Patent Publication Number: US-2010108160-A1

Title: Feather gasket for an excess flow valve

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application relates to Provisional Application No. 60/707,908 filed Aug. 12, 2005, and is a continuation-in-part application of U.S. patent application Ser. No. 12/344,430, filed Dec. 26, 2008, which is a continuation-in part of U.S. patent application Ser. No. 12/101,402 filed Apr. 11, 2008, which is a continuation-in-part application of U.S. patent application Ser. No. 11/266,457, filed Nov. 3, 2005, now issued as U.S. Pat. No. 7,380,565, which is a continuation-in-part application of U.S. patent application Ser. No. 11/220,080 filed on Sep. 6, 2005, now issued as U.S. Pat. No. 7,258,131. 
    
    
     BACKGROUND OF THE INVENTION 
     Safety valves or excess flow valves have been developed for installation in gas piping systems to shut down the flow of gas whenever there is an excess flow of gas that may indicate a leak or other problem. U.S. Pat. No. 7,258,131 describes such a safety valve. As described in U.S. Pat. No. 7,258,131, the piston assembly or moving element of this valve is relatively lighter because the piston assembly has two spaced apart pistons connected by a shaft. The safety valve closes when a stop positioned at an end of a valve stem engages a valve seat. 
     A piston assembly with a lighter mass is an advantage when trying to precisely control the flow rate at which the valve closes. However, when the valve closes at low gas pressures, there is a problem in that a small amount of gas leaks past the seal formed by the stop and the valve seat. 
     It is to solving these problems and others that the present invention is directed. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is directed to an excess flow valve having a feather gasket positioned near a valve seat. The feather gasket is made of a resilient material. The excess flow valve has a cylindrical housing through which a fluid flows. The excess flow valve also has a piston assembly with a first disc-shaped piston and a second disc-shaped piston, each piston having a diameter slightly smaller than a diameter of the housing. Each piston also has at least one orifice defined therein and has an upstream face and a downstream face. The piston assembly also has a shaft connecting the first piston and the second piston and a valve stem extending from the downstream face of the second piston. The second piston is attached to a first end of the valve stem and a stop is attached to a second end of the valve stem. The excess flow valve also has a valve seat with an opening shaped and sized to matingly receive the stop. With this configuration, the feather gasket helps to reduce any bypass flow past the valve seat. 
     The excess flow valve also has a spring positioned in the housing against the downstream face of the second piston such that the spring exerts a force on the second piston in a direction opposite the general fluid flow direction. When flow forces acting on the piston assembly sufficiently exceed the spring force acting on the second piston, the piston assembly moves in the flow direction until the stop engages the valve seat to shut off the fluid flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a safety valve positioned in the open position constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 2  is a partial cross-sectional view of a safety valve positioned in the closed position constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 3  is the cross-section  3 - 3  shown in  FIGS. 1 and 2 . 
         FIG. 4  is a cross-sectional view of a piston assembly constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 5  is a flow chart for designing a safety valve in accordance with a preferred embodiment of the present invention. 
         FIG. 6  is a flow chart for making and assembling a safety valve in accordance with a preferred embodiment of the present invention. 
         FIG. 7  is a flow chart for assembling a piston assembly on a threaded rod in accordance with a preferred embodiment of the present invention. 
         FIG. 8  is a schematic representation of a safety valve of the present invention installed on a gasoline pump. 
         FIG. 9  is a partial cross-sectional view of a fuel spill prevention system constructed in accordance a preferred embodiment of the present invention. 
         FIG. 10  is an side elevation view of a fuel spill prevention system constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 11  is a circuit schematic view of an alert system constructed in accordance a preferred embodiment of the present invention. 
         FIG. 12  is a partial cross-sectional view of a cartridge insert valve constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 13  is a partial cross-sectional view of the cartridge insert valve of  FIG. 12  installed in a segment of pipe. 
         FIG. 14  is a cross-sectional view of an alternate embodiment of a piston with orifices defined on the periphery of the piston. 
         FIG. 15  is a cross-sectional view of the cartridge insert valve constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 16  is a cross-sectional view of an excess flow valve with a feather gasket positioned near the valve seat. 
         FIG. 17  is a detailed view of the feather gasket shown in  FIG. 16 . 
         FIG. 18  is a detailed view of an alternative embodiment for the excess flow valve for which the feather gasket has been integrally formed with the safety valve housing. 
     
    
    
     It is noted that the cross-hatching of a cross section is not intended to indicate the use of a particular material for any of the drawings. 
     DESCRIPTION 
       FIGS. 1-2  show cross-sectional views of a safety valve  100 .  FIG. 1  shows the safety valve  100  in an open position and  FIG. 2  shows the safety valve  100  in a closed position. The safety valve  100  includes a housing  102  with an inner wall  104 . The housing  102  is made from a pipe of larger diameter than a diameter of inlet piping  106  upstream of the safety valve  100  and a diameter of outlet piping  108  downstream of the safety valve  100 . The safety valve  100  is connected to the inlet piping  106  by an inlet coupler  110  and to the outlet piping  108  by an outlet coupler  112 . Fluid flows through an inlet passage  111  defined in the inlet coupler  110 , into the housing  102  and, when the safety valve  100  is in the open position, though an outlet passage  113  defined in the outlet coupler  112 . The inlet coupler  110  and the outlet coupler  113  each have a hexagonal portion with six flat sides  115  to accommodate a standard wrench. 
     A disc-shaped first piston  114  and a disc-shaped second piston  116  are positioned in the housing  102 , with piston outer walls  118  and  120  having diameters slightly smaller than a diameter of the housing inner wall  104 . There are gaps between the piston outer walls  118  and  120  and the housing inner wall  104  that are generally small, but large enough to allow for free sliding of the pistons  114  and  116  within the housing  102 . The pistons  114  and  116  are separated by a shaft  122  of length L SH . The first piston  114  has a downstream face  124  and an upstream face  126 . The second piston  114  has a downstream face  128  and an upstream face  130 . 
     A spring  132  is positioned in the housing  102  between an outlet coupler inner face  134  and the downstream face  128  of the second piston  116 . The spring  132  resiliently restrains the sliding of the pistons  114  and  116  in the direction of the outlet coupler inner face  134 . The spring  132  has a spring constant K and a length L SP . 
     A valve stem  136  of length I VS  extends from the second piston downstream face  128  to support a stop  138 . The valve stem  136  is attached at a first end to the second piston  116  and at a second end to the stop  138 . As shown in  FIGS. 1 and 2 , the stop  138  may be integrally formed with the valve stem  136 , such as in a casting. The stop  138  may also be a separate piece attached to the valve stem  136 . The valve stem  136  is rigidly attached to the second piston  116  so that movement of the second piston  116  causes a like movement of the valve stem  136  and the stop  138 . A valve seat  140  is defined in the outlet coupler  112 . The valve seat  140  is shaped and sized to receive the stop  138  to form a substantial fluid seal between the stop  138  and the valve seat  140 . 
     As best seen in  FIG. 3 , orifices  142  are defined in the first piston  114 . There are two orifices  142  shown in  FIG. 3 , but if more orifices  142  are required, it is recommended that the orifices  142  be spaced equally about the circular first piston  114 . For example, if three orifices  142  are required, the orifices  142  should be spaced with centers at one hundred-twenty degree angles from one another, and with a center of each orifice  142  having the same distance from a center of the first piston  114 . The symmetry of the orifices  142  in each piston  114  and  116  is recommended so that the fluid flow field across a diameter of the housing  102  does not develop asymmetries. The second piston  116  generally has orifices  142  defined in positions identical to the first piston  114 , but more or less orifices  142  may be provided in the second piston  116  if so desired. 
     A piston assembly  144  is defined to include the first piston  114 , the second piston  116 , the shaft  122 , the valve stem  136  and the stop  138 . The orifices  142  are defined in the piston  114  and  116  to allow the flow of fluid past the pistons  114  and  116  under normal operating conditions. The selection of the size and the number of orifices  142  is discussed below in greater detail. 
     For the embodiment shown in  FIGS. 1-2 , the valve seat  140  is made from the outlet coupler  112  that has been externally threaded to fit into internal threads defined in the safety valve outer wall  104 . The valve seat  140  has been chamfered to form a geometry that more closely matches that of the stop  138 , thus forming a tighter seal between the stop  138  and the valve seat  140 . 
     In one embodiment, the stop  138  and the valve seat  140  have metallic surfaces. Thus, when the stop  138  moves into the valve seat  140 , the seal formed by the stop  138  and the valve seat  140  is not an absolute seal. However, the seal so formed substantially blocks the flow of fluid into the outlet piping  108  until a shutoff valve upstream of the safety valve  100  can be closed. 
     In another embodiment, at least one of the stop  138  and the valve seat  140  are made from a resilient material so that a tighter seal is formed between the stop  138  and the valve seat  140 . However, the resilient material selected for the stop  138  or the valve seat  140  must not degrade over time in the presence of the fluid in the piping system. Such resilient materials may include rubber, polymers, plastics, or fibrous material. 
     The materials generally used to make components for the safety valve  100  may be any suitable material for the transport of the fluid. In the case where the fluid is natural gas, such materials as steel, stainless steel, aluminum, copper, brass and various alloys thereof may be used. Generally, it is expected that the safety valve  100  may be in a natural gas pipeline for decades without the piston assembly being moved to the closed position. Thus, it is highly desirable for the material selected for use be resistant to rust and corrosion. Such materials include aluminum, stainless steel, composites and alloys thereof. Special consideration in the selection of materials must also be made when the valve is used in a high temperature or a low temperature environment and when the fluid flowing through the safety valve has a high or low operating temperature. In some applications involving high precision, the piston assembly may be made from light-weight carbon fiber materials. 
     In other applications, such as water transport, the safety valve  100  may also be made of the metallic materials listed above, but also may be made from plastic, polymers, or composite materials. 
     In operation, an excess flow rate caused by a leaky appliance or a catastrophic failure of piping downstream of the safety valve  100  creates a loss of pressure downstream of the safety valve  100 , which in turns creates a pressure difference between the upstream piping  106  and the downstream piping  108 . This pressure difference causes the pistons  114  and  116  to slide in a direction aligned with the fluid flow through the safety valve  100 . Thus, the piston  116  is pushed toward the valve seat  140  by the fluid flow forces, with the fluid forces exerted on the pistons  114  and  116  being counteracted by the force exerted by the spring  132  on the second piston  116 . When the piston  116  moves toward the outlet coupler  112 , the stop  138  in turn moves toward the valve seat  140 . As shown in  FIG. 1 , the stop  138  is generally frustro-conical in shape. However, the stop  138  may be any shape so long as it is shaped and sized to matingly engage the valve seat  140 . When the pressure difference between the inlet piping  106  and the outlet piping  108  exceeds a critical pressure difference, the stop  138  seats in the valve seat  140 , the safety valve  100  is in a closed position and fluid flow through the safety valve  100  stops, as represented in  FIG. 2 . 
     It is well known that a breaker box for an electrical supply line shut off the supply to an electrical circuit provided to a house when the current exceeds a certain level. Similarly, the safety valve  100  closes when the flow rate through the valve exceeds a certain critical flow rate. 
     The use of the first piston  114  and the second piston  116  spaced apart by a shaft  122  allows one to use a much lighter piston assembly, as compared to a solid-body piston with elongated holes, as taught by U.S. Pat. No. 5,215,113, issued to Terry on Jun. 1, 1993 (Terry). Because the pistons  114  and  116  are disc-shaped, it is also much easier to drill through the pistons  114  and  116  to create the orifices  142 , as compared to the difficulty for drilling holes in the solid-body piston. 
     The use of two spaced-apart pistons  114  and  116  attached to a shaft  122  is a more stable structure, with respect to keeping the valve stem  136  and the stop  138  in the middle of the housing  102 , as compared with using a single disc-shaped piston with a valve stem and stop. Generally, it is expected that the two disc-shaped pistons  114  and  116  will have less friction with the inner walls of the housing  104  than would the outer edges of the solid-body piston assembly taught by Terry, because the two spaced-apart pistons  114  and  116  would generally have less of a total surface area in contact with the housing inner wall  104 . 
     Another advantage over the solid-body piston is that two disc-shaped, spaced apart pistons are much lighter than a solid body occupying the same volume. This is particularly important when the safety valve  100  is used in an application requiring high precision in low pressure gas pipes. These applications occur, for example, when one wishes to monitor home appliances for excess gas flow. 
     For these applications, one wishes to know when excess gas is flowing to an appliance because it may indicate a leak in the appliance or one of the appliance&#39;s gas fittings. By using a low-mass piston assembly in conjunction with a very low spring constant, the safety valve  100  may be used to sense and respond to very small changes in pressure. It is well known in fluid mechanics that for a given flow geometry and fluid, the pressure can be directly correlated to a flow rate. Thus, the safety valve  100  may be designed to shut down the flow when a small increase in gas flow rate occurs downstream of the safety valve. 
     Reducing the mass of the piston assembly makes the safety valve  100  more sensitive to small pressure changes in part because the fluid forces acting on the piston assembly  144  must overcome inertia to move the piston assembly  144  from the open position to the closed position. Reducing the mass of the piston assembly  144  would clearly lessen the amount of force required to overcome the inertia of the piston assembly  144 . 
     It is also recommended that, for applications requiring high precision, the safety valve  100  should be installed on piping in the horizontal position. Otherwise, the weight of the piston assembly  144  may affect the flow rate at which the safety valve will close. If the valve is installed in a vertical position, the weight of the piston assembly  144  must be accounted for in selecting a spring  132  and an orifice size  142  for the first piston  114  and the second piston  116 . 
       FIG. 4  shows a cross sectional view of another embodiment of the piston assembly  144  of the safety valve  100 . In this embodiment, the piston assembly  144  is made from generally off-the-shelf mechanical hardware and piping hardware. In  FIG. 4 , a threaded rod  150  is used to form a threaded shaft  152  and a threaded valve stem  154  for the safety valve  100 . A first flat washer  156  and a second flat washer  158  acts as the pistons  114  and  116  shown in  FIGS. 1-3 . An internally threaded nose cone  160  screws onto an end of the threaded rod to act as the stop  138  to fit matingly with the valve seat  140 . The flat washers  156  and  158  are secured on the threaded rod  150  by tightening a first nut  162  against a second nut  164  on the threaded rod  150 , with two lock washers  166  and one of the flat washers  156  and  158  positioned between the two nuts  162  and  164 . 
     The flat washers  156  and  158  have holes  168  defined therein through which fluid flows. In operation, the threaded rod  150  acts identically to the valve stem and shaft shown in  FIGS. 1-2 , with the flat washers  156  and  158  being the first piston  114  and the second piston  116 . The nose cone  160  acts identically to the stop  138  of  FIGS. 1-2  to matingly engage the valve seat  140  to stop the fluid flow when the force exerted by the fluid on the flat washers  156  and  158  sufficiently exceeds the force exerted by the spring  132 . 
       FIG. 5  shows a flow chart for a method for designing the safety valve  100  of the present invention. For this flow chart, the steps of the method may be carried out in any order except where one step necessarily precedes another step. The method begins at step  200 . 
     At step  202 , a designer specifies the normal operating conditions for the safety valve  100 , such as the pipeline size, the fluid flowing in the pipeline and a range of normal pipeline pressures and normal flow rates. At step  204 , the designer specifies the material to be used for the safety valve  100  based on the fluid flowing in the pipeline. At step  206 , given the normal operating conditions, the designer specifies a desired critical pressure difference between the pressure upstream and downstream of the safety valve  100 , above which the safety valve  100  is designed to close. The designer may also express this critical pressure difference as a critical flow rate based on predetermined correlations and measurements. 
     At step  208 , the designer selects a safety valve housing nominal diameter D H  and length L H . At step  210 , the designer selects a spring  132  with spring constant K and length L SP . The spring length L SP  is selected so that the spring  132  is under a slight compression or “preload” when the spring  132  and piston assembly  144  are assembled in the housing. The exact amount of the preload will vary depending on the particular application. At step  214 , the designer selects two pistons, each having a diameter slightly smaller than the internal diameter of the housing  102 . At step  216 , the designer selects a number N of orifices  142  having diameters of D O . For a safety valve  100  of a given size, the selection of the number of orifices  142 , the orifice diameter D O , and the spring constant K determine the critical pressure difference and the critical flow rate at which the safety valve  100  will close. 
     The spring length L SP  and the spring constant K will determine the stroke S that the pistons will travel between open and closed positions of the valve for a given housing length L. The stroke S should be of a sufficient length to prevent the safety valve  100  from repeatedly opening and closing when the pressure difference across the safety valve is near the critical pressure difference. The design of the piston assembly  144 , with a relatively long valve stem length L vs  and a relatively long spring length L SP  prevent the valve from opening and closing when the valve is operating near the critical pressure difference and critical flow rate. Generally, the stroke S should be nominally 15-20% of the uncompressed length L SP  of the spring  132 , and should in all cases be less than one third of L SP . The purpose of this restriction on the stroke S is to ensure that the spring  132  deflects only in the linear range, so that the deflection of the spring  132  as a function of force can be reliably determined. 
     In selecting the number of orifices  142  and the orifice size D O , it is also generally desirable to minimize the pressure drop across the safety valve  100  with the safety valve  100  in the open position while insuring reliable operation of the safety valve  100 . It is expected that this pressure drop will increase with decreasing size of the orifice  142 , but this is a general rule subject to exceptions for particular designs. 
     In one embodiment, the first piston  114  has more than two orifices  142  and the second piston  116  has two orifices  142 , and the size of the orifices  142  in the first piston  114  is smaller than the size of the orifices  142  in the second piston  116 . In this embodiment, the first piston  114  acts as a filter to remove to remove contaminants from the fluid stream. In another embodiment, a fluid screen is placed in the pipeline upstream of the safety valve to remove contaminants before they reach the safety valve  100 . When the fluid being pumped through the pipeline is known to have contaminants, it is important to have a mechanism to remove the contaminants or the contaminants may block the orifices  142 . 
     As a rule of thumb, it has been found that the safety valve  100  operates well for a gaseous fluid when a sum of the areas of all the orifices  142  is 2 to 3 times less than the area of the inlet passage  111 . Furthermore, it is generally believed that two orifices  142  on each piston  114  and  116  are sufficient for proper operation of the safety valve  100  in a gas pipeline. 
     At step  216 , the designer must specify the length L vs  of the valve stem  136 . The length L vs  of the valve stem  136  is selected so that the spring  132  fits between the second piston  116  and the outlet coupler inner face  134 , applying a predetermined force to the second piston  116  to prevent the stop  138  from engaging the valve seat  140 . Finally, at step  218 , the designer specifies the shape of the stop  138  and valve seat  140 . The method ends at step  220 . 
     Following the method for designing a safety valve  200 , a manufacturer may conduct tests and generate a series of tables to make the selection of the safety valve  100  simply a matter of looking up in a table which safety valve  100  is required for a particular application. The designer may choose many of the design criteria based on experience, rules of thumb, and other imprecise rules of design. However, assuming that all the other criteria are determined, the choices of the number of orifices, the orifice diameters and the spring constant will determine the critical flow rate at which the safety valve  100  will close 
     For a first example, assume that all the design criteria are known except the orifice size and spring constant are known. Table 1 provides an example of the type of correlation between the orifice size, the number of orifices  142 , and the spring constant K. One can look at Table 1 and determine the orifice size required for the safety valve to close at the desired critical flow rate for various combinations of spring constant and the number of orifices. 
     Another example of the type of correlation that can be determined experimentally is shown in Table 2. For Table 2, it is assumed that all the other design criteria have been specified except the orifice size, the critical flow rate, and the spring constant. From Table 2, if one is given a particular orifice size and a critical flow rate, one can then determine the spring constant to use to cause the safety valve to close at that critical flow rate. 
     It must be noted that none of the actual numerical values given by Tables 1 and 2 have yet been determined. These tables are only meant to demonstrate the types of experimental correlations that a manufacturer can provide to designers to assist designers in the design of the safety valve  100  for particular applications. 
       FIG. 6  shows a flow chart for a method of making and assembling the safety valve  100 . The flow chart begins at step  300 . The steps for the method of making and assembling the safety valve may be performed in any order, except where one step necessarily follows another step. At step  302 , the person making and assembling the safety valve  100  (maker) provides a safety valve housing  102  larger than the size of the piping to which the safety valve  100  is attached. The safety valve housing  102  is a length of pipe made from a material suitable for the fluid being transported in the piping. 
     At step  304 , threads are defined in the safety valve housing  102 . The threads may be internal or external threads. As shown in  FIGS. 1-2 , the threads on the safety valve housing  102  that attach the housing to the inlet and outlet couplers  110  and  112  are internal. At step  306 , the maker provides an inlet coupler  110  to connect the safety valve housing  102  to the inlet piping  106 . 
     For the embodiment shown in  FIG. 1-2 , the inlet coupler  110  and the outlet coupler  112  are: (1) externally threaded on one end to connect to the internal threads on the housing and (2) are internally threaded on the other end to connect to externally threaded inlet and outlet piping. For the case where the safety valve is retrofitted to an existing fluid line, one would expect to cut a section of the existing fluid line in two places, remove the section of fluid line between the two cuts, thread ends of the existing fluid line where the cuts have been made, and install the safety valve. Various pipe couplers are available for: (1) connecting two externally threaded sections of pipe; (2) to connect an internally threaded pipe to another internally threaded pipe; or to (3) connect an externally threaded pipe to an internally threaded pipe. The choice of whether to use internal or external threads will depend largely on the application for which the piping is being used. 
     At step  308 , the maker provides an outlet coupler  112 . In some embodiments, the outlet coupler  112  is made of cast material, such as aluminum and a valve seat  140  is shaped and sized in the outlet coupler  112  when the outlet coupler  112  is cast to matingly receive the stop  138 . In other embodiments, the outlet coupler  112  is provided as an off-the-shelf item from a hardware supplier. For this embodiment, the valve seat  140  is defined in the outlet coupler  112  by using appropriate tools to chamfer an edge of a the outlet passage  113  until a portion of the valve seat is conical in shape to matingly receive the stop  138 . One appropriate tool for chamfering the edge of the passageway is a rotary grinding tool. After the valve seat  140  is defined in the outlet coupler  112 , the outlet coupler  112  is then attached to the safety valve housing at step  310   
     At step  312 , the maker provides a spring  132  designed in accordance with the method shown in  FIG. 5  and inserts the spring  132  into the safety valve housing  102  against the outlet coupler  112 . 
     At step  314 , the maker provides and assembles a piston assembly  144 . The piston assembly  144  includes the shaft  122 , the valve stem  136 , the first piston  114 , the second piston  116 , and the stop  138 . In one embodiment, the piston assembly  144  is cast as a unitary casting, with the orifices  142  defined in the casting. In a second embodiment, the piston assembly  144  is cast as a unitary casting and the orifices  142  are drilled into the unitary casting. 
       FIG. 7  is a flow chart for yet another embodiment of a method for performing step  314  in  FIG. 6 . Referring briefly to  FIG. 7 , the method begins at step  314 A. At step  314 B, a threaded rod  150  is provided for attachment of several components of the piston assembly  144 . At step  314 B, the flat washers  156  and  158  are drilled to define holes  168 . At step  314 C, the first nut  162  is screwed onto the threaded rod  150  to a predetermined thread location. At step  314 D, the lock washer  166  is inserted on the threaded rod  150  against the first nut  162 . At step  314 E, the first flat washer  156  is inserted onto the threaded rod  150  against the lock washer  166 . At step  314 F, a second lock washer  166  is inserted onto the threaded rod against the first flat washer  156 . At step  314 G, the second nut  164  is screwed onto the threaded rod  150  and tightened against the second lock washer  166 . At step  314 H, the steps  314 C through  314 G are then repeated for the second flat washer  158  to attach the second flat washer  158  to the threaded rod  150  at a predetermined location. At step  314 I, the internally threaded nose cone  160  is screwed onto the threaded rod  150 . The method stops at step  314 J and the making of the piston assembly  144  is complete. 
     Returning to  FIG. 6 , at step  316 , the maker inserts the piston assembly  144  into the safety valve housing  102  against the spring  132 . At step  318 , the inlet coupler  110  is screwed into the safety valve housing  102 . A length L PA  of the piston assembly  144  and the spring length L SP  should be selected so that the inlet coupler  110  slightly compresses the spring  132  when the inlet coupler  110  is screwed into the safety valve housing  102 . It is in attaching the inlet coupler  110  that the preload is applied by the amount that the inlet coupler  110  is screwed into the threads on the housing  102 . 
     At step  320 , the safety valve  100  is attached to the inlet piping  106  and the outlet piping  108  to complete the installation of the safety valve  100  in the piping system. For each attachment of the safety valve  102  to the inlet coupler  110 , and the outlet coupler  112 , and for the attachment of the inlet coupler  110  and the outlet coupler  112  to the upstream piping  106  and the downstream piping  108 , it is recommended that the attachment be made using a wrench that fits onto two of the flat sides  115  of the hexagonal portion of the inlet coupler  110  and the outlet coupler  112 . The method stops at step  322 . 
       FIG. 8  shows a schematic representation of a gasoline dispensing system  401 , with a safety valve  400  of the present invention installed at a gasoline pump  402 . Gasoline exits the gasoline pump  402  via the pump exit piping  404 . The exit piping  404  acts as the inlet piping  106  to the safety valve  100  shown in  FIGS. 1-2 . Gasoline exits the safety valve  400  through the safety valve outlet piping  406 , which is connected to a flexible hose. The safety valve  400  is configured internally exactly like the safety valve  100 , and acts to shut off the flow of gasoline when there is a break in the piping downstream of the safety valve  400 . Thus, a catastrophic failure occurs downstream of the safety valve  400  occurs when a motorist drives off from the gasoline pump  402  with a fill nozzle still in his automobile gasoline tank. When the catastrophic failure occurs, the safety valve  400  shuts down the flow of gasoline to the nozzle. 
     Although the example shown in  FIG. 8  and discussed in the preceding paragraph is for a gasoline pump, the same principles apply to a fill station for an LP gas tank. 
       FIG. 9  is a partial cross-sectional view of a spill prevention system  501  having a safety valve  100  installed in one of two fuel tanks  500  of a truck, where the safety valves  100  are in an opposed relationship with one another. The safety valve  100  shown in  FIG. 9  is identical to the embodiment shown in  FIGS. 1-2  between the inlet coupler  110  and the outlet coupler  112 . The inlet piping  106  of  FIGS. 1-2  is not necessary because the safety valve  100  is immersed in the fuel of the fuel tank  500 . Furthermore, instead of being connected to outlet piping  108  as in  FIGS. 1-2 , the outlet coupler  112  is connected to a reducer fitting  504  that penetrates a wall  502  of the fuel tanks  500 . 
     The reducer fitting  504  is threaded externally at both ends and is threaded internally at the end having a larger diameter than its other end. The outlet coupler  112  screws into the internal threads of the reducer fitting  504 . The reducer fitting  504  screws into threads defined in a tank wall  502  and connects to a connecting line  510 . The connecting line  510  is connected to the reducer fitting  504  by a hose coupler  508 , which is in turn attached to the connecting line  510 . 
     As best seen in  FIG. 10 , the connecting line  510  extends from the reducer fitting  504  on one fuel tank  500  to a like reducer fitting  504  on a second fuel tank  500 . The second fuel tank  500  has a safety valve  100  configured exactly like the safety valve  100  on the first fuel tank  500  shown in  FIG. 9 . A trip wire  512  is attached to the connecting line  510 . The trip wire  512  is connected to the electrical system of the truck and is configured to indicate to a truck driver in a cab of the truck whenever the trip wire  512  is broken. 
     In operation, fluid is allowed to pass through the safety valves  100  and the connecting line  510  under normal operating conditions to evenly draw fuel from both tanks  500 . This is desirable because an unbalanced load on a truck may cause the truck to have an accident. This allows fuel to flow from one tank  500  to another tank  500  if there is a difference in the amount of fuel in each tank  500 . The pressure driving the flow would be the incremental static head pressure that occurs in one tank  500  when that tank  500  has more fuel than the other tank  500 . The restriction due to opposing safety valves  100  causes the fuel transfer rate to be lower than the critical flow rate that would cause one of the safety valves  100  to close. 
     However, if the connecting line  510  breaks or is detached from one of the reducer fittings  504 , each safety valve  100  senses the change in flow rate through each safety valve  100 , by way of the increased the pressure difference between each safety valve&#39;s inlet coupler  110  and its respective outlet coupler  112 , which causes each safety valve  100  to close. As discussed above for the safety valve  100 , with all other dimensions of the safety valve  100  being constant, the spring constant K, the number of orifices  142 , and the size of the orifices  142  can be varied to produce a safety valve  100  that closes at a precise flow rate. The precision with which the safety valve  100  operates is largely due to the fact that the piston assembly  144  can be designed to have a very low mass. 
     For the embodiment shown in  FIGS. 9-10 , the fuel tanks  500  could contain any liquid and would be referred to simply as liquid storage tanks if the liquid storage tanks did not contain fuel. Furthermore, the fuel tanks  500 , the connecting line  510  and the spill prevention system  501  may be collectively referred to as a fuel tank system or a containment system. 
     The connecting line  510  may be a flexible hose or a rigid conduit. The material which the connecting line  510  is made from is determined by the particular liquid that is contained in the liquid storage tanks. If the connecting line  510  is a flexible hose, suitable materials from which to make the flexible hose include rubber, plastic, and other flexible materials. If the connecting line  510  is a rigid conduit, suitable materials for the rigid conduit include steel, aluminum, rigid plastics, and various alloys thereof. 
       FIG. 11  shows an electrical schematic for an alert system  520  to be used in conjunction with the trip wire  512  to alert a driver of the truck that the trip wire  512  has been broken. In  FIG. 11 , electrical wiring for the alert system circuit  520  is shown by solid lines, while the fuel tanks  500  and the connecting line  510  extending between the fuel tanks  500  are shown by dotted lines. The alert system  520  is powered by the truck battery  522 . 
     A negative terminal  524  of the battery  522  is connected to any suitable electrical ground for the alert system circuit  520 , such as a frame element of the truck. An electrical wire  526  leads from a battery positive terminal  528  to a branch line  527  having a sensor and indicator device (indicator)  530 . Another electrical wire  532  leads from the indicator  530  to the electrical ground for the alert system circuit  520 . Yet another electrical wire  534  leads from the branch line  527  and is connected to a first end of the trip wire  512 . Still another electrical wire  536  leads from a second end of the trip wire  512  to the electrical ground. 
     In operation, in normal operating conditions when the trip wire  512  is intact, electrical current passes through the trip wire  512  and the indicator  530  remains inactive. When the trip wire  512  is broken, the indicator  530  senses an increase in electrical current and activates to alert the truck driver that the trip wire  512  is broken. The indicator  530  may include a simple light, a flashing light, or an audible signal that indicates to the driver that the trip wire  512  is broken. 
     Although only two fuel tanks  500  are shown in  FIG. 10 , it is clear to one skilled in the art that a third tank, or any number of tanks, may be added to the fuel tank system without changing the nature of the invention. This third tank may be connected to either one of the tanks  500  shown in  FIG. 10  by connecting the third tank to one of the tanks  500  by a second connecting line. Similarly, a tee connection may be added to the connecting line  510 , and the second connecting line may be connected to the connecting line  510  through the tee connection. 
       FIG. 12  shows a cross-sectional view of a cartridge insert valve  600  in an open position. The cartridge insert valve  600  (hereinafter “cartridge”) includes a housing  602  with an inner wall  604 . The housing  602  is made from a pipe of smaller diameter than a diameter of a piping segment in which the cartridge  600  is inserted. 
     A fluid such as natural gas flows through an inlet opening  606  defined in an inlet cap  608 , into the housing  602  and, when the cartridge  600  is in the open position, though an outlet opening  610  defined in a seat cap  612 . Both the inlet cap  608  and the seat cap  612  generally have a circular cross-sectional shape. 
     A disc-shaped first piston  614  and a disc-shaped second piston  616  are positioned in the housing  602 , with piston outer walls  618  and  620  having diameters slightly smaller than a diameter of the housing inner wall  604 . There are gaps between the piston outer walls  618  and  620  and the housing inner wall  604  that are generally small, but large enough to allow for free sliding of the pistons  614  and  616  within the housing  602 . The pistons  614  and  616  are separated by a shaft  622 . The first piston  614  has a downstream face  624  and an upstream face  626 . The second piston  614  has a downstream face  628  and an upstream face  630 . 
     A spring  632  is positioned in the housing  602  between a seat cap inner face  634  and the downstream face  628  of the second piston  616 . The spring  632  resiliently restrains the sliding of the pistons  614  and  616  in the direction of the seat cap inner face  634 . 
     A valve stem  636  extends from the second piston downstream face  628  to support a stop  638 . The valve stem  636  is attached at a first end to the second piston  616  and at a second end to the stop  638 . The valve stem  636  is rigidly attached to the second piston  616  so that movement of the second piston  616  causes a like movement of the valve stem  636  and the stop  638 . A valve seat  640  is defined in the seat cap  612 . The valve seat  640  is shaped and sized to receive the stop  638  to form a substantial fluid seal between the stop  638  and the valve seat  640 . Orifices  642  are formed in the first piston  614  and the second piston  616 . 
     A piston assembly  644  is defined to include the first piston  614 , the second piston  616 , the shaft  622 , the valve stem  636  and the stop  638 . The inlet cap  608  has a flange  646  that is an enlarged diameter portion, when compared to the diameter of the inlet cap shown in  FIG. 1 . The operation of the cartridge  600  in closing when there is excess fluid flow is identical to the operation of the safety valve  100  described above and further description of this operation is omitted. 
       FIG. 13  shows the cartridge  600  inserted into a pipe segment  650  of a piping system  652 . As shown in  FIG. 13 , the inlet cap flange  646  acts to restrain the cartridge  600  from sliding further into the pipe segment  650 . The flange  646  has a diameter slightly larger than an inside diameter of the pipe segment  650  in which it is positioned. In a typical installation, threads on an end of the pipe segment  650  mate with threads on one end of a pipe collar (not shown) to attach the pipe segment to an upstream portion of the piping system  652 . A joint formed between the flange  646  and an end of the pipe segment  650  should be sealed to prevent the escape of a fluid flowing through the piping system. The joint may be sealed by positioning a sealant between the flange  646  and the end of the pipe segment  650 , such as an O-ring or another appropriate sealant. Although the pipe segment is shown in a horizontal orientation in  FIG. 13 , the pipe segment may have any orientation. In particular, in one typical installation the pipe segment is a vertical riser located downstream of a gas meter. However, the cartridge may be inserted into any portion of the piping system, including an end of the piping system near a gas appliance, which is typically flexible tubing. 
     The cartridge may be made from any appropriate material, including steel, stainless steel, copper, brass, plastic, polymers, metallic alloys, composite materials and combinations thereof. 
     In the following, a method is described for installing a cartridge in a gas pipe to protect an existing residence or business (collectively referred to as “building”). The first step in the method is having an installer close a shutoff valve upstream of the gas meter. Next, the installer disconnects the gas supply pipe that normally connects the building to the meter. This connection is downstream of the gas meter. Typically, this gas supply pipe is a vertical riser attached to the upstream portion of the piping system by a collar that engages threads on the vertical riser. Thus, the installer disconnects the gas supply pipe by rotating the pipe collar. At this point the installer may simply insert the cartridge  600  into the gas supply pipe with the seat cap inserted first. The cartridge  600  is prevented from traveling too far into the pipe segment  650  by the inlet cap flange  646 . Finally, the installer may simply reconnect the gas supply pipe to the gas meter and opens the shutoff valve upstream of the gas meter. For this installation method of a cartridge  600  in a piping system, no removal of pipe segments is required and the cartridge  600  easily retrofits into a piping system. 
       FIG. 14  shows an alternate embodiment of a piston  114  for which the orifice  142  is positioned on an outer radial periphery of the piston  140 . Although only one piston  114  is shown, the alternative embodiment can be used for the piston  116  as well. The orifice  142  is not a hole with a complete circumferential boundary, but rather a void which has only a partial circumferential boundary. With this design for a piston and orifice  142 , the piston may be formed in a mold in a single manufacturing operation. For other designs for a piston  114  having orifices  142  with a complete boundary, such as that shown in  FIG. 3 , the piston  114  is formed in one operation and the orifice  142  defined in another operation, such as by drilling into the piston to form the orifice  142 . Thus, by positioning the orifice on the radial periphery of the piston, the efficiency of the overall safety valve  100  manufacturing process may be improved and the cost of the safety valve  100  may be reduced. 
       FIG. 15  shows the cartridge insert valve of  FIG. 12  for which the seat cap  612  has been formed integrally with the housing  602 . Identical parts with the same reference numerals as in  FIG. 12  also have identical functions and the description of the operation for these parts will be omitted. The stop  638  shown in  FIG. 15  is hemispherically shaped in the embodiment shown in  FIG. 15 . It has been observed that particulates have sometimes entered the piping system and that these particulates may degrade the sealing performance of the stop  638  against the seat cap  602 . In this case, the hemispherical stop  638  may provide a better seal against the seat cap  602 . Also, if the manufacturing tolerances are less precise, the sealing ability of the stop  638  may be enhanced by having the hemispherical shape seal the cylindrically shaped valve seat. This sealing may be further enhanced by making the stop from a pliant material, such as rubber, or by making the valve stem  636  from a material, such as plastic, that permits the valve stem  636  to slightly laterally deform to form the seal. 
       FIGS. 16-18  show embodiments of a feather gasket  652  located near the outlet opening  610  of an excess flow valve  600 . In  FIG. 16-18 , identical parts with the same reference number as in  FIG. 15  also have identical functions and the description for the operation of these parts will be omitted. For the embodiment shown in  FIG. 16 , however, there are a few notable differences. 
     First, the flange  646  has been defined in the housing  602  rather than the inlet cap  608 . Also, the spring  636  of  FIG. 15  has been omitted. The function of the spring has been replaced by positioning the excess flow valve  600  in a vertical orientation. Thus, gravity acting on the piston assembly  644  performs the same function that the spring performs in the embodiment shown in  FIG. 15 . It is noted that, for the embodiment of  FIG. 15 , the excess flow valve  600  may also be positioned in a vertical orientation, but the spring constant must be selected to account for the additional weight of the piston assembly  644 . For the embodiment shown in  FIG. 15 , the excess flow valve  600  closes when the flow forces acting on the piston assembly  644  sufficiently exceed the forces exerted by the spring  636  on the piston assembly  644 . For the embodiment shown in  FIG. 16 , the excess flow valve  600  closes when the flow forces acting on the piston assembly  644  sufficiently exceed the forces exerted by gravity on the piston assembly  644 . It is further noted that the excess flow valve  600  does not have to be positioned in a strictly vertical position for the forces of gravity to act on the piston assembly  644 . Rather, the excess flow valve  600  may also be positioned at some angle to the vertical, such as 45 degrees. The lack of a need for a spring in the embodiment shown in  FIG. 16  is believed to be made feasible because of the relatively light weight of the piston assembly  644 , as compared to a piston assembly with a solid body. 
     The feather gasket  652  is disposed near the outlet opening  610  and makes up a part of the valve seat  640 . When the fluid forces acting on the piston assembly  644  cause the stop  638  to engage the valve seat  640 , the stop  638  engages the resilient feather gasket  652 , causing a tighter seal between the stop  638  and the valve seat  640 . The feather gasket  652  is necessary in part because of the relatively lighter weight of the piston assembly  644 , as compared to a solid body piston assembly. As noted above, the piston assembly  644  is relatively lighter because it is made from the two pistons  614 ,  616  separated by the shaft  622 , rather than a single heavier piston element. 
       FIGS. 17 and 18  are detailed views of the area near the feather gasket  652 . In  FIG. 17 , the feather gasket is attached to the inside of the seat cap  612 . In  FIG. 18 , the feather gasket  652  is an integral part of the housing  602  and made from the same material as the housing  602 . In  FIG. 17 , the feather gasket  652  has the shape of a washer with an annular, triangle-shaped wedge formed along an inner diameter of the washer. In  FIG. 18 , the feather gasket  652  has the shape of an annular, triangle-shaped wedge formed integrally with the housing  602  along an inner diameter of the valve seat  640 . 
     While the embodiment shown of the excess flow valve in  FIG. 16  contemplates an upward direction of gas flow so that gravity acts on the piston assembly  644  rather than a spring, the excess flow valve  600  could also be installed with gas flowing in a downward direction. However, in such as case, the spring rate must be selected slightly larger than it would be in a horizontal orientation to account for the effects of gravity acting on the piston assembly  644 . 
     The material that forms the feather gasket  652  may be any resilient material compatible with the gas or liquid flowing through the excess flow valve  600 . In one embodiment, the resilient material is Sevrene™. In other embodiments, the resilient material may be rubber, neoprene, polyvinyl chloride (PVC), a thermoplastic elastomer or a silicone compound. As mentioned above, the stop  638  may also be made of a pliant or resilient material to further enhance the seal formed between the stop  638  and the valve seat  640 . 
     With a feather gasket  652  and housing  602  made of Sevrene™, it has been found that a bypass flow rate for gas leaking past the seat cap is less than 0.5 cubic feet per hour (0.0142 cubic meters per hour). 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 DIAMETER OF EACH ORIFICE (cm) 
               
            
           
           
               
               
               
            
               
                   
                 Number of Orifices 
                 Spring Constant (N/m) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 in Each Piston 
                 0.5 
                 1.0 
                 2.0 
               
               
                   
                   
               
               
                   
                 2 
                 A 
                 B 
                 C 
               
               
                   
                 3 
                 D 
                 E 
                 F 
               
               
                   
                 4 
                 G 
                 H 
                 I 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 SPRING CONSTANT [N/m] 
               
            
           
           
               
               
               
            
               
                   
                   
                 ORIFICE 
               
               
                   
                 CRITICAL FLOW RATE 
                 SIZE (SQ. CM.) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 [CUBIC METERS/HR] 
                 0.2 
                 0.4 
                 0.5 
               
               
                   
                   
               
               
                   
                 5 
                 A 
                 B 
                 C 
               
               
                   
                 6 
                 D 
                 E 
                 F 
               
               
                   
                 7 
                 G 
                 H 
                 I