Jimei university Lesson 16 Marine boilers Marine boilers.

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Lesson 16Lesson 16

Marine Marine boilersboilers

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There are two There are two distinctdistinct types of types of marine boilers in use on board marine boilers in use on board ship, ship,

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the fire-tube boiler(the fire-tube boiler( 烟管锅炉、烟管锅炉、火管锅炉火管锅炉 )) in in whichwhich the hot ga the hot gases from the ses from the furnacesfurnaces pass thro pass through the tubes while the water iugh the tubes while the water is on the outside, s on the outside,

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and and the water-tube boiler(the water-tube boiler( 水水管锅炉管锅炉 )) in in whichwhich the water fl the water flows through the inside of the tows through the inside of the tubes while the hot furnace gasubes while the hot furnace gases pass around the outside.es pass around the outside.

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The water-tube boilerThe water-tube boiler is employ is employed ed forfor high pressure, high temperahigh pressure, high temperatureture, , high capacity steam applicatihigh capacity steam applicati

onsons, , e.g.e.g. providing steam for mai providing steam for main propulsion turbines or n propulsion turbines or cargo pucargo pump(mp( 货油泵货油泵 )) turbines. turbines.

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

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Firetube boilers were commonly uFiretube boilers were commonly used with steam reciprocating engised with steam reciprocating engines on board ship, but because of nes on board ship, but because of their their comparativecomparative poor efficiency poor efficiency and low and low power/weight ratio(power/weight ratio( 功率功率重量比重量比 )),, these systems are no lon these systems are no longer installed for propulsion plant.ger installed for propulsion plant.

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

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Water tube boilers have distinWater tube boilers have distin

ct advantages ct advantages overover firetube b firetube boilers. oilers.

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

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withoutwithout the danger experienced i the danger experienced in firetube boilers having larger ten firetube boilers having larger temperature differences betweenmperature differences between totop and bottom parts of the boiler cp and bottom parts of the boiler causing unequal expansion ausing unequal expansion resultinresulting ing in mechanical straining(mechanical straining( 机械变机械变形形 ).).

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(ii) The steam and water (ii) The steam and water drumsdrums(( 锅筒锅筒 )) are small in diameter are small in diameter comcompared with the large pared with the large shell(shell( 锅壳锅壳 )) oof the firetube boilersf the firetube boilers, therefore the, therefore they are stronger and suitable for muy are stronger and suitable for much higher steam pressure. ch higher steam pressure.

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(iii) Less mass of water is (iii) Less mass of water is

carried in the boiler, carried in the boiler, hencehence a a saving in weight and less spacesaving in weight and less space is is taken uptaken up for the same for the same output. output.

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The watertube boiler The watertube boiler doesdoes howev however require greater er require greater skill skill in operatinin operating and maintainingg and maintaining ，， especially especially wiwith regard toth regard to the purity of the feed the purity of the feed water, and its water, and its upkeep (upkeep ( 维修费维修费 )) is is more more costlycostly than the firetube boilethan the firetube boiler.r.

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A later development of water-A later development of water-tube boilers is the tube boilers is the bentbent tube tube design.design.

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This boiler has two drums, an This boiler has two drums, an integral furnace and is often integral furnace and is often referred to asreferred to as the ‘D’ type the ‘D’ type because of its shape.because of its shape.

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

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These waterwall tubes are conThese waterwall tubes are connectednected eithereither to upper and low to upper and lowerer headers(headers( 联箱联箱 )) oror a lower h a lower header and the steam drum. eader and the steam drum.

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Between the steam drum and tBetween the steam drum and the smaller water drum below, lhe smaller water drum below, large numbers of smaller diamearge numbers of smaller diameter ter generating tubes(generating tubes( 蒸发管蒸发管束束 )) are fitted. are fitted.

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These provide the main heat These provide the main heat transfer surfaces for steam transfer surfaces for steam generation.generation.

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Large bore pipes or Large bore pipes or downcomedowncomers(rs( 下降管下降管 )) are fitted between are fitted between the steam and water drums to the steam and water drums to ensure good natural circulatioensure good natural circulation of the water.n of the water.

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A A donkey boiler(donkey boiler( 辅助锅炉辅助锅炉 )) is is an auxiliary boiler sometimes an auxiliary boiler sometimes iincluded in a steamship’s installncluded in a steamship’s install

ationation forfor supplying steam to t supplying steam to the auxiliaries in port,he auxiliaries in port,

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and in and in motorshipsmotorships with steam with steam

auxiliariesauxiliaries forfor supplying supplying steam steam at seaat sea and and in portin port..

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

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Composite boilers(Composite boilers( 组合式锅炉组合式锅炉 )) a are often fitted re often fitted whereinwherein the gener the generation of steam can be maintained ation of steam can be maintained by oil firing when the exhaust gas by oil firing when the exhaust gas temperature falls temperature falls due to slow runndue to slow running of the enginesing of the engines, or , or cease when tcease when the engines are stoppedhe engines are stopped..

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Boiler Boiler mountings(mountings( 附件附件 ))

Certain Certain fittings(fittings( 附件附件 )) are nec are necessary on a boiler to ensure its essary on a boiler to ensure its safe operation. safe operation.

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They are usually referred to as They are usually referred to as boiler mountings. The boiler mountings. The mountings mountings usually found on a usually found on a boilerboiler are: are:

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Safety valvesSafety valves

These are mounted These are mounted in pairsin pairs to to protect the boiler protect the boiler against(from) against(from) overpressureoverpressure. .

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Once the valve lifting pressure Once the valve lifting pressure is set is set in the presence ofin the presence of a a SurveSurveyor(yor( 验船师验船师 )) it is locked and c it is locked and cannot be changed. annot be changed.

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

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Main steam stop valveMain steam stop valve

This valve is fitted in the main This valve is fitted in the main steam supply line and is steam supply line and is usually of the non-return type.usually of the non-return type.

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Auxiliary steam stop valveAuxiliary steam stop valve

This is a smaller valve This is a smaller valve fitted in fitted in the auxiliary steam supply linethe auxiliary steam supply line, , and is usually of the non-and is usually of the non-return type.return type.

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Feed checkFeed check or control valve or control valve((给水止回阀给水止回阀或控制阀或控制阀 ))

A pair of valves are fitted: A pair of valves are fitted: oneone is the main valve, is the main valve, the otherthe other the the auxiliary or standby. They are auxiliary or standby. They are non-return valves and must non-return valves and must give an indication of their open give an indication of their open and closed position.and closed position.

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Water level gaugeWater level gauge

Water level gauges or ‘gauge Water level gauges or ‘gauge glasses’ are fitted in pairs, glasses’ are fitted in pairs, at at opposite ends of the boileropposite ends of the boiler, the , the construction of the level gauge construction of the level gauge depends on the boiler depends on the boiler pressure.pressure.

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Air release cockAir release cock

These are fitted in the headers, These are fitted in the headers, boiler drum, etc., to release air boiler drum, etc., to release air when filling the boiler or when filling the boiler or initially initially raisingraising steamsteam..

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SamplingSampling connection connection

A water outlet cock and A water outlet cock and cooling arrangement is cooling arrangement is provided for the sampling and provided for the sampling and analysisanalysis of feed water. of feed water.

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Blow down valve(Blow down valve( 排污阀排污阀 ))

This valve enables water to be This valve enables water to be blow down or emptied from blow down or emptied from the boiler. It may be used the boiler. It may be used when partially or completely when partially or completely emptying the boiler.emptying the boiler.

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Scum valve(Scum valve( 上排污阀上排污阀 ))

A A shallowshallow dish dish positioned at the positioned at the normal water levelnormal water level is connected is connected to the scumto the scum valve. valve.

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

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Automatic feed water Automatic feed water regulatorregulator

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

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Sootblowers(Sootblowers( 吹灰器吹灰器 ))

Operated by steam or Operated by steam or compressed air, they are act to compressed air, they are act to blow away blow away sootsoot and the and the products of combustion from products of combustion from the tube surfacesthe tube surfaces..

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Waste heat Waste heat recoveryrecovery system system (W. H. R)(W. H. R)

Generation of electric power from Generation of electric power from a a steam turbo-generatorsteam turbo-generator and the s and the supply of upply of saturated steam(saturated steam( 饱合蒸饱合蒸汽汽 )) to heating services by to heating services by exhausexhaust gas heat recovery systemst gas heat recovery systems is e is effected in many ships. ffected in many ships.

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The type of W. H. R. unit The type of W. H. R. unit mainmainly fitted nowadays for medium tly fitted nowadays for medium to large outputso large outputs is made of is made of horizhorizontal extended(ontal extended( 水平分布水平分布 ) sur) surface steel tubesface steel tubes, ,

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force-circulated with water takforce-circulated with water taken from an oil fired boiler and en from an oil fired boiler and returned as water/vapour returned as water/vapour emuemulsionlsion. .

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

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Additional heat Additional heat extraction fromextraction from(( 提取提取 )) the exhaust gases the exhaust gases can be ef can be effected by adding an fected by adding an economizereconomizer(( 省煤器，经济器省煤器，经济器 )) section section to the to the unit in plants where the arrangemunit in plants where the arrangement permits the supply of cooled cient permits the supply of cooled circulating water or feed water at trculating water or feed water at the economizer inlet.he economizer inlet.

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

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

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

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that any combustion material that any combustion material from the exhaust gasfrom the exhaust gas which which deposited on the heating deposited on the heating surfacessurfaces is thoroughly cooled is thoroughly cooled below the ignition point.below the ignition point.

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Circulation should be stopped Circulation should be stopped and the system and the system securedsecured only if only if a long stay in port is expecteda long stay in port is expected ..

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When leaving an When leaving an intermediate intermediate port(port( 中途港中途港 )), , if the system has if the system has been been depressurized(depressurized( 减压减压 )),, cir circulation of the W. H. R. unit shculation of the W. H. R. unit should be commenced before the ould be commenced before the main engines are started, main engines are started,

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and any air and any air liberatedliberated by the hot by the hot circulating watercirculating water (should be) (should be) released from the header air released from the header air vents.vents.

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

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Intervals Intervals between blowingbetween blowing may may be extended when observation be extended when observation of the of the funnelfunnel shows that soot shows that soot discharge is small. discharge is small.

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

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

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The drain The drain provided at the provided at the lowest point of the inlet trunk lowest point of the inlet trunk immediatelyimmediately below the W. H. R. below the W. H. R. unitunit must be proved clear must be proved clear prior prior to and during the washing to and during the washing operationoperation. .

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

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The output can be reduced by The output can be reduced by bypassing the feed bypassing the feed heaterheater on on either the feed water or either the feed water or circulating water sides, circulating water sides,

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soso increasing the temperature increasing the temperature at the economizer inlet and at the economizer inlet and reducing gas/water reducing gas/water temperature differences in this temperature differences in this heat exchanger section. heat exchanger section.

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Other automatic or manual coOther automatic or manual control arrangements may be control arrangements may be controlling flow through ntrolling flow through an exhaan exhaust gas bypass ductust gas bypass duct by means o by means of f dampers(dampers( 档板档板 )), ,

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

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Valves must be fully open or Valves must be fully open or fully shut and fully shut and not left in an not left in an intermediate positionintermediate position. .

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READING MATERIALREADING MATERIAL

A. EXTERNAL EXAMINATIONS DURING

AUXILIARY BOILER SURVEYSSURVEYS

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Commencing the external examCommencing the external examination at the boiler crownination at the boiler crown it is it is sometimes found that sometimes found that wastagewastage(( 损坏损坏 )) exists exists beneath beneath dampdamp(( 潮湿的潮湿的 ) lagging() lagging( 保温层保温层 )) ar around leaking mountings, ound leaking mountings,

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and in and in riveted(riveted( 铆钉铆钉 )) boilers a boilers any ny bulges(bulges( 凸出部分凸出部分 )) in the la in the lagging should always be gging should always be investiinvestigated(gated( 详细检查详细检查 )), as often the, as often they are an indication of a leakage y are an indication of a leakage and wasted steam. and wasted steam.

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In boilers with smoke tubes,In boilers with smoke tubes, the the smoke box should be opened asmoke box should be opened and tube ends examined for leand tube ends examined for leakage, kage, particular attention being particular attention being paid to the paid to the stay tubes(stay tubes( 牵条管牵条管 ))..

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Thin Thin plain tubesplain tubes, if leaking at , if leaking at their ends, are best renewed, their ends, are best renewed, and in the case of leakage at and in the case of leakage at the the threadsthreads of stay tubes, of stay tubes, providedprovided the tubes are of the tubes are of round section round section andand

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andand a a fillet weld(fillet weld( 填角焊接填角焊接 )) of of tt+3.2 mm (1/8 in) (where +3.2 mm (1/8 in) (where tt is th is the tube thickness) can be appliee tube thickness) can be applied, then this, where d, then this, where practicablepracticable(( 能实行的能实行的 )), is the obvious re, is the obvious remedy. medy.

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Further down the boiler it is soFurther down the boiler it is sometimes found that metimes found that leaking leaking mumudhole(dhole( 出泥渣孔出泥渣孔 )) doors doors have c have caused wastage of the door opeaused wastage of the door opening and also the internal jointning and also the internal jointing faces of the shell. ing faces of the shell.

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Building up of Building up of the shell openingthe shell opening b by welding as repair is not y welding as repair is not feasiblefeasible(( 可行的可行的 )) as, after welding, it is p as, after welding, it is practically impossible ractically impossible in most small in most small boilersboilers to face up the inner jointin to face up the inner jointing surface. g surface.

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

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In the In the firebox(firebox( 烟箱烟箱 )) or furnac or furnace, of either the cylindrical or e, of either the cylindrical or hhemispherical(emispherical( 半球状的半球状的 )) type, type, particular attention must be giparticular attention must be given to any signs of ven to any signs of distortiondistortion((变形变形 )), ,

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and and toto thermal cracking at positio thermal cracking at positions where it is judged heat concentns where it is judged heat concentrations are likely to occur rations are likely to occur when thwhen the boiler is being firede boiler is being fired, or , or through through sscale encrustations(cale encrustations( 结垢结垢 )) being pr being present on the water sideesent on the water side. .

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The The flueflue pipe outlet from pipe outlet from Cochran boilersCochran boilers and the flat and the flat horizontal tube plate of some horizontal tube plate of some Spanner type boilersSpanner type boilers are two are two examples—examples—

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

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In the case of riveted vertical bIn the case of riveted vertical boilers, leakage and wastage sooilers, leakage and wastage sometimes occur metimes occur in way ofin way of the the ththree-ply riveting(ree-ply riveting( 三层铆焊三层铆焊 )), ,

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which is embodied in the constwhich is embodied in the construction, at the junction of the ruction, at the junction of the double riveted lap double riveted lap seam(seam( 缝缝 )) o of the shell and the f the shell and the ogee ring(Sogee ring(S形环形环 )). .

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

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inserting new piece of plate aninserting new piece of plate and raising a d raising a butt welded(butt welded( 对接对接焊接焊接 )) vertical seam on either s vertical seam on either side of it and, if necessary, part ide of it and, if necessary, part renewing the ogee ring. renewing the ogee ring.

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If this repair is If this repair is contemplated(contemplated( 计计划划 )), it will be appreciated that the , it will be appreciated that the two new vertical welded seams mtwo new vertical welded seams must be done under Class I welding ust be done under Class I welding conditions conditions as far asas far as practicable, w practicable, with full X-ray and local stress relieith full X-ray and local stress relief.f.

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B. STARTING OF AN B. STARTING OF AN AUXILIARY BOILERAUXILIARY BOILER

Prior to starting the boiler, Prior to starting the boiler, attention should be paid to the attention should be paid to the following items and through following items and through checking should be made on checking should be made on them. them.

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BoilerBoiler

1. All pressure parts are free 1. All pressure parts are free from foreign material.from foreign material.

2. All gas side-heating surfaces 2. All gas side-heating surfaces are clean and all are clean and all refractorsrefractors are are in good condition. in good condition.

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3. The furnace bottom and the 3. The furnace bottom and the burner wind box have been burner wind box have been cleaned of oil and other cleaned of oil and other foreign material.foreign material.

4. All personnel are clear. 4. All personnel are clear.

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

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6. Inspect the safety valve and 6. Inspect the safety valve and see that the see that the gag(gag( 压板、塞盖压板、塞盖 )) has been removed and the has been removed and the EasiEasing lever(ng lever( 安全阀的开启杆安全阀的开启杆 )) is i is in good condition.n good condition.

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7. Open the 7. Open the root valvesroot valves for for all all instruments and controls instruments and controls connected to the drumconnected to the drum. .

8. Open the vent valve of the 8. Open the vent valve of the boiler drum.boiler drum.

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

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10. Check and close all 10. Check and close all blow-ofblow-off valves(f valves( 排污阀排污阀 )) and drain va and drain valves.lves.11. Fill the boiler till water lev11. Fill the boiler till water level appears 25 to 50 mm high in el appears 25 to 50 mm high in the gauge glass. the gauge glass.

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

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a) When the boiler has been a) When the boiler has been lailaid up(d up( 贮存贮存 , , 搁置搁置 )) wet (Refer wet (Refer to 'Boiler protection during to 'Boiler protection during ououtagetage'), drain the boiler till the '), drain the boiler till the water level falls down to the bowater level falls down to the bottom of the gauge glass, ttom of the gauge glass,

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

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

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Use Use distilled water(distilled water( 蒸馏水蒸馏水 )) fo for feed water whenever possible.r feed water whenever possible. Boiler compoundBoiler compound shall be add shall be added as instructed by the expert ed as instructed by the expert of boiler water treatment. of boiler water treatment.

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Fuel Burning EquipmentFuel Burning Equipment

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

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3. Make sure that all 3. Make sure that all burner burner aiair registers(r registers( 调风器调风器 )) are clean are clean and move smoothly.and move smoothly.

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Lighting off and Pressure Lighting off and Pressure RaisingRaising

1. Make sure that the boiler 1. Make sure that the boiler water level in the water level water level in the water level gauge is correct. gauge is correct.

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

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

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3. Prior to lighting off, purge t3. Prior to lighting off, purge the furnace thoroughly by operhe furnace thoroughly by operating the forced ating the forced draft(draft( 通风装通风装置置 )) fan, as explosive gases ma fan, as explosive gases may be present in the furnace. y be present in the furnace.

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4.In the start-up from cold con4.In the start-up from cold condition, oil flow to the burner shdition, oil flow to the burner should be controlled to a minimuould be controlled to a minimum to m to sustain(sustain( 维持，支撑维持，支撑 )) bur burning for about 30 minutes aftening for about 30 minutes after lighting off. r lighting off.

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

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

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

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

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C. C. VACUUM DISTILLATION SVACUUM DISTILLATION SYSTEMYSTEM ((真空蒸馏系统真空蒸馏系统 ))

Distillation is the production of Distillation is the production of

pure water from sea water pure water from sea water byby evaporation(evaporation( 蒸发蒸发 )) and re- and re-concondensing(densing( 冷凝冷凝 )): :

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On motor ship, On motor ship, distilled waterdistilled water is usually produced is usually produced as a result as a result ofof evaporating sea water by a evaporating sea water by a vvacuum boiling(acuum boiling( 真空沸腾真空沸腾 )) or a or a vacuum flash(vacuum flash( 真空闪发真空闪发 )) proc process. ess.

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The machine used is called an The machine used is called an ‘‘evaporator(evaporator( 蒸发器、造水蒸发器、造水机机 ))’, although the word ‘’, although the word ‘distilldistiller(er( 蒸馏器蒸馏器 ))’ is also used. ’ is also used.

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

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the higher the pressure the higher the saturation temperature, the lower the pressure the lower the saturation temperature.

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

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Vacuum boiling evaporatorVacuum boiling evaporator((真空沸腾式造水机真空沸腾式造水机 ))

AA single-effect single-effect vacuum boiling vacuum boiling evaporator consists of a evaporator consists of a evaporating section and a evaporating section and a condensing section. condensing section.

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Vacuum boiling evaporator

5—Brine pump

6—Air ejector

2—Sea pump

4—Distillate pump

Jacketwater

Sea water

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The lower part of the evaporatThe lower part of the evaporating section is the heater which ing section is the heater which houses the houses the stack(stack( 管束管束 )) of sea of sea water tubes. water tubes.

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

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

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The steam The steam produced produced rises to the urises to the upper part of the evaporating sectipper part of the evaporating section, and passes through a water seon, and passes through a water separator or parator or demister(demister( 汽水分离器汽水分离器 )) which prevents water droplets passiwhich prevents water droplets passing throughng through. .

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In the condensing section the sIn the condensing section the steam becomes pure water, whiteam becomes pure water, which is drawn off by ch is drawn off by a distillate pa distillate pump(ump( 凝水泵凝水泵 )). .

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The remainderThe remainder of the sea water of the sea water fefedd, the brine, is drawn out by a , the brine, is drawn out by a bribrine pump(ne pump( 排盐泵排盐泵 )). An. An air ejectorair ejector(( 喷射泵式真空泵喷射泵式真空泵 )) draws out the draws out the air from the evaporator to maintaair from the evaporator to maintain the vacuum in it. in the vacuum in it.

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Flash evaporator(Flash evaporator( 闪发蒸发闪发蒸发器器 ))

Flush evaporation is the result Flush evaporation is the result of liquid of liquid containing a reasonabcontaining a reasonable amount of le amount of sensible heat(sensible heat( 显显热热 )) at a particular pressure at a particular pressure beibeing admitted to a chamber at long admitted to a chamber at lower pressure. wer pressure.

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

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A single stage flash evaporator A single stage flash evaporator consists of a flash chamber, a dconsists of a flash chamber, a demister and a vapour condenseemister and a vapour condenser. r.

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

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

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The demister removes any The demister removes any water droplets from the steam water droplets from the steam as it risesas it rises. .

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The vapour is then condensed, The vapour is then condensed, collected and pumped out by tcollected and pumped out by the distillates pump. he distillates pump.

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Any unevaporated sea water is Any unevaporated sea water is pumped out by the brine pumpumped out by the brine pump.p.

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