Patent Publication Number: US-8522729-B2

Title: Contoured flat stud and stud arrangement for cyclone slag taps

Description:
FIELD AND BACKGROUND OF INVENTION 
     The subject invention pertains in general to cyclone furnaces for burning ash-containing fuels, and in particular to studs used to protect the water-cooled tubes forming the slag tap outlet. 
     Cyclone furnaces were developed by The Babcock &amp; Wilcox Company (B&amp;W) in the 1940&#39;s. These cyclone furnaces have the ability to burn high-ash low-fusion temperature coals, which are particularly troublesome in pulverized coal boilers.  FIG. 1  shows a cyclone furnace assembly  100 , which comprises a generally horizontal barrel, typically 6 to 10 feet in diameter, attached to the side of a boiler furnace. The cyclone barrel is made up of water-cooled tubes  24 , arranged in tangent-tube construction. Crushed coal is introduced through crushed coal inlet  32 . Crushed coal and some air (primary  41  and tertiary  43 ) enter the cyclone through one or more specially designed burners on the front of the cyclone, such as radial burner  30 . 
     In the main cyclone barrel, a swirling motion is created by the tangential addition of secondary air in the upper cyclone barrel wall through secondary air velocity dampers  45 . A unique combustion pattern and circulating gas flow structure result. The products of combustion eventually leave the cyclone furnace through a re-entrant throat  60 , which includes water cooled tubes  22  adapted to form a slag tap opening or outlet  20 . A molten slag layer develops and advantageously coats the inside surface of the cyclone barrel. The slag drains to the bottom of the cyclone and is discharged through slag tap  20 . 
     Cyclone furnaces are an integral part of the boiler heat absorbing circuitry and allow for a smaller boiler since about 70-90% of the original fuel ash is captured in the slag tapped out of the furnace. 
     For additional details of the design and operation of cyclone furnaces, the reader is referred to U.S. Pat. Nos. 2,357,301, and 5,878,700, assigned to the assignee of the present invention, and to Chapter 15 of  Steam/Its Generation and Use,  41st Edition, The Babcock &amp; Wilcox Company, Barberton, Ohio, U.S.A., © 2005, the texts of which are hereby incorporated by reference as though fully set forth herein. 
     Erosion and corrosion within the Cyclone are two critical issues which require routine maintenance measures. As shown in  FIG. 1 , a protective wear liner, made up of replaceable wear blocks or liners  51 , is used to prevent excessive erosion at coal inlet  32 . The material used for these blocks is normally comprised of metal, ceramic or a combination of the two. Knuckle studs  52  are placed next to wear blocks  51 . 
     The cyclone&#39;s wet slagging environment produces a potentially corrosive iron sulfide attack on the pressure part tubing. Referring now to  FIG. 2 , in areas coated by the molten slag  72 , water-cooled tubes  22 ,  24  are typically protected by a refractory layer  74  held in place by cylindrical pin studs  53 . The pin studs  53  are welded to the outside surface of the tubes  22  in a very dense pattern. For example, the “super dense” pin studding offered by The Babcock &amp; Wilcox Company may include ¾ inch long studs with 360 or more studs per square foot. 
     In addition to retaining the refractory, the pin studs cool the refractory surface in contact with the corrosive slag and help retard the corrosive action. The pin studs hold the refractory in place, thereby improving the refractory life span, and the refractory in turn helps protect the pin studs. This insulation maintains the cyclone at a high enough temperature to permit adequate slag tapping from the bottom of the unit, and significantly reduces erosion and corrosion potential. 
     To further reduce maintenance, The Babcock &amp; Wilcox Company developed a flat, staggered stud design  54 , shown in  FIG. 1 , which includes a hand applied fillet weld. The flat staggered stud design  54  offered the following advantages: 1) more precise stud manufacturing and closer spacing, 2) minimum potential for channeling and accelerated wear between the studs, 3) excellent heat transfer which reduces metal temperature and erosion rates, and 4) thicker stud sizes to extend life. 
     To reduce erosion and corrosion in the slag tap region of a cyclone furnace, The Babcock &amp; Wilcox Company developed a contoured flat stud, designed for the tubes that make up the cyclone slag tap. The contoured design was developed to better fit a flat stud into the slag tap region. This original contoured stud was made of B&amp;W 800 material, and had a generally arcuate shape, being designed as an annular or ring-like segment. Referring now to  FIG. 3A ,  FIG. 3A  is a cross-sectional view of two original contoured studs  1  applied to a water-cooled tube  22 . Each original contoured stud  1  had an inner circumferential edge  2  that was contoured to contact the associated tube  22 . Original contoured stud  1  had an upper side  6  and a lower side  8  each of which were flat and parallel to one another. Outer circumferential edge  4  connected upper side  6  to lower side  8  at right angles and ran parallel to inner edge  2 . Outer circumferential edge  4  was exposed to slag and flue gas flowing through the slag tap opening when in use. 
     Original contoured stud  1  had a projection  9  extending from upper side  6  and lower side  8  and terminating in inner circumferential edge  2 . Projection  9  had a concave weld recess  10  (shown as concave up in  FIG. 3A ) connecting upper side  6  to inner edge  2 . Weld recess  10  was adapted to receive a weld  12 , and had a weld recess depth  11  ( FIG. 4A ). Projection  9  also had a concave recess  14  (shown as concave down in  FIG. 3A ) located opposite recess  10  and connecting lower side  8  to inner edge  2 . Concave recess  14  had a recess depth  15 . Recess  14  of stud  1  was symmetric with recess  10 , with weld recess depth  11  and recess depth  15  being equal. 
     In use, stud  1  was attached to tube  22  by fillet weld  12 . Weld  12  contacted water-cooled tube  22  at weld contact area  13 . Stud  1  itself contacted tube  22  at a stud contact area along inner edge  2 . As shown in  FIG. 3A , weld contact area  13  and the stud contact area of inner edge  2  were approximately equal. 
     As shown in  FIG. 5A , original contoured studs  1  were arranged along the water-cooled tubes using an original contoured stud arrangement  90  in which a plurality of original contoured studs  1  were disposed about water cooled tubes  22  in a vertically staggered arrangement, i.e. staggered as the studs  1  were attached vertically, not offset in the direction of slag and flue gas flow  55 . 
     While initial trials of original contoured stud  1  and original stud arrangement  90  showed some improvement, further reductions of erosion and corrosion rates were still desirable. 
     SUMMARY OF THE INVENTION 
     High maintenance and a high rate of tube failures in the cyclone slag tap region require that improved protection in this critical region be developed. Although the original contoured stud design and arrangement moved closer to improving this condition, an improved stud and stud arrangement was still warranted. The present invention provides additional protection in this critical area, thereby reducing maintenance costs and preventing tube leaks that occur in the region when studs and refractory do not adequately protect the tubes. 
     The present invention extends the life of the studs in the slag tap region by redesigning the stud arrangement, redesigning the stud itself to improve heat transfer between the stud and tube, and improving the material of the stud. Longer stud life in turn provides the benefits of reduced maintenance costs and longer overall tube life in the highly erosive and corrosive region of the cyclone slag tap. 
     Accordingly, one aspect of the invention is drawn to a stud for protecting a water-cooled tube. The stud has an annular segment having first and second sides and an outer edge connecting the sides. A projection extends from the sides and terminates in an inner edge contoured to contact a tube. The inner edge has an associated inner edge depth. The projection has a surface adapted to receive a weld between the inner edge and one of the sides. The weld surface has an associated weld depth, and the weld depth of the weld surface is greater than the inner edge depth. 
     Another aspect of the invention is drawn to a stud for protecting a water-cooled tube. The stud is an annular segment having parallel, flat sides connected by a circumferential outer edge perpendicular to the sides. A projection extends from the sides and terminates in a circumferential inner edge opposite and parallel to the outer edge. The inner edge is contoured to contact the tube along a stud contact area. The projection has a weld recess adapted to receive a weld between the inner edge and one of the sides. A weld fills the weld recess, thereby physically and thermally connecting the stud to the tube along a weld contact area, which is greater than the stud contact area. 
     Yet another aspect of the invention is drawn to a slag outlet. The slag outlet has a plurality of water-cooled tubes adapted to form a slag tap opening to discharge flowing flue gas and slag. The slag outlet also has a plurality of studs. Each stud is contoured to contact an adjacent water-cooled tube at a stud contact area and has a weld surface adapted to receive a weld. A weld fills each weld surface and contacts an adjacent water-cooled tube at a weld contact area, thereby physically and thermally connecting the stud to the tube. The flowing flue gas and slag define a flow direction, and the studs are staggered in a direction perpendicular to this flow direction. 
     The various features of novelty, which characterize the invention, are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, forming a part of this specification: 
         FIG. 1  is a partial sectional perspective view of a cyclone furnace assembly where the present invention may be used; 
         FIG. 2  is a partial sectional view of a known cyclone furnace stud and stud arrangement; 
         FIGS. 3A and 4A  are side sectional views of an original contoured stud; 
         FIGS. 3B and 4B  are side sectional views of an improved contoured stud; 
         FIG. 5A  is a side view of an original contoured stud arrangement; 
         FIG. 5B  is a side view of an improved contoured stud arrangement; 
         FIG. 6A  is a perspective view, from a view point within a furnace looking into the cyclone assembly, of an improved contoured stud arrangement applied to a forced circulation cyclone; 
         FIG. 6B  is a perspective view, from a view point within a cyclone assembly looking out towards the furnace, of an improved contoured stud arrangement applied to a forced circulation cyclone; and 
         FIG. 7  is a perspective view, from a view point within a furnace looking into the cyclone assembly, of an improved contoured stud arrangement applied to a natural circulation cyclone. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To further reduce erosion and corrosion in the slag tap region of a cyclone furnace, the present invention comprises an improved contoured stud design and an improved stud arrangement. 
     An improved contoured stud  101  has been developed, as shown in  FIG. 3B , which depicts two improved contoured studs  101  applied to a water-cooled tube  22 . Similar to original contoured stud  1 , improved contoured stud  101  has a generally arcuate shape, being designed as annular or ring-like segment. Improved contoured stud  101  preferably subtends an angle of about 30 to 45 degrees, i.e. so that a ring of about 12 to 8 studs would completely surround an associated tube  22 . 
     Also similar to original contoured stud  1 , improved contoured stud  101  has an inner circumferential edge  102 . Inner edge  102  is contoured to contact the associated water-cooled tube  22 , with inner edge  102  having a diameter slightly greater than the outer diameter of tube  22 . Outer edge  104  of improved contoured stud  101  connects upper side  106  to lower side  108  at right angles thereto and runs parallel to inner edge  102 . Upper side  106  and lower side  108  are preferably flat and parallel to one another. Outer circumferential edge  104  is exposed to flowing slag and flue gas when in use. 
     Referring now to both  FIGS. 3B and 4B , also similar to original contoured stud  1 , improved contoured stud  101  has a projection  109  extending from upper side  106  and lower side  108  and terminating in inner circumferential edge  102 . Inner circumferential edge  102  has an inner edge depth  116 . Projection  109  has a concave weld recess  110  (shown as concave up in  FIG. 3B ) connecting upper side  106  to inner edge  102 . Recess  110  is adapted to receive a weld  112  and has a weld recess depth  111 . Projection  109  also has a second concave recess  114  (shown as concave down in  FIG. 3B ) connecting lower side  108  to inner edge  102 . Concave recess  114  has a recess depth  115 . 
     In use, improved contoured stud  101  is attached to tube  22  by weld  112 , which is preferably a fillet weld, as is known in the art. Weld  112  contacts water-cooled tube  22  at weld contact area  113 , and improved stud  101  contacts the water-cooled tube  22  along inner edge  102 . 
     The fillet weld design of original contoured stud  1 , shown in  FIG. 3A , left a large area of the original contoured stud  1  un-cooled by the water-cooled tube  22 , since the weld area  13  was low relative to the stud contact area of inner edge  2 . Contact resistance between the original contoured stud  1  and tube  22  thus reduced the ability of the water flowing in the tube  22  to cool the stud  1 . As shown in  FIGS. 3B and 4B , improved contoured stud  101  has been modified to maximize the weld contact area  113  between the stud  101  and the tube  22 . In contrast with original contoured stud  1 , the recesses of improved contoured stud  101 , i.e. weld recess  110  and recess  114 , are asymmetric, with weld recess depth  111  of recess  110  being greater than, and preferably substantially greater than, the recess depth  115  of recess  114 . Weld contact area  113  is greater than the stud contact area along inner edge  102 , thereby improving heat transfer between the water-cooled tube and improved stud  101 . The larger weld contact area  113  thus helps maintain a lower operating stud temperature, which reduces stud overheating and oxidation conditions that would normally lead to shortened stud life. The depth  117  of outer edge  104  of improved stud  101  is also preferably reduced compared to depth  17  of outer edge  4  of original stud  1 . 
     Improved contoured flat stud  101  preferably is made of ASTM 297A, to further improve wear life. 
       FIGS. 5B ,  6 A,  6 B and  7  show an improved stud arrangement  290  in which a plurality of studs  1 ,  101  are disposed about the water cooled tubes  22  of slag tap opening  20  in a more horizontally staggered arrangement compared to original stud arrangement  90  of  FIG. 5A . In improved arrangement  290  the studs are stacked in vertical columns, with each column vertically offset from the adjacent vertical column. Staggering the studs in this way allows the stagger to face the flow of hot flue gas and molten running slag out of the tap, as indicated by flue gas and slag flow direction  55 . In arrangement  290  studs  1 ,  101  are offset in a direction perpendicular to the flow direction  55 , in contrast with stud arrangement  90 , where the studs  1  are offset in a direction parallel to flow direction  55 .  FIGS. 6A and 6B  show modified stud arrangement  290  applied to a forced circulation cyclone assembly.  FIG. 7  shows modified stud arrangement  290  applied to a natural circulation cyclone assembly. Improved stud arrangement  290  may be used with original contoured studs  1 , or preferably with improved contoured studs  101 . 
     While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles. For example, while the subject invention is particularly useful for retrofit applications, it is equally applicable to new installations. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims.