Marine Auxiliary Machinery 7E Episode 2 potx

30 Main propulsion services and heat exchangers Figure 1.25 Package feed system for tanker 1. Extraction pumps 4. De-aerator 2. Steam jet air ejectors 5. Flash chamber 3. L.P. heater and drain cooler 6. De-aerated water storage tank Main propulsion services and heat exchangers 31 Figure 1.26 Typical de-aerator (Weir Pumps Ltd) It will be apparent that de-aerators of this type must be installed at such a height in the engine room that the pressure head at the extraction or feed pump suction is greater than that corresponding to the water temperature. The maintenance required is the control of corrosion, the cleaning of the nozzles, the renewal of those showing signs of erosion (which will seriously impair the efficiency), the overhaul of fittings and the maintenance of the safety valve, Devaporizers If the de-aerator cannot be vented to atmosphere or to a gland condenser satisfactorily, a devaporizer (Figure 1.27 is connected to the vapour outlet condensing the vapour vented with the non-condensable gases and cooling these gases before they are discharged. In the process the feed water is raised slightly in temperature. In design and construction devaporizers are similar to the other small heat exchangers working at moderate pressures. Leaving the devaporizer the feed water enters the de-aerator header. 32 Main propulsion services and heat exchangers Figure 1.27 Sectional view of devaporizer (Weir Pumps Ltd) Weir multi stage turbo-feed pump The water tube boiler, with its low storage capacity in relation to its steaming capability, demands a steady supply of pure feed water. The robust and reliable turbine driven centrifugal pump, is ideal for use as a water tube boiler feed pump. Additionally its uncontaminated exhaust can be used for feed heating and this improves cycle efficiency. The successful development of water lubricated bearings and their use in the Weir turbo-feed pump, permitted the turbine and pump to be close-coupled in a very compact unit (Figure 1.28). During normal running, a multi-plate restriction orifice allows feed water from the first stage impeller discharge to flow through a two-way non-return valve and a strainer to the bearings. A relief valve is incorporated. A secondary supply of lubricating water is introduced through the two way non-return valve from an outside source such as the main condensate extraction pumps, to protect the bearings from damage during starting, stopping, or stand-by periods of duty. Figure 1.29 shows the arrangement of the lubricating water system. The o verspeed trip is triggered by a spring loaded unbalanced bolt mounted in the shaft between the two journals. Pressure governor A discharge-pressure operated governor (Figure 1.30) and a rising head/capacity curve from full load to no load gives inherent stability of operation. The main feature of the governor is that if the pump loses suction the steam ports are opened wide, allowing the pump to accelerate rapidly to the speed at which the Figure 1.28 Weir multi-stage turbo-feed pump 34 Main propulsion services and heat exchangers Figure 1.29 Lubricating water system (Weir Pumps Ltd.) 1. Multi-plate restriction orifice 8. Bearing 2. Relief valve 9. Balance piston 3. Two-way non-return valve 10. 1st stage impeller 4. Strainer 11, 2nd stage impeller 5. Pressure gauge 12. Baffle plate 6. Balance chamber leakage 13. Drain valve 7. Leak-off control valve 14. Drain trap emergency trip acts. An adjusting screw, the collar of which bears against a platform in the casing, is threaded into the upper spring carrier and allows the compression of the spring to be altered by varying the distance between the upper and lower spring carriers. The piston, fitted with an O-ring and a spiral back-up ring, slides in a close fitting liner. A flange on this liner locates in a recess in the governor casing cover and is sealed by an Armco iron joint ring. When the pump is started, the throttle valve moves upwards due to the increasing discharge pressure under the piston until the desired pressure is reached. At this point, the upward force exerted on the governor spindle by the piston is equal to that being applied downwards by the spring, and the throttle valve is admitting the correct quantity of steam to the turbine to maintain the desired discharge pressure. With an increased demand on the pump the discharge pressure falls, allowing the pressure governor piston to move down under compulsion of the spring until the throttle valve opens to provide steam to satisfy the new demand. The reverse action takes place when the demand decreases. Main propulsion services and heat exchangers 35 Figure 1.30 Weir discharge-pressure operated governor Safety (overspeed) trip The 'Bolt' type overspeed trip (Figure 1.31) consists essentially of a spring-loaded stainless steel bolt which, due to its special design, is heavier at one end than the other. The rotary motion of the turbine shaft tends to move the bolt outwards, while the spring retains it in its normal position until the turbine speed reaches a pre-determined safety level. At this speed the centrifugal force exerted by the heavier end of the bolt overcomes the spring opposing it and the bolt moves outwards to strike the trip trigger. This in turn disengages the trip gear, allowing the steam stop valve to shut. 36 Main propulsion services and heat exchangers Figure 1.31 Overspeed trip (bolt type) Turbine driven oil-lubricated pump Prior to the introduction of the water lubricated turbo-feed pumps an oil-lubricated pump with a relatively long horizontal shaft was commonly used. In this particular pump the overspeed trip (Figure 1.32) was of the ring type. The pump was mounted on a taper at the end of the turbine shaft adjacent to the trip gear and was secured by a mild steel set bolt tapped into the end of the shaft. This set bolt was locked in place by a copper lock washer. The overspeed mechanism consisted of a case-hardened steel ring which, bored eccentrically, was weighted off-centre. The ring, however was spring loaded to maintain concentricity with the shaft until the speed of the turbine reached a predetermined safety limit. At that speed the centrifugal force exerted by the ring would overcome the force of the opposing spring and would then move outwards to strike the trip trigger. Hydraulic balance mechanism To control the axial movement of the rotating assembly, a balance piston (Figure 1.33) is arranged to counteract the effect of the thrust of the turbine and impellers. The arrangement keeps the rotating assembly in its correct position under all conditions of loading. Water at the approximate pressure of the pump discharge passes from the last stage of the pump between the impeller hub and the balance restriction bush C into the annular space B dropping in pressure as it does so. The pressure of water in the chamber B tends to push the balance piston towards the turbine end. When the thrust on the balance piston overcomes the turbine and the impeller thrust, the gap A between the piston and balance ring widens and allows water to escape. This in turn has the effect of lowering the pressure in chamber B allowing the rotating assembly to move back towards the pump end. Main propulsion services and heat exchangers 37 Figure 1.32 Overspeed trip (ring type) Figure 1.33 Hydraulic balance Theoretically this cycle will be repeated with a smaller movement each time until the thrust on the balance piston exactly balances the other axial forces acting on the assembly. In practice the balancing of the forces is almost instantaneous and any axial movement of the shaft is negligible. 33 Main propulsion services and heat exchangers Weir electro-feeder The Weir electro-feeder (Figure 1.34) is a multi-stage centrifugal pump mounted on a common baseplate with its electric motor. The number of stages may vary from two to fourteen depending upon the capacity of the pump and the required discharge pressure. The pump body consists of a number of ring sections fitted with difrusers and held in position between a suction and discharge casing by a ring of steel tie bolts. The unit is supported on pads on the baseplate by two feet on each of the end casings, these feet being drilled to accommodate the holding down bolts. Tapered dowels are used to maintain the correct alignment, and the driving torque from the electric motor is transmitted through a flexible coupling. The shaft assembly is supported on two ring lubricated white metal lined journal bearings bedded into plurnmer blocks, the lower sections of which form oil sumps. An internal hydraulic balancing arrangement similar to that found in the turbo-feed pump automatically maintains the shaft assembly in its correct axial position at all loads during running. To avoid excessive wear on this balancing arrangement when starting the pump, it is essential that the discharge pressure be built up quickly, and for this purpose, and to eliminate the possibility of reverse flow, the pump is fitted with a spring loaded non-return discharge valve. Condensate cooled stuffing boxes packed with high quality packing, are used for shaft sealing and these can be additionally cooled by water-circulated cooling jackets in the suction casing and the balance chamber cover. A pressure-operated cut-out switch may be fitted which will automatically isolate the driving motor from its supply if the first stage discharge pressure falls to a predetermined value due to loss of suction pressure, cavitation or other reason. Figure 1.34 Electrically-driven multi-stage feed pump (Weir Pumps Ltd) Main propulsion services and heat exchangers 39 Further reading Cotton, J. B. and Scholes, I .R. (1972) Titanium in marine engineering, Trans I Mar E, 84, paper 16. Conde, ]. F. G. (1985) New materials for the marine and offshore industry, Trans I Mar E, 97, paper 24. Shone, E. C, and Grim, G. C. (1985) 25 Years experience with sea water cooled heat transfer equipment in shell fleets, Trans I Mar E, 98, paper 11. [...]... In 30 25 2) 7 6 Cul Out 34 29 24 7 6 6 7 Cut In 29 24 20 66 5 7 lead Follow Figure 2. 8 Automatic operation of air compressors (Hamworthy Ltd) pressure switch on the air receiver The 'follow' machine is arranged to back-up the lead' machine during manoeuvring, cutting in after the lead' machine when the receiver pressure falls below a pre-set value (see the table in Figure 2. 8) When the pressure switch... Figure 2. 8 shows a scheme for the automatic starting and stopping of two machines Either machine can be selected as lead' machine This will run preferentially during manoeuvring and at other times, automatically stopping and starting under the control of a 48 Machinery service systems and equipment Pressure Switch Sailing B.f Momma Working Pressure 35 30 26 8 7 Cut Out 35 30 25 8 7 Coi In 30 25 2) 7... starting air compressor The compressor illustrated in Figure 2. 5 is a Hamworthy 2TM6 type which was designed for free air deliveries ranging from 183 m3 per hour at a discharge pressure of 14 bar to 367m3 per hour at 42 bar The crankcase is a rigid casting which supports a spheroidal graphite cast Figure 2. 5 Hamworthy 2TM6 air compressor Machinery service systems and equipment 45 iron crankshaft in... Figure 2. 2 Air compressor configurations Machinery service systems and equipment 43 Figure 2. 3 Compressor indicator diagram (courtesy Ham worthy Engineering Ltd) the cycle To minimize the temperature rise, heat must be removed Although some can be removed through the cylinder walls, the relatively small surface area and time available, severely limit the possible heat removal and as shown in Figure 2, 4... machines have been used These are illustrated in Figure 2. 14 and Table 2. 1 The obsolete tubular bowl machine was physically able to withstand higher angular velocities than the wide or disc bowl type, hence a higher centrifugal Figure 2. 14 Comparison of narrow and wide bowl types (Penwalt Ltd) 60 Machinery service systems and equipment Table 2. 1 Details of tubular and disc type centrifuges used for... and the process can be represented mathematically by Clearly in a standing vessel the acceleration cannot be altered to enhance the Figure 2. 12 Centrifuges arranged in series (courtesy Alpha-Laval) Figure 2. 13 Centrifuges arranged in parallel (courtesy Alpha-Laval) Machinery service systems and equipment 59 separation force Fs, but by subjecting the operation to centrifugal force the above expression... caused severe engine wear when notdetected and removed by slow purification in the ship's fuel treatment system Figure 2, 10 Oil Refinery processes Machinery service systems and equipment 53 Fuel testing Bunkers are classified as Gas Oil, Light and Marine Diesel Oil, Intermediate Fuel Oil and Marine or Bunker (C) Fuel Oil The delivery note specifies the type of fuel, amount, viscosity, specific gravity, flash... Figure 2. 16 continuous separation will take place Since the arrangement is a very crude U-tube containing two liquids of different specific gravities, the height of the liquid in the two legs will have the relationship where: pl — density of oil ph = density of water Machinery service systems and equipment 61 Figure 2. 15 Conical disc stack Separator bowl: (courtesy Alpha-Laval) 1 Bowl hood 2 Lock... usually the dimension h which is varied, and this is done by the use of dam rings (sometimes called gravity discs) of different diameters Normally a 62 Machinery service systems and equipment Figure 2. 16 Hydrostatic situation in gravity settling tank Figure 2. 17 Hydrostatic seal in a disc type centrifugal bow (Penwalt Ltd) table is provided in the instruction book for the machine, giving the disc diameter... temperatures The constituents or fractions are collected separately in a 52 Machinery service systems and equipment distillation process Crude oil contains gaseous fuels, gasoline (petrol), kerosene (paraffin), gas oils, distillate diesel fuels and lubricating oils which can be collected from the fractionating tower (Figure 2. 10) where they condense out at the different levels maintained at appropriate . a 48 Machinery service systems and equipment Pressure Switch Sailing B.f Momma Working Pressure lead Follow Cut Out Coi In Cul Out Cut In 35 35 30 34 29 30 30 25 29 24 26 25 2) 24 20 8 8 7 7 . will reduce the volumetric efficiency of Figure 2. 2 Air compressor configurations Machinery service systems and equipment 43 Figure 2. 3 Compressor indicator diagram (courtesy Ham worthy Engineering. (19 72) Titanium in marine engineering, Trans I Mar E, 84, paper 16. Conde, ]. F. G. (1985) New materials for the marine and offshore industry, Trans I Mar E, 97, paper 24 . Shone,