Abstract:
A pressure responsive valve assembly is disclosed as having a frangible burst disk that largely controls actuation of the valve. The valve assembly includes a housing having an inlet and an outlet and a valve seat therein. An actuating unit includes a collapsible fluid chamber and a moveable pressure responsive valve member. The valve member is engageable with the valve seat, positioned to experience the pressure conditions at the inlet, and operably coupled with the mechanism to cause collapsing of the fluid chamber when the valve member moves relative to the valve seat to establish or close communication between the inlet and outlet. The rupture disk normally blocks fluid flow from the chamber and thereby prevents collapsing of the chamber and corresponding movement of the valve member until the valve member experiences a predetermined maximum pressure at the inlet. When the overpressure is sensed at the inlet, the valve member moves relative to the valve seat and causes the disk to rupture and the fluid chamber to collapse. The valve assembly is particularly useful in bypass arrangements, wherein the valve assembly is arranged to relieve pressure in a primary line caused by failure of a primary valve. A new rupture disk assembly for use in the valve assembly is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to pressure responsive valve assemblies which may be of the pressure relief or shutdown type. More particularly, the present invention concerns pressure responsive valves that are equipped with a rupture disk and a pressure-actuated unit which serves to both actuate the valve and to rupture the disk when the preselected valve set point pressure is exceeded. The valve assembly is particularly suitable for use in a bypass arrangement. The present invention also concerns a rupture disk apparatus for use in the valve assembly. 
     2. Discussion of Prior Art 
     Those ordinarily skilled in the art will appreciate that conventional pressure responsive valve assemblies present numerous problems. Particularly, pressure responsive valves tend to be expensive and, in some instances (e.g., inline rupture disks), large and unwieldy. Several conventional pressure responsive valves utilize structure for controlling valve actuation that essentially makes it impractical to use a single valve design for various set pressures. That is to say, a number of conventional valve designs require considerable modification to reconfigure the valve for actuation at different set pressure points. Some conventional valve designs present the risk of spillage and consequent plant contamination, particularly when replacement or reclosure of the valve requires disassembly of the piping to which the valve is connected. There are also concerns that some traditional valve actuating units provide only partial, incomplete valve actuation and that such actuation is probably insufficient to allow rapid and complete venting of the protected conduit or the like. These and other problems are identified in our copending application for U.S. Pat. Ser. No. 09/276,426, filed Mar. 25, 1999, entitled RUPTURE DISK CONTROLLED MECHANICALLY ACTUATED PRESSURE RELIEF VALVE ASSEMBLY, assigned of record to the assignee of the present invention. 
     Our prior application is directed to a valve assembly design that utilizes a rupture disk to control valve actuation. This arrangement permits the use of a small, easily replaceable rupture disk that can be manufactured to achieve relatively precise, reliable pressure set points for the valves over a wide range of set points and valve sizes. However, our prior application focused on the use of a mechanical actuating unit that contacted and caused the disk to rupture when the valve member experienced an elevation in pressure beyond the predetermined set pressure. In this respect, although the mechanically actuated valve design disclosed in our prior application addresses virtually every one of the problems associated with conventional pressure responsive valves, it has been determined that this design presents a few practical limitations. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Responsive to these and other problems, an important object of the present invention is to provide a rupture disk controlled pressure responsive valve assembly that addresses the problems normally associated with conventional pressure responsive valves. Another important object of the present invention is to provide a rupture disk controlled pressure responsive valve assembly that is operable at extremely high set pressures. It is also an important object of the present invention to provide a rupture disk controlled pressure relief valve assembly that can be quickly and relatively effortlessly reclosed after disk rupture, even when there is a substantial restrictive force against such reclosure. Yet another object of the present invention is to provide a pressure responsive valve design that utilizes a single rupture disk configuration to control a wide range of set pressures, such that the same rupture disk configuration may be used on a wide range of valve sizes and configurations. 
     In accordance with these and other objects evident from the following description of the preferred embodiment, the present invention concerns a rupture disktype pressure responsive valve assembly (either of the relief or shutdown variety) which can be used in a variety of contexts and may be easily repaired or retrofitted in the field without the need for extensive dismantling of associated piping or the like. Generally speaking, the inventive rupture disk controlled pressure responsive valve assembly includes a housing having an inlet and an outlet and a valve seat therein. An actuating unit includes a mechanism for defining a collapsible fluid chamber. The unit also includes a moveable pressure responsive valve member that is engageable with the valve seat, positioned to experience the pressure conditions at the inlet, and operably coupled to the mechanism to cause collapsing of the fluid chamber when the valve member moves relative to the valve seat to establish or close communication between the inlet and outlet. The rupture disk is associated with the mechanism to prevent collapsing of the chamber and corresponding movement of the valve member until the valve member experiences a predetermined maximum pressure at the inlet, whereupon the valve member moves relative to the valve seat and causes the disk to rupture and the fluid chamber to collapse. 
     The rupture disk is preferably connected to the fluid chamber opening so that fluid exiting the chamber impinges against the disk and eventually causes bursting of the disk once the maximum pressure is experienced by the valve member. It is believed that the use of fluid pressure (preferably hydraulic pressure) to burst the disk provides reliability and set pressure capabilities that have heretofore been unavailable. Those ordinarily skilled in the art will appreciate that rupture disks are traditionally designed for use in fluid conditions. It is believed that the natural environment provided by the present invention consequently yields relatively greater predictability and higher set pressures. This is likely attributable to, among other things, the fact that the fluid pressure is distributed evenly across the exposed face of the disk, as opposed to being a concentrated axial force. 
     The mechanism for defining the fluid chamber preferably comprises a cylinder and an internal, slidable piston. The size of the cylinder and piston may be adjusted relative to the valve member so as to vary the hydraulic pressure experienced by the rupture disk. In this respect, a single standard rupture disk configuration may be used for virtually any valve design and valve size. 
     The actuating unit is preferably provided with a fluid line providing selective access to the fluid chamber. Once the valve is actuated to rupture the disk, the disk may be replaced and pressurized fluid may then be supplied to the fluid chamber via the line. This will cause expansion of the chamber and eventually complete return of the valve member to its initial position. In the case of a pressure relief valve, this arrangement permits virtually effortless valve reclosure even when there is a restrictive force inhibiting such reclosure (e.g., when fluid continues to flow through the valve during reclosure). 
     The present invention also concerns the use of the valve assembly in a bypass arrangement and a rupture disk apparatus for use in the valve assembly. 
    
    
     Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein: 
     FIG. 1 is a vertical sectional view of a pressure responsive relief valve assembly constructed in accordance with the principles of the present invention, particularly illustrating the valve member in the closed position to block communication between the inlet and outlet; 
     FIG. 2 is a top plan view of the valve assembly, with the valve housing being removed; 
     FIG. 3 is a vertical sectional view of the valve assembly similar to FIG. 1, but illustrating the valve member shifted away from the valve seat so as to establish communication between the inlet and outlet, with the fluid chamber being collapsed and the disk being ruptured; 
     FIG. 4 is a side elevational view of the replaceable rupture disk apparatus constructed in accordance with the principles of the present invention and used in the valve assembly shown in FIG. 1; 
     FIG. 5 is a horizontal cross-sectional view taken along line  5 — 5  of FIG. 4, particularly illustrating the diametrically opposed fluid passageways projecting outwardly from the bore and the intersecting score lines on the convex face of the rupture disk; 
     FIG. 6 is an enlarged, fragmentary vertical sectional view of the upper end of the valve assembly, particularly illustrating the structure for defining the relief chamber, the first and second fluid lines providing selective access to the respective fluid and relief chambers, and the rupture disk before valve actuation; 
     FIG. 7 is an enlarged, fragmentary vertical sectional view of the upper end of the valve assembly similar to FIG. 6, but illustrating the disk after it has been ruptured; 
     FIG. 8 is a fragmentary vertical sectional view of a second embodiment of the present invention, wherein the valve assembly is provided with a compression spring for biasing the valve member to its initial position prior to disk rupture; 
     FIG. 9 is a vertical sectional view of a third embodiment of the present invention, wherein the fluid chamber and rupture disk are mounted astride the housing and the piston and valve member are operably intercoupled by a linkage outside the housing; 
     FIG. 10 is a vertical sectional view of a fourth embodiment of the present invention, wherein the valve assembly is a shutoff valve designed to close when the set pressure is experienced by the valve member; and 
     FIG. 11 is a schematic view illustrating the valve assembly shown in FIGS. 1-7 as part of a bypass assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning initially to FIGS. 1 and 3, the valve assembly  20  selected for illustration is designed to provide pressure relief and thereby permit fluid flow therethrough once a predetermined set point pressure is exceeded. The valve assembly  20  broadly includes a chamber defining a housing  22  having an inlet  24 , an outlet  26  and a valve seat  28 . It is noted that the housing  22  is generally similar in construction to the valve housing disclosed in our prior, which is identified above and hereby incorporated by reference to facilitate a complete understanding of the present invention. 
     It shall therefore be sufficient to explain that the housing  22  is essentially a hollow body having a primary wall  30  with end caps  32  and  34 . As shown, the cap  32  supports an elongated, tubular inlet pipe  36  that defines the inlet  24  and is connectable to a process pipe. The inboard end of the inlet pipe  36  within the confines of the housing  22  has an inner surface  38  (see FIG. 3) defining the valve seat  28 . The end cap  34  similarly supports a tubular outlet pipe  40  that defines the valve outlet  26  and communicates with the interior of the housing  22 . The outlet pipe  40  is also connectable to a process pipe. The upper end of the housing  22  remote from the end cap  32  has an annular mounting block  42  affixed thereto. An apertured plate  44  is fastened to the mounting block  42  by threaded screws  46  and serves to close off the upper end of the housing  22 . A circumferential O-ring  48  provides a positive seal between the block  42  and plate  44 . Projecting inwardly from the underside of the plate  44  is a stop wall  50 , for purposes which will subsequently be described. 
     An elongated, tubular bonnet  52  is secured to the plate  44  by screws  54  and presents an elongated, central, rod-receiving passageway  56 . The passageway  56  is equipped with two O-rings  58  and  60 . A circumferential O-ring  62  is also provided at the interface defined between the top face of the plate  44  and the underside of the bonnet  52 . 
     The valve assembly  20  further includes a rupture disk  64  and a pressure responsive actuating unit  66  which operates when the valve set point pressure is exceeded to communicate the inlet  24  and outlet  26  and burst the disk  64 . As will be described below, the illustrated actuating unit  66  utilizes hydraulic pressure to cause disk rupture. 
     Turning first to the actuating unit  66 , a piston  68  is provided for seating within the valve seat  28  and thereby blocking fluid flow between the inlet  24  and outlet  26 . The piston  68  is configured to conform with the surface  38  as shown, and has an O-ring seal  70  which sealingly engages the surface  38 . An elongated valve stem or actuator rod  72  is secured to the piston  68  and extends through passageway  56  to project upwardly beyond the bonnet  52 . At the upper end of the rod  72  is a second piston  74  that is relatively smaller than the valve piston  68 , such that the upper exposed surface area of the piston  74  is significantly less than the lower exposed surface area of the valve piston  68 . The second piston  74  is preferably circular in cross-sectional shape and is provided with an O-ring seal  76  extending about its circumference (e.g., see FIG.  7 ). The actuator rod  72  consequently interconnects and causes corresponding movement of the pistons  68  and  74 . Moreover, the pressure experienced by one of the pistons is necessarily translated to the other. 
     The actuating unit  66  further includes an apertured mounting disk  78  that is supported on the bonnet  52  and presents a central rod-receiving opening  80  through which the actuator rod  72  projects. A hollow cylinder  82  is located in axial alignment with the actuator rod  72  and is fit within an uppermost stepped shoulder defined about the central opening  80  of the disk  78 , with an O-ring seal  84  also being provided in the shoulder area of the opening  80  between the disk  78  and cylinder  82 . The cylinder  82  presents a circular cross-sectional shape and an inner diameter to match the diameter of the piston  74 , such that the piston  74  is slidable within the cylinder  82  yet sealing contact is provided therebetween. In this respect, the piston  74  and cylinder  82  cooperatively define a collapsible and expandable fluid chamber  86  that is located above the piston  74 . The upper end of the cylinder  82  defines a chamber opening  88  through which fluid flows as the chamber volume changes. It is, however, entirely within the ambit of the present invention to utilize various other structure for defining the fluid chamber (e.g., a bladder connected to the actuator rod  72 ) or to modify the cylinder and piston shape (e.g., a cylinder and piston having a rectangular crosssectional shape may be used). The chamber  86  is preferably filled with liquid, such as hydraulic fluid, although a compressible gas may be used. 
     At the upper end of the cylinder  82  is a cylindrically-shaped block  90  that presents a central, lower conduit  92  extending from the chamber opening  88 , as perhaps best shown in FIGS. 6 and 7. In generally the same manner as its lower end, the upper end of the cylinder  82  is fit within a shoulder defined about the conduit  92 , and an O-ring seal  94  is provided between the cylinder  82  and block  90 . For purposes which will subsequently be described, the upper portion of the conduit  92  has an increased diameter relative to the lower portion and is provided with internal threads. A relief chamber  96  is defined in the block  90  between the conduit  92  and the stepped upper margin of the block  90 . The relief chamber  96  presents a volume that is at least equal to, but preferably greater than, the volume of the fluid chamber  86 . With the relief chamber  96  being larger than the fluid chamber  86 , the former is preferably provided with a small reserve of fluid (when liquid is used). 
     The open top of the block  90  is covered by an apertured plate  98 . Located along a common diametrical line extending across the cover plate  90  are a so called “schrader valve”  100  (e.g., similar to a valve stem used with most automobile tires) and a normally open vent valve  102  (see FIG.  2 ). The illustrated valves  100  and  102  threadably engage the cover plate  90  and, as will subsequently be described, provide selective communication with the relief chamber  96 . The block  90  further includes a pair of upper and lower radial fluid lines  104  and  106 , with the upper line  104  projecting from and thereby communicating with the relief chamber  96  and the lower line  106  projecting from the conduit  92  and thereby communicating with the fluid chamber  86 . Each of the lines  104  and  106  is provided with a standard quick disconnect valve  108  and  110 , respectively, which permit flow through the line only when the corresponding male connector is attached thereto. 
     In the illustrated embodiment, the assembly above the bonnet  52  (i.e., the mounting disk  78 , the cylinder  82  and the block  90 ) is secured to the housing  22  by a plurality of externally threaded, long bolts  112  spaced about the circumference of the block  90  (e.g., see FIG.  2 ). The bolts  112  pass through the block  90  and the mounting disk  78  and are threadably received in internally threaded openings (not shown) defined in the bonnet  52 . The set of bolts  112  may be variously tightened when the assembly is attached to the housing  22 , and it is believed that this arrangement is capable of facilitating alignment of the cylinder  82  with the actuator rod  72  so as to reduce the risk of binding of the piston  74  within the cylinder  82 . 
     It is particularly noted that the rupture disk  64  is located between the fluid chamber  86  and relief chamber  96  and thereby prevents flow out of the fluid chamber  86  until ruptured. In this respect, the disk  64  normally serves as a restriction against movement of the piston  74  and against corresponding unseating of the valve piston  68 . However, once the disk  64  ruptures, fluid may flow to the relief chamber  96 , which consequently permits collapsing of the fluid chamber  86  and movement of the valve piston  68  away from the valve seat  28 . 
     The rupture disk  64  is preferably part of an apparatus  114  that is removably coupled to the block  90 . The rupture disk apparatus  114  also includes an elongated, generally cylindrical body  116  that is axially aligned with the conduit  92  when the apparatus  114  is attached to the block  90  (see FIGS.  6 - 7 ). The body  116  presents a lower smooth tip  118  that is circular in cross-sectional shape and configured to fit snugly within the lower portion of the conduit  92  (see FIGS.  4  and  6 - 7 ). An externally threaded section  120  spaced from the tip  118  of the body  116  threadably engages the internally threaded portion of the conduit  92  when the apparatus  114  is attached to the block  90 . The body includes a small chamfer  122  extending between the tip  118  and threaded section  120 . As perhaps best shown in FIGS. 6 and 7, the tip  118 , threaded section  120 , and chamfer  122  cooperatively provide sealing engagement with the block  90 . A generally smooth, circular outer surface  124  projects upwardly from the threaded section  120  and through the cover plate  98  when the apparatus  114  is attached to the block  90 . An O-ring seal  126  is preferably provided between the cover plate  98  and smooth outer surface  124 . A hexagonal shaped, oversized head  128  is defined adjacent the upper end of the body  116  so as to facilitate the threaded engagement and disengagement of the body  116  and block  90 . A bore  130  projects upwardly from the lower end of the body  116 . The bore  130  terminates short of the upper end of the body  16 , but a pair of diametrically opposed fluid passageways  132  and  134  extending radially between the outer surface  124  and the bore  130  serve to expose the bore  130  at locations spaced above the threaded section  120  (e.g., see FIG.  5 ). As shown in FIGS. 6 and 7, the passageways  132  and  134  are at generally the same elevation as the relief chamber  96 , and accordingly, the relief chamber  96  and the bore  130  freely intercommunicate via the passageways  132  and  134 . 
     In the illustrated embodiment, the rupture disk  64  is securely attached by suitable means (e.g., welding or soldering) against the lower end of the body  116 . Particularly, the illustrated disk  64  presents an outer, annular, flat flange and a concavo-convex burst area, with the former facilitating attachment of the disk  64  to the body  116  and the latter projecting slightly into the bore  130  (see FIGS.  6  and  7 ). Referring to FIG. 5, it will be observed that the convex face of the burst area is provided with a pair of intersecting score lines  135  which ensure more reliable operation of the disk  64 . The disk  64  is preferably formed of metal, although various other materials may be used. The principles of the present invention are also equally applicable to various other disk configurations (e.g., a reverse buckling disk), as noted in our prior application. 
     In operation of the valve assembly  20 , incoming pressurized fluid encounters the valve piston  68  and urges the piston  68  to move away from the valve seat  28 . This is translated to the upper piston  74  by the actuator rod  72  and the fluid chamber  86  is consequently urged toward a collapsed condition. However, the rupture disk  64  blocks fluid flow from the chamber  86  and thereby prevents movement of the the upper piston  74  and valve piston  68  until the pressure in the fluid chamber  86  exceeds a relatively reliable, predictable amount (referred to herein as the predetermined burst pressure value of the disk  64 ). Because the pressure within the fluid chamber  86  is proportional to the pressure within the inlet  24 , it may be said that the characteristics of the rupture disk  64  largely determine the set point pressure for the valve assembly  20 . In any case, once the bias against the valve piston  68  causes the pressure within the fluid chamber  86  to exceed the predetermined burst value of the disk  64 , the disk  64  will rupture (as shown in FIGS. 3 and 7) and fluid will consequently be permitted to flow out of the chamber  86  through the opening  88 , into the conduit  92 , pass the ruptured disk  64 , into the bore  130 , through the passageways  132  and  134  and into the relief chamber  96  (note, the normally open vent valve  102  permits air to escape from the relief chamber  96  as it fills with fluid). Thus, rupturing of the disk  64  allows the fluid chamber  86  to collapse and thereby permits movement of the pistons  68  and  74 . The valve piston  68  will eventually unseat from the surface  38  and allow fluid to flow between the inlet  24  and outlet  26 . Moreover, because movement of the valve piston  68  is no longer prevented, the piston  68  will shift away from the valve seat  28  until it engages the stop wall  50 . This ensures that full and complete communication will be established between the inlet and outlet when the disk  64  is ruptured. It is noted that the fluid passageways  132  and  134  permit only limited fluid flow therethrough and therefore serve to reduce the risk of damaging impact as the valve piston  68  engages the stop wall  50 . If necessary, the cross-sectional size of the fluid passageways  132  and  134  may be changed to provide different rates of deceleration for the valve piston  68 . 
     It is noted that the illustrated fluid chamber  86  has a smaller cross-sectional area than the cross-sectional area of the inlet  24 , and there is consequently a step up in pressure from the inlet  24  to the fluid chamber  86 . In other words, the pressure within the fluid chamber  86  is greater than the pressure in the inlet  24 . However, the illustrated arrangement is still capable of providing extremely high and predictable set pressure points. It is believed that this is primarily attributable to the fact that the rupture disk  64  is being used in a fluid environment. In addition, it is possible in a given valve application to vary the set pressure simply by changing the size of the fluid chamber (e.g., by providing a different piston and cylinder size). Accordingly, a rupture disk of a given size and configuration may be used for a wide range of valve sizes and set pressures. 
     Replacement of the rupture disk  64  and resetting of the valve to the closed position is relatively effortless and easily accomplished. Assuming pressurized fluid flow to the inlet  24  has stopped so that there is virtually no restriction to reclosure of the valve, the vent valve  102  is preferably first closed. The relief chamber  96  is then pressurized by supplying pressurized air thereto via the schrader valve  100 , whereby fluid is forced from the relief chamber  96 , back through the ruptured disk  64  and into the fluid chamber  86 . Expansion of the chamber  86  will consequently cause the pistons  68  and  74  to slide downwardly and eventually the valve piston  68  will sealingly engage the valve seat  28 . If desired, the valve housing  22  may be provided with a window that permits visual inspection of the actuating unit  66  to ensure that the valve piston  68  is properly seated. Alternatively, a sensor may be provided to ensure that the pistons  68 , 74  and actuator rod  72  have sufficiently shifted to place the valve piston  68  in sealing contact with the valve seat  28 . In any case, the rupture disk apparatus  114  is thereafter removed simply by unscrewing it from the block  90 , and a new rupture disk apparatus may subsequently be installed. The vent valve  102  is reopened and the valve assembly  20  is again operational. In the illustrated embodiment, with the relief chamber  96  being larger than the fluid chamber  86  and having a small reserve of liquid contained therein, there is relatively no risk of trapping gas below the rupture disk  64  during its replacement and reclosure of the valve. 
     On the other hand, if there is something that might restrict expansion of the fluid chamber  86  during disk replacement and valve reclosure (e.g., fluid continues to flow through the valve housing  22 ), the rupture disk apparatus  114  is first removed from the block  90  and a new replacement apparatus is installed. Fluid is preferably then removed from the relief chamber  96  by applying a suction source to the upper fluid line  104 . The removed fluid or new fluid is then pressurized and supplied to the fluid chamber  86  via the lower fluid line  106 . This causes expansion of the fluid chamber  86  and eventually reclosure of the valve. If necessary, the lower fluid line  106  may be provided with a pressure gauge to reduce the risk of over-pressurization of the fluid chamber  86  and premature failure of the new rupture disk. 
     Referring to FIG. 11, a bypass assembly  136  employing the valve assembly  20  is illustrated. The bypass assembly  136  is used as a safety measure with a primary flow conduit  138  having a control valve  140  therein. Particularly, it will be observed that a bypass inlet pipe  142  leads from the primary conduit  138  upstream of the valve  140  to the inlet pipe  24  of the valve assembly  20  and is coupled thereto by a union  144 . Similarly, a bypass outlet pipe  146  is connected to the outlet pipe  40  via a union  148 , with the bypass outlet pipe  146  leading from the pipe  40  back to the primary conduit  138  downstream of the valve  140 . 
     In the event of a control valve failure, fluid is delivered to the valve assembly  20  of the bypass assembly  136 . When the valve assembly  20  actuates, the fluid is delivered via the pipes  40  and  146  back to the primary conduit  138 . In the event of such a sequence, it is a simple matter to replace the disk  64  and reclose the valve assembly  20 . 
     FIGS. 8 and 9 illustrate various other relief valve assembly embodiments constructed in accordance with the present invention. These embodiments employ many of the same basic components as the previously described valve assembly  20 , and accordingly, the descriptions thereof will focus primarily on the distinctions. 
     Turning first to FIG. 8, the valve assembly  200  includes an elongated, compressible reclosure spring  202  located within the cylinder  204  and retained between the block  206  and the piston  208 . The spring  202  assists in holding the valve in its closed position such that the spring pressure exerted on the piston  208  supplements the rupture disk  210  in providing the valve set pressure. Upon actuation of the assembly  200 , the valve will be reclosed by the spring  202  when the pressure at the valve inlet (not shown) falls below the spring pressure. A valve of this design may be useful in applications where it is desired to limit the exposure of process fluids to the atmosphere after valve actuation, i.e., the valve serves to minimize pollution problems. Further, the spring  202  may be used to reduce the impact load when the valve piston (not shown) reaches the end of its travel proximal to the stop wall (also not shown). 
     The embodiment shown in FIG. 9 concerns a pressure relief valve assembly  300  having an actuator rod  302  that extends outside of the valve housing  304  and is coupled with an exterior linkage  306  permitting actuation of the rupture disk (not shown) mounted within the block  308  outside and astride of the housing  304 . In detail, the apertured plate  310  has a lateral extension  312  which supports an upright pivot leg  314  as well as an apertured mounting block  316 . The block  316  is secured to the underside of extension  312 , and has a passageway  320 . An elongated actuator rod  322  extends through the passageway  320  as shown and is equipped with a piston (not shown). The piston cooperates with the cylinder  324  to define a fluid chamber upstream from the rupture disk (not shown). A slotted crank arm  326  is operably coupled to the ends of the rods  302  and  322 , and is pivotally supported by the leg  314 . This design places the rupture disk (not shown) out of axial alignment with the valve piston  328  and rod  302 . It will be appreciated that in this unit the pivot point for the arm  326  may be varied to achieve different levels of force multiplication at the piston supported on the rod  322 . To this end, the arm may be straight, angled or of virtually any other desired configuration. 
     Turning now to FIG. 10, a shutdown valve assembly  400  is shown. In this case, the assembly  400  has an elongated tubular chamber-defining housing  402  presenting an integral tubular inlet  404  and an integral tubular outlet  406 . A valve seat  408  is formed within the housing  402  between the inlet and outlet. One end of the housing  402  has an annular plug  410  having an inner O-ring seal  412 . The opposite end of the housing  402  has an annular bonnet  414  secured to the housing  402  by screws  416 . Similar to the embodiment shown in FIGS. 1-7, the actuating unit  415  includes a mounting disk  418 , a cylinder  420  and a block  422 , all of which are secured to the bonnet  414  by bolts  424 . In addition, the actuating unit  415  has a pressure responsive piston  426  mounted on an actuating rod  428 . A passageway  430  provided with O-ring seals  432  is defined in the bonnet  414  and is configured to slidably receive the actuating rod  428 , along with the plug  410 . Although not shown, it will be appreciated that the actuating rod  428  is provided with a piston that cooperates with the cylinder  420  to define a fluid chamber upstream from the rupture disk mounted in the block  422 . 
     In the assembly  400 , the piston  426  is held at a precise off-seat position so as to establish a minor pressure drop across the piston during normal flow of fluid through the valve assembly  400 . At the desired valve set point, the increased pressure drop generates an axial force serving to push the actuating rod  428  and thereby rupture the disk and collapse the fluid chamber defined within the cylinder  420 . This allows the piston  426  to come into seating engagement with the valve seat  408  so as to close the valve and eliminate further fluid flow therethrough. This type of shutdown valve can be used in many applications such as in the tubes of tube and shell heat exchangers. If a tube ruptures, the valve assembly will close at an increased flow rate. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.