Abstract:
A buckstay system is described comprising horizontal buckstays for the walls of a steam generator, for example in plural vertically space assemblies tied with vertical buckstays, in which a buckstay extends generally horizontally across each wall such as to form a connected pair with an adjacent buckstay at each corner; an elongate tie bar formation extends across each wall such as to form a fixedly mounted pair with an adjacent tie bar formation at each corner; an anchor assembly associated with each buckstay and providing engagement means by which each buckstay engages with a respective tie bar; and each horizontal buckstay is split to comprise at least two rigid elongate buckstay elements mounted together to be relatively slideable in a buckstay longitudinal direction.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to buckstay systems for steam generation apparatus, for example for use with large boilers that are supported by a frame. 
     2. Description of Related Art 
     Boilers are commonly constructed of tube banks forming side walls, and typically planar side walls defining a structure of polygonal and usually rectangular section. As the system reaches operating temperature, the walls expand vertically and horizontally. Additionally, furnace pressure variations, pressure differential between fireside and ambient, may produce additional flexing of the tube walls either inwardly or outwardly. 
     To accommodate gas pressure differential and like effects the boiler walls are typically supported on the outside by an arrangement of members that surround the boiler to provide additional support to the boiler wall and limit the deformation of the wall in a horizontal direction attributable to pressure variations. The arrangement typically uses both vertical and horizontal members that are respectively known as vertical and horizontal buckstays. 
     Typically, horizontal buckstays are disposed in bands around the perimeter of the boiler walls at vertically spaced intervals. Horizontal buckstays surrounding the boiler at a given level walls are mechanically tied. Thus as the boiler flexes in a horizontal direction the reaction of one buckstay is resisted by the reactions of the buckstay on the opposing wall. Vertical buckstays are provided to connect series of adjacent horizontal buckstays and complete a buckstay support structure. These may be adapted at least at some points with a connection that permits a sliding action to allow relative movement between the wall and the buckstays. As the boiler expands in a vertical direction this accommodates a variable effect on the various levels of buckstays. 
     The typical boiler has planar walls meeting to form corners. There is a requirement to effect a connection between horizontal buckstay members where a first wall meets a second wall at an angle to form a corner. Conventionally, horizontal buckstays are continuous elongate structural members such as I-beams spanning the length of an associated wall with buckstays associated with adjacent walls extending at the corner formed by the adjacent walls to be connected by means of corner assemblies. The corner assemblies require potentially complex arrangements of links and brackets to accommodate differential expansion between a “hot” boiler wall and “cold” buckstays. An example of such a corner assembly can be seen in  FIGS. 1 and 2 . 
     SUMMARY OF THE INVENTION 
     According to the invention in a first aspect there is provided: a buckstay system for a wall of a steam generator having a first wall section which meets a second wall section at an angle to form a corner, the system comprising:
         a buckstay extending generally horizontally across each wall section such as to form a connected pair at the said corner;   an elongate tie bar formation extending across each wall section such as to form a fixedly mounted pair at the corner; for example by means of an end connection corner angle reinforcement tie welded at the corner to the pair of tie bars;   an anchor assembly associated with each buckstay and providing engagement means by which each buckstay engages with a respective tie bar;   wherein each such buckstay is split to comprise at least two rigid elongate buckstay elements mounted together to be relatively slideable in a buckstay longitudinal direction.       

     A pair of buckstays in accordance with the invention as most broadly stated are associated together at a corner corresponding to a point where a first boiler wall meets a second boiler wall at an angle to form a corner, the associated buckstays being dimensioned and configured for an associated boiler to support such boiler walls in familiar manner. Each buckstay extends across an associated boiler wall. Tie bars are provided in generally conventional manner, and each buckstay engages with a respective tie bar in familiar force transferring manner so that a buckstay forming part of a buckstay assembly can react to horizontal loading in the wall and tend to prevent dishing. 
     However, a buckstay in accordance with the invention is particularly characterised in that it is split to comprise at least two rigid elongate buckstay elements which are slideably mounted together in mechanical association in a longitudinal direction. This slideable configuration enables each buckstay, in itself, to accommodate expansion in a longitudinal (that is, in use, horizontal) direction, and in particular to accommodate differential expansion between conditions imposed by the difference between a hot boiler wall and cold buckstays. Since the buckstay itself, by being variable in length via such a mechanical means, accommodates this expansion, the requirement in the prior art to provide potentially complex arrangements of links and brackets between a pair of buckstays at each corner is reduced or eliminated. 
     Instead of such a complex connection, a simple connection may be provided between the horizontal extensions of each buckstay at a corner, for example in the form of a simple mechanical connection between respective formations on each buckstay which extend beyond a said corner. For example, a fixed mechanical engagement between a bracket portion on an end of a first such buckstay and a receiving portion on an end of a second such buckstay may be provided. This joint need not provide for any expansion in a buckstay longitudinal, or horizontal, direction. Optionally, the joint may be adapted to provide for a relative variation in angle between the two buckstays meeting at the joint. Alternatively, the joint may simply be fixed, for example bolted or welded. 
     An anchor assembly is associated with each buckstay to enable the buckstay to engage with a tie bar of its associated wall and thus enable the buckstay arrangement in use to transmit bending forces which tend to bend each wall section to each respective buckstay, which therefore resists such bending forces in generally conventional manner. 
     An anchor assembly for example comprises a support formation such as a plate formation fixedly engaged with, and for example welded to, a buckstay element, and a bearing surface located to bear upon and engage in use with a surface of a tie bar. A buckstay element may comprise an anchor housing, for example including a co-operably shaped recess, to receive a support formation. In a particular preferred embodiment, two rectangular support plates, comprising an upper and a lower support plate, are deployed above and below a buckstay element to comprise an anchor assembly. The support formation(s) of the anchor assembly preferably comprise additional stiffening plates, for example in a direction parallel to and/or perpendicular to a longitudinal direction of the buckstay. 
     To minimise loading balances, it is preferable that each anchor assembly is provided on a buckstay element closely towards a corner formed by its associated wall and the adjacent wall. For example, each buckstay anchor assembly is preferably within 2 m and more preferably within 600 mm of such a corner. 
     In a more complete aspect, a buckstay system is provided for a wall of a steam generator having plural wall sections which meet adjacent wall sections at an angle to form a polygonal steam generator structure, and in particular at orthogonal angles to form a rectangular-sectioned steam generator structure. The system comprises at least one buckstay assembly comprising a plurality of buckstays as above described disposed surroundingly around the plural wall sections, the assembly thereby surrounding the perimeter of the wall of the steam generator, the buckstays configured and connected in the manner above described. That is, each buckstay in the assembly comprises at least two rigid elongate buckstay elements mounted together to be relatively slideable in a buckstay longitudinal direction. 
     Preferably each buckstay assembly is disposed generally horizontally. Preferably a plurality of such horizontal buckstay assemblies spaced vertically up a steam generator structure are provided. 
     Vertical support means may be provided to support and space such plural horizontal buckstay assemblies vertically up the steam generator, for example in the form of vertical buckstays in familiar manner. Vertical buckstays may engage with horizontal buckstays by means of engagement which is fixed in a vertical direction, or which permits movement in a vertical direction for example in sliding manner. Such buckstay arrangements will be familiar. 
     The distinctive feature of the present invention is in the split horizontal buckstay, providing two or more buckstay elements to comprise the horizontal buckstay, with horizontal sliding engagement provided between the elements to accommodate expansion by giving the buckstay an inherent capacity to vary in length. Conveniently, vertical supports such as vertical buckstays are provided in the vicinity of some or all of the points where buckstay elements of a horizontal buckstay make sliding engagement. For example, at a sliding engagement point between two horizontal buckstay elements, a vertical support is provided having a fixed engagement with one said element, and having a sliding engagement with the other said element whereby said element may slide horizontally relative to the vertical support and therefore relative to the other element. 
     In the preferred embodiment, a buckstay system comprises horizontal and vertical buckstays which can embody generally familiar principles of design. The buckstay system may be restrained and the weight carried by a support frame, for example in that load carrying restraints are provided between a buckstay and a frame member at a number of horizontal restraint levels. The buckstay system is distinctly characterised in that the horizontal buckstays are split into at least two buckstay elements to accommodate horizontal expansion, most preferably by the vertical buckstays. This dispenses with the need for complex corner bracket arrangements between adjacent horizontal buckstays at a wall corner, and can also confer advantages of flexibility of design, for example in reducing the number of buckstay fixed levels which might be required compared with typical prior art structures. 
     A horizontal buckstay element conveniently comprises a rigid elongate structural member, for example of a suitable structural metal such as structural steel. A buckstay conveniently comprises a rolled member. A buckstay element is for example a rolled steel member. A horizontal buckstay element may for example have a web shaped surface. 
     A sliding engagement between two buckstay elements is made in any suitable manner that permits sliding to vary length in a buckstay elongate direction but tends to maintain the rigidity of the structure of the buckstay to resist bending forces out of this buckstay elongate direction. For example, buckstay elements may be provided with end formations which engage in side to side sliding manner, in telescoping manner etc. For example, a sliding connection between buckstay elements may be contained within a housing to maintain rigidity of the structure out of the buckstay elongate direction. In a preferred embodiment, a buckstay element is a web structure, and a sliding engagement connection is provided which permits sliding movement in a longitudinal direction of the two buckstay elements by means of relative sliding of the web surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will now be described by way of example only with reference to  FIGS. 1 to 7  with the accompanied drawings, in which: 
         FIG. 1  is a plan section of a prior art horizontal buckstay assembly at a restraint level; 
         FIG. 2  is a more detailed section through the buckstay corner bracket arrangement of  FIG. 1 ; 
         FIG. 3  is a plan section of a horizontal buckstay assembly according to an embodiment of the invention at a restraint level;  FIG. 4  is a transverse section through and elevation of the buckstay corner region of  FIG. 3 ; 
         FIG. 5  is a more detailed section through the buckstay anchor assembly of  FIG. 4 ; 
         FIG. 6  is a section through B-B of  FIG. 5 . 
         FIG. 7  is a side elevation of a boiler with a buckstay system suitable to embody the principles of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An arrangement of horizontal buckstays at a restraint level in a typical prior art buckstay system is shown in  FIG. 1 , with a corner assembly shown in greater detail in  FIG. 2 . 
     Boiler walls  5  of a rectangular boiler are surrounded by an arrangement of horizontal buckstays  4  and vertical buckstays  6 . Buckstays are of any suitable known construction, for example comprising steel I beams. The arrangement in  FIG. 1  is illustrated at a restraint level, and restraints are provided to transmit load to a support framework  1 . 
     As can be seen in particular detail in  FIG. 2 , a complex arrangement of brackets and links is required to accommodate horizontal expansion as the thermal regime changes. Each horizontal buckstay  4  comprises a single monolithic elongate structural member. Each buckstay  4  is provided with an end bracket  10  which is typically welded to the web portion  12  of the I beam comprising the buckstay. Elongate tie bars  9  are provided. A corner tie  13  and corner bracket  14  are welded to a pair of adjacent tie bars  9  at a corner. A link is provided between the corner bracket  14  which is a fixed part of the tie bar system and the end bracket  10  fixed to each adjacent buckstay  4  by means of the elongate link plates  16  and pin connections  18 . The assembly is necessarily complex as it is required thereby to accommodate relative lateral movement of the respective buckstays and tiebar assembly as the conditions change between cold and hot operation. 
       FIG. 3  illustrates a typical plan view at a restraint level of an arrangement in accordance with an example embodiment of the invention. This shows a steel support framework comprising horizontal  20  and vertical  19  steel girders which surrounds a boiler wall  25 . Horizontal buckstays  24  surround the wall sections and provide a means of reacting to an expansion load within the boiler. 
     Where the arrangement differs in accordance with the invention is that a buckstay  24  does not comprise a single monolithic whole, but is instead comprised of multiple (in the example two) rigid elongate elements which are relatively slideable at a split point  27 . The result of this split is that length changes in a horizontal direction can be accommodated inherently in the horizontal buckstay  24  itself, as the sliding action varies its overall length, which can simplify corner structures as these no longer need to accommodate this. 
     Vertical buckstays  26  are provided in generally familiar manner. At least one, and depending on the size of the boiler more than one, vertical buckstay may be provided. Preferably, a vertical buckstay is linked to a horizontal buckstay in the vicinity of a split point  27 . For example, one of the two vertical buckstays on each long side of the illustrated in  FIG. 3 , and the single vertical buckstay on each short side in  FIG. 3 , are so located. Restraints  22  tie the buckstay assembly, again conveniently at these points, to the steel girders which make up the support framework. 
     This arrangement produces a simplification of the corner structure, which is illustrated in greater detail in  FIGS. 4 to 6 . 
       FIG. 4  illustrates a corner portion of a buckstay assembly in accordance with the invention. A pair of horizontal buckstays  24   a ,  24   b  meet at a wall corner. 
     The direct connection between the two adjacent buckstays meeting at the corner is much simplified. A first horizontal buckstay  24   a  is provided with an elongate buckstay bracket member  30 . This engages with a flange surface of a second buckstay  24   b  by means of a cut away of the flange  32  in the vicinity of this bracket. A simple single pin connection  34  in the illustrated embodiment, or any other suitable simple fixed or rotating connection, is all that is needed to tie the two buckstays together at the corner. 
     A tie bar assembly at the corner comprises the end portions of each respective elongate main tie bar  42  or  46  to which is welded a respective stub tie bar  43  or  47 , the assembly being completed by a welded corner reinforcement angle formation  50  that completes the corner, and connects the two tie bars. Buckstays are located on and engage with the tie bar assembly via respective anchors  44  or  48  no more than 600 mm (measure by anchor centre line) inboard of the corner. Buckstay clips  49  engage with the tie bars. 
     The anchoring arrangement of a first anchor  44  is additionally illustrated with reference to a side elevation representing a view along A-A of  FIG. 4   a  as illustrated in  FIG. 4   b .  FIG. 4   c  illustrates the use of a cheek plate  40 . This anchor formation is illustrated in greater detail in transverse sectional view  FIG. 5  (with bearing plates omitted for clarity) and in section through B-B in  FIG. 6 . 
     Engagement between the buckstay  24   b  and the tie bar assembly illustrated in  FIG. 4  is achieved by the anchoring means  44 , and in particular by engagement of a bearing surface  52  on the anchoring means and a bearing pad  54  at an adjacent engagement end of the stub tie bar  43 . 
     A horizontal buckstay  24   b  is brought into a load transferring engagement with the tie bar assembly by means of a pair of anchor plates  56  to be received in an anchor housing  53  in a pair of recesses  55  located above and below the horizontal buckstay. The anchor plate comprises a plate having a 20 mm thickness, 250 mm wide. It is provided with secondary vertical  58  and horizontal  59  stiffening plates which are 10 mm in thickness. It is fixedly mounted, for example by welding, into a corresponding recess  55  so as to be in fixed relationship with the buckstay  24   b . A forward facing bearing surface  52  then makes a bearing engagement with corresponding bearing surfaces  54  on an adjacent tie bar. Thus, loads may be transmitted to the buckstay system allowing the buckstay system to react against them. 
       FIG. 7  illustrates an elevation of a buckstay system used in association with a boiler  61 . The buckstay system is designed to transmit transient pressure loading to the boiler support structure via a suitable restraint link system in generally familiar manner, for example as illustrated in  FIG. 3 . 
     To that end, the buckstay system generally comprises a framework of horizontal and vertical elongate structures. Horizontal buckstays are provided at a plurality of buckstay levels as illustrated and labelled respectively L 1  to L 20   a  on the left hand side of  FIG. 7 . Vertical members comprising continuous O/T posts towards the corners of the boiler structure, and vertical buckstays therebetween, tie with the horizontal buckstays to complete the buckstay assembly. 
     These vertical buckstays are used in accordance with the invention to split horizontal buckstays which would otherwise extend in continuous manner between boiler corners. The number of vertical buckstays to be used is dependent on many factors including boiler width, depth, sootblowers etc. As a general guide, a single vertical buckstay might be appropriate for a wall width of less than 17 m, two for a wall width of up to 24 m, and three for a wall width of an excessive 24 m. Typically, the vertical buckstays will be positioned such so that they split horizontal buckstays into equal lengths. 
     Usually vertical buckstays will span from the first buckstay level L 1  to the transition header level  70 . Buckstays are anchored to wall via a tie bar and anchor arrangement such as described above. Tie bars may be anchored to the boiler wall in such manner as to allow the tie bar to be free to move vertically with the buckstay to provide a vertical buckstay sliding joint  67 , or may be anchored to the tube wall to allow no such vertical movement to provide for a buckstay fixed level joint  65  at a buckstay fixed level  66 . 
     Buckstays are linked to the supporting steelwork at the bracing levels only, labelled on the right side of the figure respectively B 1  to B 6 . Where possible, the distance between the horizontal bracing levels above the boiler knuckle is to be no greater than 12 m. The top horizontal bracing level B 1  is in line with buckstay level L 2 . 
     Each vertical buckstay may be anchored vertically by the horizontal buckstay closest to the mid span of the vertical buckstay. A typically maximum length of the vertical buckstay is likely to be limited by the differential expansion between the wall and the horizontal buckstay closest to the end. In the illustrated embodiment this differential expansion should not exceed 100 mm. 
     In the illustrated embodiment tie bars are anchored at each horizontal restraint level only. 
     Most of the horizontal buckstays create a support structure that extends around the perimeter of the boiler, and in particular that is required to extend around the points where two faces of the boiler connect to form a corner, for example in the manner illustrated in  FIG. 3 . However, other buckstay structures can be noted in  FIG. 7 , including mini buckstays  64   a  on the burner centre line  71  and mini buckstays  64   b  local to the arch, as will be familiar. 
     A further possible advantage of the design is that the number of buckstay fixed levels BFL required can be reduced relative to conventional buckstay arrays. In the illustrated embodiment, only three such fixed levels are necessary.