Propulsion

Basic Construction

This design allows the use of lower quality feed. It is easy to clean and easy to maintian and replace tubes. Disadvantages are the large number of handhole doors and the extensive brickwork. The drum is all welded and the casing bolted

Front fired studded wall refractory covered boiler


Roof fired membrane wall modern Radiant heat boiler



Components

Steam drum

In the early designs the drums were riveted or solid forged from a single ingot, but for modern boilers the drum is generally fabricated from steel plate of differing thicknesses and welded. The materials used are governed by classification society rules. Test pieces must be provided.

The cylindrical drum is normally constructed from four plates. Two dished End plates, a thick wall tube plate ( thicker to accommodate the holes drilled in it without increased stress) and completed with a thinner wrapper plate.

Construction takes the form of rigidly clamping the descaled, bent wrapper and tube plates together. In addition test pieces cut from the original material are attached to the construction in such away that the longitudinal weld extends either sided of the join. These pieces are later removed and shaped test shapes cut out from specified areas including across the weld.

The longitudinal weld is critical ( taking twice the circumferential stress) and is normally carried out by specialised automatic machinery using submerged arc techniques.

The dished end pieces are accurately aligned and welded.

On completion the construction is cleaned and non-destructive testing- such as x-ray photography, carried out. Final machining is carried out and any stub pieces and doublers attached. The now complete drum is heat treated at 600 to 650'C.

The final process is hydraulic testing to classification requirements. Natural circulation within a boiler is due to the differing specific gravities of the water at the differing temperatures, the steam drum provides a reservoir of cool water to give the gravitational head necessary for natural circulation. Cool water entering the steam drum via the feed lines provides the motive effect for the circulation distributing it to the downcomers.

Also the space within the drum provides for the separation of the steam and water emulsions formed in the water walls and the generating tubes. Water droplets entrained with the separated steam are removed by separating components fitted in the drum as well as the perforated baffle plates fitted at the water line.

The space above the water line provides for a reserve steam space needed to maintain plant stability during manoeuvring conditions.

Also fitted are the chemical injection distributing pipe and the scuming plate.

The smaller the drum is made, the less thickness of material that is required. However, the limitation to how small is that sufficient space must be allowed for the separation of water from the steam before passing out to the superheater space otherwise dryers must be used. Also, due to the smaller reserve of water, larger fluctuations in water level occur during manoeuvring.

Water drum

Distributes feed water from the downcomers to the headers and generating tubes. Provides a space for accumulating precipitates and allows them to be blown down.

Water drum size is limited to that required to receive the generating tubes, for modern radiant heat boilers with only a single bank of screen tubes and no generating tubes between the drums, the water drum has been replaced by a header and the downcomers fed straight to the waterwall headers. With system blow down is done at the steam drum. Too small a water drum can cause problems of maintaining ideal water level and little steam reserve

Headers

These have a similar purpose to the water drum but are smaller in size. Due to their reduced size they may have a square cross section without resorting to exceptional thickness. .

Generating tubes

Consists of a large number of small diameter tubes in the gas flow, more commonly found in boilers of an older design

For roof fired boilers the generating bank may consist of one or two rows of close pitched tubes. For a modern radiant heat boiler the generating bank has been omitted to allow the replacement of the water drum by a distribution header, a bare tube economiser is fitted generating 5% of the steam capacity. The generation bank is normally heated by convection rather than radiant heat.

For a set water circulation the tube diameter is limited to a minimum as the ratio of steam to water can increase to a point where the possibility of overheating could occur due to the lower heat capacity of the steam.

The number of tubes is limited to prevent undercooling of the gas flow leading to dew point corrosion

Screen tubes

These are larger bore tubes receiving the radiant heat of the flame and the convective heat of the hot gasses. The large diameter keeps the steam/water ratio down hence preventing overheating. There main duty is to protect the superheater from the direct radiant heat. On a modern marine radiant heat boiler the screen wall is formed out of a membrane wall

Waterwall tubes

Contains the heat of the heat of the furnace so reducing the refractory and insulation requirements.

Comes in four designs

Mono and membrane walls greatly improve on the older designs in being gas tight



Advantages of roof firing over side firing

Ligament  (ligature) Cracking Mechanics

Generally associated with failure of refractory plug located beneath steam drum. The ligature is the space between the tube plate holes. Classification rules typically allow isolated ligature cracks to be gouged and re-welded. For continuous cracks repairs are not normally allowed and a new drum/tube plate may be required

Hot gasses acting on the thick section tube plate set up a temperature gradient leading to creep, plastic flow to relief thermal stress and high tensile stress on the surface at cool down. In addition grain growth leads to the metal becoming brittle

A more severe form may lead to distortion of the entire drum in two possible directions. The thick section tube plate is exposed to the heat of the furnace and is subject to overheating. Thermal distortion takes place leading to stressing. This stressing is relieved by creep . When the drum cools a set distortion is in place

The distortion may occur in three ways, in a radial or axial direction as shown below


Advantages of Water tube boilers over smoke tube (Tank)

Advantages over tank

Disadvantages


Downcomers

These are large diameter unheated i.e. external to the furnace, their purpose is to feed water from the steam drum to the water drum and bottom headers.

Riser/Return tubes

These return steam from the top water wall headers to the steam drum.

Superheater tubes

These are small diameter tubes in the gas flow after the screen tubes. Due to the low specific heat capacity of the saturated steam they require protection from overheating in low steam flow conditions, say when flashing.

Superheater support tubes

These are large diameter tubes designed to support part of the weight of the superheater bank of tubes.

Material requirements

Tube temperatures for the water cooled sections is considered to be saturation temperature plus 15oC. Solid drawn mild steel is generally used.

Tube temperatures for convection superheater sections is considered to be final superheat temperatures plus 30oC. For Radiant heat a higher temperature is considered.

For Superheater tubes operating above 455oC a Chrome Molybdenum alloyed steel is required.



Advantages of membrane/monowalls

These were originally introduced in land power stations after experience had been gained in making the lower parts of the furnace sufficiently tight to hold liquid ash. This was achieved by welding steel strips between the floor tubes. Further development resulted in completely gas tight furnace wall panels being constructed by welding together either finned tubes or normal plane tubes with steel strips in between and welded. In both methods he longitudinal welds are done by automatic processes and panels of the required size are built up in the factory ready for installation into the boiler in one piece.


A disadvantage would be that tube replacement following failure is more difficult. Also, the possibility of entire walls parting from the drum can occur during a furnace explosion.


Smoke Tube Boiler

These were the most common form of boiler design before the introduction of water tube designs. See Comparisons of water tube and Smoke tube boilers.

This style of boiler still see active service were low quantities of low quality steam are required, such as for cargo and fuel tank heating when in port.

This style of boiler is relatively cheap, supplied as a packaged unit and requires less stringent feed water conditioning and level control.

Design

Consists of a shell wrapper plate to which is welded ( or for later designs riveted, end plates.. Pressure is naturally container in the shell plate due to is cylindrical design. The flat end plates, however, must be 'stayed' to prevent buckling and distortion.

The combustion chamber is of similar section and is also 'stayed'.

The boiler shown above is a single furnace, two pass design. Larger boilers may have multiple furnaces and have multiple passes by replacing the exhaust stack with a return chamber and fitting another bank of tubes.

The smoke tubes may be plain or threaded to act as stays. There are one stay tube for every three plain tubes approx.



These were the most common form of boiler design before the introduction of water tube designs. See Comparisons of water tube and Smoke tube boilers.

This style of boiler still see active service were low quantities of low quality steam are required, such as for cargo and fuel tank heating when in port.

This style of boiler is relatively cheap, supplied as a packaged unit and requires less stringent feed water conditioning and level control.

Design

Consists of a shell wrapper plate to which is welded ( or for later designs riveted, end plates.. Pressure is naturally container in the shell plate due to is cylindrical design. The flat end plates, however, must be 'stayed' to prevent buckling and distortion.

The combustion chamber is of similar section and is also 'stayed'.

The boiler shown above is a single furnace, two pass design. Larger boilers may have multiple furnaces and have multiple passes by replacing the exhaust stack with a return chamber and fitting another bank of tubes.

The smoke tubes may be plain or threaded to act as stays. There are one stay tube for every three plain tubes approx.



Operation

Although the maintenance of the water level is not so critical as with water tube designs, it should not be allowed to fall too much as overheating of furnace and combustion spaces leads to catastrophic failure due to component collapse. The content of the boiler is then expelled via the furnace door.

Similarly , although water treatment is not so critical scale must not be allowed to build up which can lead to overheating of material

Authors Note

Although package boilers of this design are fairly robust it should not be forgotten the potential for danger a poorly maintained unit can be.

The author carried out a supposed routine inspection on one such unit. Opening the upper manhole revealed that the unit has been 'wet layed' with water left at normal working level rather than being pressed up . Severe pitting was present at and just below the water level.
The lower man hole door beneath the furnace was opened after draining the water. Heavy wastage could be seen later measured at 60 % of the shell plating with pitting on top of this. Evidently no oxygen scavenger, such as Sodium Sulphite, had been added before laying up.

In this condition, operation at full load would almost certainly have led to catastrophic failure.



Marine Radiant Reheat Boilers

To increase plant efficiency reheat systems are used. In this the exhaust from the HP Turbine is led back to the furnace and reheated to superheat temperatures. This allows the steam to be expanded to lower pressures in the LP turbine with reduced need for blade taper twist and other efficiency degrading designs to cope with the steam wetness.




The boiler design is of a standard roof fired radiant furnace with a gas tight membrane water wall and single row of screen tubes. The convection space is divided into a superheater and reheater section and a section containing superheat and reheat temperature control by-pass economisers. Gas dampers allow cooling air from the windbox to pass over the reheater section during astern manoeuvres to prevent overheating and thermal shocking when the plant is moved ahead.

An alternate design using water drum attemperation for superheat control rather than by-pass economisers splits the convective zone into two parts



The superheaters in the reheat section ensure that gas is cooled sufficiently to prevent overheating in the reheat in the event of gas dampler leakage. Provision is made to allow windbox air into the reheat space in the event of damper failure.

Furnace Gas tight


Dual Pressure ( Double Evaporation) Boiler

The main reason for the adoption of this design of boilers is to allow use of modern high efficiency watertube boilers without fear of damage through contamination by cargo or fuel oils.


The basic design consists of a D-Type boiler design upon which is mounted a Steam/Steam generator drum. The steam generated by the main boiler heats water in the Steam/Steam generator which produces steam requirements.


The primary drum is initially filled with high quality feed water and suitably dosed. Make up is limited to small amounts due to leakage therefore the feed pump may be of simple design. An example could be a steam or air driven reciprocating pump. The chemical treatment is simple with little requirement for addition or blowdown.

The above design shows the fitting of a superheater. These are normally only fitted where the generated steam will be required to power turbine operated machinery most typically an alternator.

Secondary drum.

The U-tube heating elements are passed through the manhole dorr and expanded into headers welded into the dished end of the drum. The tubes are well supported. A manhole may be fitted at the lower part of the shell allows access to the heating elements.

The drum is generally mounted integral, supports are attached to the structure of the primary boiler. The secondary drum also acts as a steam receiver for the exhaust gas boiler.Typical pressures are 63 bar for the primary circuit and 23.5 for the secondary.

The author has not sailed with pressures anywhere near this with this design. Primary pressures of 35bar and secondary pressures nearer 15 bar have proven sufficient even to drive an alternator. Of note is that these designs are obviously more expensive than a normal single steam drum plant even taking into account the improved efficiency. They are therefore generally associated with larger motor powered plant with large waste heat units capable of supplying all requirements including an alternator. However the author has sailed on this plant on a 20,000 tonne product tanker.

Where these boilers are installed in Motorships a "simmering coil" may be fitted. This is located in the primary drum and is supplied from the exhaust economiser to keep both circuits warm thereby preventing any possible damage due to lay-up.

Mountings are those typically found on any boiler with low level water alarms and low/low level shut off on both boilers. The accumulation of pressure test for the safety valves fitted to the secondary drum are calculated with the primary boiler firing at maximum rate generating maximum heating steam supply.

Typical circuit incorporating Dual Pressure Boiler


Under port conditions the main boiler is fired to providing heating steam for the secondary drum. From this steam is supplied for tank heating or to a turbo-alternator via a superheater.

When the vessel is underway the main boiler may stop firing. A waste heat circulating pump passes water from the secondary drum via the waste heat unit back to the drum. The steam produced is again available for tank heating and powering a turbo-alternator.

Cross over valves are fitted for Harbour and sea-duty conditions.