Marine Boilers

1. Marine Boilers Rajan Bhandari

2. Marine Boilers Overview • Boilers are required for all type of vessels. • For motor vessels, boilers are required to supply steam mainly for heating and auxiliary purposes. • For steam vessels, boilers are required mainly for propulsion plant, heating and auxiliary purposes. • In modern diesel vessels steam is available in port thru oil fired auxiliary boiler and at sea through exhaust gas economiser by utilising diesel engine exhaust

3. Tutorials • State the uses of steam on board ship. • Explain how the steam supply is provided on a motor vessel while in port and at sea. • State the advantages of a water-tube boiler as compared to fire-tube type. • With a suitable sketch of a boiler label four important boilermountings and state the function of each mounting. • With reference to boiler water chemistry, state the purpose of water treatment.

4. Tutorials • List the steps to raise steam in a boiler which has been shut down. • State the function of the following with reference to steam turbine plant: • Nozzles • Vacuum condenser • Astern turbine • Reduction gearing • Cascade tank • Compounding in turbines

5. Marine Boilers Learning Objectives • Need of steam on motor vessel • Classification boilers • Fire tube boilers • Water tube boilers (Drum type) • Comparison of Fire tube & Water tube boilers • Waste heat recovery in motor ship by composite blr • Boiler mountings • Boiler water chemistry

6. Purpose of Steam on Motor V/L • Heating • M/E FW and fuel oil heater • Cargo heating, oil tank heating • Accommodation heating for Air con plant • Calorifier • Turbo-generators • Cargo pumps • Steam driven deck machineries • FWG heating medium • Tank washing in tankers • Steam smothering system for scavenge fire

7. STEAM PLANT LAYOUT ON MOTOR SHIP

8. Classification of Boilers • Smoke Tube/Fire Tube • Water Tube

9. Tank type Smoke Tube Boiler

10. Classification of Boilers Fire tube boilers • Hot flue gases from furnace pass through the fire-tube banks • Tubes are immersed in water contained in the boiler shell • Heat transfer takes place across fire-tubes, from flue gases to water • Thereby producing steam Cochran spheroid boiler

11. Classification of Boilers Fire tube boilers • Hot flue gases from furnace pass through the fire-tube banks • Tubes are immersed in water contained in the boiler shell • Heat transfer takes place across fire-tubes, from flue gases to water • Thereby producing steam Cochran spheroid boiler

12. Water Tube Boiler-Concept

13. Boiler Heat Transfer System • 1. Radiant Heat from Hot Flame • 2. Heat Conduction across tube wall • 3. Convection flow of liquid where hotter portion will rise and cooler portion fall. 2 3 1

14. OPEN FEED SYSTEM – GENERAL LAYOUT EGE Fuel oil Tank Steam distribution header Condensate return header Main steam stop valve Condenser Feed check valves FWG Boiler Feed Tank Observation Tank Circulating pump Filter Feed Pumps Cascade Tank Oil drain Feed heating

15. Water Tube Boiler

16. Classification of Boilers Water tube boilers • Hot flue gases from the furnace passes around the tubes • Water circulates through the tubes • Heat transfer takes place across firetubes, from flue gases to water • Thereby producing steam • For higher pressure & temp steam generation, water tube boilers are used • Fuel consumption is less due to high efficiency over 85% • Capable of very high output • Steam can be raised very quickly from cold condition • Compact and light weight

17. Water Tube Boiler Advantages (over Smoke Tube Boiler) • Saving in Weight – about 3:1 with same heating area • Efficiency over 85% hence reduced fuel consumption • High pressures and temperature of steam production • Capable of very high output (high evaporative rate) • Compact and light weight compare to smoke tube boiler of same capacity • High temperature & pressure makes it suitable for steam turbine operation • Steam can be raised rapidly in comparison to smoke tube because of positive water circulation

18. Water Tube Boiler Advantages (cont…) • Wider safety margin in the event of explosion (smaller amount of water) • Saving space • Less thermal stress

20. Exhaust Gas Economiser • Flexibility in boiler positioning • Better efficiency • Circulation pump circulates water through the EGE at sea to produce steam. • And Aux boiler acts as a steam storage at sea • Aux boiler generates steam in port using oil fired burner Exhaust Gas

21. Composite boiler • Natural circulation heat recovery • Boiler can be simultaneously fired on oil or run on exhaust gas • Exhaust gas flow normally controlled by dampers for flow through the boiler or is by-passed to the funnel exit.

22. Composite boiler • Combination of Aux Blr and Exh Blr • Used for waste heat recovery • 28% of heat present in DE exh gas is utilised partially to produce steam at sea in exh gas boiler • In port steam is produced by auxiliary boiler

23. Boiler Mountings • Fitted directly to boiler enabling safe and efficient operation • Allows to carry out certain important or emergency tasks • List of boiler mountings • Safety valve • Water level indicator • Burner assembly • Steam main stop valve • Manholes • Blow down valve • Soot blowers • Water sampling valve • Feed water valves • Pressure gauges and pressure switches • Water level alarms & cut-outs

24. Boiler Mountings Stop valve Safety valve Air vent cock Manhole door Scum valve Uptake Gauge glass Drain pipe Smoke box Internal feed pipe Feed valve Salinometer cock Blow down valve Fire hole Furnace Mud hole

25. Corrosion, Scale & Overheating

26. Steam Boiler Top Blowdown Feed water Chemical Dosing Tank Bottom Blowdown Water Treatment

27. Boiler Water Chemistry Main problems in boiler • Corrosion • Scale formation Water treatment will minimise the above problems and • will maintain a clean, scale-free heat-transfer surface • prevent metal loss due to corrosion Various chemicals dosed into feed water system to remove oxygen and neutralise scale forming compounds. Boilers must be blown down partially everyday to remove sludge formed due to chemical reaction from above chemicals

28. Steam Turbine Classification: • Condensing type • steam is exhausted into vacuum condenser to improve power output and thermal efficiency • Non-condensing type • steam is exhausted at a pressure higher than or near atmospheric pressure • Less expensive as condenser is not required

29. Steam Turbine According to principle action of steam, turbine can be classified as: • Impulse turbine • Reaction turbine

30. Impulse turbine • Steam pressure drop takes place in fixed nozzles, generating kinetic energy • High velocity steam jets directed on to the blades of the wheel and develops driving force on the wheel • For optimum efficiency, the velocity of steam at the nozzle exit must equal to twice the velocity of turbine blades

31. Reaction turbine • There is no fixed nozzles • Entire pressure drop takes place in the moving element itself • Kinetic energy is generated entirely in the moving element itself

32. Steam Turbine Nozzles It is a specially shaped passage. When steam flows through it, pressure drop is created across it and causes increase of kinetic energy of steam. Nozzles are classified as: Convergent nozzles– can deliver steam at subsonic velocity Convergent-divergent nozzles – can deliver steam at supersonic velocity

33. Steam Turbine Compounding / Multi-staging Necessary to reduce rotational speed of turbines without sacrificing power developed or efficiency. Velocity compounding High velocity steam is delivered from the nozzles and no expansion occurs in the blade system. Cargo pump turbines on tankers are of this type Speed reduction gear box Used to get lower shaft speed to match the propeller speed

34. Main propulsion turbines of steam ships comprises of: • H.P. Ahead turbine • L.P. Ahead & Astern turbine mounted on L.P. turbine shaft