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
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.
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 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.
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 reserveThese 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. . 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.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 Contains the heat of the heat of the furnace so reducing the refractory and insulation requirements.
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
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.
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.
To aid circulation the tubes are arranged in vertical rows to offer minimum resistance.
Fuel is combusted in the corrugated watercooled furnace. The corrugations increase the surface area and allow a degree of flexibility to allow for expansion and contraction.
The hot gas passes to the water cooled combustion space though to the smoketubes. The upper portion of the combustion chamber lies close to the water level and is therefore liable to distortion due to in correct water level maintenance.
Access to the boiler is via a manhole door on the upper shell plate. In addition a smaller door may be fitted below the furnace to allow inspection and scale/sludge removal.
The design is simialr to the scothch boiler other than the combustion chamber wich requires no stays. This design is a three pass design
Similarly , although water treatment is not so critical scale must not be allowed to build up which can lead to overheating of material
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.
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.
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.
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.