Theoretical Knowledge of Construction and Operation of Marine Boilers Including Materials Used
Total Page:16
File Type:pdf, Size:1020Kb
Competence No.6 (Course covered 6.2) 6.2/9 . Theoretical knowledge of construction and operation of marine boilers including materials used. A boiler is a closed pressure vessel wherein steam is generated by boiling distilled water / feed water under pressure. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water. Boilers are fitted with one steam drum and one water drum each to ensure steam and water can be separated. There must also be a variety of fittings and controls to ensure that fuel oil, air and feed water supplies are matched to the demand for steam. Fig 1 Requirement of a Marine Boiler: • It should be capable of generating the maximum quantity of steam with minimum fuel consumption • It should be light in weight and should not occupy much space • It should have safe working conditions • The initial cost, installation cost and maintenance cost of the boiler should be low. • It should be accessible for easy inspection and repair. • It should be capable of quick starting and should be able to meet rapid variations of load. Types of boilers: • Fire tube boilers • Water tube boilers Fire tube boilers: In fire tube boilers, the hot gases pass though the tubes that are surrounded by water. The water is heated up and converted into steam. E.g. Cochran, Scotch boiler & Clarkson boiler. The FTB is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feed water of medium quality is used. Water tube boilers: In water tube boilers, water is circulated though the tubes and hot flue gases flow outside the tubes. e.g. Bobcock & Wilcox, Admiralty three drum, Y-160 and Foster wheeler D-type. The water tube boiler is employed for high pressure, high temperature, high capacity steam applications, e.g. providing steam for main propulsion turbines of cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low pressure stem on diesel engine powered ships. Fig.2 a. Smoke uptake m. Footing b. Economizer n. Waterwall A heat exchanger that transfers Tubes welded together to form a heat from Boiler Flue Gases to Boiler Feedwater. wall. c. Steam outlet o. Waterwall Header Saturated steam from the Steam Distribution box for waterwall and Drum to the Superheater downcomers. d. Cyclone p. Back side waterwall A device inside the steam drum that q. Boiler hood is used to prevent water and solids r. Waterwall Header from passing over with the steam. Collecting box for waterwall and e. Stay tube risers. for superheater s. Riser f. Stays Tubes in which steam is generated for superheater tubes due to high convection or radiant g. Superheated steam outlet heat. The water-steam emulsion h. Superheater rises in these tubes toward the A bank of tubes, in the exhaust gas steam drum. duct after the boiler, used to heat t. Downcomer the steam above the saturation A tube through which water flows temperature. downward. These tubes are i. Superheater Headers normally not heated, and the boiler Distribution and collecting boxes for water goes through them to supply the superheater tubes. the generating tubes. j. Water drum u. Steam drum k. Burner separates the steam from the l. Waterwall Header water. Distribution box for waterwall and v. Economizer Header downcomers. Distribution box for the economizer tubes. Construction of boilers: The construction of water tube boiler, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than that of the fire tube boiler and steam raising process is much quicker. This boiler has two drums namely steam drum (bigger in size) and water drum (smaller in size) and an integral furnace. This furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. Between these two drums, large numbers of smaller diameter generating tubes are fitted. Tubes neighboring furnace are called fire row tubes of screen tubes which act as an upriser. Large bore down comer pipes are fitted between steam drum and water drum to ensure good natural circulation of water. In this arrangement, super heater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brick work is used on the furnace floor and the burner wall. The double casing of the boiler provides a passage for the combustion air to the control of register surrounding the burner. The furnace side, the floor and roof tubes are welded into the steam and water drums. The lower water wall headers are connected by external down comer from the steam drum and upper water wall header are connected to the steam drum by riser tubes. The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over primary superheater and the economizer before passing to exhaust. Fig.3 Water circulation: In the steam generation process the feed water enters the boiler where it is heated and becomes steam. The feed water circulates from the steam drum to the water drum and is heated in the process. The water from the water drums rise up to the steam drum due to the thermo-convection current through uprisers consisting of generator row tubes and screen tubes. The downcomers fitted on each boiler bring down the relatively cooler water from the steam drum to the water drum to establish a positive circulation during normal operation. Some of the feed water passes through tubes surrounding the furnace, i.e. water wall and floor tubes, where it is heated and returned to the steam drum (in case furnace is cooled by water filled tubes). Large bore downcomer tubes are used to circulate feed water between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums. The steam is produced in a steam drum and drawn of as a ‘saturated steam’ which contains small quantities water particles. Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and dries, i.e. all traces of water are converted into steam. This superheater steam then leaves the boiler for use in the system. The temperature of this steam will be above that of the steam in the drum. An attemperator may be fitted in the system to control the temperature of the superheated steam as per requirements. Materials Used in Boiler Construction. Component Material Composition & Description Boiler Casing Mild steel plates (low C, Si, Mn, (Hot finished rolled plates) carbon) Steam Drum Mild steel plates (low C, Si, Mn (Hot finished) carbon) TS-430-490MN/m2 Fire Row & Gen. Cold drawn seamless (low C, Si, Mn, S, P (cold drawn) Row Tubes carbon steel) S/H Tubes Cr, Molybdenum alloy steel Cr, Mo, C, Si, Mn, Mi (cold finished) S/H Tube Support Heat resistant austenitic C, Si, Mn, Ni, Cr, P, S (hot finished) steel Steam Piping (S/H Cr-Mo low carbon alloy C, Si, Mn, P, S, Ni, Cr, Mo (cold range upto 9500F) steel finished) Economiser Tubes Cold drawn seamless steel C, Si, Mn, P (stud resistance welded to tubes) Water Drum Mild steel plates (low C, Si, Mn (hot finished rolled plate) carbon) 6.2/10. List the services provided by boilers and the typical pressures used. • For main engine propulsion/turbines (in case of steam ships) • For power generation (to run steam turbo generators) • For running auxiliaries (in case of steam ships) • For soot blowing and for the steam atomized burners. • For fresh water generation (Evaporators) • For fire major fighting (steam drenching) • For heating duties (ME fuel oil heater, Galley supply, Purifier, Calorifier, Galley, Accommodation heating, Sea chests tracer lines for pipeline heating) • For cargo heating • For fuel treatment plant tank coil heating • For deck machineries • For running Cargo pump turbines • For operating bilge, stripping and other steam driven pumps. • For tank washing in tanker ships and general cleaning. • For using as a steam ejector media for ejector pumps and vacuum devices • For Driving steam driven deck machineries like winches etc., Pressures used: The working pressure used in marine boilers will vary from boiler to boiler as required. Still the normal working pressure of boilers used is as below:- For Steam ships High pressure 60 bar and above For Motor ships Low pressure 6-15 bar Medium pressure 17-30 bar For Tanker Vessels Medium pressure 17-30 bar. 6.2/11. Define a) Fire tube boiler b) Water tube boiler c) Packaged boiler and briefly explain the differences and why one type of boiler is preferred over other Fire tube boilers: In fire tube boilers, the hot gases pass though the tubes that are surrounded by water. The water is heated up and converted into steam. e.g. Cochran, Scotch boiler & Clarkson boiler. The FTB is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feed water of medium quality is used. Water tube boilers: In water tube boilers, water is circulated though the tubes and hot flue gases flow outside the tubes.