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1、Lesson 16 Marine boilersThere are two distinct types of marine boilers in use on board ship, the fire-tube boiler(煙管鍋爐、火管鍋爐) in which the hot gases from the furnaces pass through the tubes while the water is on the outside, and the water-tube boiler(水管鍋爐) in which the water flows through the inside

2、of the tubes while the hot furnace gases pass around the outside.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(貨油泵) turbines.Fire-tube boilers are used for auxiliary purposes to

3、 provide smaller quantities of low pressure steam on diesel engine powered ships.Firetube boilers were commonly used with steam reciprocating engines on board ship, but because of their comparative poor efficiency and low power/weight ratio(功率重量比), these systems are no longer installed for propulsio

4、n plant.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. Water tube boilers have distinct advantages over firetube boilers. (i) Circulation of the water is natural and immediate on lighting up,

5、 thus steam can be raised from cold water in a shorter period of time without 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(機(jī)械變形). (ii) The steam and wate

6、r drums(鍋筒) are small in diameter compared with the large shell(鍋殼) of the firetube boilers, therefore they are stronger and suitable for much higher steam pressure. (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. The watert

7、ube boiler does however require greater skill in operating and maintaining，especially with regard to the purity of the feed water, and its upkeep (維修費(fèi)) is more costly than the firetube boiler.A later development of water-tube boilers is the bent tube design.This boiler has two drums, an integral fur

8、nace and is often referred to as the D type because of its shape.The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes.These waterwall tubes are connected either to upper and lower headers(聯(lián)箱) or a lower header and the steam drum. Between the steam drum and the

9、 smaller water drum below, large numbers of smaller diameter generating tubes(蒸發(fā)管束) are fitted. These provide the main heat transfer surfaces for steam generation.Large bore pipes or downcomers(下降管) are fitted between the steam and water drums to ensure good natural circulation of the water.A donkey

10、 boiler(輔助鍋爐) is an auxiliary boiler sometimes included in a steamships installation for supplying steam to the auxiliaries in port,and in motorships with steam auxiliaries for supplying steam at sea and in port.In motor ships, an exhaust gas boiler (sometimes called a waste-heat boiler) is often us

11、ed to recover some of the heat carried in the exhaust gases from the main engines. Composite boilers(組合式鍋爐) 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 stoppe

12、d.Boiler mountings(附件)Certain fittings(附件) are necessary on a boiler to ensure its safe operation. They are usually referred to as boiler mountings. The mountings usually found on a boiler are:Safety valvesThese are mounted in pairs to protect the boiler against(from) overpressure. Once the valve li

13、fting pressure is set in the presence of a Surveyor(驗船師) it is locked and cannot be changed. The valve is arranged to open automatically at the pre-set blow off pressure.Main steam stop valveThis valve is fitted in the main steam supply line and is usually of the non-return type.Auxiliary steam stop

14、 valveThis is a smaller valve fitted in the auxiliary steam supply line, and is usually of the non-return type.Feed check or control valve(給水止回閥或控制閥)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

15、 open and closed position.Water level gaugeWater 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.Air release cockThese are fitted in the headers, boiler drum, etc., to release air when filling the b

16、oiler or initially raising steam.Sampling connectionA water outlet cock and cooling arrangement is provided for the sampling and analysis of feed water.Blow down valve(排污閥)This valve enables water to be blow down or emptied from the boiler. It may be used when partially or completely emptying the bo

17、iler.Scum valve(上排污閥)A shallow dish positioned at the normal water level is connected to the scum valve. This enables the blowing down or removal of scum and impurities from the water surface.Automatic feed water regulatorFitted in the feed line prior to the main check valve, this device is essentia

18、l to ensure the correct water level in the boiler during all load conditions.Sootblowers(吹灰器)Operated by steam or compressed air, they are act to blow away soot and the products of combustion from the tube surfaces.Waste heat recovery system (W. H. R)Generation of electric power from a steam turbo-g

19、enerator and the supply of saturated steam(飽合蒸汽) to heating services by exhaust gas heat recovery systems is effected in many ships. The type of W. H. R. unit mainly fitted nowadays for medium to large outputs is made of horizontal extended(水平分布) surface steel tubes, force-circulated with water take

20、n from an oil fired boiler and returned as water/vapour emulsion. The boiler is fired whenever the demand for steam exceeds that available from exhaust gas heating. Additional heat extraction from(提取) the exhaust gases can be effected by adding an economizer(省煤器，經(jīng)濟(jì)器) section to the unit in plants wh

21、ere the arrangement permits the supply of cooled circulating water or feed water at the economizer inlet.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.Circulation of the W. H. R. plan

22、t(equipment) must be maintained at all times when the main engine is on load.Also it must be continued for at least two hours after the engine has been stopped to ensure that that any combustion material from the exhaust gas which deposited on the heating surfaces is thoroughly cooled below the igni

23、tion point. *Circulation should be stopped and the system secured only if a long stay in port is expected.When leaving an intermediate port(中途港), if the system has been depressurized(減壓), circulation of the W. H. R. unit should be commenced before the main engines are started, and any air liberated

24、by the hot circulating water (should be) released from the header air vents.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.Intervals between blowing may be e

25、xtended when observation of the funnel shows that soot discharge is small. A guide to their effectiveness is given by a change in the funnel exhaust temperature immediately after soot blower operation.Water washing of the heating surface must also be carried out at intervals of three months, or such

26、 lesser intervals as may be decided upon from sighting of the surface during intermediate inspections. 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. At a constant main engine power level,

27、W.H.R. systems have a rising pressure characteristic on a falling steam demand. The output can be reduced by bypassing the feed heater on either the feed water or circulating water sides, so increasing the temperature at the economizer inlet and reducing gas/water temperature differences in this hea

28、t exchanger section. Other automatic or manual control arrangements may be controlling flow through an exhaust gas bypass duct by means of dampers(檔板), and, when sub-divided inlet headers are provided, by closing one or more of the isolating valves supplying circulating water to separate sections of

29、 the steam generator. Valves must be fully open or fully shut and not left in an intermediate position. READING MATERIAL A. EXTERNAL EXAMINATIONS DURING AUXILIARY BOILER SURVEYS Cochran spheroid vertical boilerSpanner vertical boilerCommencing the external examination at the boiler crown it is somet

30、imes found that wastage(損壞) exists beneath damp(潮濕的) lagging(保溫層) around leaking mountings, and in riveted(鉚釘) boilers any bulges(凸出局部) in the lagging should always be investigated(詳細(xì)檢查), as often they are an indication of a leakage and wasted steam. In boilers with smoke tubes, the smoke box should

31、 be opened and tube ends examined for leakage, particular attention being paid to the stay tubes(牽條管). 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 andand a fillet weld(填角焊接) of t+3.2 mm

32、 (1/8 in) (where t is the tube thickness) can be applied, then this, where practicable(能實行的), is the obvious remedy. Further down the boiler it is sometimes found that leaking mudhole(出泥渣孔) doors have caused wastage of the door opening and also the internal jointing faces of the shell. Building up o

33、f the shell opening by welding as repair is not feasible(可行的) as, after welding, it is practically impossible in most small boilers to face up the inner jointing surface. A satisfactory repair can however be made by positioning inside the boiler a false seating ring for the door, and then fillet wel

34、ding it in position working from the outside. In the firebox(煙箱) or furnace, of either the cylindrical or hemispherical(半球狀的) type, particular attention must be given to any signs of distortion(變形), and to thermal cracking at positions where it is judged heat concentrations are likely to occur when

35、the boiler is being fired, or through scale encrustations(結(jié)垢) being present on the water side. The flue pipe outlet from Cochran boilers and the flat horizontal tube plate of some Spanner type boilers are two examplesin the case of the former, when the cracking is extensive, the usual method of repa

36、ir is to weld in a new flue pipe with a flanged extension which takes in part of the furnace crown. In the case of riveted vertical boilers, leakage and wastage sometimes occur in way of the three-ply riveting(三層鉚焊), which is embodied in the construction, at the junction of the double riveted lap se

37、am(縫) of the shell and the ogee ring(S形環(huán)). In extreme cases a satisfactory repair can be made by cutting out the double riveted seam in the lower course of the boiler shell, inserting new piece of plate and raising a butt welded(對接焊接) vertical seam on either side of it and, if necessary, part renewi

38、ng the ogee ring. If this repair is contemplated(方案), 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.B. STARTING OF AN AUXILIARY BOILER Prior to starting the boiler, attention

39、should be paid to the following items and through checking should be made on them. Boiler1. All pressure parts are free from foreign material.2. All gas side-heating surfaces are clean and all refractors are in good condition. 3. The furnace bottom and the burner wind box have been cleaned of oil an

40、d other foreign material. 4. All personnel are clear. 5. The manhole cover is securely tightened. All access doors on the casing are closed tight.6. Inspect the safety valve and see that the gag(壓板、塞蓋) has been removed and the Easing lever(平安閥的開啟桿) is in good condition.7. Open the root valves for al

41、l instruments and controls connected to the drum. 8. Open the vent valve of the boiler drum.9. Open all pressure gauge valves, check and see all valves in the pressure gauge tubing are open.10. Check and close all blow-off valves(排污閥) and drain valves.11. Fill the boiler till water level appears 25

42、to 50 mm high in the gauge glass. The boiler shall be filled in the following procedure and the feed water line shall be inspected simultaneously.a) When the boiler has been laid up(貯存, 擱置) wet (Refer to Boiler protection during outage), drain the boiler till the water level falls down to the bottom

43、 of the gauge glass, and bring the water level up again to about 50 mm high in the gauge glass.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. Use distilled water(蒸餾水) for feed water whenev

44、er possible. Boiler compound shall be added as instructed by the expert of boiler water treatment. 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. 3. Make sure that all burner ai

45、r registers(調(diào)風(fēng)器) are clean and move smoothly.Lighting off and Pressure Raising1. Make sure that the boiler water level in the water level gauge is correct. 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.

46、 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. 3. Prior to lighting off, purge the furnace thoroughly by operating the forced draft(通風(fēng)裝置) fan, as explosive gases may be present in the furnace. 4.In the start-up from col

47、d condition, oil flow to the burner should be controlled to a minimum to sustain(維持，支撐) burning for about 30 minutes after lighting off. It is desirable to bring the boiler to the normal pressure in at least 2.5 to 3 hours. When, the furnace is sufficiently hot, however, this period can be shortened

48、 to 0.5 to 1 hour. 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. 5. Close the air vent valve if it begins to blow steam.6. When the boiler is brought nearl

49、y to normal pressure, crack the main steam valve to warm the steam line gradually. C. VACUUM DISTILLATION SYSTEM (真空蒸餾系統(tǒng))Distillation is the production of pure water from sea water by evaporation(蒸發(fā)) and re-condensing(冷凝): On motor ship, distilled water is usually produced as a result of evaporating

50、 sea water by a vacuum boiling(真空沸騰) or a vacuum flash(真空閃發(fā)) process. The machine used is called an evaporator(蒸發(fā)器、造水機(jī)), although the word distiller(蒸餾器) is also used. The saturation temperature of water or steam is the temperature at which water evaporates or steam condenses and this depends upon t

51、he pressure, the higher the pressure the higher the saturation temperature, the lower the pressure the lower the saturation temperature. 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.Vacuum boiling evaporator(真空沸騰式造水機(jī))A single-effect vacuum boiling evaporator consists of a evaporating section and a condensing section. Vacuum boiling evaporator5Brine pump6Air ejector2Sea pump4Distillate pumpJacketwaterSea waterThe l