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
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
