Basic Boiler Construction

Header

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 three designs
  • water cooled with refractory covered studded tubes
  • Close pitched exposed tubes
  • Membrane Wall

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 15^oC. Solid drawn mild steel is generally used. Tube temperatures for convection superheater sections is considered to be final superheat temperatures plus 30^oC. For Radiant heat a higher temperature is considered.
For Superheater tubes operating above 455^oC 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.

• Entire walls may be prefabricated
• Maintenance costs, particularly of insulation are lower
• Lower quality fuels may be used due to the much reduced amount of insulation reducing problems of slagging
• Simplified water washing procedures
• Due to gas tight seal there is no corrosion of outer casing.

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.

Advantages of roof firing over side firing

• Increased efficiency due to the longer length allowed for the flame giving more time for complete combustion. This also allows more heat to be released as radiant rather than convective cutting down the required number of screen wall generating tubes
• The longer period allowed for complete combustion means that less excess air is required, this has the knock on effect of lowering the Dew Point of the flue gasses.
• Equal length flames
• Better gas flow
• For roof fired the effect of each flame is the same, foir side firing it differs. To keep within the design limitations the boiler must be operated to the highest effect flame with the other two operating at reduced effect

Ligament Cracking Mechanics

Generally associated with failure of refractory plug located beneath steam drum.

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

The Direction of the cracking indicates how it occurred