Abstract:
According to some embodiments, a pressure relief valve system comprises a valve body coupled to a pressure chamber. The valve body defines a vapor flow path between the pressure chamber and the atmosphere. A valve orifice in the valve body is configured to allow vapor to flow between the pressure chamber and the valve body. A vapor conduit comprises a vapor conduit first end coupled to the valve orifice and a second end opening into the pressure chamber. The vapor conduit forms a passageway between the vapor conduit first and second end. The vapor conduit also comprises a vapor conduit orifice near its first end. The vapor conduit orifice is configured to allow the vapor to flow from the pressure chamber into the vapor conduit. A gravity-operated valve positioned near the vapor conduit first end is configured to control the flow of vapor through the vapor conduit orifice.

Description:
RELATED APPLICATIONS 
     This application is a U.S. National Stage Filing under 35 U.S.C. §371 of International Patent Application Serial No. PCT/US2015/010171 filed Jan. 5, 2015 and entitled “Improved Pressure Relief Valve for Railroad Tank Cars” and claims priority to U.S. Provisional Application Ser. No. 61/924,146, entitled “IMPROVED PRESSURE RELIEF VALVE FOR RAILROAD TANK CARS,” filed Jan. 6, 2014. 
    
    
     TECHNICAL FIELD 
     This disclosure generally relates to pressure relief valve systems, and more particularly to a pressure relief valve system for railroad tank cars. 
     BACKGROUND 
     Railroad tank cars transport bulk liquids such as oil or ethanol. Regulations require most railroad tank cars operating in North America be equipped with pressure relief devices. Pressure relief devices protect the tank from events such as product surges, over fills, and commodity reactions, but the primary purpose is preventing catastrophic tank failure by regulating pressure within the tank during a fire condition. The contents of the tank exist in a two-phase state—the liquid phase tank lading (commodity for transport) and the vapor phase above the lading. For normal ambient temperature liquid commodities the vapor phase is approximately atmospheric pressure. In a fire situation, heat input to the tank drastically increases the vapor pressure until it exceeds the pressure relief device safety setting and vapor is expelled. 
     Existing pressure relief devices are generally located near the center of the tank along its longitudinal center line. This location places the pressure relief device in communication with the vapor space above the tank lading when the tank is oriented in its normal operating position. A properly sized relief device is effective at relieving pressure during a fire situation by discharging vapor at a rate greater than or equal to the rate at which the pressure is generated. 
     In a derailment and roll-over situation, the tank car is no longer oriented in its normal operating position and the pressure relief device may no longer be in communication with the vapor space. Instead, the pressure relief device may be in communication with the liquid space. In this situation, the pressure relief device will expel liquid instead of vapor. Expelling liquid is problematic for several reasons. If the liquid is flammable, it may continue to fuel the fire and accelerate the temperature increase. Furthermore, because the pressure buildup is mainly caused by the vapor mass, expelling liquid does not relieve pressure at the same rate as expelling vapor. Additionally, a decrease in liquid mass increases heat transfer to the tank, further accelerating the pressure buildup. 
     SUMMARY 
     According to some embodiments, a pressure relief valve system comprises a valve body coupled to a pressure chamber. The valve body defines a vapor flow path between the pressure chamber and the atmosphere. A valve orifice in the valve body is configured to allow a vapor to flow between the pressure chamber and the valve body. A vapor conduit comprises a vapor conduit first end coupled to the valve orifice and a vapor conduit second end opening into the pressure chamber. The vapor conduit forms a passageway between the vapor conduit first end and the vapor conduit second end. The vapor conduit also comprises a vapor conduit orifice near the vapor conduit first end. The vapor conduit orifice is configured to allow the vapor to flow from the pressure chamber into the vapor conduit. A gravity-operated valve positioned near the vapor conduit first end is configured to control the flow of the vapor through the vapor conduit orifice. The gravity-operated valve operable to unseal the vapor conduit orifice when the valve body is positioned above a horizontal axis of the pressure chamber and substantially seal the vapor conduit orifice when the valve body is positioned below the horizontal axis of the pressure chamber. 
     In particular embodiments, the gravity-operated valve comprises a vapor conduit sleeve slidably positioned around the vapor conduit first end. The vapor conduit sleeve is configured to control the flow of the vapor through the vapor conduit orifice. The vapor conduit sleeve is operable to slide to a first position unsealing the vapor conduit orifice when the valve body is positioned above a horizontal axis of the pressure chamber and slide to a second position substantially sealing the vapor conduit orifice when the valve body is positioned below the horizontal axis of the pressure chamber. 
     Certain embodiments may provide one or more technical advantages. In some embodiments, the pressure relief system improves the survivability of tank cars in fire situations. The vapor conduit and conduit sleeve allow the pressure relief valve system to be in communication with the vapor space whether in an upright or roll-over position, efficiently relieving pressure in either orientation. Another advantage of some embodiments is that tank car owners can increase their existing tank&#39;s fire survivability time without retrofitting expensive insulating materials to the tanks. 
     As a result, particular embodiments of the present disclosure may provide numerous technical advantages. Particular embodiments the present disclosure may provide some, none, all, or additional technical advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete and thorough understanding of the particular embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
         FIG. 1A  is a side view of an example railway tank car; 
         FIG. 1B  is a cross-sectional side view of an example pressure relief valve system of a railway tank car; 
         FIG. 2  is a cross-sectional view of the pressure relief valve system of  FIG. 1B  in an upright position, in accordance with particular embodiments; 
         FIG. 3  is a cross-sectional view of the pressure relief valve system of  FIG. 1B  in a roll-over position, in accordance with particular embodiments; 
         FIG. 4  is a cross-sectional view of an embodiment of a pressure relief valve system in an upright position and fitted with a Y-shaped vapor conduit; 
         FIG. 5  is a cross-sectional view of an embodiment of a pressure relief valve system in a roll-over position and fitted with a Y-shaped vapor conduit; 
         FIG. 6  is a cross-sectional view of a pressure relief valve system in an upright position and fitted with a Y-shaped vapor conduit, in accordance with particular embodiments; 
         FIG. 7  is a cross-sectional view of an embodiment of a pressure relief valve system with a conduit sleeve internal to a vapor conduit; 
         FIG. 8  is a cross-sectional view of an embodiment of a pressure relief valve system with a pressure relief valve spring assembly; and 
         FIG. 9  is a cross-sectional side view of an example pressure relief valve system retro-fitted to a railway tank car. 
     
    
    
     DETAILED DESCRIPTION 
     Particular embodiments and their advantages are best understood by reference to  FIGS. 1A through 9  wherein like reference numbers indicate like features. 
       FIG. 1A  is a side view of an example railway tank car. The railway tank car comprises tank  102 . Tank  102  is a railroad car tank designed to transport liquid commodities. Tank  102  may be pressurized or non-pressurized, insulated or non-insulated, and may be designed for single or multiple commodities. Tank  102  may be constructed of any suitable material such as carbon steel, aluminum alloy, high alloy steel, or nickel plate steel. The contents of tank  102  comprise a liquid phase tank lading (the commodity for transport) and a gaseous vapor phase above the lading. 
       FIG. 1B  is a cross-sectional side view of an example pressure relief valve system of a railway tank car.  FIG. 1B  illustrates pressure relief valve system  100  comprising tank  102 , pressure relief valve housing  104 , and vapor conduit  106 . 
     Pressure relief valve housing  104  is coupled to tank  102 . In the illustrated embodiment, pressure relief valve housing  104  is located near the center of tank  102  along its longitudinal center line. Pressure relief valve housing  104  houses a pressure relief valve constructed of any suitable material compatible with the lading of tank  102 . The pressure relief valve is sized to permit sufficient flow capacity to prevent pressure build up in tank  102  under fire conditions. As an example, Appendix A of the Association of American Railroads Specification defines a flow rating pressure for tank cars. In the illustrated embodiment, pressure relief valve housing  104  houses a reclosing safety relief valve. In some embodiments, pressure relief valve housing  104  may house other pressure relief devices such as rupture disks, safety vents, and combination devices. The pressure relief device protects tank  102  from events such as product surges, over fills, commodity reactions, and prevents catastrophic failure of tank  102  by regulating pressure within tank  102  during a fire condition. Pressure relief valve housing  104  and the pressure relief components housed within may generally be referred to as the valve body. 
     Vapor conduit  106  is coupled to pressure relief valve housing  104  and extends into tank  102 . In some embodiments, vapor conduit  106  extends from the bottom of relief valve housing  104  to within a few inches of the opposite interior wall of tank  102 . The length of vapor conduit  106  may vary depending on the size of tank  102  and the commodity contained in tank  102 . In some embodiments, vapor conduit  106  may comprise a plurality of threaded sections. The threaded sections may be coupled together to form varying lengths of conduit. Vapor conduit  106  may be constructed of stainless steel or any other material compatible with the lading of tank  102  and suitable for creating a vapor passageway. 
       FIG. 2  is a cross-sectional view of the pressure relief valve system of  FIG. 1B  in an upright position, in accordance with particular embodiments.  FIG. 2  illustrates cover plate  202  of pressure relief valve housing  104  coupled to mounting flange  202  of tank  102 . Pressure relief valve housing  104  comprises cover plate  204 , valve orifice  206 , and spring loaded valve assembly  208 . Tank  102  contains liquid  210  and vapor  212 . Vapor conduit  106  is coupled to cover plate  204  and extends into tank  102 . Vapor conduit orifice  214  is disposed in vapor conduit  106  near the end of vapor conduit  106  closest to pressure relief valve housing  104 . Conduit sleeve  216  is slidably coupled to vapor conduit  106  and positioned near vapor conduit orifice  214 . Sleeve stop  218  is coupled to vapor conduit  106 . 
     Under normal operating pressure, spring loaded valve assembly  208  is in sealing contact with cover plate  204  preventing vapor  212  from escaping tank  102 . Under high pressure conditions such as a fire situation, heat input to tank  102  increases the vapor pressure in tank  102  until it exceeds the safety setting of spring loaded valve assembly  208 . The high vapor pressure causes spring loaded valve assembly  208  to open and expel vapor  212  from tank  102  through valve orifice  206 . The diameters of valve orifice  206  and vapor conduit  106  depend on the flow rating of the pressure relief device housed in pressure relief valve housing  104  as determined based on the particular specifications of tank  102 . 
     In normal upright operating position, as illustrated, pressure relief valve housing  104  is located on top of tank  102 . Vapor conduit  106  extends longitudinally from cover plate  204  towards the bottom of tank  102 . In the upright position, gravity holds conduit sleeve  216  against sleeve stop  218 . When conduit sleeve  216  is resting against sleeve stop  218 , vapor  212  is able to pass from the vapor portion of tank  102  to vapor conduit  106  through conduit orifice  214  (illustrated by arrows marked FLOW). 
     Conduit sleeve  216  is an example of a gravity-operated valve operable to substantially seal or unseal conduit orifice  214 . Conduit sleeve  216  is free to slide along the longitudinal axis of vapor conduit  106 . Conduit sleeve  216  may be constructed of nylon, ultra high molecular weight (UHMW) polyethylene, or any other material compatible with the lading of tank  102  and suitable for sliding along vapor conduit  106 . In some embodiments, conduit sleeve  216  may comprise weights to aid in overcoming friction between conduit sleeve  216  and vapor conduit  106 . Some embodiments may comprise O-rings disposed between conduit sleeve  216  and vapor conduit  106  to form a seal between conduit sleeve  216  and vapor conduit  106 . Conduit sleeve  216  is sized to substantially prevent liquid  210  from entering conduit orifice  214  when conduit sleeve  216  is positioned along the portion of vapor conduit  106  comprising conduit orifice  214 . One of skill in the art will appreciate other configurations of gravity-operated valves operable to substantially seal or unseal conduit orifice  214 , such as swing valves or flapper valves. 
     Sleeve stop  218  limits the movement of conduit sleeve  216 . Sleeve stop  218  may be constructed of stainless steel or any other material compatible with the lading of tank  102  and suitable for preventing conduit sleeve  216  from sliding on vapor conduit  106 . In some embodiments, sleeve stop  218  may comprise a bolt passing horizontally through vapor conduit  106 . Other embodiments may include detents coupled to the exterior wall of vapor conduit  106  and capable of limiting the motion of conduit sleeve  216 . Sleeve stop  218  is positioned sufficiently below conduit orifices  214  so that conduit sleeve  216  does not block conduit orifices  214  when conduit sleeve  216  is resting against sleeve stop  218 . 
     Conduit orifice  214  allows vapor  212  to flow between tank  102  and vapor conduit  106 . Conduit orifice  214  is sized to permit at least the amount of vapor flow required by the pressure relief device housed in pressure relief valve housing  104 . In some embodiments, conduit orifice  214  comprises a plurality of circular openings in vapor conduit  106 . For example, four circular conduit orifices  214  may be evenly spaced around the circumference of vapor conduit  106 . Other embodiments may include any shape openings in any configuration sized to provide the required amount of vapor flow without compromising the structural integrity of vapor conduit  106 . Conduit orifice  214  is disposed in the top end of vapor conduit  106  closest to valve orifice  206 . 
     In operation under high pressure conditions, pressure relief valve system  100  will expel vapor  212  from tank  102 . In the illustrated embodiment, the expelled vapor flows through conduit orifice  214 . 
       FIG. 3  is a cross-sectional view of the pressure relief valve system of  FIG. 1B  in a roll-over position, in accordance with particular embodiments. In the roll-over position, pressure relief valve housing  104  may be located below the horizontal center line of tank  102 . Vapor conduit  106  extends longitudinally from cover plate  204  towards the top of tank  102 . In this position, gravity holds conduit sleeve  216  against cover plate  204 . When conduit sleeve  216  is resting against cover plate  204 , conduit sleeve  216  substantially prevents liquid  210  from passing through conduit orifice  214 . In some embodiments, gravity may hold conduit sleeve  216  against a portion of mounting flange  202  or an additional sleeve stop (not illustrated) similar to sleeve stop  218 . 
     In the roll-over position, the open radial end of vapor conduit  106  extends into the vapor space of tank  102 . Vapor  212  is able to pass from the vapor portion of tank  102  to vapor conduit  106  through the open radial end of vapor conduit  106  (illustrated by arrow marked FLOW). At the same time, conduit sleeve  216  substantially prevents liquid  210  from entering vapor conduit  106  and passing through valve orifice  206 . 
     In operation under high pressure conditions, pressure relief valve system  100  will expel vapor  212  from tank  102 . In the illustrated embodiment, the expelled vapor flows through the open radial end of vapor conduit  106 . 
     During a derailment, liquid  210  may enter vapor conduit  106  through the open radial end of vapor conduit  106  as tank  102  moves from an upright to a roll-over position. Similarly, liquid  210  may enter vapor conduit  106  through conduit orifice  214  before gravity fully seals conduit orifice  214  with conduit sleeve  216 . During a high-pressure situation, pressure relief valve system  100  will initially expel liquid  210  that may be contained in vapor conduit  106 . After initial expulsion of liquid  210 , pressure relief valve system  100  will then expel vapor  212  from the vapor portion of tank  102 . 
     Advantages of particular embodiments include various safety benefits. During a roll-over under high pressure conditions, such as a fire situation, pressure relief system  100  substantially prevents spring loaded valve assembly  208  from expelling potentially flammable liquid. Expelling liquid  210  is hazardous because it may further fuel the fire causing accelerated temperature and pressure increases within tank  102 . Furthermore, expelling vapor  212  releases pressure in tank  102  quicker and more efficiently than expelling liquid  210 . Additionally, a decrease in mass of liquid  210  in tank  102  increases heat transfer to tank  102 , further accelerating the pressure buildup. Efficiently releasing pressure within tank  102  may prevent a catastrophic failure of tank  102  and enhances safety of nearby property as well as people such as emergency responders and bystanders. 
     In particular embodiments, exposed portions of pressure relief valve system  100 , such as pressure relief valve housing  104 , may be susceptible to damage in a roll-over situation. For example, an emergency event such as a roll-over may shear pressure relief valve housing  104  from tank  102 . With a conventional pressure relief valve system, the opening remaining where the conventional housing used to attach to the tank will likely be in communication with the liquid portion of the tank and would likely expel potentially dangerous liquid. In pressure relief valve system  100 , even if exposed portions of the system are damaged in an emergency event, vapor conduit  106  will likely remain intact within tank  102 . Thus, any opening resulting from damage to the exposed portions of pressure relief valve system  100  will likely be in communication with the vapor portion of tank  102  and would likely expel vapor  212 . 
     In some embodiments, conduit sleeve  216  may not completely seal conduit orifice  214  during a roll-over event. Particular orientations of tank  102  may also prevent conduit sleeve  216  from completely sealing conduit orifice  214 . In some orientations of tank  102 , the open radial end of vapor conduit  106  may be disposed partially in the vapor portion of tank  102  and partially in the liquid portion of tank  102 . In these embodiments or orientations, pressure relief valve system  100  may expel both vapor  212  and liquid  210 . The advantages of pressure relief valve system  100  described herein may still be realized and such advantages may be proportional to the amount of vapor expelled relative to the amount liquid expelled. 
       FIG. 4  is a cross-sectional view of an embodiment of a pressure relief valve system in an upright position and fitted with a Y-shaped vapor conduit.  FIG. 4  illustrates a pressure relief valve system similar to the system depicted in  FIG. 2  where vapor conduit  406  is similar to vapor conduit  106 . In  FIG. 4 , vapor conduit  406  also comprises left vapor conduit leg  408  and right vapor conduit leg  410 . Vapor conduit  406  extends longitudinally from cover plate  204  towards the bottom of tank  102  where vapor conduit  406  branches into left vapor conduit leg  408  and right vapor conduit leg  410 . Other embodiments may contain any suitable number of conduit legs. A suitable number and configuration of conduit legs may be at least partially determined by the size of tank  102  and the ratio of liquid  210  to vapor  212  in tank  102 . 
     Left vapor conduit leg  408  comprises left conduit orifice  412 , left conduit sleeve  418 , and sleeve stops  420  and  422 . Right vapor conduit leg  410  comprises right conduit orifice  414 , right conduit sleeve  416 , and sleeve stops  420  and  422 . The conduits, conduit orifices, conduit sleeves, and sleeve stops are of similar size and construction as vapor conduit  106 , conduit orifice  214 , conduit sleeve  216 , and sleeve stop  218 , respectively, described in accordance with  FIG. 2  above. 
     Sleeve stops  420  limit the movement of conduit sleeves  416  and  418  away from conduit orifices  412  and  414 , respectively. Sleeve stops  422  are positioned to prevent conduit sleeves  416  and  418  from sliding off the ends of vapor conduit legs  408  and  410 , respectively. In some embodiments, sleeve stops  422  may also seal the radial ends of vapor conduit legs  408  and  410  to prevent liquid  210  from entering the conduits through their radial ends. 
     In normal upright operating position, as illustrated, pressure relief valve system  100  operates similar to the system described in  FIG. 2  above. Gravity holds conduit sleeve  216  against sleeve stop  218  and vapor  212  is able to pass from the vapor portion of tank  102  to vapor conduit  406  through conduit orifice  214  (illustrated by arrows marked FLOW). In operation under high pressure conditions, pressure relief valve system  100  expels vapor  212  from tank  102 . In the illustrated embodiment, the expelled vapor flows through conduit orifice  214 . 
       FIG. 5  is a cross-sectional view of an embodiment of a pressure relief valve system in a roll-over position and fitted with a Y-shaped vapor conduit. In the roll-over position, pressure relief valve housing  104  is located below the horizontal center line of tank  102 . Vapor conduit  406  extends longitudinally from cover plate  204  towards the top of tank  102  where vapor conduit  406  branches into left vapor conduit leg  408  and right vapor conduit leg  410 . In this position, similar to the system described in  FIG. 3  above, gravity holds conduit sleeve  216  against cover plate  204  and substantially prevents liquid  210  from passing through conduit orifice  214 . 
     Right vapor conduit leg  410  is also submerged within the liquid portion of tank  102 . Gravity holds right conduit sleeve  416  against sleeve stop  422 . When right conduit sleeve  416  is resting against sleeve stop  422 , right conduit sleeve  416  substantially prevents liquid  210  from passing through right conduit orifice  414 . 
     Left vapor conduit leg  408  extends into the vapor portion of tank  102 . Gravity holds left conduit sleeve  418  against sleeve stop  420 . When left conduit sleeve  418  is resting against sleeve stop  420 , vapor  212  is able to pass from the vapor portion of tank  102  to left vapor conduit leg  408  and vapor conduit  406  through left conduit orifice  412  (illustrated by arrow marked FLOW). 
     In operation under high pressure conditions, pressure relief valve system  100  will expel vapor  212  from tank  102 . In the illustrated embodiment, the expelled vapor flows through left conduit orifice  412 . 
     The embodiment illustrated in  FIG. 5  depicts a roll-over in the clockwise direction. A roll-over in the counter-clockwise direction may result in left vapor conduit leg  408  extending into the vapor portion of tank  102  and right vapor conduit leg submerged in the liquid portion of tank  102 . In such a scenario, gravity would seal left conduit orifice  412  with left conduit sleeve  418  and pressure relief valve system  100  would draw the expelled vapor from tank  102  through right conduit orifice  414 . An advantage of a Y-shaped vapor conduit is the increased chance that one of the conduit legs will extend into the vapor space during a roll-over situation. 
     In another scenario, a vapor conduit orifice may not initially be in communion with the vapor space during a roll-over situation. In this case, the pressure relief valve will expel liquid. After some amount of time, the pressure relief valve may expel enough liquid that at least one vapor conduit orifice is in communion with the vapor space and the pressure relief valve is then able to more efficiently release pressure in the tank by expelling vapor. An advantage of a Y-shaped vapor conduit is the decreased time required for one of the conduit legs to extend into the vapor space during such a scenario. 
       FIG. 6  is a cross-sectional view of a pressure relief valve system in an upright position and fitted with a Y-shaped vapor conduit, in accordance with particular embodiments.  FIG. 6  illustrates a pressure relief valve system similar to the system depicted in  FIG. 4  without the conduit sleeves or sleeve stops on left vapor conduit leg  408  or right vapor conduit leg  410 . 
     In operation under high pressure conditions in the upright position, pressure relief valve system  100  will expel vapor  212  from tank  102 . In the illustrated embodiment, the expelled vapor flows through conduit orifice  214 . In operation under high pressure conditions in a roll-over scenario, pressure relief valve system  100  may expel vapor  212  from tank  102  either through left conduit orifice  412  or right conduit orifice  414 . Because neither left conduit orifice  412  or right conduit orifice  414  is sealed with a conduit sleeve, pressure relief valve system  100  may expel both vapor  212  and liquid  210  in some orientations of tank  102 . The pressure relief valve system depicted in  FIG. 6  may realize the same advantages as the pressure relief valve system depicted in  FIG. 4  with the additional advantage of using fewer moving parts. 
       FIG. 7  is a cross-sectional view of an embodiment of a pressure relief valve system with a conduit sleeve internal to a vapor conduit.  FIG. 7  illustrates a pressure relief valve system similar to the system depicted in  FIG. 2  except that conduit sleeve  716  is slidably coupled to the interior of vapor conduit  106 . Sleeve stop  718  is coupled to vapor conduit  106 . 
     Conduit sleeve  716  is free to slide along the longitudinal axis of vapor conduit  106 . Similar to conduit sleeve  216 , conduit sleeve  716  may be constructed of any material compatible with the lading of tank  102  and suitable for sliding along vapor conduit  106  and may comprise weights to aid in overcoming friction between conduit sleeve  716  and vapor conduit  106 . Some embodiments may comprise O-rings disposed between conduit sleeve  716  and vapor conduit  106  to form a seal between conduit sleeve  716  and vapor conduit  106 . 
     Sleeve stop  718  limits the movement of conduit sleeve  716 . Similar to sleeve stop  218 , sleeve stop  718  may be constructed of any material compatible with the lading of tank  102  and suitable for preventing conduit sleeve  716  from sliding on vapor conduit  106 . In some embodiments, sleeve stop  718  may comprise a bolt passing horizontally through vapor conduit  106 . Other embodiments may include detents coupled to the interior wall of vapor conduit  106  and capable of limiting the motion of conduit sleeve  716 . 
     Particular embodiments may comprise a combination of conduit sleeves both internal and external to the vapor conduit. 
       FIG. 8  is a cross-sectional view of an embodiment of a pressure relief valve system with a pressure relief valve spring assembly.  FIG. 8  illustrates a pressure relief valve system similar to the system depicted in  FIG. 7  except that spring loaded valve assembly  808  is positioned not to be in contact with the commodity contained in tank  102 . In other embodiments, pressure relief valve housing  104  may house any pressure relief device suitable for relieving pressure in tank  102 . 
       FIG. 9  is a cross-sectional side view of an example pressure relief valve system retro-fitted to a railway tank car. In the illustrated embodiment, pressure relief valve system  100  is installed adjacent to an existing assembly for removing or expelling the contents of the tank car. In some embodiments, pressure relief valve system  100  may function simultaneously with a traditional pressure relief valve. In some embodiments, the traditional pressure relief valve may be removed or plugged when pressure relief valve system  100  is retro-fitted to the tank car. In some embodiments, a vapor conduit comprising conduit orifices along with conduit sleeves and sleeve stops such as those pictured in  FIGS. 1-8  may be coupled to a previously installed pressure relief device. 
     Although embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the following claims. For example, although the disclosure describes particular embodiments with reference to a straight or Y-shaped vapor conduit, various embodiments contemplate flexibility in the configuration and dimensions of the vapor conduit. Additionally, while the disclosure describes certain embodiments with respect to a railroad tank car, particular embodiments may be used for a variety of pressure chambers or other vessels capable of transporting liquids such as a tanker truck. 
     Other advantages of particular embodiments include increasing the fire survivability rating of a railroad tank car less expensively than alternative methods of thermal protection such as insulation.