while embodiments will be described below with reference to the accompanying drawings , the specific structures and descriptions which follow are illustrative and exemplary of a broad scope , and are not to be construed as limiting embodiments . as used here , the terms “ above ” and “ below ”; “ up ” and “ down ”; “ upper ” and “ lower ”; “ upwardly ” and “ downwardly ”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments . however , when applied to equipment and methods for use in wells that are deviated or horizontal , such terms may refer to a left to right , right to left , or diagonal relationship as appropriate . a gas lift valve can operate or actuate ( open and close ) by a pneumatic process that allows pumped or injected lift gas or fluid to mix with crude oil or well fluid in a production tubing , thereby reducing the density of the crude oil or well fluid , and enhancing the production rate of the well . the injection gas or fluid is provided to an annulus between the production tubing and wellbore , and injected into the valve via one or more mandrels ( e . g ., side pocket ) distributed along the production tubing . the valve controls the flow of the injection gas or fluid as it mixes with crude oil or well fluid in the production tubing . when the annulus pressure of the injection gas or fluid exceeds a predefined threshold , the valve opens to allow the injection gas or fluid to be injected into the production tubing . when the annulus pressure is below the threshold , the valve is closed , thus at least substantially preventing injection gas or fluid from being injected into the production tubing . gas lift valves can include a bellows - type actuation device that uses a combination of forces from the production tubing and annulus to regulate and selectively open or close the valve , often using a square edged orifice choke mechanism or a venturi style orifice . gas lift valves can include a reverse - flow check valve mechanism , often of the velocity check - type , to prevent well fluids from flowing in a reverse direction through the valve . however , a reverse - flow check valve mechanism can be relatively unprotected from the injection gas or fluid since they are included within the flow path , and thus can be subject to unacceptable erosion , corrosion , and other conditions that lead to gas leakage over time , causing hydrocarbons to be inadvertently released into the environment when well shut - in is required . accordingly , some embodiments described herein relate to a valve with a long - term , positive sealing system to provide systems with zero or minimal gas release when the system is closed . fig1 - 3 depict schematic diagrams of a gas lift valve 100 . fig1 shows the valve 100 in a closed position . the valve 100 includes a ball stem and bellows assembly 110 , venturi orifice 120 , hydraulic system 130 , tubular device 140 , flapper system 150 , and flow - thru latch 160 . the ball stem and bellows assembly 110 is positioned at one end of the valve 100 . the ball stem and bellows assembly 110 includes a ball stem 11 2 , which interfaces with the venturi orifice , and bellows 114 . the bellows 114 is filled with nitrogen charged gas . the ball stem 112 and bellows 114 are connected to form the ball stem and bellows assembly 110 , which is moveable as a single unit . the tip of the ball stem 112 may be positioned to interface with an entrance of the venturi orifice 120 . the position of the ball stem and bellows assembly 110 relative to the entrance of the venturi orifice 120 determines whether the valve 100 is open or closed , i . e ., whether injection gas or fluid is allowed to flow through the valve 100 . as described below in more detail , when the tip of the ball stem 112 interfaces with the entrance of the venturi orifice 120 so as to close the passageway , injection gas or fluid is prevented from flowing through the valve 100 . conversely , when the tip of the ball stem 112 is not integral with the entrance of the venturi orifice 120 , the valve is to some extent open , and injection gas or fluid may flow through the valve 100 . the venturi orifice 120 is shaped to allow pressure to be reduced at a stable rate , which is advantageous in a variety of applications , e . g ., increasing flow through the orifice . other orifices , such as a square edge orifice , may also be used . the end of the venturi orifice 120 opposing the entrance is in communication with the hydraulic system 130 . the hydraulic system 130 includes tubular device bellows 132 , 134 . the tubular device bellows 132 , 134 are filled with liquid silicon , and are in communication with each other . the hydraulic system 130 provides a force on the tubular device 140 when the tubular device bellows 132 , 134 expand and contract . other hydraulic pressure systems may be used in place of , or in addition to , the use of tubular device bellows 132 , 134 , such as a system utilizing a piston . the illustrative hydraulic system 130 utilizing tubular device bellows 132 , 134 operates like a piston . the hydraulic system 130 is bounded by a flow channel 136 , which transports the injection gas or fluid from the venturi orifice 120 to the tubular device 140 . the end of the hydraulic system 130 opposing the venturi orifice 120 is connected to the tubular device 140 . the tubular device 140 slides within the valve 100 to allow the flapper system 150 to open and close . the tubular device 140 is encased by a spring 142 , which when pressed upon , allows the tubular device 140 to translate . the spring 142 biases the tubular device 140 toward the venturi orifice 120 . when the valve 100 is in the closed position , as in fig1 , the tubular device 140 is pressed against the flapper system 150 , with the flapper system 150 blocking the flow path of the injection gas or fluid , preventing the tubular device 140 from translating along the axis of the valve 100 , and sealing the valve 100 . the flapper system 150 is a type of reverse - flow check valve mechanism , serving to prevent well fluids from flowing in a reverse direction through the valve 100 . the flapper system 150 may include a flapper 150 , soft seat 152 , and hard seat 154 . the seats 152 , 154 of the flapper system 150 are positioned outside of the flow path and tubular device 140 . thus , when the tubular device 140 is moved to the left in the figures , the flapper 150 and seats 152 , 154 are not subjected to the flow of the injection gas or fluid , which causes deterioration . in this regard , the flapper system 150 can provide a long - term , positive valve closure and sealing , with zero or minimal gas release after its closure . in the illustrative example , the flapper 152 is formed of a metallic material , and is opened and closed using a hinge . the soft seat 154 is formed of a non - metallic material , such as a polymer . the hard seat 156 is formed of a metallic material . the optional soft seat 154 allows for sealing at minimal pressure differentials . one having ordinary skill in the art will appreciate that alternative materials may be used . in the illustrative example , the primary sealing is the metal - to - metal contact between the flapper 152 and the hard seat 156 . the housing of the flapper system 150 is connected to the flow - thru latch 160 , which is positioned on the end of the valve 100 opposing the ball stem and bellows assembly 110 . when the valve 100 is in an open position , injection gas or fluid flows through the flow - thru latch and into the production tubing , where it mixes with crude oil or other fluid . the operation of the valve 100 will now be described . as described above , the illustrative valve 100 controls the flow of injection gas or fluid that is mixed with crude oil or well fluid in a production tubing to reduce the density of the crude oil or well fluid , thus enhancing the production rate of the well . the injection gas or fluid is provided to the valve 100 via an annulus between the production tubing and well . alternatively , the injection gas or fluid could be provided from control line connected with surface . the valve 100 connects to the production tubing via one or more mandrels distributed along the line . the injection gas or fluid enters the valve 100 through inlet 170 . seals 180 provide the valve 100 with an isolation area between the seals 180 , channeling the injection gas or fluid to the inlet 170 . the bellows 114 of the ball stem and bellows assembly 110 may be filled , for example , with nitrogen charged gas . when the pressure of the injected gas or fluid exceeds the pressure in the nitrogen charged bellows 114 , the nitrogen charged bellows contracts , and the ball stem 112 , moving in conjunction with the bellows 114 , is positioned so that the injection gas or fluid is able to enter the venturi orifice 120 . conversely , when the pressure of the injected gas or fluid is less than the pressure of the nitrogen charged bellows 114 , the nitrogen charged bellows 114 expands , and the ball stem 112 mates with the opening of the venturi orifice 120 , preventing the injection gas or fluid from entering the venturi orifice 120 . when the valve 100 is in the closed position , as depicted in fig1 , no injection gas or fluid flows through the venturi orifice 120 . with no flow through the venturi orifice 120 , the hydraulic system 130 is not actuated . in this state , the tubular device 140 , connected to the hydraulic system 130 , is positioned in the valve 100 towards the end with the ball stem and bellows assembly 110 , as depicted in fig1 . the flapper system 150 is closed , with the flapper 152 being in the path of the tubular device , positively sealing the valve 100 . with the flapper system 150 closed , the valve is protected from crude oil or well fluid flowing in the valve in the reverse direction from the flow path of the injection gas or fluid . fig2 depicts a schematic diagram of the valve of fig1 when the valve 100 is in a half - open position . in this state , the pressure of the injected gas or fluid exceeds the pressure in the nitrogen charged bellows 114 , moving the ball stem 112 , in conjunction with the contracted bellows 114 , away from the entrance of the venturi orifice 120 , although the pressure of the injected gas or fluid is not so great as to completely avoid obstructing the entrance . the injection gas or fluid flows through the venturi orifice 120 and actuates the hydraulic system 130 . the entrance area of the hydraulic system , operating as a piston , may include a fluid filtering system to minimize the intrusion of contaminants to the operating piston sealing systems , thereby providing an increased sealing system operational life . potential forms of filtering include sintered metal and wire mesh systems . the flow from the venturi orifice 120 causes the tubular device bellows 134 of the hydraulic system 130 to contract , thereby forcing fluid into the tubular device bellows 132 which causes the tubular device bellows 132 to expand , resulting in a net translational expansion of the bellows 132 , 134 . consequently , the hydraulic system 130 , which is connected to the tubular device 140 , forces the tubular device 140 to translate axially within the valve 200 , in the direction towards the flapper assembly 150 . after the injection gas or fluid leaves the venturi orifice and actuates the hydraulic system 130 , the injection gas or fluid disperses through a flow channel 136 encasing the hydraulic system 130 , and then recombines as it enters the tubular device 140 . the hydraulic system 130 including the tubular device bellows 134 operating as a piston and may contain one or more sealing elements or systems in one or more locations of its length . the sealing elements may be dynamic or static in nature , and may be of a metal , elastomeric , or plastic material , of a combination thereof . the sealing elements may be configured as o - rings , t - rings , or other pressure energized or non - pressure energized sealing designs . the translation of the tubular device 140 can open the flapper system 150 . alternatively , the flow can open the flapper valve . alternatively , the tubular device 140 and the flow can together open the flapper system 150 . as shown in fig2 , the valve 100 is only partially open , and so the pressure actuating the hydraulic system 130 , and the translation of the tubular device 140 , are consequently not at a maximum . accordingly , as depicted in fig2 , in this state the flapper system 150 is partially open , with the tubular device 140 forcing it open part way . the closing force of the valve 100 may be a mechanical spring or a pressure containing chamber such as a bellows or a combination thereof . an additional closure motivator is a pressure differential on the hydraulic system 130 in the direction to allow the flapper 152 to shift to the closed position via its torsion spring . while the flapper system 150 is partially open , the valve 100 is protected from crude oil or well fluid from the production tubing flowing through the valve 100 in the reverse direction because the tubular device 140 is seated integral with the housing of the valve 100 . with the flapper system partially open 150 , the injection gas or fluid is able to traverse the flow - thru latch 160 and ultimately combine with crude oil or well fluid in the production tubing . fig3 depicts a schematic diagram of the valve of fig1 when the valve 100 is in an open position . in this state , the pressure of the injected gas or fluid exceeds the pressure in the nitrogen charged bellows 114 to the extent that the ball stem 112 is positioned away from the entrance of the venturi orifice 120 to allow the injected gas or fluid to enter . as described above , the pressure of the injection gas or fluid that has traversed the venturi orifice 120 actuates the hydraulic system 130 . in this state , the combination of the tubular device bellows 132 , 134 causes the tubular device 140 to translate through to the flapper 142 and completely open the flapper system 150 . the injection gas or fluid flows through the tubular device 140 , and the valve 100 is protected from reverse - flowing crude oil or well fluid by the integral tubular device seating within the housing of the valve 100 . from the tubular device 140 , the injection gas or fluid traverses the flow - thru latch 160 and ultimately combines with crude oil or well fluid in the production tubing . fig4 - 6 depict schematic diagrams of a gas lift valve 200 according to an embodiment . fig4 shows the valve 200 in a closed position . the valve 200 includes a ball stem and bellows assembly 110 , venturi orifice 120 , flow deflecting system 230 , tubular device 140 , flapper system 150 , and flow - thru latch 160 . aside from the configuration and operation of the flow deflecting system 230 , the remaining components of the valve 200 may be identical to corresponding components described with respect to illustrative valve 100 . the exit of the venturi orifice 120 is in communication with the flow deflecting system 230 . the flow deflecting system 230 includes a flow deflector , e . g ., a dart 235 , that is shaped to obstruct / deflect the flow of the injection gas or fluid . the dart can have a rounded shape , but can also have many other profiles . the dart 235 is connected to the tubular device 140 . when the flow deflecting system 230 is subjected to the flow of the injection gas or fluid , the dart 235 provides a force on the tubular device 140 , causing it to translate axially within the valve 200 , and allowing the tubular device 140 to open and close the flapper system 150 . other flow deflecting systems may be used in place of , or in addition to , the use of the dart 235 . in fig4 , the pressure of the injected gas or fluid is less than the pressure in the nitrogen charged bellows 114 , and thus the valve 200 is closed . in this state , the ball stem 112 is mated with the entrance of the venturi orifice 120 , preventing the injection gas or fluid from flowing throughout the valve 200 . in this state , the flow deflecting system is not actuated , the tubular device is positioned towards the end of the valve 200 with ball stem and bellows assembly 110 , and the flapper system 150 is closed . fig5 depicts a schematic diagram of the valve 200 of fig4 when the valve 200 is in a half - open position . as described with respect to fig2 , in this state the pressure of the injected gas or fluid exceeds the pressure in the nitrogen charged bellows 114 , and the ball stem 112 is positioned so that the injection gas or fluid is able to enter the venturi orifice 120 , although the ball stem 112 is not completely clear from the entrance . the injection gas or fluid flows through the venturi orifice 120 , with the pressure being reduced at a stable rate , and actuates the flow deflecting system 230 . the flow deflects from dart 235 , providing the force for the tubular device 140 to translate axially within the valve 200 in the direction towards the flapper system 150 . as described above , the translation of the tubular device 140 and or the flow partially opens the flapper 152 , and the injection gas or fluid traverses the flow - thru latch 160 and ultimately combines with crude oil or well fluid in the production tubing . fig6 depicts a schematic diagram of the valve of fig4 when the valve 200 is in an open position . as described above with respect to fig3 , in this state the pressure of the injected gas or fluid exceeds the pressure of the nitrogen charged bellows 114 , and the ball stem 112 is positioned sufficiently away from the entrance of the venturi orifice 120 to allow the injected gas or fluid to enter more freely than as depicted in fig5 . as described above , the injection gas or fluid flows through the venturi orifice 120 , with the pressure being reduced to a stable rate , and actuates the flow deflecting system 230 , providing the force for the tubular device 140 to translate axially and fully open the flapper 152 , and allowing the injection gas or fluid to traverse the flow - thru latch 160 and ultimately combine with crude oil or well fluid in the production tubing . fig7 is a flow diagram depicting the flow path 300 of the injection gas or fluid as it traverses the valve 100 or valve 200 , as described above . the injection gas or fluid enters valve 100 or valve 200 through inlet 170 ( step 310 ). if the pressure of the injection gas or fluid exceeds the pressure of the nitrogen charged bellows 114 , the injection gas or fluid flows through the venturi orifice 120 ( step 320 ). if , however , the pressure of the injection gas or fluid does not exceed the pressure of the nitrogen charged bellows 114 , the injection gas or fluid does not flow through the venturi orifice 120 ( step 330 ) because the entrance is blocked by the ball stem 112 , closing the valve 100 . where the hydraulic system 130 is used , from the venturi orifice 120 the injection gas or fluid flows through flow channel 136 encasing the hydraulic system 140 ( step 340 ). where the flow deflecting system 230 is used , from the venturi orifice 120 the injection gas or fluid is deflected by and around the dart 235 ( step 350 ). in both situations , the injection gas or fluid next flows through the tubular device 140 ( step 360 ) and passes through the flapper system 150 . the injection gas or fluid then flows through the flow - thru latch 160 ( step 370 ), ultimately mixing with crude oil or well fluid in the production tubing . the illustrative valves 100 , 200 described above are able to be independently and selectively operated , with benefits similar to those of a surface controlled subsurface safety valve ( scssv ). the long - term , positive sealing flapper system 150 allows zero or minimal gas or fluid release upon closure , thereby providing a cost - effective , positive closing valve to dramatically reduce the potential for inadvertent hydrocarbon releases into the environment when well shut - in is required . moreover , the annulus pressure operated designs are retro - fitable into wells where applicable and serviceable side - pocket mandrels are present . the above embodiments and descriptions allow the illustrative valves 100 , 200 to open and close via an applied pressure and independently of a choke , or choke - like , flow - entering , pressure differential device . moreover , the valves 100 , 200 can use hydraulic pressure applied to either open or close the valves via one or more control lines or conduits that are connected from a hydraulic power source through independent conduits to effect movement of a piston assembly integral to the valve , which either moves the mechanism to the open or closed position depending upon the conduit selected or the count of the pressure cycles on the conduit . the valves 100 , 200 can operate from a down hole casing pressure source or from a single or dual control line surface controlled conduit . the valve system can be used in all standard wireline retrievable gas lift configurations and is capable of installation in typical industry standard side pocket mandrels . the system can be installed in all standard gas lift completion configurations . while various embodiments have been described , it will be apparent to those of skill in the art that many more embodiments and implementations are possible .