Patent Publication Number: US-2017356701-A1

Title: Apparatus, systems and methods for protection against high pressure gas intrusion in shell and tube heat exchangers

Description:
FIELD 
     The present disclosure relates to shell and tube type heat exchangers, and more particularly to an apparatus, and systems and methods using the apparatus, for protecting shell and tube type heat exchangers against high pressure gas intrusion in the event of a tube fracture. 
     BACKGROUND 
     Conventional shell and tube type heat exchangers, such as those used in refineries, chemical processing and offshore oil and gas facilities, include a shell containing a shell side fluid and a plurality of tubes containing a tubeside fluid. Heat is exchanged between the shell side fluid and the tubeside fluid. In such heat exchangers, the shell side fluid or the tubeside fluid can be at a higher pressure, thus there is a high-pressure side (i.e., either the shell side or the tubeside) and a low-pressure side (i.e., the other side, either the shell side or the tubeside). Such heat exchangers frequently use rupture disks, also referred to as bursting disks, that burst at a predetermined pressure to protect the low-pressure side of the shell and tube heat exchanger from rapid transient pressure waves generated during a sudden tube rupture, also referred to as a guillotine fracture. Such ruptures, also referred to as overpressure incidents, can occur at various times including as a result of vibration, thermal shock, incorrectly installed or defective tube and/or corrosion of a tube.  FIG. 1  illustrates such a shell and tube heat exchanger  10  in which the shell side is the low-pressure side having a section of conduit  21  including a bursting disk assembly  12  to protect against overpressure. The heat exchanger  10  has a shell  6  and a number of tubes  8 . The heat exchanger  10  shown can be used, for example, to transfer heat between a high-pressure gas  2  and a low-pressure fluid  4 . High-pressure gas  2  is fed through a gas inlet into the tubes  8 . Low-pressure liquid medium  4  is fed through a shell inlet into the shell and surrounds the tubes  8 . Alternatively, not shown, high-pressure gas can be fed into the shell surrounding the tubes while low-pressure fluid can be fed into the tubes. In a typical configuration, a bursting disk assembly  12  is used including a thin metal disk (bursting disk)  12 A within a disk holder  12 B that can take any suitable form such as, for example, a seat for the metal disk  12 A located between two plates held in place by bolts and nuts. Conduit  15  leads to a closed disposal system  13 . The closed disposal system  13  is intended for disposal of gas as may be necessary during normal operation. In the event of an overpressure incident, the low-pressure liquid medium  4  will be directed to the closed disposal system  13 . 
     While bursting disks have a fast opening time to mitigate the pressure waves resulting from a sudden tube rupture, they also have several disadvantages. In certain applications, bursting disks are not practical since premature failure or opening of the bursting disk can occur potentially resulting in the entire cooling medium header being discharged into the closed disposal system  13 . Such an event may impair the operability of the disposal system, e.g., a flare system, which could have serious safety consequences and lost product opportunity costs. In cases where a rupture disk assembly  12  cannot be utilized, the design pressure of the low pressure side must be increased such that fluid from the high pressure side cannot damage it. In practice, if the high pressure fluid stays below a hydrostatic test pressure of the low pressure side, it is assumed that no damage will occur and the possibility of an overpressure incident can be disregarded. Hydrostatic test pressure is typically 130% of the exchanger design pressure. Increasing the design pressure of an exchanger may require thicker shell plating, additional bolting materials, and larger piping flanges, all of which will undesirably add to the cost and weight of the exchanger installation. It would be desirable to have an alternative means for mitigating rapid transient pressure waves caused by sudden tube rupture in shell and tube heat exchangers that would avoid the aforementioned problems. 
     SUMMARY 
     In one aspect, an apparatus is provided for protection against high pressure gas intrusion in the event of a tube fracture in a shell and tube heat exchanger having a shell and a plurality of tubes contained within the shell where the shell or the tubes normally contain low-pressure fluid and the shell or the tubes normally contain high-pressure fluid, such that there is a high-pressure side and a low-pressure side of the heat exchanger. The apparatus includes a conduit having a first end and a second and, the first end capable of being attached to the shell or the channel head in communication with the tubes, whichever contains the low-pressure fluid, such that the conduit is in fluid communication with the shell or channel head. The conduit also has a rupture disk assembly therein, the rupture disk assembly having a rupture disk conduit having a first face directed towards the first end of the conduit and a second face directed towards the second end of the conduit, the rupture disk formed from a material having a predetermined burst pressure such that the rupture disk will rupture when subjected to a pressure exceeding the predetermined burst pressure. The conduit also has a hydraulic surge chamber located in the conduit between the second face of the rupture disk and the second end of the conduit, the hydraulic surge chamber having a length and diameter resulting in a predetermined volume of the hydraulic surge chamber. The apparatus has a pressure relief valve located at the second end of the conduit capable of opening over time in response to a pressure increase caused by fluids flowing through the conduit as well as closing in response to a pressure decrease. 
     In another aspect, a system is provided for transferring heat between a high-pressure gas and a low temperature fluid using the shell and tube heat exchanger described above while protecting against high pressure gas intrusion in the event of a tube fracture in the heat exchanger. The first end of the conduit of the apparatus described above is connected to the low-pressure side of the heat exchanger, either to the shell or the channel head, whichever contains the low-pressure fluid. 
     In another aspect, a process is provided for transferring heat between a high-pressure gas and a low temperature fluid using the shell and tube heat exchanger described above while protecting against high pressure gas intrusion in the event of a tube fracture in the heat exchanger. The first end of the conduit of the apparatus described above is connected to the low-pressure side of the heat exchanger, either to the shell or the channel head, whichever contains the low-pressure fluid. 
     In yet another aspect, a process is provided for retrofitting a shell and tube heat exchanger having a shell, a plurality of tubes contained within the shell and an existing rupture disk assembly in communication with the low-pressure side of the heat exchanger and containing a rupture disk configured to burst at a predetermined burst pressure. The existing rupture disk assembly is replaced with the above-described apparatus for protection against high pressure gas intrusion in the event of a tube fracture in the heat exchanger. The process includes removing the existing rupture disk assembly from the heat exchanger, attaching the first end of the conduit of the above-described apparatus to the shell or channel head of the heat exchanger, whichever contains the low-pressure fluid, such that the conduit is in fluid communication with the low-pressure side of the heat exchanger, and connecting the pressure relief valve to a disposal system for disposal of liquid and/or gas. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become better understood with reference to the following description, appended claims and accompanying drawings. The drawings are not considered limiting of the scope of the appended claims. The elements shown in the drawings are not necessarily to scale. Reference numerals designate like or corresponding, but not necessarily identical, elements. 
         FIG. 1  is a simplified view illustrating a shell and tube heat exchanger according to the prior art. 
         FIG. 2  is a simplified view illustrating a shell and tube heat exchanger according to one exemplary embodiment. 
         FIGS. 3A-D  are simplified views illustrating the operation of a shell and tube heat exchanger according to one exemplary embodiment. 
         FIG. 4  is a schematic diagram illustrating a method of retrofitting a shell and tube heat exchanger with an apparatus according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  illustrates a shell and tube heat exchanger according to one exemplary embodiment. In one embodiment, an apparatus  20 , also referred to herein as a surge mitigation device  20 , is provided for protection against high pressure gas intrusion in the event of a tube fracture in a shell and tube heat exchanger  10  having a shell  6  and a plurality of tubes  8  contained within the shell  6 . The shell  6  or the tubes  8 normally contain low-pressure liquid  4 , also referred to herein as low-pressure fluid. Thus the shell side or the tubeside can be the low-pressure side. The other side, whichever of the shell side or the tubeside having a higher pressure, is then the high-pressure side. The low-pressure liquid  4  can be any suitable liquid such as glycol, water, oil, a refrigerant or mixtures thereof as would be apparent to one of ordinary skill in the art. In some embodiments, the low-pressure liquid is from 50 psig to 275 psig. Any fluid requiring a 150 pound flange rating can be considered low-pressure. The shell  6  or the tubes  8  can normally contain high-pressure gas  2 . The high-pressure gas  2  can be, for example, hydrogen, hydrogen sulfide, methane, ethane, propane, natural gas and combinations thereof. In some embodiments, the high-pressure gas  2  is at a pressure of greater than 1000 psig. Such high-pressure gas can require 600 pound flange systems. The apparatus  20  includes a conduit  22  having a first end  22 A to attach to the shell  6  or the channel head  7 , whichever contains the low-pressure fluid, such that the conduit is in fluid communication with the low-pressure side and having a second end  22 B, a rupture disk assembly  12  including a rupture disk  12 A located in the conduit  22 , the rupture disk having a first face directed towards the first end of the conduit and a second face directed towards the second end of the conduit. The rupture disk  12 A is formed from a material having a predetermined burst pressure such that the rupture disk  12 A will rupture when subjected to a pressure exceeding the predetermined burst pressure. A hydraulic surge chamber  14  is located in the conduit  22  between the rupture disk assembly  12  and the second end of the conduit  22 B provided to negate or lessen any water hammering effect created by the sudden opening of the bursting disk. In one embodiment, the hydraulic surge chamber  14  is an integral part of the conduit  22 , e.g., a bulge where the conduit diameter increases. The hydraulic surge chamber  14  has a length and diameter resulting in a predetermined volume of the hydraulic surge chamber  14 . In one embodiment, the hydraulic surge chamber  14  has a diameter of from 3 inches to 12 inches. In one embodiment, the hydraulic surge chamber  14  has a volume of from 0.5 ft. 3  to 2.5 ft. 3    
     A pressure relief valve  16  is located at the second end of the conduit  22 B and is capable of reversibly opening over time in response to a pressure increase caused by fluids flowing through the conduit  22 , meaning that the relief valve  16  opens in response to a pressure increase caused by fluids flowing through the conduit  22 , and closes in response to the pressure decreasing. The relief valve  16  can have a number of mechanical components including, for example, a spring (not shown) and a metal seat (not shown). In one embodiment, the pressure relief valve  16  is a spring loaded pressure relief valve having a spring controlling the position of a disk relative to a seat. In another embodiment, the pressure relief valve  16  is a pilot operated pressure relief valve having a piston and a remote pilot controlling the position of a disk relative to a seat. 
     In one embodiment, the pressure relief valve  16  is connected to a closed disposal system  13  that may include a storage tank, a vent or a flare system in fluid communication with the pressure relief valve. Gas passing through the pressure relief valve  16  can thus be collected and disposed of by any suitable method, such as storing temporarily, venting for flaring. 
       FIGS. 3A-D  illustrate the operation of the shell and tube heat exchanger  10  utilizing the surge mitigation device  20  in which high-pressure gas is fed to the tubeside and low-pressure liquid is fed to the shell side. This is for illustration purposes only, and it is to be understood that the high-pressure gas could be fed to the shell side and the low-pressure liquid could be fed to the tubeside.  FIG. 3A  illustrates the initiation of an overpressure incident in which a tube  8  fractures at a location A. This can be a guillotine type fracture in the tube caused by vibration or corrosion. This results in a sudden, explosive pressure spike in the low-pressure side within the shell  6 . As shown in  FIG. 3B , as the high-pressure gas  2  expands from the point of the fracture A, gas  2  displaces liquid  4  and the low-pressure liquid  4  expands from the shell and into the surge mitigation device  20 . The liquid  4  causes the rupture disk  12 A to burst, and then expands into the surge chamber  14  where the liquid accumulates. At this point, the relief valve  16  has not yet deployed. 
     Upon further pressure increase in the surge mitigation device, the relief valve  16  opens. The relief valve  16  has a slower response or opening time than the bursting disk  12 A, due to the relatively large number of mechanical components present in the relief valve  16 . Sequential operation of these components is necessary to open the relief valve  16 , i.e., in the case of a spring-loaded pressure relief valve, the spring must sense the pressure, compress, and lift the valve disk from the seat, etc. As shown in  FIG. 3C , the relief valve  16  opens fully, allowing liquid  4  to pass into conduit  15 . The transient pressure wave causes displaced liquid  4  to move through the relief valve  16  and the conduit  15 , ultimately leading to a liquid storage drum (not shown). As shown in  FIG. 3D , liquid  4  will be initially displaced in the low-pressure side (within the shell  6 ) of the heat exchanger  10 . Gas  2  is sent through the relief valve  16  at steady-state conditions. Once the high pressure gas is isolated, pressure in the low pressure side will decrease and the pressure relief valve  16  will reseat or close in response to the pressure decrease. 
     In one embodiment, an existing shell and tube heat exchanger  10  is retrofitted with the surge mitigation device  20 . As shown in  FIG. 4 , the existing section of conduit  21  including the rupture disk assembly  12  can be removed from the shell  6  of the heat exchanger  10  such that an opening is provided accessible to the shell side of the heat exchanger  10 . A first end of the conduit of the above-described surge mitigation device  20  is then attached to the shell  6  such that the conduit  22 , including the rupture disk assembly  12  within the conduit  22 , the surge chamber  14  and the relief valve  16 , is in fluid communication with the interior of the shell  6 . Lastly, the pressure relief valve  16  of the surge mitigation device  20  is connected to a disposal system for disposal of liquid and/or gas. In one embodiment, the existing section of conduit  21  including the rupture disk assembly  12  can remain in place and a new section of conduit  22  including a surge chamber  14  and a relief valve  16  can be installed. The disposal system (not shown) can include a knockout drum for liquids and/or a flare or vent for gases. In the embodiment shown, high-pressure gas is fed to the tubeside and low-pressure liquid is fed to the shell side. This is for illustration purposes only, and it is to be understood that the high-pressure gas could be fed to the shell side and the low-pressure liquid could be fed to the tubeside, in which case the surge mitigation device  20  would be attached to the channel head  7  (in turn in communication with the plurality of tubes) rather than the shell. By retrofitting existing refinery heat exchangers, in the event of overpressure incidents, rupture disks can be tied into a closed flare system, increasing safety and reliability. 
     Through the use of the surge mitigation device disclosed herein, overpressure protection can be provided to shell and tube heat exchangers such that lower pressure and lower cost exchangers can be used and validated from a safety and a pressure vessel coding perspective. This is because the surge mitigation device allows the low-pressure side of the heat exchanger to be rated for lower pressure while still ensuring safe operation, thus reducing the requirements for size, thickness, bolts, weight, materials and the like. Thus the use of the surge mitigation device allows safer operation at a lower cost (and reduced space and weight) for large shell and tube heat exchangers that are currently reliant on other means of overpressure protection such as higher design pressures or burst disks. 
     EXAMPLE 
     An example of how the hydraulic surge chamber  14  can be sized will now be described. In this nonlimiting example, a volumetric flow rate of 12.8 ft 3 /s is assumed based on flow through a single 0.75″ tube located within a heat exchanger. In this case, the normal operating pressure of the high pressure side is 390 psig, and the maximum allowable accumulation pressure of the low pressure side is 82.5 psig. Thus, the pressure drop across the tube is 300 psig. The fluid composition is sour hydrogen gas used in a distillate hydrotreater reactor loop. 
     Assuming a typical opening time of the pressure relief valve  16  of 100 msec, the volume needed for the surge chamber  14  can be calculated as: 12.8 ft 3 /s×100 msec×(1 sec/100 msec)=1.28 ft 3 . 
     It should be noted that only the components relevant to the disclosure are shown in the figures, and that many other components normally part of a heat exchanger are not shown for simplicity. 
     For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present invention. It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” include plural references unless expressly and unequivocally limited to one referent. 
     Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope is defined by the claims, and can include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. All citations referred herein are expressly incorporated herein by reference. 
     From the above description, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.