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Safe Boiler Operation- Preparations for Raising Steam

A boiler is used to heat feed water in order to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced.

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.

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sketch of a boiler fuel oil system
Sketch of a boiler fuel oil system

A drum must be provided where steam and water can separate. There must also be a variety of fittings and controls to ensure that fuel oil, air and feedwater supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler.

In the steam generation process the feedwater enters the boiler where it is heated and becomes steam. The feedwater circulates from the steam drum to the water drum and is heated in the process. Some of the feedwater passes through tubes surrounding the furnace, i.e. waterwall and floor tubes, where it is heated and returned to the steam drum. Large-bore downcomer tubes are used to circulate feedwater between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums.

Boiler arrangement
General cargo ship boiler arrangement

The steam is produced in a steam drum and may be drawn off for use from here. It is known as 'wet' or saturated steam in this condition because it will contain small quantities of water, Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and 'dried', i.e. all traces of water are converted into steam. This superheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. An 'attemperator', i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature.

The hot gases produced in the furnace are used to heat the feedwater to produce steam and also to superheat the steam from the boiler drum. The gases then pass over an economiser through which the feedwater passes before it enters the boiler. The exhaust gases may also pass over an air heater which warms the combustion air before it enters the furnace. In this way a large proportion of the heat energy from the hot gases is used before they are exhausted from the funnel. The arrangement is shown in Figure

Two basically different types of boiler exist, namely the watertube and the firetube. In the watertube the feedwater is passed through the tubes and the hot gases pass over them. In the firetube boiler the hot gases pass through the tubes and the feedwater surrounds them.

The procedure adopted for raising steam will vary from boiler to boiler and the manufacturers' instructions should always be followed. A number of aspects are common to all boilers and a general procedure might be as follows.

The uptakes should be checked to ensure a clear path for the exhaust gases through the boiler; any dampers should be operated and then correctly positioned. All vents, alarm, water and pressure gauge connections should be opened. The superheater circulating valves or drains should be opened to ensure a flow of steam through the superheater.

All the other boiler drains and blow-down valves should be checked to ensure that they are closed. The boiler should then be filled to slightly below the working level with hot de-aerated water. The various header vents should be closed as water is seen to flow from them. The economiser should be checked to ensure that it is full of water and all air vented off.

The operation of the forced draught fan should be checked and where exhaust gas air heaters are fitted they should be bypassed. The fuel oil system should be checked for the correct positioning of valves, etc. The fuel oil should then be circulated and heated.

Raising steam

The forced draught fan should be started and air passed through the furnace for several minutes to 'purge' it of any exhaust gas or oil vapours. The air slides (checks) at every register, except the lighting up' burner, should then be closed. The operating burner can now be lit and adjusted to provide a low firing rate with good combustion. The fuel oil pressure and forced draught pressure should be matched to ensure good combustion with a full steady flame.

The superheater header vents may be closed once steam issues from them. When a drum pressure of about 210kPa (2.1 bar) has been reached the drum air vent may be closed. The boiler must be brought slowly up to working pressure in order to ensure gradual expansion and to avoid overheating the superheater elements and damaging any refractory material. Boiler manufacturers usually provide a steamraising diagram in the form of a graph of drum pressure against hours after flashing up.

The main and auxiliary steam lines should now be warmed through and then the drains closed. In addition the water level gauges should be blown through and checked for correct reading. When the steam pressure is about 300 kPa (3 bar) below the normal operating value the safety valves should be lifted and released using the easing gear. Once at operating pressure the boiler may be put on load and the superheater circulating valves closed. All other vents, drains and bypasses should then be closed. The water level in the boiler should be carefully checked and the automatic water regulating arrangements observed for correct operation.

Shipboard best practices
The following need to be carried out:

Accumulation test of the boiler – A boiler test to ensure that the safety valves can release steam fast enough to prevent the pressure rising by 10%. The main steam stop valve is closed during the test.

List of safety fittings in a boiler fuel oil 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 maker’s recommendations. Each test is to be recorded with the signature of the Engineer Officer who conducted the test....

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