Abstract:
A coke drum skirt to minimize the stresses experienced by the coke drum and the supporting structure of the coke drum is described. The skirt includes one circumferential horizontal plate attached to the coke drum, and the circumferential horizontal plate is slidingly sandwiched between a lower supporting plate that supports the weight of the drum through the circumferential horizontal plate and an upper retaining plate that prevents the coke drum from tipping or falling over. The upper retaining plate may be embodied as a series of retaining clips that also keep the coke drum centered and prevent rotation of the coke drum. The upper and lower plates are anchored to a concrete support base. The sliding connection of the plates allows the coke drum to thermally expand and contract while reducing stresses and metal fatigue from the typical fixed securing of the coke drum to the support base.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a coke drum mounting and support skirt, and more particularly to a novel support skirt that allows for expansion and contraction of the coke drum during the extreme temperature changes experienced by the coke drum during the delayed coking processes. The described support skirt securely supports the coke drum and prevents tipping of the drum, while allowing thermal contraction and expansion without undue stress to the support or drum. 
         [0003]    2. Background and Related Art 
         [0004]    Many oil refineries recover valuable products from the heavy residual hydrocarbons (commonly referred to as resid or residuum) that remain following initial refining by a thermal cracking process known as delayed coking. The processing of crude oil into gasoline, diesel fuel, lubricants, and the like, as well as many other petroleum-refining operations, produces byproducts that have very little value. However, the value of these byproducts can be substantially increased when they are heated for a long enough time at a temperature sufficient to cause “destructive distillation.” During the process of destructive distillation, a portion of the byproducts is converted to usable hydrocarbon products. The remainder is transformed into a solid carbon product called coke. In the refining industry, this process is commonly known as delayed coking. 
         [0005]    Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. The unvaporized portion of the heated heavy feed settles out in the coke vessel where the combined effect of retention time and temperature causes the formation of coke. Vapors from the top of the coke vessel, which typically consist of steam, gas, naphtha and gas oils, are returned to the base of the fractionation unit for further processing into desired light hydrocarbon products. The operating conditions of delayed coking can be quite severe. Normal operating pressures in coke vessels typically range from 25 to about 50 pounds per square inch and the heavy feed input temperature may vary between 800 degrees Fahrenheit and 1000 degrees Fahrenheit. 
         [0006]    Coke drums are typically large, cylindrical vessels commonly 19 to 30 feet in diameter and up to 120 feet tall having a top head and a funnel shaped bottom portion fitted with a bottom head and are usually present in pairs so that they can be operated alternately. The size, shape, and configuration of the coke drum may vary considerably from one installation to another. Coke is formed and accumulates in the vessel until it is filled to a safe margin, at which time the heated feed is switched to the empty “sister” coke vessel. This use of multiple coke drums enables the refinery to operate the fired heater and fractionation tower continuously. Thus, while one coke vessel is being filled with heated residual oil, the other vessel is being cooled and purged of coke (between 500 and 1200 tons) formed in the vessel during the previous recovery cycle. The full vessel is isolated, steamed to remove hydrocarbon vapors, cooled by filling with water, drained, opened, and the coke is removed. The drums typically operate on a cycle, switching every 10 to 30 hours. 
         [0007]    Coke removal, also known as decoking, begins with a quench step in which steam and then water are introduced into the coke-filled vessel to complete the recovery of volatile, light hydrocarbons and to cool the mass of coke. The vessel is then drained and vented to atmospheric pressure then opened (unheaded or deheaded) in preparation for decoking. Decoking is accomplished at most plants using a hydraulic system consisting of a drill stem and drill bit that direct high pressure water jets into the coke bed. This cuts the coke into small pieces which fall out the opened bottom of the coke drum. Once it is decoked, the drum is closed (re-headed), purged of air, leak tested, warmed-up, and placed on stand-by, ready to repeat the 10- to 30-hour cycle. 
         [0008]    The coke drums are largely vertical, with heights from three to four times their diameters to facilitate the delayed coking process and the decoking process. This large height/diameter ratio makes the coking drums susceptible to tipping due to forces such as those from strong winds. Further compounding this problem, the coke drums must be elevated to some extent to allow room underneath the coke drums for the dislodged coke to fall out and be removed during the decoking process. This increases the susceptibility of the coke drums to winds and other forces. 
         [0009]    The coke drums must be secured against these forces. A typical coke drum is supported by a skirt which is welded to the drum near the junction of the drum shell and the lower cone of the drum. The skirt of the coke drum is then typically placed on a reinforced cylindrical or quasi-cylindrical hollow concrete base that provides support for the drum. This is necessary due to the extreme weight of a filled steel coke drum containing as much as 1200 tons of coke and built to withstand over 50 pounds per square inch of pressure at 900 degrees Fahrenheit. The coke drum&#39;s skirt is typically bolted to the concrete base with heavy bolts along the base of the skirt. 
         [0010]    This is problematic, however, for the cyclical coking/decoking process subjects the large and heavy coke drums to frequent temperature fluctuations of hundreds of degrees. The temperatures fluctuate from the decoking temperature which may approach environmental conditions of 100 to 200 degrees Fahrenheit to the operating temperature around or above 900 degrees Fahrenheit. The steel drums, of course, expand and contract as a result of the temperature changes, and this expansion and contraction can be quite severe. For example, an unsecured thirty-foot-diameter steel coke drum may increase in diameter as much as two to two and one-half inches during the 700-800-degree-Fahrenheit temperature change it experiences during delayed coking and decoking. 
         [0011]    The typical coke drum, however, is not unsecured, but is securely bolted at its base to prevent tipping. The typical bolting process severely restricts the range of expansion within which the base of the coke drum can expand. This fixed securing structure results in large forces and stresses at the base of the coke drum. The bolts securing the skirt to the concrete base may be subjected to large shear stresses as the coke drum attempts to expand, which may eventually result in failure of the bolts. In addition, the joining of the skirt to the coke drum also undergoes large stresses and is subject to failure, which may lead to rupture of the shell of the coke drum. In addition, the concrete in which the bolts are embedded may crack and fail due to the stresses incurred. Finally, another potential hazard exists. The failure of the system securing the coke drum to the concrete base may be slow and almost invisible, resulting in a gradual weakening of the support system. While the support system might appear to be fine externally, the weakened support system may no longer be able to support the drum in high winds or other lateral forces, leading to sudden, unexpected, and catastrophic failure. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    A novel coke drum skirt provides a secure connection between a coke drum and a support base while simultaneously providing for reduced-stress thermal expansion and contraction of the coke drum during operation of the coke drum during the delayed coking/decoking processes. The connection that provides for the reduced-stress thermal expansion and contraction is a horizontally-sliding floating connection between the coke drum and the fixed and anchored support structure for the coke drum. 
         [0013]    This is achieved by providing for a three-layer sandwich of metal plates surrounding the coke drum. A substantially-horizontal center plate may be attached to either the coke drum or the fixed support structure for the coke drum, so long as corresponding top and bottom plates are attached to the opposite structure: if the center plate is attached to the coke drum, the top and bottom plates are attached to the fixed support structure; if the center plate is attached to the support structure, the top and bottom plates are attached to the coke drum. When the center plate is attached to the coke drum, the bottom plate provides weight-bearing support for the center plate, which rests on, but is not attached to, the bottom plate. The top plate is attached to the bottom plate and rests over the center plate, preventing the coke drum from tipping over due to external forces from wind, earthquake, or any other lateral tipping force. Because the center plate rests between the top and bottom plates, the center plate may expand and contract with the coke drum during thermal expansion and contraction, sliding over and under the top and bottom plates, respectively, as needed. 
         [0014]    The space between the bottom and top plates may be provided by a spacer that may be slightly thicker than the thickness of the center plate. In some embodiments, the bottom plate, the center plate, the spacer, and the top plate may each be single plates that completely encircle the coke drum. In other embodiments, the top plate is replaced by a series of retaining clips that serve as a top plate to retain the coke drum from tipping. In other embodiments, the center plate may be provided as a series of individual plates to provide for further reducing the stresses on the coke drum during thermal expansion and contraction. 
         [0015]    To provide additional support to the center plate, which bears nearly the full weight of the coke drum, the center plate may be provided with a series of struts and pads attached to and extending between the center plate and the coke drum. This distributes the weight and connection between the coke drum and the center plate for maximal support. In embodiments containing the struts and retaining clips, the struts and retaining clips may engage and interact to ensure that the coke drum remains centered in the coke drum skirt support structure and to brace against rotational forces that might transfer stresses to the feed lines or other structures attached to the coke drum. In some embodiments, the lower plate may be elevated above the supporting base by a support structure, while in other embodiments the lower plate may be secured directly to the supporting base, which supporting base is typically concrete. 
         [0016]    Thus a horizontally-sliding connection is provided between the fixed and anchored support base and the floating coke drum. As the coke drum heats and expands during the delayed coking process, the center plate slides over the bottom plate and top plate within the space defined by the bottom plate, the top plate (or retaining clips) and the spacer, and fewer forces are transferred to the coke drum, the support structure, and any mounting hardware. As the coke drum cools during the quenching and decoking processes, the center plate slides inwardly as the coke drum contracts, yet the center plate still remains over the bottom plate sufficiently to provide continued support for the coke drum, and no large lateral forces are transferred between the coke drum and the support structure. 
         [0017]    The sliding motion may be facilitated by providing low-friction surfaces on the top surface of the bottom plate, on the bottom surface of the top plate or retaining clips and on the bottom and top surfaces of the center plate. The top surface of the bottom plate and the bottom surface of the center plate are most important to provide with a low-friction surface since these are the weight-bearing surfaces of the coke drum skirt. The low-friction surfaces may be provided by attaching a low-friction material to the surfaces or by grinding or polishing the surfaces. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]    The objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0019]      FIG. 1  shows a perspective view of one embodiment of a coke drum skirt; 
           [0020]      FIG. 2  shows a closer perspective view of one feature of the embodiment of a coke drum skirt of  FIG. 1 ; 
           [0021]      FIG. 3  shows a sectional view of the embodiment of a coke drum skirt from  FIG. 1 ; and 
           [0022]      FIG. 4  shows a sectional view of an alternate embodiment of a coke drum skirt. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring now to the Figures, a description of the embodiments of the present invention will be given. It is expected that the present invention may take many other forms and shapes, hence the following disclosure is intended to be illustrative and not limiting, and the scope of the invention should be determined by reference to the appended claims. 
         [0024]    The inventive coke drum skirt described herein provides for thermal expansion and contraction of the coke drum during the delayed coking and decoking processes by providing for a sliding secure connection between the coke drum and the skirt. This connection is provided by sandwiching one circular plate of metal that encircles the coke drum or by sandwiching a series of metal plates encircling the coke drum between two other circular plates of metal encircling the coke drum or between a circular plate of metal and a series of retaining clips that approximate a second circular plate of metal. The two sandwiching circular plates of metal or the circular plate of metal and retaining clips are attached to the supporting base if the sandwiched plate of metal is attached to the coke drum. Conversely, if the sandwiched plate of metal is attached to the supporting base, the other circular plate of metals or retaining clips are attached to the coke drum. The sandwiched plate or plates is not attached to the sandwiching plates or retaining clips. 
         [0025]    The various plates of metal or retaining clips are provided with low-friction surfaces that allow the coke drum to expand and contract as the various plates or clips slide past one another, greatly reducing the stresses incurred on the coke drum and support mechanism from the thermal expansion and contraction. The low-friction surface may be provided by coating the surface of the plates with a low-friction material, or it may be provided by grinding or polishing the surface of the plates to achieve as smooth a surface as desired or as possible. Regardless of the extent of thermal expansion or contraction, the sandwiched plate remains sandwiched: the coke drum is securely supported at all times and is secured against tipping forces that would otherwise rotate the coke drum away from its vertical operating configuration or cause it to fall. 
         [0026]    A representative embodiment with the sandwiched plate secured to the coke drum will now be described.  FIG. 1  shows a coke drum  30  of the type used for delayed coking. The coke drum  30  has an upper portion  32  that is substantially cylindrical and a lower portion  34  that is roughly conical. The coke drum  30  also has a shoulder  36  joining the upper portion  32  and the lower portion  34 . Attached to the shoulder  36  are a series of pads  38  symmetrically arranged around the diameter of the coke drum  30  at the shoulder  34 . The pads  38  may be attached to the coke drum  30  by any means known in the art for such joining where large weights will be supported, including welding, spot welding, strong gluing, riveting, bolting or any other such method known now or later invented. The pads  38  may also be integrally formed as part of the coke drum  30  or may optionally be omitted in other embodiments. 
         [0027]    Each pad  38  has a pair of vertical struts  40  that provide support to a sandwiched plate  42  that extends substantially horizontally from the shoulder  36 , completely encircling the coke drum  30 . The vertical struts  40  and sandwiched plate  42  may be joined to the pads  38  and/or coke drum  30  by any means known in the art, as described above, or may optionally be integrally formed with the pads  38  in any combination, i.e. the sandwiched plate  42  may be integrally formed with the struts  40  and pads  38  and the pads  38  then attached to the coke drum  30 , or the struts  40  and pads  38  may be integrally formed and attached to the sandwiched plate  42  and the coke drum  30 , etc. Although the sandwiched plate  42  is shown as being separated into individual plates associated with one or several pads  38 , with several such individual sandwiched plates encircling the coke drum  30  to provide support, it is anticipated that the sandwiched plate may be a continuous sandwiched plate  42   a , such that the contiguous plate completely encircles the coke drum  30 . 
         [0028]    The sandwiched plate  42  is “sandwiched” because it is located between two other plates, which two other plates are separated by a spacer  44 . The spacer  44  may be slightly thicker than the sandwiched plate  42  so as to provide a minimal amount of vertical play in the location of the sandwiched plate  42  to provide less friction as the sandwiched plate moves with the coke drum&#39;s  30  expansion and contraction during the delayed coking/decoking process. The spacer  44  has a smaller radial thickness than do the plates it separates, which will become apparent below. Below the spacer  44  is a lower plate  46  on which the sandwiched plate  42  rests and which supports the weight of the coke drum  32  through the sandwiched plate  42 , struts  40 , and pads  38 . Above the spacer  44  is an upper plate  48  shown as a retaining clip (hereafter referred to as “retaining clip  48 ,” when referring to the specific embodiment displayed in  FIGS. 1-3  and as “upper plate  48 ” when referring to the embodiment displayed in  FIG. 4 ). 
         [0029]    The retaining clip  48  in the embodiment shown in  FIG. 1  does not support the weight of the coke drum  30  but rather retains the coke drum  30  against tipping or other similar forces against the coke drum  30  due to winds, earthquakes, or other events that would otherwise cause the coke drum  30  to tip or fall over. In the event such a force is encountered that is strong enough to overcome the coke drum&#39;s  30  weight, the coke drum  30  will shift a very slight amount corresponding to the difference in thickness of the sandwiched plate  42  and the spacer  44 , until the upper surface of the sandwiched plate  42  engages the lower surface of the retaining clips  48 . At that point, the retaining clips  48  prevent further tipping motion of the coke drum  30  until the tipping force passes and the coke drum  30  settles back into its resting position. In part because the retaining clips  48  do not typically experience large forces, the retaining clips  48  need not completely enclose every point on the sandwiched plate  42 , but may placed to selectively engage certain points surrounding the coke drum  30  instead, as is depicted in  FIG. 1 . As may be appreciated by reference to  FIG. 1 , the illustrated embodiment of the retaining clips  48  are approximately shaped like a capital “E” so as to slidingly engage the struts  40 . This provides the additional benefits of providing some support against rotational forces that might cause stress to or breakage of attached feed lines and other structures attached to the coke drum  30  and keeping the coke drum  30  centered on the support structure. 
         [0030]    The retaining clips  48  may be attached to spacer  44  and lower plate  46  by any means commonly known in the art, as described above in reference to the pads  38 . It may be desirous to use a method of reversible attachment such as bolting in some instances to allow easy replacement of damaged or stressed retaining clips. The lower surface of the retaining clips  48 , the upper surface of the lower plate  46 , and the lower and upper surfaces of the sandwiched plate  42  are desirably manufactured to have a low coefficient of friction. This allows the sandwiched plate  42  to easily slide in and out as the coke drum  30  expands and contracts during the delayed coking and decoking processes. The interaction between the various plates may be appreciated further by reference to  FIGS. 2 and 3 . 
         [0031]      FIG. 2  shows a more-detailed close-up perspective view of the interaction of the various plates. As may be appreciated from the foregoing description and from reference to  FIG. 2 , a radial expansion space  50  is defined by the outer edge of the sandwiched plate  42  and the inner edge of the spacer  44  to allow for proper expansion and contraction of the coke drum  30 . As the coke drum  30  heats and expands, the radial expansion space  50  decreases in size. Conversely, during cooling of the coke drum  30 , the coke drum  30  contracts, the sandwiched plate  42  moves away from the spacer  44  and the radial expansion space  50  increases in size. To allow for full expansion of the coke drum  30  and attached structures, the inner diameter of the spacer  44  (when measured across the widest diameter of the coke drum  30 ) should be chosen so as to be no less than the maximum expected expanded diameter of the coke drum  30 . 
         [0032]      FIG. 3  shows a cross-sectional view of the embodiment of the coke drum skirt shown in  FIGS. 1 and 2 , taken through the middle prong of one of the E-shaped retaining clips  48  shown in those Figures. As may be readily appreciated, as the coke drum skirt is essentially radially symmetrical, the cross section from  FIG. 3  is similar to the cross section taken at any one of the retaining clips  48  shown in  FIGS. 1 and 2 . It is anticipated, however, that an asymmetrical coke drum skirt would provide the same functionality.  FIG. 3  illustrates how the lower plate  46  and sandwiched plate  42  provide support for the coke drum  30 . The sandwiched plate  42  extends over the lower plate  46  sufficiently so that even in when the coke drum  30  is in its maximally-contracted state, or even when the coke drum  30  is not in use and is at an environmental temperature, the sandwiched plate  42  still rests on the lower plate  46  and provides support for the coke drum  30 . The support is transferred through the struts  40  to the pads  38 , and thus to the coke drum  30 . 
         [0033]    From the Figures, it may be recognized that the struts  40  provide an additional function besides transferring the support of the lower plate  46  to the coke drum  30 . Because the struts  40  are interlaced with the arms of the retaining clips  48 , the struts  40  prevent two potentially-troublesome occurrences. First, as mentioned above, the struts  40  prevent rotational forces from being transferred to feed lines and other structures on the coke drum  30 . Additionally, the struts  40  and interlacing arms of the retaining clips  48  also keep the coke drum  30  centered on lower plate  46  where the support for the coke drum  30  is strongest. Thus, the coke drum  30  will not slide laterally until it is unsupported on one side and prone to tipping due to its large weight. Optionally, the sandwiched plate  42  and lower plate  46  may be sized so as to prevent such tipping even if the coke drum  30  were to slide laterally to a maximal extent. This type of sizing is particularly helpful in embodiments where the struts  40  are omitted. 
         [0034]    The embodiment of the coke drum skirt depicted in  FIGS. 1-3  also includes additional structure linking the lower plate  46 , spacer  44 , and retaining clips  48  to the concrete supporting pad (not shown). While it is envisioned that in some embodiments the lower plate  46 , spacer  44 , and retaining clips  48  may be directly mounted to the concrete supporting pad, other embodiments include an additional elevated supporting structure such as that shown in the Figures. The supporting structure shown includes a riser  52  formed from an inner plate  54  and an outer plate  56 . The riser  52  supports the lower plate  46  and rests on and is attached to a mounting structure  58 . The mounting structure  58  may be bolted to the concrete supporting pad (not shown) and the bolting may be facilitated by access holes  60  placed in the outer plate  56  over the locations where the bolts are to be used. Optionally, another mounting method other than bolting may be used, as long as it secures the coke drum skirt to the supporting pad. 
         [0035]    One method by which the coke drum skirt may be provided and mounted will be described now. The site where the coke drum  30  will be placed is prepared to receive the coke drum skirt. The elevated supporting structure may then be placed on the site and firmly attached to the concrete support base, whether it be a pad, tube, or other structure that facilitates the delayed coking/decoking process. The elevated supporting structure at this point may include the lower plate  46  and spacer  44 , but it does not include the retaining clips  48  as they would interfere with the placement of the coke drum  30  on the supporting structure of the coke drum skirt. The coke drum  30  is prepared by attaching the pads  38 , struts  40  and sandwiched plate  42 , as described above. Then, the coke drum is lifted up and vertically lowered into the supporting structure until the sandwiched plate  42  rests on the lower plate  46  of the supporting structure and approximately centered within the spacer  44 . 
         [0036]    The retaining clips  48  may then be mounted to retain the coke drum by any means known in the art as described above. As this takes place, the coke drum  30  may continue to be supported by whatever method was used to lift the coke drum  30  in place, which may allow the coke drum  30  to be shifted laterally as the retaining clips  48  are mounted to properly align the coke drum  30 . The feed lines and other structures that need to be mounted on the coke drum  30  may then be mounted and operation of the coke drum  30  may begin. The retaining clips  48  may be removed if necessary for repairs or if the coke drum  30  is to be removed completely for any reason. 
         [0037]    Alternatively, the process illustrated above may occur in a different order. Rather than mount the coke drum skirt supporting structure to the concrete supporting structure and then lift the coke drum  30  in place, the entire structure depicted in  FIGS. 1-3  may be mounted to the coke drum  30  while the coke drum  30  is not in place (such as with the coke drum in a horizontal position on its side). Then, the coke drum  30  and the entire structure may be lifted into place and the supporting structure discussed above merely bolted to the concrete support structure or pad through the mounting structure  58 , as described above. 
         [0038]    If the coke drum skirt is to be used with an existing installation, it may be less than practical or desirable to completely remove the coke drum  30 , lower it to its side, and proceed as above. In such a situation, it may be desirous and advantageous to simply lift the coke drum  30  from its operating location, remove the old coke drum skirt or support structure, place the entire supporting structure depicted in the Figures underneath the coke drum  30 , whether assembled in place or placed after fully assembled, and then lower the coke drum  30  into the new coke drum skirt. The pads  38 , struts  40 , and sandwiched plate  42  could then be attached to the coke drum  30  and operation resumed. Thus it may be understood that the illustrated coke drum skirt is flexible in its installation and ability to be retrofitted to old installations. 
         [0039]    Although the illustrated embodiment discussed above shows the coke drum skirt located at approximately the shoulder  36  of the coke drum  30 , it may be readily appreciated that the coke drum skirt may be located at other vertical locations of the coke drum  30  as desired without affecting its function of supporting the coke drum. Other changes may also be made and still come within the meaning and range of equivalency of the claims below. For example, as discussed above, the various plates forming the sandwich of plates may be continuous or may be divided into discrete elements. For example, the sandwiched plate  42 , the lower plate  45  and the spacer  44 , which are illustrated in  FIG. 1  as all being continuous plates forming a circle around the coke drum  30 , may be divided into smaller individual sections if desired to improve ease of attachment or to provide reduced stresses during expansion and contraction of the coke drum  30 . The sandwiched plate  42 , particularly, may be divided into individual sections corresponding to single pads  38 , two pads  38 , or any number of pads  38 . 
         [0040]    Although the illustrated embodiment is shown as being largely circularly symmetrical, it is envisioned that the coke drum skirt and/or its individual structures may be asymmetrical or only partially symmetrical without affecting its primary purpose. In addition, the exact number of supporting structures (pads  38  and struts  40 ) encircling the coke drum  30  is not deemed important as long as the coke drum  30  is provided with sufficient support and retention. Indeed, it is envisioned that the pads  38  and struts  40  may be eliminated in some embodiments and that different structures may be used to provide linking support from the coke drum  30  to the sandwiched plate  42 . In some embodiments, a sandwiched plate  42  may be provided that either has a bend in it that conforms to the coke drum  30 , or a sandwiched plate  42  may be provided that is sufficiently strong, in and of itself, to not require any additional supporting structure attached to the coke drum  30 . 
         [0041]      FIG. 4  shows an alternate embodiment of the coke drum skirt. In the embodiment depicted in  FIG. 4 , the sandwiched plate  42  is mounted to the supporting structure rather than to the coke drum  30 . This means that the upper plate  48 , spacer  44 , and lower plate  46  are mounted to the coke drum  30 . In this embodiment, the weight-supporting interaction occurs between the upper plate  48  and sandwiched plate  42 , while the lower plate  46  provides the function provided by the retaining clips  48  in the embodiment illustrated in  FIGS. 1-3 . The primary sliding functionality of the coke drum skirt is maintained, and the embodiment of  FIG. 4  illustrates another way in which the invention may be modified and still maintain its primary functionality. 
         [0042]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.