Abstract:
A ceramic ferrule assembly is provided, including a head including at least one tab protruding outwardly from an inner peripheral surface thereof, and a stem having an annular flange about an outer peripheral surface thereof and having at least one slot formed therein, the slot being adapted to receive the tab and retain the tab in a stationary position when the stem is rotated relative the head, whereby the at least one tab and the at least one slot mechanically engage one another to secure the ferrule stem and ferrule head to prevent axial disassociation from one another without counter rotation.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to ceramic ferrules for use in connection with tube sheets of waste heat boilers in general, and specifically, to a two-piece ceramic ferrule assembly including a ferrule head and a ferrule stem that are mechanically mated to one another prevent axial and radial disassociation during use. 
       BACKGROUND OF THE INVENTION 
       [0002]    Waste heat boilers are commonly used for the recovery of thermal energy from process gas in industrial applications. Waste heat boilers typically include an array of metallic tubes that are secured on each end by comparatively thick plates that are referred to in the art as tubesheets. The tubesheets are surrounded by a shell to form a pressure vessel. Hot process gas is introduced to the waste heat boiler at one end, and flows down the inside diameters of the tubes. Boiler feed water is fed to the other side of the tubes within the pressure vessel. Heat is transferred by conduction through the tube walls from the hot process gas to the boiler feed water, thereby producing pressurized steam for use in other processes. 
         [0003]    Since the tubesheet has a greater thickness than that of the the boiler tube wall, heat transfer is retarded in this region, and tubesheet temperatures at the inlet end can reach levels where certain modes of corrosion are problematic. Accordingly, it is often necessary to protect the front plate of the waste heat boiler and the inlet ends of the tubes from such high temperatures and/or highly corrosive atmospheres. This is commonly achieved using a combination of ceramic ferrules and castable refractory pieces. 
         [0004]    One common ceramic ferrule design is a two-piece design including a ferrule head piece, which protects the front face of the waste heat boiler, and a ferrule stem piece, which protects the tube inlet. During assembly, the ferrule stem is inserted into and passes through the ferrule head, and a sealing gasket of refractory ceramic fiber sleeve fills the space between the head and stem to secure the connection through compression of the fiber. One of these ferrule assemblies is inserted into each tube in the waste heat boiler and secured in place by additional fiber provided around the ferrule stem outside diameter and between the boiler tube inside diameter. 
         [0005]      FIG. 1  is a perspective view of such a conventionally known two-piece ceramic ferrule, and  FIG. 2  is a cross-sectional view of the two-piece ceramic ferrule shown in  FIG. 1 . As shown, the two-piece ferrule  10  includes a ferrule stem  2  that is inserted into the ferrule head  1 . The ferrule stem  2  is held in place by the compressive forces attributed to the location of the ceramic fiber braided rope  4  and the ceramic fiber gasket  5 , along with the heat resistant fiber sleeve  3 B, which is provided on a portion of the outer diameter (OD) of the stem  2  that is located within the central bore of the head  1  (see  FIG. 2 ). The ferrule stem  2  also includes a flange  21  at one end thereof, which is seated against an annular rim  122  within the central bore of the head  1  to prevent the stem  2  from passing completely through the head  1 . A heat resistant fiber sleeve  3 C is provided on a portion of the outer diameter of the ferrule stem  2  that remains extended from the ferrule head  1 , and the heat resistant fiber sleeve  3 A is provided on a portion of the outer surface of the head  1 . 
         [0006]    As described above, a piece of compressed high temperature ceramic fiber is used to secure the connection between the ferrule head land the ferrule stem of the two-piece ferrule  10 . However, when the ferrules  10  are used in service, the forces present can cause separation between the respective heads  1  and the stems  2 , resulting in the failure of that ferrule  10  from its intended use. At the minimum, this requires refractory repair and replacement of the ferrules  10  causing production down time, which is costly not only in terms of manpower and parts, but in terms of process down-time and lost productivity. More likely, however, is the potential for catastrophic failure of the boiler and tube due to the thermal degradation and corrosion that results from the ferrules not being properly in place. This scenario is significantly more costly than mere refractory replacement. 
         [0007]    One mode of failure of the above-described two-piece design occurs when the stem  2  disassociates from or “backs out” of the head  1 , which can be caused by vibrations in the system or any kind of resultant back pressure, which is common during cleaning operations. Once the stem  2  is unseated from its properly assembled position, it is possible for the process gas to bypass the ferrule  10  by transitioning along the stem  2  outer diameter (OD), contacting the waste heat boiler tubesheet area and causing unacceptable heating and subsequent corrosion. 
       SUMMARY OF THE INVENTION 
       [0008]    The object of the present invention is to provide a two-piece ceramic ferrule that is not susceptible to the above-described drawbacks associated with the prior art two-piece ferrule design. In view of the above, the present invention aims to provide a two-piece ferrule assembly including mechanical features on the ferrule head that work in conjunction with corresponding mechanical features on the ferrule stem to secure the two pieces of the ferrule assembly together in order to prevent the ferrule stem and head from disassociating during service due to vibrations or other external forces. 
         [0009]    According to one embodiment of the present invention, a ceramic ferrule assembly is provided, comprising a ferrule head and a ferrule stem. The ferrule head comprises at least one tab protruding outwardly from an inner peripheral surface thereof, and the ferrule stem comprises an annular flange about an outer peripheral surface thereof and has at least one slot formed therein. The slot is adapted to receive the tab and retain the tab in a stationary position when the ferrule stem is rotated relative the ferrule head, whereby the at least one tab and the at least one slot mechanically engage one another to secure the ferrule stem and the ferrule head to one another and to prevent axial and radial disassociation form one another without otherwise providing specific counter rotation. 
         [0010]    According to another aspect of the present invention, a ceramic ferrule assembly is provided, comprising a ferrule head having a hexagonal outer peripheral shape and extending from a first end to an opposed second end and defining a length thereof The ferrule head has a central bore extending from a hexagonal opening at the first end of the ferrule head to a circular opening at the second end of the ferrule head along the length thereof defining a longitudinal extension axis (central axis) thereof The central bore has an inner peripheral surface defined by a first section having a tapered hexagonal shape extending from the first hexagonal opening to an annular ridge, and a second section having a cylindrical shape extending from the annular ridge to the circular opening at the second end. At least one tab is provided, extending from a portion of the inner peripheral surface of the ferrule head. The ceramic ferrule assembly also includes a ferrule stem extending from a first end toward an opposed second end and defining a length thereof along a longitudinal extension axis (central axis) of the ferrule stem. A first radial flange is spaced a distance away from the second end of the stem along the longitudinal extension direction of the ferrule stem and has a slot formed therein. A second radial flange is located proximate and defining the second end of the ferrule stem. The at least one tab of the ferrule head resides in the slot in a rotatably engaged position, securing the ferrule stem to the ferrule head without adhesive or additional sealing means, and the second flange of the ferrule stem is seated on the annular ridge of the inner peripheral surface of the ferrule head. 
         [0011]    Preferably, the outer diameter of the second flange of the ferrule stem is greater than an outer diameter of the first flange of the ferrule stem, and the outer diameter of the first flange is greater than an outer diameter of the ferrule stem. 
         [0012]    According to one aspect, the opening of the at least one slot is parallel with respect to the longitudinal extension axis of the ferrule stem. According to another aspect, the opening of the at least one slot is helically-shaped. When the opening/slot is helically-shaped, it is preferred that the slot has an angle of 45° or less with respect to the longitudinal extension axis of the ferrule stem. 
         [0013]    According to another aspect of the present invention, the at least one tab comprises two tabs diametrically opposed to one another on the inner peripheral surface of the ferrule head. In accordance with this embodiment, it is preferred that the at least one slot likewise comprises at least two slots diametrically opposed to one another on the first annular flange. According to one aspect of this embodiment, the openings of the two slots are parallel with respect to the longitudinal extension axis of the ferrule stem. According to another aspect of this embodiment, the two slots are helically-shaped. 
         [0014]    The ceramic ferrule head and ferrule stem are preferably formed from ceramic materials including, but not limited to alumina, mullite and silicon carbide, for example. The slot and tab features can be integrally formed with the respective ferrule stem and ferrule head pieces when the respective pieces are initially formed by casting, molding or other suitable ceramic forming processes, such as extruding and machining Alternatively, the slots and tabs can both be subsequently formed by known ceramic machining processes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a better understanding of the nature and object of the present invention, reference should be made to the following detailed description of a preferred mode of practicing the invention, read in connection with the accompanying drawings, in which: 
           [0016]      FIG. 1  is a perspective view of a prior art two-piece ceramic ferrule; 
           [0017]      FIG. 2  is a cross-sectional (cut) view of the prior art two-piece ceramic ferrule shown in  FIG. 1 ; 
           [0018]      FIGS. 3A-3D  are perspective views of ceramic ferrule stems  200  and  210  according to the present invention, wherein  FIGS. 3A and 3B  are perspective views of a ceramic ferrule stem  200  having an annular flange  211  with an L-shaped slot  220  having an opening  221  that extends parallel to the longitudinal extension axis of the ferrule stem according to one aspect of the present invention, and  FIGS. 3C and 3D  are perspective views of a ceramic ferrule stem  210  having an annular flange  211 ′ with a slot  230  having a helically-shaped opening according to another aspect of the present invention; 
           [0019]      FIG. 4  A is a perspective view of a ceramic ferrule head  100  according to the present invention; 
           [0020]      FIG. 4B  is a cross-sectional (cut-view) of the ceramic ferrule head  100  shown in  FIG. 4A ; 
           [0021]      FIGS. 5A-5C  are assembly views showing the steps for joining the ceramic ferrule head  100  and ceramic ferrule stem  200  together to form a ceramic ferrule assembly  500  according to the present invention; 
           [0022]      FIG. 6A  is a cut-view showing the ferrule assembly  500  in  FIG. 5C ; 
           [0023]      FIG. 6B  is a cross-sectional (cut view) showing a ferrule assembly  600  according to another aspect of the present invention; 
           [0024]      FIG. 7  is a bottom view of the ferrule assembly  500  shown in  FIGS. 5C and 6A ; and 
           [0025]      FIG. 8  is a bottom view of a ferrule assembly  800  according to another aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIGS. 3A and 3B  are perspective views of a ceramic ferrule stem  200  according to one aspect of the present invention. The ferrule stem  200  extends from a first end  201  to an opposed second end  202  along the length thereof, defining the longitudinal extension axis (central axis) of the ferrule stem  200 . A substantially cylindrical central bore  203 , having a substantially cylindrical inner diameter (ID), extends along the length of the ferrule stem  200  from a circular opening at the first end  201  to a circular opening at the second end  202 . A first annular flange  211  is provided, circumscribing the outer diameter (OD) of the ferrule stem  200  and spaced a distance away from the second end  202 , and a second annular flange  212  is provided, circumscribing the OD of the ferrule stem  200  proximate and defining the second end  202  of the ferrule stem  200 . 
         [0027]    The outer diameter of the second flange  212  is greater than the outer diameter of the first flange  211 , and the outer diameter of the first flange  211  is greater than the OD of the ferrule stem  200 . Preferably, the outer diameter of the first flange  211  is 1.5 inches, the outer diameter of the second flange  212  is 1.3 inches and the OD of the ferrule stem  200  is 1.1 inches. It is also preferred that the first flange  211  is spaced a distance of 1.0 inch, more preferably 0.250 inches, from the second end  202  of the ferrule stem  200  in the longitudinal extension direction. 
         [0028]    The annular flange  211  has a slot  220  formed therein with an opening portion that extends parallel to the longitudinal extension axis of the stem  200 , and has a general L-shape according to one aspect of the present invention. That is, as shown in  FIG. 3B , the slot  220  has a slot first part  221  that extends parallel to the longitudinal extension axis (central axis) of the ferrule stem  200 , and a slot second part  222  that is substantially perpendicular with respect to the first part  221 , and represents a circumferential extension portion of the slot  220  in which a tab T on the ferrule head  100  (see, e.g.,  FIGS. 4A and 4B ) is positionally located once the ferrule stem  200  is rotatably positioned with respect to the ferule head  100  to define the ferrule assembly  500 , as described below (see, e.g.,  FIG. 6A ). In connection with the above, the dimensions (i.e., length and width of the slot first and second parts  221  and  222  must at least matingly correspond to the dimensions of the tab T, so that the T is able to pass through the slot first part  221  and then reside within the slot second part  222  without becoming dislodged during operative use. 
         [0029]      FIGS. 3C and 3D  are perspective views of a ceramic ferrule stem  210  according to another aspect of the present invention. The ferrule stem  210  extends from a first end  201 ′ to an opposed second end  202 ′ along the length thereof, defining a longitudinal extension axis (central axis) of the ferrule stem  210 . A substantially cylindrical central bore  203 ′ having a substantially cylindrical inner diameter (ID) extends along the length of the ferrule stem  210  from a circular opening at the first end  201 ′ to a circular opening at the second end  202 ′ thereof A first annular flange  211 ′ is provided, circumscribing the outer diameter (OD) of the ferrule stem  210  and spaced a distance away from the second end  202 ′, and a second annular flange  212 ′ is provided, circumscribing the outer diameter OD of the ferrule stem  210  proximate and defining the second end  202 ′ of the ferrule stem  210 . 
         [0030]    The outer diameter of the second flange  212 ′ is greater than the outer diameter of the first flange  211 ′, and the outer diameter of the first flange  211 ′ is greater than the OD of the ferrule stem  210 . Like with the embodiment described in connection with  FIGS. 3A and 3B , preferably, the outer diameter of the first flange  211 ′ of the ferrule stem  210  is 1.5 inches, the outer diameter of the second flange  212 ′ is 1.3 inches and the OD of the ferrule stem  210  is 1.1 inches. It is also preferred that the first flange  211 ′ is spaced a distance of 1.0 inch, more preferably 0.250 inches, from the second end  202 ′ of the ferrule stem  210  in the longitudinal extension direction. 
         [0031]    The annular flange  211 ′ of the ferrule stem  201  has a slot  230  having a helically-shaped opening  231  (slot first part), rather than the axially parallel-oriented opening  221  (slot first part) shown in  FIGS. 3A and 3B . In this case, the angled shape of the opening  231  twists helically around the axis of the ferrule stem  210  for even greater security and radial blocking, but still engages the tab T in a similar manner to accept and retain the tab T on the ferrule head  100  in the slot second part  232  after the ferrule stem  210  is rotated into place with respect to the ferrule head  100  to define the assembly  600 , as described below in connection with  FIG. 6B . 
         [0032]    Preferably, the angle of the helically-shaped opening (slot first part  231 ) of the slot  230  extends at an angle in a range of 25-35°, more preferably not more than 45° with respect to the longitudinal extension axis (central axis) of the ferrule stem  210 . In connection with the above, the dimensions of the slot first part  231  (i.e., width and length) must at least matingly correspond to the dimensions of the tab T, so that the T is able to pass through the angled slot first part  231  of the helically-shaped slot  230  and then reside within the adjacent slot second part  232 , in a similar manner as that described above in connection with the slot  220  shown in  FIGS. 3A and 3B , without becoming dislodged during operative use without otherwise purposefully providing the required counter rotation needed to disengage the mechanically interlocked ferrule pieces. 
         [0033]      FIG. 4A  is a perspective view of a ceramic ferrule head  100  according to the present invention, and  FIG. 4B  is a cross-sectional (cut-view) of the ceramic ferrule head  100  shown in  FIG. 4A . The ferrule head  100  has a substantially hexagonal outer peripheral surface  103  and extends from a first end  101  to an opposed second end  102  along the length thereof, defining a longitudinal extension axis (central axis) thereof The ferrule head  100  has a central bore  110  that extends from a first hexagonal opening  111  in the first end  101  to a circular opening  114  in the second end  102 . The central bore  110  has a first section  112  having a tapered hexagonal inner peripheral shape that extends from the first hexagonal opening  111  toward an annular ridge  122 , which defines a transition point of the central bore  110 . A second section  113  of the central bore  110  having a cylindrical inner peripheral shape extends from the annular ridge  122  toward the circular opening  114  at the second end  102  of the ferrule head  100 . 
         [0034]    The varied shape of the inner peripheral surface of the ferrule head  100  is provided for several reasons. One such reason is that having a hexagonal opening  111  at one end and a hexagonal inner peripheral surface  112  that transitions in a tapered manner toward the circular opening  114  decreases thermal stresses and provides an improvement in pressure loss as opposed to having a straight, open central bore construction along the length of the ferrule head  100 . Another reason is that the inner annular ridge  122  provides not only a geometrical transition point between the tapered hexagonal section  112  and the cylindrical section  113  of the central bore  110 , but also provides a seat surface for enabling at least some degree of axial retention of the ferrule stem  200  in the insertion direction (see, e.g.,  FIGS. 5A-5C and 6A ). The annular ridge  122  also cooperates with the first annular flange  212  of the ferrule stem  200  to prevent process gas from penetrating therebehind. 
         [0035]    At least one locking member, such as tab T, is provided on a portion of the inner peripheral surface of the second section  113  of the ferrule head  100 , preferably proximate the second end  102  thereof The preferred location for the tab(s) is in the lower section of the ferrule head  100  so as to engage the slot  220  on the first flange  211  of the ferrule stem  200 , although if wall thickness restrictions dictate, it is possible to locate the tab(s) in the upper section of the ferrule head  100  to instead engage a similar slot that can likewise be provided in the second flange  212  of the ferrule stem  200 . 
         [0036]    As shown, the tab T extends from the cylindrical inner peripheral surface of the second section  113  of the central bore  110  of the ferrule head  100  inwardly, substantially perpendicular to the longitudinal extension axis of the ferrule head  100 . The tab T on the inside of the ceramic ferrule head  100  may have any suitable shape that appropriately engages the appropriate slot in the stem  200 , but is preferably round, trapezoidal, or square. In order to ensure adequate strength of the tab T, the aspect ratio of the minimum radial width (extending along the inside diameter of the ferrule head  100 ) to the height (outwardly projecting distance dimension) of the tab T should be at least 2:1. 
         [0037]      FIGS. 5A-5C  are assembly views showing the steps for joining the ceramic ferrule head  100  and ferrule stem  200  to form the ceramic ferrule assembly  500  according to the present invention. As shown in  FIG. 5A , the first end  201  of the ferrule head  200  is inserted into the hexagonal opening  111  of the central bore  110  of the ferrule head  100  in the direction indicated by the arrow, so that the longitudinal extension axis (central axis) of the ferrule stem  200  is aligned with the longitudinal extension axis of the ferrule head  100  (coaxial alignment). The ferrule stem is continually inserted downward (in the arrow direction shown), as shown in  FIG. 5B , until the tab T of the ferrule head  100  passes through the first part  221  of the slot  220  of the ferrule stem  200 . 
         [0038]    The ferrule stem  100  is then rotated about its longitudinal extension axis (central axis), thereby directing the radially oriented tab T of the ferrule head  100  into the circumferential extension portion  222  of the slot  220  in the ferrule stem  200 . Because of the interlocking mechanical relationship or interference between the tab T and the slot  220 , the ferrule stem  200  cannot then be removed from the ferrule head  100  by a single force in the axial direction. That is, once tab T passes through the slot  220 , the ferrule stem  200  is rotated into a position such that tab T cannot pass back through the slot  220  without purposefully performing a precise counter rotation. 
         [0039]    The requirement for rotation, as well as axial extraction of the ferrule stem  200  to disassemble the ferrule assembly  500  significantly reduces the chances that the ferrule stem  200  might undesirably disassociate from the ferrule head  100  during operation. The ferrule stem  200  is preferably rotated about 0.25 inches or about 10°, depending on the precise dimensions of the slot  220  and tab T, so that the tab T resides in the correct position within the second portion  222  of the slot  220  (see, e.g.,  FIGS. 6A and 7 ). 
         [0040]    As shown in  FIG. 5C , in the ferrule assembly  500 , the first end  201  of the ferrule stem  200  extends beyond the second end  102  of the ferrule head  100 , and the second end  202  of the ferrule stem  200  is housed within the central bore  110  of the ferrule head  100 . 
         [0041]      FIG. 6A  is a cross-sectional (cut-view) showing the assembly  500  in  FIG. 5C , and  FIG. 7  is a bottom view of the assembly  500  shown in  FIGS. 5C and 6A . The tab T cannot be seen in  FIG. 6A  given the rotated state of the ferrule head  200 , because the annular flange  211  obscures the view of the tab T. This is clear based on the bottom-view shown in  FIG. 7 , where only a portion of the tab T extending from the cylindrical surface  113  can be seen behind the annular flange  211  in a location corresponding to the second portion  222  (not shown) of the slot  220 . The bottom face of the first flange  212  of the ferrule stem  200  is seated on the annular ridge  122  within the central bore  110  of the ferrule head  100 , as described above. This prevents further axial movement in the insertion direction indicated by the arrows shown in  FIGS. 5A-5C , and provides the thermal and processing benefits described above. 
         [0042]      FIG. 6B  is a cut view showing an assembly  600 , where the first annular flange  211 ′ of the ferrule stem  210  has a helically-shaped slot (see, e.g.,  FIGS. 3C and 3D ). The tab T of the ferrule head  100  cannot be seen in  FIG. 6B  given the rotated state of the ferrule head  210 , because the annular flange  211 ′ obscures the view of the tab T. The bottom face of the first flange  212 ′ of the ferrule stem  210  is seated on the annular ridge  122  within the central bore  110  of the ferrule head  100 , as described above. This prevents further axial movement in the insertion direction indicated by the arrows shown in  FIGS. 5A-5C . 
         [0043]      FIG. 8  is a bottom view of an assembly  800  according to another aspect of the present invention. In this embodiment, the ferrule head  100  includes are two tabs, T 1  and T 2 , diametrically opposed to one another about the inner circumference of the cylindrical peripheral surface  113  of the central bore  110  of the ferrule head  100  (i.e., separated by 180°. Likewise, the radial flange  211  of the ferrule stem  200  includes two diametrically opposed slots  220   a  and  220   b,  which rotatably engage the respective Tabs T 1  and T 2  in the same manner described above when the ferrule stem  200  is inserted into the ferrule head  100  and rotated into the locked position. 
         [0044]    Although no fiber wraps or gaskets are shown in the accompanying Figures, it should be readily understood that they may still included in the overall design, but were removed for simplicity of view and explanation. Such fiber wraps and gaskets are not needed in order to functionally secure the ferrule stem  200  and head  100  to one another, as in the prior art design, however. 
         [0045]    While the present invention has been shown and described above with reference to specific examples, it should be understood by those skilled in the art that the present invention is in no way limited to these examples, and that variations and modifications can readily be made thereto without departing from the scope and spirit of the present invention.