet carrier G and through the shaft, 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 current generator. Exhaust gas boilers The original exhaust gas boilers or economizers were of simple construction and produced, from the low powered engines of the time, a very moderate amount of steam. As large slow speed engine powers increased, the larger quantity of steam that could be generated from otherwise wasted exhaust energy, was sufficient finally for provision of the ships entire electrical power requirement through a turbo-alternator, plus any necessary heating steam. Slow-speed diesel power development has increased engine efficiency but actually reduced the waste heat available to an exhaust gas boiler. Waste heat systems have become more sophisticated (Figure 7.13) in order to continue to supply the electrical requirement and to obtain other economies. Auxiliary steam turbines Auxiliary steam turbines are used in turbo-generator sets and also for cargo pump and fan drives. Power outputs vary up to about 1.5 MW for generator sets. The single cylinder turbines can be arranged horizontally or vertically. Both condensing and back pressure turbines have been used, being designed for steam conditions ranging from about 6 bar to about 62 bar at 510°C The layout for a closed feed system featured in Chapter 1 shows how turbo-generators and turbine driven cargo pumps are incorporated into a steamship system. Turbo-generators are also fitted in many motor ships in conjunction with waste heat recovery schemes, based on using the exhaust from very large and powerful slow-speed diesels. Diesel engine builders have developed engines with greater powers in response to the shipowners demand and also in competition with steam turbines, for propulsion. Diesels are now used almost exclusively for modern ships. Only for liquefied gas carriers where the gas boil-off can be burned in the boilers, are steam turbines still being installed. Auxiliary power 233 Figure 7.13 Typical waste heat recovery system (courtesy of Sulzer) Turbo-generator construction For electrical power generation, turbines are conventionally horizontal axial flow machines of the impulse reaction type. They may exhaust either to an integral condenser (invariably underslung) or to a separate central auxiliary condenser or the ship's main condenser. A typical auxiliary condensing turbine 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 plain journal bearings are cast mild steel or, for temperatures exceeding 460°C they are of 0.5% molybdenum steel, with cast or fabricated mild steel for parts not subject to high temperatures. Solid gashed rotors of chrome-molybdenum alloy steel are usual though some may be encountered having rotor spindles of this alloy, with shrunk and keyed bucket wheels. Blades may be of stainless iron, stainless steel or monel metal, with shrouded tips, fitted into the rotors in a number of root forms. Depending on steam conditions and power the turbine will have a two row velocity compounded stage followed by a suitable number, probably five or more, single row pressure compounded stages, each separated by a cast steel nozzle. Steam enters the turbine at the free end via a cast steel nozzle box and flows towards the drive end which is connected to the pinion of the reduction gearing by a fine tooth or other flexible coupling designed to accommodate Auxiliary power 235 longitudinal expansion of the rotor. Typical rotating speed of the rotor is about 6S00rev/rnin. The diaphragms separating each stage are split horizontally and fitted in grooves in the casing, to which they are securely fixed so as not to be disturbed when the top half casing is lifted. The diaphragms may be of steel or cast iron depending on the stage pressure. Interstage leakage, where the rotor shaft passes through the diaphragm, is minimized by labyrinth glands of a suitable non-ferrous alloy such as nickel-bronze. Labyrinth packing may also be used for the turbine shaft/casing glands which are steam-packed. In some turbines contact seals utilizing spring-loaded carbon segments as the sealing media, are used instead of the labyrinth 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 annulus for the gland sealing steam; this flows inwards into the turbine and some escapes through the centre labyrinth into the outer pocket. The supply of sealing steam is regulated to keep the pressure in the outer pocket just above atmospheric. Surplus steam in the outer pocket is usually led to a gland steam condenser. The gland at the high pressure end of the turbine is subject to a considerable pressure range from sub-atmospheric at low load to considerably above 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 the leakage steam to rejoin the main stream and do further work while the outermost pocket, connected to the gland condenser, prevents excessive leakage to atmosphere. The labyrinth packings at both ends of the turbine and in the diaphragms are retained 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 the T-head prevent inward movement and the arrangement permits temporary outward 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 to arrange a thrust bearing on the rotor shaft and it is normal to make this integral with the high-pressure end journal bearing. Sometimes the thrust is of multi-collar design but is more frequently a Michell-type tilting pad bearing. Governing Unlike propulsion turbines, generator turbines work at constant speed and must be governed accordingly. Classification Society rules require that there must be only a 10% momentary and a 6% permanent variation in speed when full load is suddenly taken off or put on. On an alternating current installation it is required that the permanent speed variations of machines intended for parallel operation must be equal within a tolerance of ±0.5%. In addition to the constant speed governor an overspeed governor or emergency trip is also fitted. Speed-governing system Speed governing systems consist of three main elements: 1 A speed sensing device, usually a centrifugal flyweight type governor driven through worm and bevel gearing from the turbine shaft. 2 A linkage system from the governor to the steam and throttle valve; on larger turbines this is an oil operated relay consisting of a pressure balanced pilot 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 and electrical 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, is 3% between no load and full load. If the full load is suddenly removed, there will be a momentary speed increase to a value of 7—10% above normal before it returns to a value of 3% above normal above the full load speed (the droop 238 Auxiliary power value). Similarly if the full load is suddenly applied, a momentary fall in speed of 4-7% below normal will occur before recovery. Figure 7.17 is a simplified schematic arrangement of a typical speed governing system from which the sequence of events during load changes may be more easily followed. In the diagram the throttle valve is operated via lever Y. Overspeed trip Overspeed occurs when the load is suddenly thrown off and while this is normally rectified by the speed governor, an emergency trip is always fitted. A common type is illustrated in Figure 7.18. 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 remains below tripping speed. If the speed increases 10—15% above the turbine rated speed, the centrifugal effect on the trip valve, overcomes the spring force and the valve lifts rapidly from the valve seat. This allows lubricating oil, fed to the centre of the shaft extension through an orifice plate, to escape. Oil system pressure drops to zero downstream from the orifice and this causes the low pressure oil trip to operate and drain oil from the relay cylinder. The relay cylinder spring raising the relay piston and closing the throttle valve cuts off the steam supply to the inlet of the turbine. It is vital to maintain the trip gear in good 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 to steam turbines and frequently a back pressure trip (Figure 7.19) is fitted. [...]... pressed into the groove between the end of the sleeve and the nut The grip of the coupling is checked by measuring the diameter of the outer sleeve before and after tightening The diameter increase should agree with the figure stamped on the sleeve To disconnect the coupling, oil pressure is brought to a set pressure in the hydraulic space Then with the shafts supported, oil is forced between the sleeves... Figure 8.12 (left) Rubber stave bearing (right) Lignum vitae bearing (Glacier Metal Co.) The propeller shaft Figure 8.13 261 Sea-water lubricated stern tube Wastage of the vulnerable steel shaft is prevented by a shrunk-on bronze liner and rubber seal sandwiched between the propeller hub and the liner end It is essential that the rubber has freedom to flow when nipped between the hub and liner Excessive... the sleeves The outer sleeve slides off the inner at a rate controlled by release of the hydraulic oil pressure 260 The propeller shaft Stern tubes The propeller shaft (or tailshaft) is supported in a stern tube bearing of one of a number of designs The bearing, being at the end of the shaft, is affected by the overhanging weight of the propeller The propeller mass pulls the outer end of the shaft... The SKF coupling (Figure 8.11) consists basically of two steel sleeves The thin inner sleeve has a bore slightly larger than the shaft diameter and its outer surface is tapered to match the taper on the bore of the outer sleeve The nut and sealing ring close the annular space at the end of the sleeves When the coupling is in position, the outer sleeve is hydraulically driven on to the tapered inner... Length 1250 mm Figure 8.11 SKF (muff) coupling When the outer sleeve has been driven on to a predetermined position, the forced lubrication pressure is released and drained Oil pressure is maintained in the hydraulic space until the oil between the sleeves drains and normal friction is restored After disconnecting hoses, plugs are fitted and rust preventive applied to protect exposed seatings A sealing... failures have been traced to alignment errors which should have been detected and remedied during installation 246 The propeller shaft After fitting the stern tube and propeller shaft, the propeller is mounted The considerable weight of the propeller however, causes droop in the tailshaft and potential edge loading of the stern tube bearing Arching tends to lift the inboard end of the propeller (or tail)... sometimes bronze) bush Oil retention and exclusion of sea water necessitated the fitting of an external face type seal The stuffing box was retained in many early oil-lubricated stern tubes, at the inboard end In oil-lubricated bearings the shaft does not require a full length protective bronze sleeve Simplex type stern tube The later designs of oil-lubricated stern tube (Figure 8.15) are fitted in a stern... system, to give a reference for cutting through bulkheads and machining of the aperture in the stern frame One method employs a telescope with crosswires, set up on the shaft centre line at the forward end of the double bottom engine platform with a plain cross wire target on the same axis at the after end of the engine seating With both in use, the centre of engine room and aft peak bulkheads can be... suitable outboard sealing arrangement and design, permits the two halves of the bearing to be drawn into the ship, exposing the shaft and the white metal bearing, Glacier-Herbert stern bearing In the Glacier-Herbert system (Figure 8.16) the two completely symmetrical bearing halves are flanged along the horizontal centre line and held together by bolts The after end of the bearing carries a spherical... the surface when push up is applied to the propeller and where there is any transmission of torque from the shaft via the key to the propeller hub Torque causes a deformation which tends to open the keyway The rubber seal sandwiched by the propeller hub and protective bronze liner, prevents ingress of sea water which would act as an electrolyte to promote galvanic corrosion of the exposed part of the . with waste heat recovery schemes, based on using the exhaust from very large and powerful slow-speed diesels. Diesel engine builders have developed engines with greater powers in response . steam. Slow-speed diesel power development has increased engine efficiency but actually reduced the waste heat available to an exhaust gas boiler. Waste heat systems have become more sophisticated. frequency through main engine speed changes, Mechanical constant speed drive from variable speed engine The system shown (Figure 7. 12) uses speed increasing gears to deliver drives from