Abstract:
An unheading and containment system for unheading and heading a pressure vessel includes an unheading apparatus for removing a cover from a pressure vessel in an unheading operation and a modular enclosure, mechanism to substantially enclose the cover during the unheading operation. The unheading apparatus includes a cover removably secured to the pressure vessel, a lock plate cooperating with the cover, and a cover moving mechanism capable of moving the cover vertically and laterally.

Description:
FIELD OF THE INVENTION 
     This invention relates to a pressure vessel, and, more particularly, to a modular containment system for removing and replacing a cover of a pressure vessel. This invention is particularly applicable to removing and replacing bottom covers of coke drums. 
     BACKGROUND OF THE INVENTION 
     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 what is known as “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.” The temperature at which destructive distillation normally occurs is about 900 degrees F. Generally, an industrial furnace is used to incur destructive distillation. 
     Conventionally, a large pressure vessel known as a coke drum is provided at a furnace outlet for a sufficient amount of time to allow for a complete destructive distillation reaction. A typical coke drum is a large, vertical metal vessel with top and bottom closures. The actual size, shape, and configuration of the coke drum, however, can vary considerably from one installation to another. The bottom closure typically includes a relatively large and heavy removable cover that is secured to the drum by dozens of bolts. Disengagement and reengagement of the removable cover, known as unheading and heading, respectively, can be quite labor intensive, given the mass of the cover and the numerous bolts that hold it in place. 
     During the refining process, petroleum byproducts are deposited in the coke drum as a hot liquid slurry. Typically, the slurry enters the drum through an opening in the bottom closure. Lighter hydrocarbons, the products of destructive distillation, flow out the top of the coke drum while heavier material remains inside the drum. 
     After a coke drum is filled to the desired capacity and the flow of slurry into the drum ceases, the drum is cooled. This typically involves injecting steam into the drum to strip useful hydrocarbon vapors from the solid material and then injecting water into the drum to further cool the coke. The liquid mass remaining in the coke drum is substantially full of coke that, as it cools, hardens into solid material. This solid coke must be removed from the drum before the drum can be reused. The process of removing coke from a drum is referred to as “decoking.” 
     A typical decoking process involves several steps. First, any water remaining in the drum is drained through piping to allow for removal of the cover from the bottom closure of the drum. In a hydraulic operation, as opposed to a manual operation, the cover is supported by a hydraulic lifting mechanism to detension the joint. Next, the cover must be unlocked from the coke drum and disengaged in a controlled manner by manipulating the bolts attaching the cover to a flange on the drum. The cover then is lowered by the hydraulic mechanism. As can be appreciated, unheading a coke drum can be a time consuming process. After unheading is complete, the coke in the drum is cut out of the drum by high pressure water jets. The operation is reversed to resecure the cover on the drum. 
     To help streamline the unheading process, oil refineries frequently use automated unheading devices. Such unheading devices typically are provided at the lower end of the coking drums for automatic and semi-automatic heading and unheading and are capable of being remotely operated. An example of a known remotely operated unheading device for a coking drum is disclosed in U.S. Pat. No. 4,726,109 to Malsbury et al. In that patent, a platform device is provided beneath the coking drum for lowering the header unit (or cover), moving the header unit laterally to one side, and tipping it to facilitate cleaning of the header unit. 
     Other examples of remotely-operated removable closures are shown in U.S. Pat. Nos. 4,820,384, 5,290,072, and 5,221,019. For example, in the &#39;384 patent a remotely-operated vessel cover assembly includes a cover which can be attached to a flange surrounding an opening-in the vessel. When the cover is raised into position, a series of connector pins fits through corresponding holes in a force ring and keyhole-shaped holes in a lock ring. The lock ring then is rotated so that heads of the connector pins are locked behind the lock ring. Fluid pressure then is applied to a force actuator, pressurizing inner and outer annular rings, which expand to pre-stress the pins and the cover. A ramp ring then is rotated until a series of ramps thereon firmly contacts a complementary series of ramps on the cover. The force actuator is then depressurized. According to this patent, the angle of inclination of the ramps is sufficiently shallow that friction between the ramp ring and the cover prevents slippage. 
     Automatic and semi-automatic unheading devices that confine the flow of discharge from the drum to a storage arrangement by means of a chute are also known in the art. For example, U.S. Pat. No. 6,039,844 to Malik discloses a containment system for coke drums including a safety shield, a removable cover, a plurality of actuators, and a system to vertically position the shield. The Malik patent also discloses an inner shield telescopically disposed within the safety shield to channel discharge from the coke drum to a switch deck floor. 
     Despite the current state of the art, there is a need in the art for a system that reliably contains and controls the entire unheading and heading process. There is a further need for such a system that is conducive to remote actuation. There is a still further need for such a system that is modular in construction so as to be easily transported and removably disposed around the bottom closure of a coke drum. There is also a need in the art for a system that confines drum discharge when the cover is disengaged from the flange and simplifies the process of delivering the discharge to an unheading deck floor. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to provide an improved unheading and containment system for a pressure vessel. 
     According to one aspect of the present invention, an unheading and containment system is provided having an unheading apparatus for removing a cover from a pressure vessel in an unheading operation, and a modular enclosure mechanism. The unheading apparatus includes a cover removably secured to the pressure vessel, a lock plate that cooperates with the cover, and a cover moving mechanism capable of moving the cover vertically and laterally. The modular enclosure mechanism includes a chassis that substantially encloses the cover during the unheading operation. The unheading apparatus may also include a flange mounted to a headed end of the pressure vessel and having a plurality of fasteners cooperating with the lock plate to secure the cover to the pressure vessel when the cover is brought into aligned contact with the flange. 
     According to another aspect of the present invention, a pressure vessel unheading and containment system is provided having a removable cover closing a pressure vessel bottom outlet and a chassis substantially enclosing an area between the bottom outlet and a support surface, with the cover contained within the enclosed area in an open position and a closed position. The system may also include a flange mounted to the pressure vessel at the bottom outlet and a lock plate cooperatively connected to the cover. The flange and the lock plate cooperate to secure the cover to the bottom outlet. 
     According to yet another aspect of the present invention, a pressure vessel unheading and containment system is provided having an unheading means for unheading a cover from a bottom outlet of the pressure vessel and an enclosure means for substantially enclosing the unheading operation performed by said unheading means. The system may also include a cover moving means adapted to move the cover vertically and laterally. 
     The unheading means may comprise a removable cover, a lock plate cooperating with the cover, and a cover moving mechanism capable of moving the cover vertically. The system may also include a flange mounted to the pressure vessel near its bottom outlet and having a plurality of fasteners cooperating with the lock plate to secure the cover to the pressure vessel. 
    
    
     These and other objects, features, and advantages of the present invention will be more clearly understood from the following discussion with reference to the following drawings, in which like reference numerals refer to like elements throughout. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be further described with reference to the following drawings, in which: 
     FIG. 1 shows a vertically-oriented coke drum with a removable cover and a chassis of the present invention provided at a lower end of the drum; 
     FIG. 2 shows an elevation view of a headed end of the coke drum; 
     FIG. 3 shows a perspective view of a flange of the present invention attached to a lower end of the drum; 
     FIG. 4 shows a top plan view of a removable cover of the present invention; 
     FIG. 5 shows a perspective view of a bottom of the removable cover of the present invention; 
     FIG. 6 shows a top plan view of a lock plate of the present invention; 
     FIG. 7 shows a perspective view of the chassis of the present invention; 
     FIG. 8 is an enlarged elevation view showing a top casing of the present invention mounted to a flange of the present invention; 
     FIG. 9 shows a top plan view of the unheading and containment system of the present invention; 
     FIG. 10 shows a side elevation view of the unheading and containment system of the present invention; 
     FIG. 11 shows an enlarged view of one of the keyhole-shaped holes in the lock plate; 
     FIG. 12 shows an cross-sectional view of a slot in the lock plate; 
     FIG. 13 shows a cross-sectional view along lines I—I of FIG. 12 with a piston rod of a short-stroke horizontal piston actuator engaged in the slot; 
     FIG. 14 is an elevation view showing the unheading and containment system of the present invention at the start of the unheading process; 
     FIG. 15 is an elevation view showing the bolts extended by the bolt tensioners; 
     FIG. 16 is an elevation view showing the lock plate moved laterally to its unlocked position; 
     FIG. 17 is an elevation view showing the removable cover lowered from the coke drum onto rails; 
     FIG. 18 is an elevation view showing the cover moved laterally away from the coke drum to a position under a hood; and 
     FIG. 19 is an elevation view showing the hood raised to permit cleaning of the cover. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As generally shown in FIG. 1, a vertically-oriented coking drum  100  is supported by a support structure  102  that includes a support deck  104  provided near the lower end of the drum  100 . The coking drum  100  has a conical lower portion  106  narrowing toward an open end  108 . Shown beneath the coking drum  100  is a chassis  500  which, as described below, encloses components for heading and unheading the drum  100 . 
     Referring to FIG. 2, the drum  100  is shown headed by three primary components which cooperate with each other in a manner to be described below: a flange  200 ; a removable cover  300 ; and a lock plate  400 . The flange  200  is secured to the drum  100  by welding, for example. The cover  300  and the lock plate  400  are removably secured to the flange  200  in a manner discussed in detail below. The flange  200 , the cover  300 , and the lock plate  400  are enclosed within the chassis  500 , a top casing  504  of which is removed in FIG. 2 for illustration purposes, but can be seen in FIG.  7 . 
     Referring to FIG. 3, the flange  200  is disposed around, and substantially flush with, the open end  108  of the drum  100 . The periphery of the flange  200  includes a step portion  200 ( a ). The flange  200  may be constructed of a high-strength thermally-stable material such as low-alloy carbon steel, for example. Other suitable materials may also be substituted, as is well known in the art. Holes  202 , for receiving bolts  606  (shown in FIG.  8 ), are spaced circularly and substantially evenly around the flange  200 . A second set of holes  203  are spaced circularly and evenly around the step portion  200 ( a ) of the flange  200 . The holes  203  receive bolts  511  for securing the top casing  504  of the chassis  500  to the flange  200 , as shown in FIG.  8 . There is an opening  204  in the center of the flange  200  to permit coke and quench water to exit through the open end  108  of the drum  100  during a decoking process. Preferably, the opening  204  is generally circular, which is an advantageous configuration for a drum of circular cross-section. Alternatively, the opening can be other shapes. In the illustrated embodiment, the drum  100  extends slightly beyond the flange  200  to form a lip  206  which helps to achieve a tight seal between the open end  108  of the drum  100  and the removable cover  300 , as will be discussed in more detail below. 
     Turning to FIGS. 4 and 5, the removable cover  300  is shown to be generally rectangular in shape, as viewed from above. The cover  300  has an elongated end  300 ( a ) extending from the left side thereof and an upper surface  302  adapted to mate and tightly seal with the flange  200 . To this end, a groove  306  is provided on the upper surface  302  of the cover  300  and is configured to accept a gasket (not shown). In the illustrated embodiment, the groove  306  is circular. However, it is to be understood that the shape of the groove  306  may be varied. The cover  300  also includes through-holes  304 , shoulder bolts  310 , and a pair of wing-like projections  312 . In the illustrated embodiment, the through-holes  304  are spaced circularly and substantially evenly around the periphery of the groove  306 , but the location and configuration of the through-holes  304  may be varied as will be discussed below. 
     The wing-like projections  312  are located at the elongated end  300 ( a ) of the cover  300 . In the illustrated embodiment, the projections  312  are integrally formed with the cover  300  and have the same thickness as the cover  300 . Pivotable connectors  314  extend from the projections  312 . Each pivotable connector  314  is located and configured to engage the distal end of a piston rod  802  (shown in FIG.  9 ). Thus, the removable cover  300  remains engaged to the actuator associated with the rod  802  at all times. 
     The shoulder bolts  310 , best seen in FIG. 5, extend from the underside  308  of the removable cover  300 . As discussed below, the shoulder bolts  310  allow for slidable mounting of the lock plate  400  with respect to the cover  300 . The number and configuration of the shoulder bolts  310 , it is to be understood, may vary, depending on, inter alia, various design choices of the cover  300 , the lock plate  400 , and the shoulder bolts  310  themselves. The cover  300  also includes a lateral conduit  316 , attached to its underside  308 , for feeding hydrocarbon, steam, and water into the drum  100  through its open end  108 , as well as for draining water and other byproducts from the drum  100 . 
     FIG. 6 illustrates the lock plate  400  of the present invention. The lock plate  400  is a truncated annulus having an annular portion  400 ( a ), a squared end  400 ( b ), and straight sections  406 . This annular shape is an advantageous configuration for a drum of circular cross-section. However, the lock plate can be other shapes. The lock plate  400  is dimensioned to travel within the chassis  500  in a manner discussed in detail below. The straight sections  406  allow for an annular portion  400 ( a ) of greater diameter while still permitting the lock plate  400  to travel within the chassis  500 . 
     Slotted holes  402  and holes  404  are spaced circularly and substantially evenly around the lock plate  400 . The slotted holes  402  are dimensioned to accept, and to cooperate with, the shoulder bolts  310  extending from the removable cover  300 . Thus, the number and location of the slotted holes  402  corresponds to the number and location of the shoulder bolts  310 . The holes  404  are positioned for alignment with the holes  304  of the removable cover  300  and the holes  202  of the flange  200  when the drum  100  is headed by the cover  300 . 
     The lock plate  400  also includes slots  408  on its squared end  400 ( b ). Each slot  408  is located and configured to engage and removably retain a complimentary configured distal end of a piston rod  904 , as shown in FIG.  13 . 
     The lock plate  400  is slidably mounted to the underside of the removable cover  300  by the shoulder bolts  310 . The shoulder bolts  310  extend through, and cooperate with, the slotted holes  402  so as to allow limited slidable movement of the lock plate  400  relative to the cover  300 . 
     FIG. 7 shows the chassis  500  that encloses the open end  108  of the drum  100 , the flange  200 , the removable cover  300 , and the lock plate  400 . The chassis  500  includes a chassis body  502 , a top casing  504 , a hood  506 , a skirt  508 , and a box beam  510 . 
     In the preferred embodiment shown, the chassis  500  is generally rectangular in cross-section. The chassis  500  is dimensioned to enclose an area wider than the removable cover  300 . The skirt  506  extends through an opening in the floor  110 . The top casing  504  is disposed on an upper side of the chassis  500  and includes a drum opening  505  for receiving the lower end of the drum  100 . Holes  509  are spaced around the opening and aligned with the holes  203  in the flange  200 . Bolts  511  extend through the holes  509  and  203  to secure the chassis  500  to the flange  200 . 
     The hood  506  is hinged to the chassis  500  by hinges  507  (shown in FIG.  9 ). An actuator assembly  514  (also shown in FIG. 9) pivots the hood  506  about the hinges  507 . In the closed position shown in FIG. 7, the hood  506  is seated against the chassis body  502  and the box beam  510 . The periphery of the hood  506  is fitted with a gasket (not shown) to ensure that the hood/chassis and hood/box beam interfaces are sealed to prevent drum discharge from leaking during the unheading and decoking process. A second gasket (not shown), which is circular in shape, is provided between the cover  300  and the flange  200  and normally rests in the groove  306  of the cover  300 . After the unheading and decoking procedures are completed, the hood  306  may be pivoted upwardly about the hinges  507  to permit cleaning of the cover  300 . While the hood is raised, the second gasket, which preferably is a double metal jacketed gasket, can be replaced. The hood is wider than the top casing to provide openings  513  for accommodating piston rods  802  (shown in FIG.  9 ). 
     The skirt  508  is attached to a lower side of the chassis body  502 . The skirt  508  is preferably formed of a relatively high-strength material, such as a low alloy carbon steel, for example. Other suitable materials, well known in the art, can be substituted. The skirt  508  extends downwardly from the chassis body  502  through the floor  110 . Thus, the skirt  508 , the box beam  510 , the hood  506 , the top casing  504 , and the chassis body  502  cooperate to substantially enclose the space beneath the open end  108  of the drum  100  so as to confine any discharge from the drum  100  during the unheading and decoking process. 
     As illustrated in FIG. 8, the step portion  200 ( a ) of the flange  200  is dimensioned to overlap a portion of the top casing  504  adjacent to the drum opening  505 . The holes  203  of the step portion  200 ( a ) are aligned with the holes  509  of the top casing  504 . Bolts  511  extend through the holes  203  and  509  to secure the chassis  500  to the flange  200 . In this manner, the flange  200  supports the weight of the chassis  500 . 
     FIG. 9 shows a plan view of the present invention with the drum  100 , the hood  506 , and the top casing  504  removed for purposes of illustration. Rails  512  extend laterally within the chassis  500  along its length and are configured to allow the removable cover  300  to travel thereon. Each of the rails  512  can be a single element or, alternatively, a series of aligned shorter rails. Also shown in FIG. 9 is an actuator assembly  514  that is used to pivot the hood  506  about hinges  507 . The actuator assembly  514  is connected at one end to the box beam  510  and at the other end to the hood  506 . 
     A number of actuators are mounted to the chassis  500  for moving the cover  300  and the lock plate  400  within the chassis from a headed position, where the removable cover  300  is mated to the flange  200 , to an unheaded position, where the cover  300  is positioned adjacent to the drum  100 . The term “actuator” broadly includes any mechanical, electrical or hydraulic device suitable for movably positioning the cover  300  and/or the lock plate  400 . 
     More specifically, as best seen in FIG. 9, the chassis  500  includes four vertically-oriented piston actuators  700  for lowering and raising the cover  300  during unheading and heading. Each piston actuator  700  is mounted to the underside of the chassis body  502  above the skirt  508 . The piston actuators  700  are capable of providing sufficient lifting force to maintain the removable cover  300  in the mated position during the unheading and heading process. 
     Each piston actuator  700  includes a piston rod  702  configured to engage the underside  308  of the removable cover  300 . In the illustrated embodiment, the piston actuators  700  are arranged to engage the corners of the cover  300  when the cover  300  is aligned with the drum  100 . The piston actuators  700  lower the cover  300  onto the rails  512  of the chassis  500 , preferably at a level plane such that the upper surface  302  of the cover  300  is parallel to the flange  200  throughout the unheading process. As will be appreciated, the location and configuration of the piston actuators  700  can be varied without changing their function. 
     Still referring to FIG. 9, the chassis  500  also includes dual horizontally-oriented long-stroke piston actuators  800  adapted for controllably moving the cover  300  laterally within the chassis  500  along the rails  512 . Each long-stroke piston actuator  800  includes a piston rod  802  slidably disposed in a cylinder  804 . The distal end  802 ( a ) of each rod  802  is configured to pivotably engage the pivotal connectors  314  of the cover  300 . This engagement may be achieved through any number of means typical in the art, including, for example, a hinge arrangement. This pivotable engagement allows the long-stroke piston actuators  800  to remain engaged with the removable cover  300  throughout the heading and unheading cycle. The long-stroke piston actuators  800  are attached to the chassis  500  by hinges  806 . 
     After the cover  300  is lowered onto the rails  512  by the vertically-oriented piston actuators  700 , the horizontally-oriented piston actuators  800  are actuated to extend the rods  802  and move the cover  300  from an aligned position under the drum  100  to an offset position relative to the drum  300 . When extended, the rods  802  pass through the openings  513  in the chassis  500 . The openings  513  receive the rods  802  but can otherwise be sealed to prevent discharge from the drum  100  from escaping during the unheading and decoking process. 
     Multiple bolt tensioning units  600 , shown in FIG. 8, are mounted on the top side of the flange  200 . The tensioning units  600  may be mounted to the flange  200  by any conventional means, such as, for example, mounting brackets and bolts or the like. In the illustrated embodiment, the tensioning units  600  are circularly and substantially evenly spaced around the periphery of the flange  200 . However, the number and location of the tensioning units  600  may vary, depending on, inter alia, the construction of the flange  200  and the pressure rating of the drum  100 . The locations of the tensioning units  600  correspond to the locations of the holes  202  of the flange  200 . 
     Each tensioning unit  600  is constructed and operated similarly. As best seen in FIG. 8, each tensioning unit  600  comprises a cylinder  604  and a bolt  606 . The tensioning units  600  are usually operated by a suitable hydraulic pressure source. U.S. Pat. Nos. 6,223,925 and 6,085,929 to Malsbury, et al., each of which is incorporated by reference herein, disclose bolt tensioning units that can be utilized for the purposes described herein. 
     Each bolt  606  is slidably disposed in the cylinder  604  of a respective one of the tensioning units  600 . As best shown in FIG. 10, each bolt  606  includes a shank  606 ( a ) and a head  606 ( b ). The head  606 ( a ), which either can be secured to the bolt or integrally formed therewith, is larger in diameter than the shank  606 ( a ) in cross section, but is smaller in diameter than the holes  202  of the flange  200 , the holes  304  in the removable cover  300 , and, as is discussed in detail below, a portion of the holes  404  in the lock plate  400 . Preferably, the bolt head  606 ( a ) is a hex nut or the like, which is threaded onto the bolt shank  606 ( b ). This provides a convenient means for making fine adjustments to the location of the head  606 ( a ) relative to the other elements of the mechanism. 
     As best seen in FIG. 11, each of the holes  404  in the lock plate  400  includes at least two different-sized regions that are alternately alignable with the bolts  606 —a narrowed portion  404 ( a ), through which the heads  606 ( b ) of the bolts  606  cannot fit longitudinally, and an enlarged portion  404 ( b ), through which the heads  606 ( b ) of the bolts  606  can fit longitudinally. The holes  404  can be shaped in any of several ways to achieve this result. In the illustrated embodiment, each hole  404  is shaped like a key hole. However, one of ordinary skill in the art will recognize that other configurations are also possible. It is noted that the portion of the lock plate  400  surrounding the narrowed portion  404 ( a ) of each hole  44  provides a bearing surface for a respective bolt head  606 ( b ). 
     As discussed above, the lock plate  400  is slidably mounted to the underside  308  of the removable cover  300  by the shoulder bolts  310 . The shoulder bolts  310  extend through the slots  402  in the lock plate  400 , which slots are configured to allow limited selective lateral movement of the lock plate  400  relative to the cover  300 . Thus, the lock plate  400  can be moved from a “locked” position, in which the narrowed portions  404 ( a ) of the holes  404  are aligned with the bolt heads  606 ( b ), to an “unlocked” position, in which the enlarged portions  404 ( b ) of the holes  404  are aligned with the bolt heads  606 ( b ). 
     As shown in FIG. 10, when the drum  100  is headed, the cover  300  is sandwiched between the flange  200  and the lock plate  400 . In this condition, the lock plate  400  is in the locked position and the bolts  606  extend through the holes  202  in the flange  200 , the holes  304  in the cover  300 , and the narrowed portions  404 ( a ) of the holes  404  in the lock plate  400 . 
     The slots  408  of the lock plate  400 , shown in cross section in FIGS. 12 and 13, are configured to automatically engage the complimentary configured distal ends  904 ( a ) of the piston rods  904  of the horizontally-oriented short-stroke piston actuators  900  when the removable cover  300  is raised in a manner discussed in detail below. In this illustrated embodiment, each slot  408  includes parallel sidewalls  410  extending from a slot opening  408 ( a ) and a narrowing portion  412  where the sidewalls  410  taper inwardly toward a capture portion  414  at the distal end of the slot  408 . 
     The horizontally-oriented short-stroke piston actuators  900  are positioned on the chassis  500  to move the lock plate  400  from the locked position to the unlocked position and vice versa. Each short-stroke piston actuator  900  comprises a cylinder  902  and a rod  904 . As FIG. 13 illustrates, the distal end  904 ( a ) of each rod  904  is provided with extensions  906  for engagement with a respective slot  408  of the lock plate  400 . In the illustrated embodiment, the distal end  904 ( a ) is greater in diameter than the remainder of the rod  904  and the extensions  906  are formed by a circumferential groove in the distal end  904 ( a ). However, one of ordinary skill in the art will recognize that other configurations are possible, such as laterally-extending wings. The short-stroke piston actuators  900 , when actuated, selectively retract or extend the rods  904 , thus moving the lock plate  400  with respect to the cover  300 , as described below. 
     As the removable cover  300  is raised by the vertically-oriented piston actuators  700 , the distal ends  904 ( a ) of the rods  904  slide down the slots  408  toward the capture portions  414 . As the removable cover  300  continues to rise, each rod  904  slides to a final position seated in the capture portion  414 . Thus, the extensions  906 , in cooperation with the slots  408 , allow the rods  904  to engage with, and disengage from, the lock plate  400  when the cover  300  is in the aligned position and is raised or lowered by the vertically-oriented piston actuators  700 . 
     The lock plate  400  is moved by the horizontally-oriented piston actuators  900 , preferably by at least two bi-directional piston actuators. 
     In a particularly advantageous application, the present invention may be used with what is commonly referred to as an “unheading deck floor.” An unheading deck floor typically has an opening that leads to a coke pit below. In a conventional unheading deck floor installation two chutes are required—a “first chute” extending from the drum opening to the floor opening, and a “second chute” extending from the floor opening to the coke pit. 
     When employed with an unheading deck floor, the chassis  500  and the skirt  508 , which enclose the area from past the open end  108  of the drum  100  through the floor  110 , may be used to channel water and coke exiting the open end  108  of the drum  100  in place of the “first chute.” Consequently, the need for any additional structures such as a coke chute extending from the drum to the floor is obviated. 
     Referring now to FIGS. 14-19, which show the present invention with the top casing  504  removed for clarity, an unheading operation is described. As shown in FIG. 14, where the rod  802  is removed for clarity, the drum  100  is shown headed, with the cover  300  secured thereto. In this condition, the vertically-oriented piston actuators  700  apply a lifting force to the cover  300  and the lock plate  400  via the rods  702 . In turn, the rods  702  transfer the full load of the removable cover  300 , as well as a portion of the load bearing on the cover  300  by the drum&#39;s contents, to the chassis  500 . Next, as shown in FIG. 15, the tensioning units  600  are actuated to extend the bolts  606 . In this condition the flange-cover joint is detensioned allowing slidable movement of the lock plate  400 . 
     Then, as FIG. 16 illustrates, the horizontally-oriented short-stroke piston actuators  900  are actuated to retract the rods  904  and horizontally move the lock plate  400  engaged thereto from a locked position to an unlocked position. In the unlocked position, the enlarged portions  404 ( b ) of the holes  404  are aligned with the bolts  606 , thereby allowing the lock plate  400 , and thus the removable cover  300 , to be separated from the flange  200 . Thereafter, the vertically-oriented piston actuators  700  decrease the amount of lifting force applied to the removable cover, allowing the weight of the removable cover  300 , the lock plate  400 , and the contents of drum  100  to gradually and controllably overcome the lifting force. This net downward force retracts the rods  702 , thus controllably lowering the removable cover  300  onto the rails  512  of the chassis  500 , as shown in FIG.  17 . 
     It is to be appreciated that several mechanical actions take place during the lowering of the removable cover  300 . First, as the cover  300  is lowered, the distal ends  904 ( a ) of the rods  904  slide up and out of the slots  408  of the lock plate  400 . Second, the horizontally-oriented long-stroke piston actuators  800 , pivoted upward by virtue of the position of the projections  312  and the pivotable connectors  314 , pivot to a substantially horizontal position about the hinge  804 . In this position the horizontally-oriented long-stroke piston actuators  800  are parallel to the rails  512 . 
     As best seen in FIG. 18, when the removable cover has been lowered onto the rails  512  and the vertically-oriented piston actuators  700  retracted, the horizontally-oriented long-stroke piston actuators  800  are actuated to extend the rods  802  and move the cover  300  laterally aside to a position adjacent to the drum  100 . As illustrated, the cover  300  is completely under the hood  506 . The rods  802  extend through openings  513  while the cover  300  travels on the rails  51 . 2 . 
     Lastly, as FIG. 19 illustrates, if desired, and after all of the drums contents, including the coke, have been removed, the hood  506  may be tilted by the actuator assembly  514  about the hinges  507  to gain access to the cover  300  for cleaning. In this position, it is also possible (and usually desirable) to replace the gasket between the cover  300  and the flange  200 . 
     It should be appreciated that throughout the unheading operation the removable cover  300  remains within the chassis  500 . In addition, it is to be understood that to head the drum  100 , the aforesaid operations are performed in reverse order. 
     While the present invention has been described with respect to what are at present considered to be the preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. To the contrary, as exemplified above, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, rather than employing hydraulic pressure to actuate the pistons and/or move the lock plate, various mechanical drive mechanisms can be used instead, as will be appreciated by those skilled in the art. Additionally, the inventive closure mechanism can be employed in other environments, such as autoclaves or other pressure vessels. Therefore, the scope of the following claims is intended to be accorded the broadest reasonable interpretations so as to encompass all such modifications and equivalent structures and functions.