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(11) | EP 0 187 542 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Spiral to vertical furnace tube transition |
| (57) A furnace for a once-through steam generator comprises a lower section of inclined
tubes (13), an upper section of vertical tubes (15) and a support system in a transition
zone intermediate the sections. The support system comprises support straps (22) supported
on finger plates (30) whereby the lower inclined tubes (13) are supported by the upper,
vertical tubes (15). The upper section of vertical tubes (15) overlaps the lower section
of inclined tubes (13). |
[0002] In such furnace tube wall enclosures defining the periphery of a furnace, a lower section of the furnace comprises spiral-wound inclined tubes and an upper section comprises load-carrying vertical tubes which support the lower section. Hereinafter, "inclined tubes" will define the spiral-wound tubes in the lower furnace and "vertical tubes" the load-carrying vertical tubes in the upper portion of the furnace. The invention is applicable to subcritical or supercritical once-through steam generators having all-welded membrane furnace walls.
[0003] A major problem in the design of such furnace tube wall enclosures is in the support system and in the tube arrangement in a transition zone from inclined tubes to vertical tubes. The inclined tubes are sloped up to about 30 degrees from the horizontal and spiral up the furnace making several turns. The inclined tubes terminate in a header below a furnace arch for transition to the vertical tubes in the upper section of the furnace. Because a near-horizontally oriented tube can usually carry only a limited amount of static load in the vertical direction, the inclined tube section of the furnace requires an external support system to transfer load to the vertical tubes.
[0004] Relevant prior art includes United States Patent No. US-A-3 027 882 which discloses vertical support bars connecting inclined tubes to vertical tubes in a furnace; United States Patent No. US-A-3 400 689, which discloses vertical tension members connected at their lower ends to the furnace walls and supported by springs at their upper ends; and United States Patent No. US-A-4 116 168, which discloses inclined tubes in an intermediate furnace section connected by bifurcates to upper and lower vertical tube portions of the furnace.
[0005] According to the invention there is provided a furnace tube wall enclosure defining a furnace for a once-through steam generator comprising, a lower furnace section of inclined tubes which spiral upwards to a transition zone, an upper furnace section of load-carrying vertical tubes in fluid communication with the lower furnace section, vertical external support straps spaced transversely about the lower furnace section and weldably connected to the inclined tubes of the lower furnace section for support thereof, and means for transfer of the static load of the lower furnace section from the support straps to the vertical tubes of the upper furnace section, characterised in that the upper furnace section extends below the transition zone to overlap a portion of the lower furnace section.
[0006] The lower section of inclined tubes is fed in parallel from a ring header at the bottom of the furnace. The inclined tubes spiral upwards at an angle of up to about 30 degrees from the horizontal, making at least one full turn around the furnace, and terminating in vertical manifold headers located at spaced distances around the furnace. The upper furnace section load-carrying vertical tubes are in fluid communication with the lower section at a transition zone. The vertical and inclined tubes are spaced from one another and weldably interconnected by membranes forming a gas-tight enclosure. the vertical external support straps are flush mounted and weldably attached to the tubes of the lower furnace section for transfer of the static load to the tubes of the upper furnace section. Causing the tubes of the upper section to overlap the tubes of the lower section can eliminate eccentric loading and simplify the tube arrangement at the transition zone.
[0007] The invention is diagrammatically illustrated by way of example with reference to the accompanying drawings, in which: -
Figure 1 is a schematic sectional side view of a once-through steam generator having inclined and vertical furnace tube sections;
Figure 2 is an isometric view of a portion of the inclined tube lower furnace walls showing an external support strap with extending bars;
Figure 3A is a sectional view of a previously proposed support at the transition zone;
Figure 3B is a sectional view of a support used in a furnace tube wall enclosure according to the invention;
Figure 4 is a view from the furnace side showing the previously proposed tube arrangement and closure at the transition zone;
Figure 5 is a view from the furnace side showing the tube arrangement at the transition zone in the furnace tube wall enclosure of Figure 38;
Figure 6 is a sectional plan view of Figure 5;
Figure 7 Is a sectional view taken on line 7-7 of Figure 5;
Figure 8 is an external view showing the support system of Figures 3B, 5, 6 and 7 in greater detail; and
Figure 9 is a sectional elevation taken on line 9-9 of Figure 8.
[0008] Referring to Figure 1, a once-through steam generator 10 is shown having a lower furnace section 12 of inclined tubes 13 which are membraned and spiral upwards to a transition zone 16 below a furnace arch 18. From the transition zone 16, the inclined tubes 13 are in fluid communication with an upper furnace section 14 comprising vertical membraned tubes 15 in panels for upward flow to outlet headers 20. The inclined and vertical tubes 13 and 15 are shown in greater detail in subsequent drawings.
[0009] Figure 2 illustrates the method of support of the inclined tubes 13 in the lower furnace section 12 and one of several support straps 22 with bars 24 which extend the full width of the support strap 22 and weldably interconnect membranes 26 of the inclined tubes 13 with the support strap 22. The support strap 22 are mounted flush with the outside surface of the inclined tubes 13 and are spaced at suitable distances around the periphery of the lower furnace section 12 to accommodate the static load of the inclined tubes 13 and to transfer load to the vertical tubes 15 in the upper furnace section 14. The upper ends (shown in Figures 5 and 8) of the support straps 22 are flared to a greater width for transfer of load to a greater number of vertical tubes 15.
[0010] Figure 3A shows a previously proposed design of the transition zone and the load transfer from the support straps 22 to the vertical tubes 15 by means of finger plates 30. As indicated, the longitudinal centrelines of the vertical tubes 15 are in line with the centrelines of the inclined tubes 13. Since the static load of the lower furnace section 12 is carried by the support straps 22, a turning moment is produced due to eccentricity. To eliminate this moment, a complex, expensive pinned linkage system 28 is employed between the support straps 22 and vertical buckstays 32. Also shown are the membranes 26 of the inclined tubes 13, membranes 27 of the vertical tubes 15, and the bars 24 interconnecting the membranes 26 with the support strap 22.
[0011] An improved design is shown in Figure 3B. The vertical tubes 15 are located to overlap the inclined tubes 13. The longitudinal centrelines of the vertical tubes 15 are approximately in line with the centrelines of the support straps 22 thereby eliminating the moment due to eccentric loading while also eliminating the vertical buckstays 32 and the linkage system 28 associated with the previously proposed design shown in Figure 3A. A seal 34 between the uppermost tubes 13 and the vertical tube membranes 27 ensures the gas tight integrity of the furnace enclosure. Other advantages of this improved design will become apparent as it is described with reference to subsequent drawings. Also shown is one of the finger plates 30 and the inclined tube membranes 26.
[0012] Figure 4 is a view from the furnace side showing the closure at the transition zone of the previously proposed design. The inclined tubes 13 leave the furnace at the same horizontal plane and terminate in a horizontal outlet header, not shown, exterior to the furnace enclosure. Since the longitudinal centrelines of the vertical tubes 15 are in line with the inclined tube centrelines, the vertical tubes 15 leave the furnace at varying elevations, following the slope of the inclined tubes 13. Accordingly, the vertical tube membranes 27 also terminate at varying elevations. To provide a gas-tight furnace, closure plates 36 of various shapes are required to seal the areas between the terminations of the vertical membranes 27 and the uppermost inclined tubes 13. As is evident from Figure 4 this is an expensive design requiring much hand fitting and welding in the field during erection. In addition, the vertical tubes 15 require individual hand bending because of the varying elevations where they leave the furnace and terminate in an external header, not shown.
[0013] In contrast, Figures 5, 6 and 7 illustrate in detail the improved design and its advantages. In the embodiment shown, the inclined tubes 13 leave the furnace in vertical groups of five, although a greater or lesser number of the tubes 13 may be grouped. The groups penetrate to outside the furnace through spaces between the vertical tubes 15 and terminate in outlet vertical manifolds 38 spaced at suitable distances around the furnace. The manifolds 38 are connected to horizontal inlet headers 40 for fluid flow communication. Flow is then upward in the vertical tubes 15 to the outlet headers 20 (shown in Figure 1). The vertical tubes 15 overlap the inclined tubes 13 and bend outwardly in a horizontal plane below the inclined tube exit groups, terminating in the horizontal headers 40. Expensive closure plates are eliminated and field hand welding is reduced. The vertical tubes 15 are machine pack-bent in panels eliminating the individual hand bending of the previous design. Also shown are the finger plates 30 and the support straps 22.
[0014] Figures 8 and 9 are similar to Figures 5 and 7 but are viewed from outside the furnace. Shown is the flared end of one of the support straps 22 having ten vertical finger plates 30 welded thereto and the upper ends of the finger plates welded to adjacent pairs of the vertical tubes 15 for load transference. A greater or lesser number of finger plates 30 may be employed, depending on the magnitude of the static load of the lower furnace section 12 and the width of the flared upper ends of the support straps 22. Also shown are the inclined tubes 13, and the inclined and vertical tube membranes 26 and 27, respectively.
1. A furnace tube wall enclosure defining a furnace for a once-through steam generator
comprising, a lower furnace section (12) of inclined tubes (13) which spiral upwards
to a transition zone (16), an upper furnace section (14) of load-carrying vertical
tubes (15) in fluid communication with the lower furnace section (12), vertical external
support straps (22) spaced transversely about the lower furnace section (12) and weldably
connected to the inclined tubes (13) of the lower furnace section (12) for support
thereof, and means for transfer of the static load of the lower furnace section (12)
from the support straps (22) to the vertical tubes (15) of the upper furnace section
(14), characterised in that the upper furnace section (14) extends below the transition
zone (16) to overlap a portion of the lower furnace section (12).
2. A furnace tube wall enclosure according to claim 1, wherein the support straps
(22) are mounted flush with the outside surface of the inclined tubes (13) with bars
(24) weldably interconnecting the inclined tubes (13) to the support straps (22).
3. A furnace tube wall enclosure according to claim 1 or claim 2, wherein the upper
ends of the support straps (22) are flared to a greater width.
4. A furnace tube wall enclosure according to any one of claims 1 to 3, in which the
load transfer means includes a multiplicity of vertical finger plates (30) having
their lower ends welded to the flared ends of the support straps (22) and their upper
ends welded to adjacent pairs of the vertical tubes (15).
5. A furnace tube enclosure according to any one of claims 1 to 4, wherein the longitudinal
centrelines of the vertical tubes (15) are approximately in line with the centrelines
of the support straps (22).
6. A furnace tube wall enclosure according to any one of claims 1 to 5, wherein the
inclined tubes (13) which spiral upwards make least one complete turn around the periphery
of the furnace.
7. A furnace tube wall enclosure according to any one of claims 1 to 6, wherein the
inclined tubes (13) leave the furnace at the transition zone (16) in vertical groups
through spaces between the vertical tubes (15) and terminate in vertical manifolds
(38) spaced around the furnace.
8. A furnace tube wall enclosure according to claim 7, wherein the vertical tubes
(15) bend outwardly at the transition zone (16) in a horizontal plane below the inclined
tube exit groups and terminate in horizontal headers (40) in flow communication with
the vertical manifolds (38).
9. A furnace tube wall enclosure according to claim 1, wherein the tubes (13, 15)
in both the lower (12) and upper (14) furnace sections are interconnected by membranes
(26, 27) welded to adjacent tubes to form a gas-tight structure.
10. A furnace tube wall enclosure according to claim 1, wherein the tubes (13) of
the lower section (12) spiral upwards at an angle up to about 30 degrees from horizontal.