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How to control water level in a marine boiler ?

Modern high-pressure watertube boiler
: 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.



When a boiler is operating the water level in the gauge glass reads higher than when the boiler is shut down. This is because of the presence of steam bubbles in the water, a situation which is accepted in normal practice. If however there occurs a sudden increase in steam demand from the boiler the pressure in the drum will fail.

Some of the water present in the drum at the higher pressure will now 'flash off and become steam. These bubbles of steam will cause the drum level to rise. The reduced mass of water in the drum will also result in more steam being produced, which will further raise the water level. This effect is known as "swell', A level control system which used only level as a measuring element would close in the feed control valve—when it should be opening it.

When the boiler load returns to normal the drum pressure will rise and steam bubble formation will reduce, causing a fall in water level. Incoming colder feed water will further reduce steam bubble formation and what is known as 'shrinkage' of the drum level will occur.

The problems associated with swell and shrinkage are removed by the use of a second measuring element, 'steam flow'. A third element, 'feed water flow', is added to avoid problems that would occur if the feed water pressure were to vary.

Boiler water level control

Fig:Boiler water level control


A three element control system is shown in Figure . The measured variables or elements are 'steam flow', 'drum level' and 'feed water flow'. Since in a balanced situation steam flow must equal feed flow, these two signals are compared in a differential relay. The relay output is fed to a two-term controller and comparator into which the measured drum level signal is also fed. Any deviation between the desired and actual drum level and any deviation between feed and steam flow will result in controller action to adjust the feed water control valve. The drum level will then be returned to its correct position.

A sudden increase in steam demand would result in a deviation signal from the differential relay and an output signal to open the feed water control valve. The swell effect would therefore not influence the correct operation of the control system. For a reduction in steam demand, an output signal to close the feedwater control valve would result, thus avoiding shrinkage effects. Any change in feed water pressure would result in feed water control valve movement to correct the change before the drum level was affected.



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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