Home || Diesel engines
||Steam turbines ||Fuel treatment ||Pumps ||Valves ||Refrigeration ||
Development of Watertube boiler - how they work
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.
The construction of watertube boilers, which use small-diameter tubes
and have a small steam drum, enables the generation or production of
steam at high temperatures and pressures. The weight of the boiler is
much less than an equivalent firetube boiler and the steam raising
process is much quicker.
Design arrangements are flexible, efficiency is
high and the feedwater has a good natural circulation. These are some
of the many reasons why the watertube boiler has replaced the firetube
boiler as the major steam producer.
Early watertube boilers used a single drum. Headers were connected
to the drum by short, bent pipes with straight tubes between the headers.
The hot gases from the furnace passed over the tubes, often in a single
Fig:Water tube boiler arrangement
A later development was the bent tube design. This boiler has two
drums, an integral furnace and is often referred to as the 'D' type
because of its shape . The furnace is at the side of the two
drums and is surrounded on all sides by walls of tubes. These waterwall
tubes are connected either to upper and lower headers or a lower header
and the steam drum. Upper headers are connected by return tubes to
the steam drum. Between the steam drum and the smaller water drum
below, large numbers of smaller-diameter generating tubes are fitted.
These provide the main heat transfer surfaces for steam generation.
Large-bore pipes or downcomers are fitted between the steam and water
drum to ensure good natural circulation of the water.
arrangement shown, the superheater is located between the drums,
protected from the very hot furnace gases by several rows of screen
tubes. Refractory material or brickwork is used on the furnace floor, the
burner wall and also behind the waterwalls. The double casing of the
boiler provides a passage for the combustion air to the air control or
register surrounding the burner,
The need for a wider range of superheated steam temperature
control led to other boiler arrangements being used. The original
External Superheater 'D' (ESD) type of boiler used a primary and
secondary superheater located after the main generating tube bank
. An attemperator located in the combustion air path was
used to control the steam temperature.
ESD II & ESD III type boilers
The later ESD II type boiler was similar in construction to the ESD I
but used a control unit (an additional economiser) between the primary
and secondary superheaters. Linked dampers directed the hot gases
over the control unit or the superheater depending upon the superheat
temperature required. The control unit provided a bypass path for the
gases when low temperature superheating was required.
In the ESD III boiler the burners are located in the furnace roof,
which provides a long flame path and even heat transfer throughout the
Fig:ESD III monowall boiler
In the boiler shown in Figure above, the furnace is fully
water-cooled and of monowali construction, which is produced from
finned tubes welded together to form a gaslight casing. With monowali
construction no refractory material is necessary in the furnace.
The furnace side, floor and roof tubes are welded into the steam and
water drums. The front and rear walls are connected at either end to
upper and lower water-wall headers. The lower water-wall headers are
connected by external downcomers from the steam drum and the upper
water-wall headers are connected to the steam drum by riser tubes.
The gases leaving the furnace pass through screen tubes which are
arranged to permit flow between them. The large number of tubes
results in considerable heat transfer before the gases reach the
secondary superheater. The gases then flow over the primary
superheater and the economiser before passing to exhaust.
pipe is located in the steam drum to obtain reasonably dry saturated
steam from the boiler. This is then passed to the primary superheater
and then to the secondary superheater. Steam temperature control is
achieved by the use of an attemperator, located in the steam drum,
operating between the primary and secondary superheaters.
Radiant-type boilers are a more recent development, in which the
radiant heat of combustion is absorbed to raise steam, being transmitted
by infra-red radiation. This usually requires roof firing and a
considerable height in order to function efficiently. The ESD IV boiler
is of the radiant type. Both the furnace and the
outer chamber are fully watercooled. There is no conventional bank of
The hot gases leave the furnace through an opening at
the lower end of the screen wall and pass to the outer chamber. The
outer chamber contains the convection heating surfaces which include
the primary and secondary superheaters. Superheat temperature
control is by means of an attemperator in the steam drum. The hot
gases, after leaving the primary superheater, pass over a steaming
economises This is a heat exchanger in which the steam—water mixture
is flowing parallel to the gas. The furnace gases finally pass over a
conventional economiser on their way to the funnel.
Reheat boilers are used with reheat arranged turbine systems. Steam
after expansion in the high-pressure turbine is returned to a reheater in
the boiler. Here the steam energy content is raised before it is supplied
to the low-pressure turbine. Reheat boilers are based on boiler designs
such as the 'D' type or the radiant type.
Double evaporation boiler
A double evaporation boiler uses two independent systems for steam
generation and therefore avoids any contamination between the
primary and secondary feedwater. The primary circuit is in effect a
conventional watertube boiler which provides steam to the heating coils
of a steam-to-steam generator, which is the secondary system. The
complete boiler is enclosed in a pressurised casing.
Summarized below marine boiler detail Info pages:
Requirement for various boiler types - water tube boilers and more 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.....
- Fire tube boilers working principle and operational procedure
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....
- Exhaust Gas Boilers And Economisers working procedure
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 .....
- The use of boiler mountings
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 ....
- Purity of boiler feedwater 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 steam-to-steam generator working principle and operational procedure
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.....
- How to control combustion in a marine boiler
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.....
- Safe boiler operation - Preparations & raising steam
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.....
- Fuel oil burning process - various design burners 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.
- Boiler arrangement - combustion process - supply of air
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....
- Ordinary spring-loaded safety valve and improved high-lift safety valve for a marine boiler
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.....
- Correct working level for marine boilers - use of water level gauges
The water level gauge provides a visible indication of the water level in the boiler in the region of the correct working level.
- How to maintain water level in a marine 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......
- Safety precautions for working with marine boiler 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....
Marine machineries - Useful tags
Marine diesel engines ||Steam generating plant ||Air conditioning system ||Compressed air ||Marine batteries ||Cargo refrigeration ||Centrifugal pump ||Various coolers ||Emergency power supply ||Exhaust gas heat exchangers ||Feed system ||Feed extraction pump ||
Flow measurement || Four stroke engines || Fuel injector || Fuel oil system || Fuel oil treatment ||Gearboxes || Governor ||
Marine incinerator ||
Lub oil filters ||
MAN B&W engine ||
Marine condensers ||
Oily water separator ||
Overspeed protection devices ||
Piston & piston rings ||
Crankshaft deflection ||
Marine pumps ||
Various refrigerants ||
Sewage treatment plant ||
Starting air system ||
Steam turbines ||
Steering gear ||
Sulzer engine ||
Turbine gearing ||
Two stroke engines ||
UMS operations ||
Drydocking & major repairs ||
Critical machinery ||
Deck machineries & cargo gears
|| Control and instrumentation
||Engine room safety ||
Machinery Spaces.com is about working principles, construction and operation of all the machinery
items in a ship intended primarily for engineers working on board and those who working ashore . For any remarks please
Copyright © 2010-2016 Machinery Spaces.com All rights reserved.
Terms and conditions of use