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Jimei university Lesson 16 Marine boilers Marine boilers.

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1 Jimei university Lesson 16 Marine boilers Marine boilers

2 Jimei university There are two distinct types of marine boilers in use on board ship,

3 Jimei university the fire-tube boiler( ) in which the hot gases from the furnaces pass through the tubes while the water is on the outside,

4 Jimei university and the water-tube boiler( ) in which the water flows through the inside of the tubes while the hot furnace gases pass around the outside.

5 Jimei university The water-tube boiler is employed for high pressure, high temperature, high capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump( ) turbines.

6 Jimei university Fire-tube boilers are used for auxiliary purposes to provide smaller quantities of low pressure steam on diesel engine powered ships.

7 Jimei university Firetube boilers were commonly used with steam reciprocating engines on board ship, but because of their comparative poor efficiency and low power/weight ratio( ), these systems are no longer installed for propulsion plant.

8 Jimei university There are many types of watertube boilers in general use on board ship and each manufacturer has a variety of designs to offer depending upon requirements.

9 Jimei university Water tube boilers have distinct advantages over firetube boilers.

10 Jimei university (i) Circulation of the water is natural and immediate on lighting up, thus steam can be raised from cold water in a shorter period of time without

11 Jimei university without the danger experienced in firetube boilers having larger temperature differences between top and bottom parts of the boiler causing unequal expansion resulting in mechanical straining( ).

12 Jimei university (ii) The steam and water drums( ) are small in diameter compared with the large shell( ) of the firetube boilers, therefore they are stronger and suitable for much higher steam pressure.

13 Jimei university (iii) Less mass of water is carried in the boiler, hence a saving in weight and less space is taken up for the same output.

14 Jimei university The watertube boiler does however require greater skill in operating and maintaining especially with regard to the purity of the feed water, and its upkeep ( ) is more costly than the firetube boiler.

15 Jimei university A later development of water- tube boilers is the bent tube design.

16 Jimei university This boiler has two drums, an integral furnace and is often referred to as the D type because of its shape.

17 Jimei university

18 The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes.

19 Jimei university These waterwall tubes are connected either to upper and lower headers( ) or a lower header and the steam drum.

20 Jimei university Between the steam drum and the smaller water drum below, large numbers of smaller diameter generating tubes( ) are fitted.

21 Jimei university These provide the main heat transfer surfaces for steam generation.

22 Jimei university Large bore pipes or downcomers( ) are fitted between the steam and water drums to ensure good natural circulation of the water.

23 Jimei university A donkey boiler( ) is an auxiliary boiler sometimes included in a steamships installation for supplying steam to the auxiliaries in port,

24 Jimei university and in motorships with steam auxiliaries for supplying steam at sea and in port.

25 Jimei university In motor ships, an exhaust gas boiler (sometimes called a waste-heat boiler) is often used to recover some of the heat carried in the exhaust gases from the main engines.

26 Jimei university Composite boilers( ) are often fitted wherein the generation of steam can be maintained by oil firing when the exhaust gas temperature falls due to slow running of the engines, or cease when the engines are stopped.

27 Jimei university Boiler mountings( ) Certain fittings( ) are necessary on a boiler to ensure its safe operation.

28 Jimei university They are usually referred to as boiler mountings. The mountings usually found on a boiler are:

29 Jimei university Safety valves These are mounted in pairs to protect the boiler against(from) overpressure.

30 Jimei university Once the valve lifting pressure is set in the presence of a Surveyor( ) it is locked and cannot be changed.

31 Jimei university The valve is arranged to open automatically at the pre-set blow off pressure.

32 Jimei university Main steam stop valve This valve is fitted in the main steam supply line and is usually of the non-return type.

33 Jimei university Auxiliary steam stop valve This is a smaller valve fitted in the auxiliary steam supply line, and is usually of the non- return type.

34 Jimei university Feed check or control valve( ) A pair of valves are fitted: one is the main valve, the other the auxiliary or standby. They are non-return valves and must give an indication of their open and closed position.

35 Jimei university Water level gauge Water level gauges or gauge glasses are fitted in pairs, at opposite ends of the boiler, the construction of the level gauge depends on the boiler pressure.

36 Jimei university Air release cock These are fitted in the headers, boiler drum, etc., to release air when filling the boiler or initially raising steam.

37 Jimei university Sampling connection A water outlet cock and cooling arrangement is provided for the sampling and analysis of feed water.

38 Jimei university Blow down valve( ) This valve enables water to be blow down or emptied from the boiler. It may be used when partially or completely emptying the boiler.

39 Jimei university Scum valve( ) A shallow dish positioned at the normal water level is connected to the scum valve.

40 Jimei university This enables the blowing down or removal of scum and impurities from the water surface.

41 Jimei university Automatic feed water regulator Fitted in the feed line prior to the main check valve, this device is essential to ensure the correct water level in the boiler during all load conditions.

42 Jimei university Sootblowers( ) Operated by steam or compressed air, they are act to blow away soot and the products of combustion from the tube surfaces.

43 Jimei university Waste heat recovery system (W. H. R) Generation of electric power from a steam turbo-generator and the supply of saturated steam( ) to heating services by exhaust gas heat recovery systems is effected in many ships.

44 Jimei university The type of W. H. R. unit mainly fitted nowadays for medium to large outputs is made of horizontal extended( ) surface steel tubes,

45 Jimei university force-circulated with water taken from an oil fired boiler and returned as water/vapour emulsion.

46 Jimei university The boiler is fired whenever the demand for steam exceeds that available from exhaust gas heating.

47 Jimei university Additional heat extraction from( ) the exhaust gases can be effected by adding an economizer( ) section to the unit in plants where the arrangement permits the supply of cooled circulating water or feed water at the economizer inlet.

48 Jimei university Similar results, but with some disadvantages, can be obtained by producing low-pressure steam from a separate heat exchange unit at the tail end of the gas stream.

49 Jimei university Circulation of the W. H. R. plant(equipment) must be maintained at all times when the main engine is on load.

50 Jimei university Also it must be continued for at least two hours after the engine has been stopped to ensure that

51 Jimei university that any combustion material from the exhaust gas which deposited on the heating surfaces is thoroughly cooled below the ignition point.

52 Jimei university *********************** Circulation should be stopped and the system secured only if a long stay in port is expected.

53 Jimei university When leaving an intermediate port( ), if the system has been depressurized( ), circulation of the W. H. R. unit should be commenced before the main engines are started,

54 Jimei university and any air liberated by the hot circulating water (should be) released from the header air vents.

55 Jimei university When soot blowers are fitted to the unit, the fires should be lit in the oil-fired boiler to maintain steam pressure, the steam lines drained and the soot blowers operated once daily at sea.

56 Jimei university Intervals between blowing may be extended when observation of the funnel shows that soot discharge is small.

57 Jimei university A guide to their effectiveness is given by a change in the funnel exhaust temperature immediately after soot blower operation.

58 Jimei university Water washing of the heating surface must also be carried out at intervals of three months, or such lesser intervals as may be decided upon from sighting of the surface during intermediate inspections.

59 Jimei university The drain provided at the lowest point of the inlet trunk immediately below the W. H. R. unit must be proved clear prior to and during the washing operation.

60 Jimei university At a constant main engine power level, W.H.R. systems have a rising pressure characteristic on a falling steam demand.

61 Jimei university The output can be reduced by bypassing the feed heater on either the feed water or circulating water sides,

62 Jimei university so increasing the temperature at the economizer inlet and reducing gas/water temperature differences in this heat exchanger section.

63 Jimei university Other automatic or manual control arrangements may be controlling flow through an exhaust gas bypass duct by means of dampers( ),

64 Jimei university and, when sub-divided inlet headers are provided, by closing one or more of the isolating valves supplying circulating water to separate sections of the steam generator.

65 Jimei university Valves must be fully open or fully shut and not left in an intermediate position.

66 Jimei university READING MATERIAL SURVEYS A. EXTERNAL EXAMINATIONS DURING AUXILIARY BOILER SURVEYS

67 Jimei university Cochran spheroid vertical boiler

68 Jimei university Spanner vertical boiler

69 Jimei university Commencing the external examination at the boiler crown it is sometimes found that wastage( ) exists beneath damp( ) lagging( ) around leaking mountings,

70 Jimei university and in riveted( ) boilers any bulges( ) in the lagging should always be investigated( ), as often they are an indication of a leakage and wasted steam.

71 Jimei university In boilers with smoke tubes, the smoke box should be opened and tube ends examined for leakage, particular attention being paid to the stay tubes( ).

72 Jimei university Thin plain tubes, if leaking at their ends, are best renewed, and in the case of leakage at the threads of stay tubes, provided the tubes are of round section and

73 Jimei university and a fillet weld( ) of t+3.2 mm (1/8 in) (where t is the tube thickness) can be applied, then this, where practicable( ), is the obvious remedy.

74 Jimei university Further down the boiler it is sometimes found that leaking mudhole( ) doors have caused wastage of the door opening and also the internal jointing faces of the shell.

75 Jimei university Building up of the shell opening by welding as repair is not feasible( ) as, after welding, it is practically impossible in most small boilers to face up the inner jointing surface.

76 Jimei university A satisfactory repair can however be made by positioning inside the boiler a false seating ring for the door, and then fillet welding it in position working from the outside.

77 Jimei university In the firebox( ) or furnace, of either the cylindrical or hemispherical( ) type, particular attention must be given to any signs of distortion( ),

78 Jimei university and to thermal cracking at positions where it is judged heat concentrations are likely to occur when the boiler is being fired, or through scale encrustations( ) being present on the water side.

79 Jimei university The flue pipe outlet from Cochran boilers and the flat horizontal tube plate of some Spanner type boilers are two examples

80 Jimei university in the case of the former, when the cracking is extensive, the usual method of repair is to weld in a new flue pipe with a flanged extension which takes in part of the furnace crown.

81 Jimei university In the case of riveted vertical boilers, leakage and wastage sometimes occur in way of the three-ply riveting( ),

82 Jimei university which is embodied in the construction, at the junction of the double riveted lap seam( ) of the shell and the ogee ring(S ).

83 Jimei university In extreme cases a satisfactory repair can be made by cutting out the double riveted seam in the lower course of the boiler shell,

84 Jimei university inserting new piece of plate and raising a butt welded( ) vertical seam on either side of it and, if necessary, part renewing the ogee ring.

85 Jimei university If this repair is contemplated( ), it will be appreciated that the two new vertical welded seams must be done under Class I welding conditions as far as practicable, with full X-ray and local stress relief.

86 Jimei university B. STARTING OF AN AUXILIARY BOILER Prior to starting the boiler, attention should be paid to the following items and through checking should be made on them.

87 Jimei university Boiler 1. All pressure parts are free from foreign material. 2. All gas side-heating surfaces are clean and all refractors are in good condition.

88 Jimei university 3. The furnace bottom and the burner wind box have been cleaned of oil and other foreign material. 4. All personnel are clear. 4. All personnel are clear.

89 Jimei university 5. The manhole cover is securely tightened. All access doors on the casing are closed tight.

90 Jimei university 6. Inspect the safety valve and see that the gag( ) has been removed and the Easing lever( ) is in good condition.

91 Jimei university 7. Open the root valves for all instruments and controls connected to the drum. 8. Open the vent valve of the boiler drum.

92 Jimei university 9. Open all pressure gauge valves, check and see all valves in the pressure gauge tubing are open.

93 Jimei university 10. Check and close all blow- off valves( ) and drain valves. 11. Fill the boiler till water level appears 25 to 50 mm high in the gauge glass.

94 Jimei university The boiler shall be filled in the following procedure and the feed water line shall be inspected simultaneously.

95 Jimei university a) When the boiler has been laid up(, ) wet (Refer to 'Boiler protection during outage'), drain the boiler till the water level falls down to the bottom of the gauge glass,

96 Jimei university and bring the water level up again to about 50 mm high in the gauge glass.

97 Jimei university b) When the boiler has been laid up dry, fill the boiler to the water level 25 to 50 mm higher than the normal water level through the boiler feed water line.

98 Jimei university Use distilled water( ) for feed water whenever possible. Boiler compound shall be added as instructed by the expert of boiler water treatment.

99 Jimei university Fuel Burning Equipment 1. Make sure that all fuel oil lines including oil filters, etc. are in good condition. 2. Make sure that no oi1 residue is in the burner wind box.

100 Jimei university 3. Make sure that all burner air registers( ) are clean and move smoothly.

101 Jimei university Lighting off and Pressure Raising 1. Make sure that the boiler water level in the water level gauge is correct.

102 Jimei university The water level should fall when the drain valve of the level gauge is opened, and should return to the previous level when the valve is closed.

103 Jimei university 2. Attention must be paid not to mistake the empty level gauge for full of water or stains on the gauge glass for water level.

104 Jimei university 3. Prior to lighting off, purge the furnace thoroughly by operating the forced draft( ) fan, as explosive gases may be present in the furnace.

105 Jimei university 4.In the start-up from cold condition, oil flow to the burner should be controlled to a minimum to sustain( ) burning for about 30 minutes after lighting off.

106 Jimei university It is desirable to bring the boiler to the normal pressure in at least 2.5 to 3 hours.

107 Jimei university When, the furnace is sufficiently hot, however, this period can be shortened to 0.5 to 1 hour.

108 Jimei university The sudden rise of temperature and pressure that causes severe thermal stresses and expansions of the furnace walls and other boiler parts should be avoided to prevent damages on the boiler.

109 Jimei university 5. Close the air vent valve if it begins to blow steam. 6. When the boiler is brought nearly to normal pressure, crack the main steam valve to warm the steam line gradually.

110 Jimei university C. VACUUM DISTILLATION SYSTEM ( ) Distillation is the production of pure water from sea water by evaporation( ) and re- condensing( ):

111 Jimei university On motor ship, distilled water is usually produced as a result of evaporating sea water by a vacuum boiling( ) or a vacuum flash( ) process.

112 Jimei university The machine used is called anevaporator( ), although the word distiller( ) is also used.

113 Jimei university The saturation temperature of water or steam is the temperature at which water evaporates or steam condenses and this depends upon the pressure,

114 Jimei university the higher the pressure the higher the saturation temperature, the lower the pressure the lower the saturation temperature.

115 Jimei university The saturation temperature in the vacuum evaporator is quite low, around 40 for example, the energy from diesel engine jacket water is usually used to produce distilled water.

116 Jimei university Vacuum boiling evaporator ( ) A single-effect vacuum boiling evaporator consists of a evaporating section and a condensing section.

117 Jimei university Vacuum boiling evaporator 5Brine pump 6Air ejector 2Sea pump 4Distillate pump Jacket water Sea water

118 Jimei university The lower part of the evaporating section is the heater which houses the stack( ) of sea water tubes.

119 Jimei university Hot diesel engine jacket water passes over the stack and heats the sea water passing through the tubes.

120 Jimei university Because of the reduced pressure in the machine, some of the sea water boils as it flows through the tubes.

121 Jimei university The steam produced rises to the upper part of the evaporating section, and passes through a water separator or demister( ) which prevents water droplets passing through.

122 Jimei university In the condensing section the steam becomes pure water, which is drawn off by a distillate pump( ).

123 Jimei university The remainder of the sea water fed, the brine, is drawn out by a brine pump( ). An air ejector( ) draws out the air from the evaporator to maintain the vacuum in it.

124 Jimei university Flash evaporator( ) Flush evaporation is the result of liquid containing a reasonable amount of sensible heat( ) at a particular pressure being admitted to a chamber at lower pressure.

125 Jimei university The liquid immediately changes into steam, i.e. it flashes, without boiling taking place.

126 Jimei university A single stage flash evaporator consists of a flash chamber, a demister and a vapour condenser.

127 Jimei university An air ejector or a vacuum pump is used to keep a very low pressure in the evaporator.

128 Jimei university When the sea water from the pre-heater enters the evaporator shell, some of the water flashes off into steam.

129 Jimei university The demister removes any water droplets from the steam as it rises.

130 Jimei university The vapour is then condensed, collected and pumped out by the distillates pump.

131 Jimei university Any unevaporated sea water is pumped out by the brine pump.

132 Jimei university More than one stage of evaporation can take place by admitting the sea water into chambers with progressively lower pressures, but usually, two stage flash evaporators are used on board ship.


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