BOILER STRUCTURE AND METHOD OF ASSEMBLY
The invention relates to a boiler structure, for example for a thermal power
generation plant, and in particular to a support structure for a boiler envelope and
to a boiler apparatus incorporating such a support structure. The invention further
relates to a method of assembly of a boiler structure and in particular a method of
assembly of a support structure for a boiler envelope and of a boiler envelope
supported by such a support structure.
Thermal power plant driven by combustion of carbonaceous fuel for the generation
of steam within a boiler system (which term as used herein encompasses subcritical
and supercritical systems) remains a major source of generation of electrical power.
Large scale boiler systems, especially fuelled by combustion of carbonaceous fuel,
also have other industrial applications.
Boiler systems for a thermal power plant or other industrial apparatus on a
comparable scale are large and complex apparatus requiring complex assembly and
extensive structure. Although small industrial boilers, for example up to around 30
MW generating capacity, may practically be self-supporting, larger boilers, for
example utility boilers of 60 MW generating capacity and above, typically require
extensive supporting structures. In particular, extensive support structures are
required to provide a mechanical support for and structural stability to the boiler
envelope. Such structures are necessary to carry the high static mechanical loads
associated with the boiler envelope and to transmit those loads to the ground, and
also to provide stability and to accommodate operational load factors, such as those
which might be attributable to thermal expansion effects and environmental
influences such as weather and seismic influences on the large boiler envelope
structures. Such support structures can be expensive both in terms of the structural
materials used and in terms of the build complexity. Support structures for the
boiler envelope can have a very large footprint.
In a typical prior art construction, a boiler house is first constructed including a
large primary support structure which incorporates as part of the primary support
structure a suspension deck in the vicinity of the boiler house roof and above the
boiler envelope maximum height. The boiler envelope and associated structures are
then supported on the primary suspension deck by suitable secondary and tertiary
structural elements. The primary load carrying structure for the entire boiler
envelope, including the heating surfaces within the boiler envelope and the boiler
envelope structural elements themselves, is primarily supported via the suspension
deck, and the associated load primarily transmitted to the ground via the
suspension deck and associated supporting primary structure.
As the skilled person will understand, it is conventional in the art to consider the
elements of a boiler envelope and support structure to fall into one of three
classifications: boiler pressure parts; boiler hot structures; and boiler cold
structures. In particular it is conventional to classify the structural support elements
into boiler hot structures and boiler cold structures. The distinction will be well
understood by the skilled person but can perhaps be expressed simply by defining
boiler hot structures as those support structures which are designed to expand and
contract thermally with the boiler, and boiler cold structures as those support
structures which do not.
In the typical prior art construction above described the suspension deck comprises
a generally horizontally extending primary support structure which sits at full
height above the maximum height of the boiler envelope in the boiler house roof.
Typically, the whole boiler house primary structure goes up to the level of this full
height suspension deck, and supports the suspension deck at roof height to
complete the primary load bearing structure. Thus, in the typical design, the boiler
cold structures extend to a full height above the maximum height of the boiler
envelope whereby the boiler hot structures and thereby the boiler pressure parts
are supported.
Such an arrangement produces a boiler cold structure with a large footprint
extending fully above the height of the boiler envelope and extensively beyond the
boiler envelope footprint and requires a large quantity of high strength structural
steel to make up the extensive full height boiler cold structure.
Moreover such an arrangement typically requires the boiler house, and in particular
at least the boiler cold structure including the suspension deck, to be constructed in
its entirety first, before any boiler envelope structure, heating surfaces etc can be
brought into position and supported upon it. The prior art support system does not
lend itself to progressive parallel construction from the ground upwards.
The invention is directed at providing a support structure for a boiler envelope and
a method of assembly of a boiler support structure which mitigate some or all of the
above disadvantages.
In a particular preferred case the invention is directed at the provision of a support
structure for a boiler envelope which offers simplicity of fabrication and/or a
reduction in the structural material requirement and/or a reduction in the overall
structural envelope.
In a particular preferred case the invention is directed at a method of assembling
the support structure which facilitates the progressive parallel construction of the
support structure and of a supported boiler envelope.
Thus, in accordance with the invention in a first aspect a support structure for a
boiler envelope, for example being the boiler envelope of a boiler thermal plant,
comprises:
a support platform structure, comprising a part of the primary load bearing boiler
cold structure for the boiler envelope, and adapted to engage with and thereby carry
at least a major part of the static load of the boiler envelope;
wherein the support platform structure is provided surroundingly about the boiler
envelope at a support platform level substantially below full envelope height and
provided with associated further boiler cold structure in such manner that at least
the major part and preferably substantially all of the boiler cold structure is at or
below the support platform level.
That is to say in the alternative, in accordance with the invention in a first aspect a
support structure for a boiler envelope, for example being the boiler envelope of a
boiler thermal plant, comprises:
a boiler support cold structure including a support platform structure and a boiler
support hot structure supported via the boiler support cold structure;
wherein the support platform structure is provided surroundingly about the boiler
envelope at a support platform level substantially below full envelope height and
configured such that at least the major part and preferably substantially all of the
boiler cold structure is at or below the support platform level.
The support platform structure of the invention is in broad structural terms the
equivalent to the primary load bearing structure provided in the prior art by the full
height suspension deck. The support platform structure of the invention performs
an equivalent mechanical function. It is a primary loaded bearing structure to carry
at least a static load of the boiler envelope. It is a primary load bearing part of the
boiler cold structure by means of which the boiler hot structures and thereby the
boiler pressure parts are supported.
However the support platform structure of the invention can be contrasted with a
prior art suspension deck at roof height, above the entire boiler structure, in that it
eliminates the need for boiler cold structure above the support platform level. At
least the substantial majority and preferably essentially all the boiler cold structure
is at or below the support platform level, and only elements of the boiler hot
structure and the boiler pressure parts extend above the support platform level
supported by the boiler cold structure.
Advantages can accrue over a prior art design with boiler cold structure including a
suspension deck which sits at full height above the maximum height of the boiler
envelope in the boiler house roof in two ways in particular. First, the boiler cold
structure of the present invention is all at substantially below full height. Second, in
consequence, whereas the prior art cold structure extends across and over the
boiler envelope footprint, in the present invention that the support platform
structure surrounds the boiler envelope, the boiler envelope passing through it via
an apertured portion. This can reduce the overall footprint. Both of these may
produce a significant reduction in the overall structural requirement, and in
particular in the requirement for higher strength structural materials.
The Invention further provides as an advantage that the boiler envelop may expand
thermally upward and downward from the support platform structure, providing a
qualitative change to the way boiler is supported and expanded, adding core value
to the design by removal of much of the variable and constant spring requirements
that are found in prior art designs.
The support platform structure performs otherwise a similar role, within the boiler
cold support structure, as the prior art suspension deck structure, and aspects of the
of the support platform structure of the invention may consequently be inferred
from the prior art by analogy. In particular, the support platform structure of the
invention preferably comprises a substantially horizontal framework of structural
members, for example structural steel members, extending around and supporting
the boiler envelope at the desired height substantially below the boiler maximum
height. The support platform structure forms part of the primary load bearing boiler
cold structure which boiler cold structure will include vertical structural elements
which support the boiler hot structure and transmit the primary load directly from
the support platform structure to the ground in familiar manner. Suitable secondary
and tertiary structural elements, including suitable cross members, beams, ties,
buckstays, tensile members and the like as applicable will complete the load
transmitting structure in a manner which will be generally familiar to the skilled
engineer.
In accordance with the invention, the support platform structure is provided at a
height level substantially below the full height of the boiler envelope and there is
preferably essentially no cold structure above this height. In a particular preferred
case the support platform structure is provided at a height above the burners (and
typically for example above any overfire airports if present, and typically for
example just below or level with any cage ring main if present) but below the
heating surfaces (and typically therefore below the soot blowers etc.). In a typical
boiler design embodying the principles of the invention a preferred support
platform structure level will be at around 40 to 60 per cent of the boiler envelope
maximum height, and this level may be referred herein below for convenience as
"half height". However, the skilled person will appreciate that the principles set out
above are the ones that are important for determining an ideal location in
conjunction with the particular boiler structure to which the support structure of
the invention is to be associated.
The invention takes advantage of the fact that in typical burner design a significant
majority of the equipment within the boiler house is below the half height level. By
providing a primary support platform structure at this level it may be possible to
substantially reduce or eliminate the requirement for any boiler cold structure at all
above this level. The support platform structure performs a similar role, within the
boiler cold structure, as the prior art suspension deck structure, and as a result a full
height suspension deck structure and associated boiler cold structure comparable
with that in the prior art is not required. It is not necessarily for the boiler cold
structure to extend to the full boiler house height and to the full extent of the
footprint, as is required in the prior art. The extent of the boiler cold structure can
be reduced in terms of height and/or footprint, with a potential for simplification of
construction and for reduction in the quantity of structural steel requirement. Both
of these can confer significant savings. It is possible to dispense with boiler cold
structure altogether above the level of the suspension deck structure, relying only
on boiler hot structural elements above this level.
A further potential advantage of the support structure in accordance with the
invention may accrue in relation to flexibility of construction. If, in accordance with
the invention, the boiler cold structure extends only up to the half height support
platform structure, it ceases to be necessary to construct the boiler house and all of
the associated full height structural support elements before considering assembly
of the boiler envelope and associated structures. It is possible to construct and
support the boiler assembly on the half height suspension deck and through the
aperture therein, and to develop a method of assembly of a support structure and
boiler envelope in integrated manner which is at least to some extent progressively
effected. It is not necessary to build support structures to full height of the
suspension deck structure before commencing assembly of the boiler envelope, but
merely to the half height of the support platform structure. It is necessary only to
complete that part of the support platform structure and associated cold structure
and hot structure that directly bears on the boiler envelope structure before
beginning construction of the side walls. The structure that is required before
parallel construction of the boiler can start may be relatively much reduced. The
roof can be built as a separate module away from the site. This parallel construction
option may confer significantly increased flexibility in practice.
In the preferred case, a support platform structure in accordance with the invention
is provided at a support platform level generally above the burners but below the
heating surface of a typical boiler structure. All the boiler cold structure is at or
below the support platform level. Only elements of the boiler hot structure and the
boiler pressure parts extend above the support platform level supported by the
boiler cold structure. In a typical boiler structure to which the support structure of
the invention is intended to be applied, the majority of associated functional
components within the boiler house will similarly be below this level, including for
example the secondary air duct and other secondary structures. There are some
functional structures above this level, notably including the heating surfaces and
other boiler envelope structures, soot blowers and the like which may require some
support structure to transmit residual static loads and to stabilise against dynamic
load variations, in particular, for example against lateral movement, but the
structural requirement above the half height level is substantially reduced. Any such
additional support structure need extend only across the horizontal extent of the
boiler envelope/hot structures not to the full extent of the boiler house to full height
as is the case with prior art designs. For both these reasons, there will be a
significant reduction in the structural material requirement above the half height
support platform structure. In particular, there is no requirement for a full height
suspension deck as such, as the primary static load bearing role it performs in the
prior art boiler cold structure is at least in major part performed in the invention by
the half height support platform structure.
The invention does not preclude the provision of support structure elements above
the height of the primary support platform structure, and in particular considers
that structural elements forming part of the boiler hot structure may be provided
above this height, for example to stabilise/support the envelope/heating surfaces
above mid height and/or accommodate expansion, stabilise the structure laterally
against environmental dynamic loads etc. However, the invention is characterised in
that it substantially eliminates the need for boiler cold structure above the primary
support platform structure level.
Regardless of such provision of boiler hot structure above the support platform
structure level the general principle of the invention is maintained, in which an
integrated structure is provided which seeks to take the boiler cold structure down
to half height. The skilled person will readily appreciate that the limited hot
structure which may optionally be provided above this support platform structure
level is not in engineering terms in any way equivalent to the full height suspension
deck and associated boiler cold structure in the prior art.
Additional lateral stabilisation may be provided as required in a familiar manner.
In a particular preferred case, additional structural elements associated with the
support structure of the invention may include buckstay arrangements such as are
described in International Publication WO2010/0730030.
In accordance with a more complete aspect of the invention, a boiler apparatus
comprises a support structure as hereinbefore described in conjunction with and
providing at least a major part of the static load support for a boiler envelope, in
particular of a boiler apparatus comprising part of a thermal power generation
plant.
In particular the boiler is a utility boiler for thermal power generation on a utility
scale, for example with 60 MW generating capacity and above. Such boilers in the
prior art typically have boiler cold structures which in conventional design extend
to and beyond the full boiler envelope height and have a large footprint. The
invention when applied to such boilers offers significant constructional advantages
as above described.
In a further more complete embodiment, a thermal power generation plant includes
at least one such boiler apparatus.
In accordance with the invention the further aspect, a method of assembly of a
boiler apparatus comprises the steps of:
providing a support structure for a boiler envelope in accordance with the above
described first aspect of the invention;
assembling a boiler envelope so as to be mechanically supported thereon in such a
manner that at least the major part and preferably substantially all of the boiler cold
structure is at or below the support platform level and there is essentially no boiler
cold structure above the support platform level; that is, only elements of the boiler
hot structure and the boiler pressure parts extend above the support platform level
supported by the boiler cold structure.
The invention further comprises by analogy a method of assembly of provision of a
thermal power generation plant including at least one such boiler apparatus.
As has been previously noted, it is a particular preferred feature of the support
structure in accordance with the invention, and in particular of the fact that the full
height suspension deck and associated boiler cold structure is dispensed with, and
instead a boiler cold structure at less than full height is provided which includes a
support platform apertured to surroundingly support the boiler envelope, that it
lends itself to parallel assembly of supporting structure and boiler envelope
structure.
Thus, in a preferred embodiment of the method, the steps of providing a support
structure, and assembling in a supporting manner thereon a boiler envelope
structure, are performed progressively in parallel. This can be contrasted with
typical prior art methods, where it is necessary to assemble the entire support
structure, and indeed the entire boiler house, before suspension of the boiler
envelope from the primary suspension deck structure can be commenced.
The invention will now be described by way of example only with reference to the
accompanying drawings in which:
Figure 1 is a schematic side view in elevation of a typical prior art thermal power
plant boiler and support structure;
Figure 2 is a schematic side view in elevation of an embodiment of support structure
in accordance with the invention as applied to a boiler such as that in figure 1;
Figure 3 is a plan view of the support structure of figure 2 at primary support
platform structure level;
Figure 4 is a plan view of the support structure of figure 2 at roof level;
Figure 5 is a more detailed end elevation of the framework structure below support
platform structure level;
Figure 6 is a sectional view of a possible embodiment by means of which thermal
expansion and contraction may be accommodated by the roof indicating movement
of walls, buckstays, roof, heating surface supports and galleries when the roof is
designed to move upwards when the wall expands horizontally and vertically.
Figure 1 is a representation of a typical boiler structure to which a support structure
in accordance with the invention might be applied, but shown in the case of figure 1
in side elevation with a prior art supporting structure including a conventional
suspension deck. It will be appreciated that this is an example only of a boiler
structure which the invention might be applicable, and that the invention might
confer advantages to any boiler structure with similar structural considerations. It
will also be appreciated that where example dimensions for the structures of figure
1 and figure 2 are given herein, they are for exemplification only.
Figure 1 illustrates a typical vertical boiler assembly with a typical prior art primary
support structure with the boiler cold structure extending to full height and
supporting the boiler hot structure and pressure parts from above maximum boiler
envelope height. For simplicity, most structural elements other than primary
structural elements have been omitted.
The primary structure consists of a primary boiler support structure 1 and a SCR
support structure 3 . The boiler support structure consists of a suspension deck 5 at
approximately 15 metres above the boiler roof level. In the example embodiment,
the suspension deck is thus at approximately 82 metres above the ground. The
suspension deck 5 forms the main load bearing part of the boiler cold structure. The
boiler hot structure and thereby the envelope and the heating surfaces are all
supported from this deck via slings 7 .
The deck typically consist of several very large structural steel beams, usually
around 5 metres to 6 metres deep depending on boiler width and on loads spanning
across the boiler lateral plain. Interconnecting beams, designed as secondary
structure and laid out in boiler fore and aft directions, are arranged to suit the sling
positions.
With this arrangement, the boiler vertical expansion is entirely downward from the
roof. The boiler framing designed to restrain the boiler envelope in the horizontal
plane is supported by the boiler envelope.
As can be seen from the figure, the primary structural framework making up the
boiler cold structure extends to the full extent of the boiler house and to the full
height of the suspension deck 5 at 82 metres.
The boiler cold structure as a result has a substantial footprint, and requires
primary structural elements to the full height of the boiler house. This imposes a
significant structural steel requirement. Estimated structural steel requirements for
the typical prior art design illustrated in figure 1 are in excess of 11,000 tonnes.
A side elevation of a modified support structure embodying the principles of the
invention, employed in association with an equivalent boiler to that shown in figure
1, is presented in figure 2 .
In figure 2, the full height primary structure suspension deck is dispensed with.
Instead, the primary support platform 15 is provided at mid height, above the
burners 17 but below the heating surfaces 19. This carries a substantial part of the
static load of the boiler envelope and associated structures.
In the embodiment, the boiler support platform is provided at a 42 metre level, and
the boiler cold support structure associated with the boiler support platform, and
required to transmit the load from the boiler support platform to the ground,
consequently only needs to rise to this 42 metre level, rather than, as in figure 1, to
the full height and full extent of the boiler house at the 83 metre level. This
eliminates the need for boiler cold structure above the 42 metre level. All the boiler
cold structure is at or below the 42 metre level. Only elements of the boiler hot
structure and the boiler pressure parts extend above the 42 metre level supported
by the boiler cold structure.
Secondary supporting structure 21 is provided above the 42 metre platform level.
All horizontal forces above this supporting platform level are transmitted to the
boiler envelope via braces and tie bars, and then transmitted to the structure below
support level by appropriate links. The wall lateral stiffness is maintained by a
buckstay system, such as described in International Patent Publication
WO2010/073030.
In the embodiment, boiler hot structural elements and a roof structure 23 are
shown at 75 metre level. Alternatively, at least the boiler envelope may be selfsupporting
above the 42 metre level. However, it will be appreciated even in the
embodiment where boiler hot structural elements and a roof structure 23 are
present that this is in engineering terms a very different structure from the
suspension deck 5 and associated cold structure in the prior art illustration of figure
1 . None of the structure above the 42 metre level is cold structure. Only elements of
the boiler hot structure and the boiler pressure parts extend above the 42 metre
level. There is a substantial saving, both in terms of footprint and in terms of
material requirement, above the platform level of 42 metres. In the proposed
design, the structural steel requirement may be reduced to below 8000 tonnes. This
is because the boiler cold structure has both reduced footprint and reduced height,
and the structural steel requirement above the 42 metre platform level is
substantially reduced.
Figure 3 illustrates the 42 metre level platform structure in more detail in plan view,
and in particular illustrates the manner in which it surroundingly supports the
boiler envelope. Even at this level, the design allows for a reduction in footprint.
Figure 4 illustrates in plan view the roof level support at 75 metres, and shows the
much reduced footprint and structural requirement at this level when a half height
support platform structure 1 and associated boiler cold structure at or below this
height is provided in accordance with the invention.
Figure 5 illustrates in end elevation the open frame primary structural support
arrangement by which load is transmitted from the support platform 15 at the 42
metre level to the ground. The use of cross bracings 3 in addition to conventional
horizontal and vertical framework elements opens up the framework below the
platform level, providing at its widest opening of 21.5 metres. This can be
contrasted by the conventional framework structure in figure 1 which has a
maximum dimension of 12 or 13 metres. This open framework, together with the
requirement to construct the primary support platform only to the half height 42
metre level, combined to facilitate the simultaneous progressive assembly of the
support structure and the boiler envelope. It is only necessary to develop the
support structure up to platform level to the extent sufficient to support the boiler
side walls before it becomes possible to begin to assemble the boiler envelope
within, through and upwardly from the aperture defined by the 42 metre platform
support. The possibility of jacking from below rather than below using large cranes
from above may further simplify the assembly process.
Figure 6 illustrates a possible embodiment by means of which thermal expansion
and contraction may be accommodated by incorporating the roof into the boiler hot
structure. The figure illustrates movement of walls, vertical buckstays, gallery and
roof due to boiler expansion.
With reference to figure 6, the increase in wall temperature will result in boiler wall
expansion outward. In the embodiment an assumed expansion is in the y direction
(i.e. a boiler lateral direction) and the magnitude is 63 mm and assuming that the
wall is supported at 42000 mm level and free to expand upward (as well as
downward) from this level. It follows that when the wall expand horizontally
outwards, the wall and the roof intersection will tend to expand upward by 140 mm
and the central part of the roof will need to move upward by the same amount if the
stress level at the roof and the heating surface is to be negligible.
In the illustrated embodiment this is enabled by supporting the roof and the heating
surface by angulated links L4 and L5. The diagram indicates the link cold positions
marked by C which will move to their corresponding hot positions marked by H
when the vertical buckstay is moved outward by 63 mm as a result of boiler
expansion. This is controlled by the movement of horizontal buckstays such as for
example are described in International Patent Publication WO2010/073030
incorporated herein by reference. The diagram also indicates two spring units
marked as SI and S2. However it is noted that the system will work with other
arrangements for example using other link systems. The system further indicates
gallery support and other structure which are integral part of the total system.
Thus, the design in accordance with figures 2 to 6 represents a potentially
significant improvement over the prior art design in figure 1 . There is a potential
reduction in structural weight, and an associated cost reduction, of around 30 per
cent when compared to conventional support structures having substantial boiler
cold structure to full height. Parallel construction may be enhanced leading to a
significant reduction in construction lead time. The overall footprint of the boiler
may be reduced.

CLAIMS
A support structure for a boiler envelope, for example being the boiler
envelope of a boiler thermal plant, comprising:
a support platform structure, comprising a part of the primary load bearing
boiler cold structure for the boiler envelope, and adapted to engage with and
thereby carry at least a major part of the static load of the boiler envelope;
wherein the support platform structure is provided surroundingly about the
boiler envelope at a support platform level substantially below full envelope
height and provided with associated further boiler cold structure in such
manner that at least the substantial majority of the boiler cold structure is at
or below the support platform level.
Asupport structure in accordance with claim 1 comprising:
a boiler support cold structure including a support platform structure and a
boiler support hot structure supported via the boiler support cold structure;
wherein the support platform structure is provided surroundingly about the
boiler envelope at a support platform level substantially below full envelope
height and configured such that at least the substantial majority of the boiler
cold structure is at or below the support platform level.
3 . A support structure in accordance with claim 1 or claim 2 wherein
essentially all the boiler cold structure is at or below the support platform
level and only elements of the boiler hot structure and the boiler pressure
parts extend above the support platform level supported by the boiler cold
structure.
4 . A support structure in accordance with any preceding claim wherein the
support platform structure comprises a substantially horizontal framework
of structural members extending around and supporting the boiler envelope
at a height substantially below the boiler maximum height.
A support structure in accordance with any preceding claim further
comprising suitable secondary and tertiary structural elements, including
suitable cross members, beams, ties, buckstays, tensile members and the
like.
A support structure in accordance with any preceding claim wherein the
support platform structure is provided in association with a boiler at a
height above the burners of the boiler but below the heating surfaces of the
boiler.
A support structure in accordance with any preceding claim wherein the
support platform structure is provided in association with a boiler such that
all the boiler cold structure sits at or below a maximum height level which
maximum height level is above the burners of the boiler but below the
heating surfaces of the boiler.
A support structure in accordance with any preceding claim wherein the
support platform structure is provided in association with a boiler at a
height of around 40 to 60 per cent of a boiler envelope maximum height.
A support structure in accordance with any preceding claim wherein the
support platform structure is provided in association with a boiler such that
all the boiler cold structure sits at or below a maximum height level which
maximum height level is around 40 to 60 per cent of a boiler envelope
maximum height.
10. A boiler apparatus comprising a support structure in accordance with any
preceding claim in conjunction with and providing at least a major part of
the static load support for a boiler envelope of a boiler.
11. Aboiler apparatus in accordance with claim 10 wherein the boiler is a utility
boiler for thermal power generation.
12. Athermal power generation plant including at least one boiler apparatus in
accordance with claim 10 or 11.
13. Amethod of assembly of a boiler structure comprising the steps of:
providing a support structure for a boiler envelope in accordance with the
above described first aspect of the invention;
assembling a boiler envelope so as to be mechanically supported thereon in
such a manner that at least the major part and preferably substantially all of
the boiler cold structure is at or below the support platform level.
14. The method of claim 13 wherein the boiler envelope is supported such that
there is essentially no boiler cold structure above the support platform level
and essentially only elements of the boiler hot structure and the boiler
pressure parts extend above the support platform level.
15. The method of claim 13 or 14 wherein the steps of providing a support
structure, and assembling in a supporting manner thereon a boiler envelope
structure, are performed progressively in parallel.