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Boiler arrangement & Fuel oil burning process - various designs burners

Various design burners : Marine boilers currently burn residual low-grade fuels. This fuel is stored in double-bottom tanks from which it is drawn by a transfer pump up to settling tanks. Here any water in the fuel may settle out and be drained away.

The high-pressure fuel is supplied to a burner which it leaves as an atomised spray . The burner also rotates the fuel droplets by the use of a swirl plate. A rotating cone of tiny oil droplets thus leaves the burner and passes into the furnace. Various designs of burner exist, the one just described being known as a 'pressure jet burner' .

The 'rotating cup burner' atomises and swirls the fuel by throwing it off the edge of a rotating tapered cup. The 'steam blast jet burner', atomises and swirls the fuel by spraying it into a high-velocity jet of steam. The steam is supplied down a central inner barrel in the burner.

The air register is a collection of flaps, vanes, etc., which surrounds each burner and is fitted between the boiler casings. The register provides an entry section through which air is admitted from the windbox. Air shut-off is achieved by means of a sliding sleeve or check. Air flows through parallel to the burner, and a swirler provides it with a rotating motion. The air is swirled in an opposite direction to the fuel to ensure adequate mixing . High-pressure, high output marine watertube boilers are roof fired . This enables a long flame path and even heat transfer throughout the furnace. The fuel entering the furnace must be initially ignited in order to burn.

pressure jet burner

Fig:pressure jet burner

Rotating cup burner

Fig:Rotating cup burner

Steam blast jet burner

Fig:Steam blast jet burner

Air register for side fired boiler

Fig:Air register for side fired boiler

Once ignited the lighter fuel elements burn first as a primary flame and provide heat to burn the heavier elements in the secondary flame. The primary and secondary air supplies feed their respective flames. The process of combustion in a boiler furnace is often referred to as 'suspended flame' since the rate of supply of oil and air entering the furnace is equal to that of the products of combustion leaving.

The oil from the settling tank is filtered and pumped to a heater and then through a fine filter. Heating the oil reduces its viscosity and makes it easier to pump and filter. This heating must be carefully controlled otherwise 'cracking' or breakdown of the fuel may take place. A supply of diesel fuel may be available to the burners for initial firing or low-power operation of the boiler.

From the fine filter the oil passes to the burner where it is 'atomised', i.e. broken into tiny droplets, as it enters the furnace. A recirculating line is provided to enable initial heating of the oil.

Boiler fuel oil supply system

Fig:Boiler fuel oil supply system

Summarized below marine boiler detail Info pages:

  1. Requirement for various boiler types - water tube boilers and more
  2. The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.....

  3. Fire tube boilers working principle and operational procedure
  4. The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed. The name 'tank boiler is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler are also in use....

  5. Exhaust Gas Boilers And Economisers working procedure
  6. The use of exhaust gases from diesel main propulsion engines to generate steam is a means of heat energy recovery and improved plant efficiency.The auxiliary steam installation provided in modern diesel powered tankers usually uses an exhaust gas heat exchanger at the base of the funnel and one or perhaps two watertube boilers .....

  7. The use of boiler mountings
  8. Watertube boilers, because of their smaller water content in relation to their steam raising capacity, require certain additional mountings: 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. Boilers with a high evaporation rate will use a multiple-element feed water control system ....

  9. Purity of boiler feedwater
  10. Most 'pure' water will contain some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer, which can result in local overheating and failure of the tubes. Other salts remain in solution and may produce acids which will attack the metal of the boiler. An excess of alkaline salts in a boiler, together with the effects of operating stresses, will produce a condition known as 'caustic cracking'. This is actual cracking of the metal which may lead to serious failure.....

  11. The steam-to-steam generator working principle and operational procedure
  12. Steam-to-steam generators produce low-pressure saturated steam for domestic and other services. They are used in conjunction with watertube boilers to provide a secondary steam circuit which avoids any possible contamination of the primary-circuit feedwater. The arrangement may be horizontal or vertical with coils within the shell which heat the feedwater.....

  13. How to control combustion in a marine boiler
  14. The essential requirement for a combustion control system is to correctly proportion the quantities of air and fuel being burnt. This will ensure complete combustion, a minimum of excess air and acceptable exhaust gases. The control system must therefore measure the flow rates of fuel oil and air in order to correctly regulate their proportions.....

  15. Safe boiler operation - Preparations & raising steam
  16. 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.....

  17. Fuel oil burning process - various design burners
  18. Marine boilers currently burn residual low-grade fuels. This fuel isstored in double-bottom tanks from which it is drawn by a transfer pump up to settling tanks. Here any water in the fuel may settle out and be drained away.

  19. Boiler arrangement - combustion process - supply of air
  20. Combustion is the burning of fuel in air in order to release heat energy. For complete and efficient combustion the correct quantities of fuel and air must be supplied to the furnace and ignited. About 14 times as much air as fuel is required for complete combustion....

  21. Ordinary spring-loaded safety valve and improved high-lift safety valve for a marine boiler
  22. Safety valves are fitted in pairs, usually on a single valve chest. Each valve must be able to release all the steam the boiler can produce without the pressure rising by more than 10% over a set period.....

  23. Correct working level for marine boilers - use of water level gauges
  24. The water level gauge provides a visible indication of the water level in the boiler in the region of the correct working level.

  25. How to maintain water level in a marine boiler ?
  26. A modern high-pressure, high-temperature watertube boiler holds a small quantity of water and produces large quantities of steam. Very careful control of the drum water level is therefore necessary. The reactions of steam and water in the drum are complicated and require a control system based on a number of measured elements......

  27. Safety precautions for working with marine boiler
  28. All boiler controls, regulators, alarms and trips must be tested regularly in accordance with the applicable Planned Maintenance System and makerís recommendations. Each test is to be recorded with the signature of the Engineer Officer who conducted the test....

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