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Purity of boiler feedwater- Dealing most common impurities

The use of boiler feedwater
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.



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.

The presence of dissolved oxygen and carbon dioxide in boiler feedwater can cause considerable corrosion of the boiler and feed systems. When boiler water is contaminated by suspended matter, an excess of salts or oil then 'foaming' may occur. This is a foam or froth which collects on the water surface in the boiler drum. Foaming leads to 'priming' which is the carry-over of water with the steam leaving the boiler drum. Any water present in the steam entering a turbine will do considerable damage.


Air register for roof fired boiler

Fig: Air register for roof fired boiler


Common impurities

Various amounts of different metal salts are to be found in water. These include the chlorides, sulphates and bicarbonates of calcium, magnesium and, to some extent, sulphur. These dissolved salts in water make up what is called the 'hardness' of the water. Calcium and magnesium salts are the main causes of hardness,

The bicarbonates of calcium arid magnesium are decomposed by heat and come out of solution as scale-forming carbonates. These alkaline salts are known as 'temporary hardness'. The chlorides, sulphates and nitrates are not decomposed by boiling and are known as 'permanent hardness*. Total hardness is the sum of temporary and permanent hardness and gives a measure of the scale-forming salts present in the boiler feedwater.

Boiler feed water treatment

Modern high-pressure, high-temperature marine boilers with their large steam output require very pure feedwater. Feedwater treatment deals with the various scale and corrosion causing salts and entrained gases by suitable chemical treatment. This is achieved as follows:

1. By keeping the hardness salts in a suspension in the solution to prevent scale formation.



2. By stopping any suspended salts and impurities from sticking to the heat transfer surfaces.

3. By providing anti-foam protection to stop water carry-over.

4. By eliminating dissolved gases and providing some degree of alkalinity which will prevent corrosion.

The actual treatment involves adding various chemicals into the feedwater system and then testing samples of boiler water with a test kit. The test kit is usually supplied by the treatment chemical manufacturer with simple instructions for its use.

For auxiliary boilers the chemicals added might be lime (calcium hydroxide) and soda (sodium carbonate). Alternatively caustic soda (sodium hydroxide) may be used on its own.

For high-pressure watertube boilers various phosphate salts are used, such as trisodium phosphate, disodium phosphate and sodium metaphosphate. Coagulants are also used which combine the scaleforming salts into a sludge and stop it sticking to the boiler surfaces. Sodium aluminate, starch and tannin are used as coagulants. Final de-aeration of the boiler water is achieved by chemicals, such as hydrazine, which combine with any oxygen present.

Boiler feed pump

The feed pump raises the feedwater to a pressure high enough for it to enter the boiler.For auxiliary boilers, where small amounts of feedwater are pumped, a steam-driven reciprocating positive displacement pump may be used.



Another type of feed pump often used on package boiler installations is known as an 'electro feeder'. This is a multi-stage centrifugal pump driven by a constant speed electric motor. The number of stages is determined by the feed quantity and discharge pressure.

Steam turbine-driven feed pumps are usual with high-pressure watertube boiler installations. A typical turbo-feed pump is shown in Figure above. The two-stage horizontal centrifugal pump is driven by an impulse turbine, the complete assembly being fitted into a common casing.

The turbine is supplied with steam directly from the boiler and exhausts into a back-pressure line which can be used for feed heating. The pump bearings are lubricated by filtered water which is tapped off from the first-stage impeller. The feed discharge pressure is maintained by a governor, and overspeed protection trips are also provided.

Boiler feed pump

Fig:Boiler feed pump


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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