Abstract:
Method of operating a hydrogen dispensing where pressure relief is provided through block and bleed valves. After dispensing hydrogen from a dispensing station where the hydrogen is cooled during dispensing, trapped hydrogen remains in the transfer lines. During the idle time between refueling vehicles, the temperature of the trapped hydrogen increases resulting in an increase in the pressure of the trapped hydrogen. Block and bleed valves operate to relieve the pressure in the transfer lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional application U.S. Ser. No. 62/221,425, titled “Method of Operating a Hydrogen Dispensing Unit,” filed Sep. 21, 2015, and provisional application U.S. Ser. No. 62/245,007, titled “Method of Operating a Hydrogen Dispensing Unit,” filed Oct. 22, 2015, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to a method of operating a hydrogen dispensing unit. The present invention more particularly relates to a method of operating a hydrogen dispensing unit where the hydrogen is cooled prior to dispensing the hydrogen to a hydrogen storage tank in a vehicle. 
     Hydrogen dispensing units are used to dispense high pressure hydrogen into hydrogen storage tanks in vehicles such as cars, buses, trucks, and forklifts. 
     There is growing interest to use hydrogen as a transportation fuel in cars, buses, trucks, and other vehicles. Hydrogen is generally stored in a fuel tank on-board the vehicles at high pressure. After most of the on-board hydrogen has been depleted, the pressure of the hydrogen in the fuel tank is reduced and the fuel tank must be refueled with hydrogen. 
     During refueling, hydrogen is dispensed into the fuel tank at a hydrogen dispensing station. The dispensing station includes a hydrogen supply, which can be one or more high pressure storage tanks. Hydrogen is transferred from the high pressure storage tank into the fuel tank of the vehicle. The driving force for transferring hydrogen is the pressure difference between the high pressure storage tank and the vehicle fuel tank. 
     Dispensing from a high pressure supply vessel to the lower pressure receiving vessel in a vehicle results in a temperature increase of the hydrogen that was dispensed. To compensate for the temperature rise of the dispensed hydrogen, hydrogen dispensing stations may include one or more heat exchangers to cool the hydrogen as it is being dispensed. Cooling may be provided by a refrigerant in a refrigeration cycle. The heat exchanger may comprise one or more high thermal capacity cooling blocks, which are cooled by the refrigerant and through which the hydrogen passes and is cooled. 
     The transfer line between the high pressure storage tank and the fuel tank typically includes various control valves, and block and bleed valves. A block valve blocks the flow from the high pressure storage tank and a bleed valve allows a portion of the hydrogen trapped between the block valve and the dispensing nozzle to discharge, thereby reducing the pressure at the dispensing nozzle. Accepted standards, such as SAE J2600 and ISO 17268, require that the pressure at the nozzle be less than 0.5 MPa (gauge) before the dispensing nozzle can be disconnected from the fueling receptacle on the vehicle. 
     While the transfer line between the block valve and the dispensing nozzle will be at a lower pressure after dispensing hydrogen to a vehicle, the residual hydrogen trapped in the transfer lines between the control valve and one or more block valves will still be at high pressure and a cold temperature because of being cooled in the heat exchanger. 
     As the hydrogen dispension unit sits idle waiting to refuel another vehicle, the temperature of the residual trapped hydrogen will increase resulting in a pressure increase in the lines. The resulting pressure increase may be greater than design pressure limits for the equipment. The resulting high pressure may cause the dispensing unit to trip because of protections configured to protect the vehicle from excessive pressure, or because the excessive pressure might be interpreted by the controller as some sort of pressure transducer failure. Pressure relief valves could be used to relieve the pressure when it exceeds safe limits, but pressure relief valves are known for failing to completely reseal after relieving the pressure. Use of pressure relief valves on a frequent basis is undesirable. 
     Industry desires reliable hydrogen dispensing units. 
     BRIEF SUMMARY 
     The present invention relates to a method for operating a hydrogen dispensing unit. 
     There are several aspects of the invention as outlined below. In the following, specific aspects of the invention are outlined. The reference numbers and expressions set in parentheses are referring to an example embodiment explained further below with reference to  FIG. 1 . The reference numbers and expressions are, however, only illustrative and do not limit the aspect to any specific component or feature of the example embodiment. The aspects can be formulated as claims in which the reference numbers and expressions set in parentheses are omitted or replaced by others as appropriate. 
     Aspect 1. A method of operating a hydrogen dispensing unit ( 1 ) comprising:
         dispensing hydrogen from a supply vessel ( 102 ) to a receiving vessel ( 118  or  218 ) via the hydrogen dispensing unit ( 1 ), the hydrogen dispensing unit ( 1 ) comprising a heat exchanger ( 106 ) to cool the hydrogen prior to the hydrogen being dispensed into the receiving vessel ( 118  or  218 ), said dispensing continuing until a target quantity of hydrogen is dispensed and thereupon terminating said dispensing;   wherein upon terminating said dispensing, a first quantity of hydrogen is trapped within a first one or more conduits ( 130 ,  132 ,  232 ,  233 ), the first one or more conduits operatively connecting a plurality of valves, the plurality of valves including a control valve ( 104 ) and a block valve ( 108  or  208 ), said first quantity of hydrogen being trapped upon closing said plurality of valves, at least a portion of the first quantity of hydrogen having been cooled in said heat exchanger ( 106 ), the first quantity of hydrogen exerting a pressure in the first one or more conduits ( 130 ,  132 ,  232 ,  233 );   measuring the pressure of the first quantity of hydrogen in the first one or more conduits ( 130 ,  132 );   opening and subsequently closing the block valve ( 108  or  208 ) when the pressure of the first quantity of hydrogen equals or exceeds a selected pressure thereby removing a fraction of the first quantity of hydrogen from the first one or more conduits ( 130 ,  132 ,  232 ,  233 ) and transferring the fraction of the first quantity to a second one or more conduits ( 134 ,  136  or  234 ,  236 ), the second one or more conduits ( 134 ,  136  or  234 ,  236 ) operatively connected to the block valve ( 108  or  208 ) and a bleed valve ( 114  or  214 ); and   opening and subsequently closing the bleed valve ( 114  or  214 ) thereby discharging a first quantity of vented hydrogen from the second one or more conduits ( 134 ,  136  or  234 ,  236 ).       

     Aspect 2. The method of aspect 1 wherein the first quantity of vented hydrogen comprises at least a portion of the fraction of the first quantity of hydrogen. 
     Aspect 3. The method of aspect 1 or aspect 2 wherein during the steps of opening and subsequently closing the block valve ( 108  or  208 ) and opening and subsequently closing the bleed valve ( 114  or  214 ), the block valve ( 108  or  208 ) is opened at the same time as the bleed valve ( 114  or  214 ) is opened and the block valve ( 108  or  208 ) closed at the same time as the bleed valve ( 114  or  214 ) is closed. 
     Aspect 4. The method of aspect 1 or aspect 2 wherein during the steps of opening and subsequently closing the block valve ( 108 ) and opening and subsequently closing the bleed valve ( 114 ), the bleed valve ( 114 ) is opened and subsequently closed after the block valve ( 108 ) is opened and subsequently closed. 
     Aspect 5. The method of any one of the preceding aspects wherein at least a portion of the first quantity of hydrogen has an initial temperature less than −17.5° C. or less than −33° C. upon first being trapped. 
     Aspect 6. The method of any one of the preceding aspects wherein the control valve ( 104 ) is a pressure control valve, programmable pressure regulator, or a dome loaded regulator. 
     Aspect 7. The method of any one of the preceding aspects wherein upon terminating said step of dispensing, a second quantity of hydrogen is trapped within the second one or more conduits ( 234 ,  236 ), the second one or more conduits ( 234 ,  236 ) operatively connecting the block valve ( 208 ), the bleed valve ( 214 ), and a second block valve ( 226 );
         wherein the first quantity of vented hydrogen comprises a fraction or all of the second quantity of hydrogen.       

     Aspect 8. The method of any one of aspects 1 to 6 wherein upon terminating said step of dispensing, a second quantity of hydrogen is trapped within the second one or more conduits ( 234 ,  236 ), the second one or more conduits ( 234 ,  236 ) operatively connecting the block valve ( 208 ), the bleed valve ( 214 ), and a second block valve ( 226 ), the second quantity of hydrogen exerting a pressure in the second one or more conduits ( 234 ,  236 ), the method further comprising:
         measuring the pressure of the second quantity of hydrogen in the second one or more conduits ( 234 ,  236 ); and   opening and subsequently closing the bleed valve ( 214 ) when the pressure of the second quantity of hydrogen equals or exceeds a selected pressure thereby discharging a fraction or all of the second quantity of hydrogen from the second one or more conduits ( 234 ,  236 ).       

     Aspect 9. The method of aspect 7 or aspect 8 wherein upon terminating said step of dispensing, a third quantity of hydrogen is trapped within a third one or more conduits ( 235 ), the third one or more conduits ( 235 ) operatively connecting the second block valve ( 226 ) and a dispensing nozzle ( 210 ) having an internal valve, the third quantity of hydrogen exerting a pressure in the third one or more conduits ( 235 ), the method further comprising;
         measuring the pressure of the third quantity of hydrogen in the third one or more conduits ( 235 ); and   opening and subsequently closing the second block valve ( 226 ) when the pressure of the third quantity of hydrogen equals or exceeds a selected pressure thereby removing a fraction of the third quantity of hydrogen from the third one or more conduits ( 235 ) and transferring the fraction of the third quantity to the second one or more conduits ( 234 ,  236 ); and   opening and subsequently closing the bleed valve ( 214 ) thereby discharging at least a portion of the fraction of the third quantity of hydrogen from the second one or more conduits ( 234 ,  236 ).       

     Aspect 10. The method of aspect 7 or aspect 8 wherein upon terminating said step of dispensing, a third quantity of hydrogen is trapped within a third one or more conduits ( 235 ), the third one or more conduits ( 235 ) operatively connecting the second block valve ( 226 ) and a dispensing nozzle ( 210 ) having an internal valve, the method further comprising:
         opening and subsequently closing the second block valve ( 226 ) thereby removing a fraction of the third quantity of hydrogen from the third one or more conduits ( 235 ) and transferring the fraction of the third quantity to the second one or more conduits ( 234 ,  236 ),   wherein during the steps of opening and subsequently closing the second block valve ( 226 ) and opening and subsequently closing the block valve ( 208 ), the second block valve ( 226 ) is opened at the same time as the block valve ( 208 ) is opened and the second block valve ( 226 ) is closed at the same time as the block valve ( 208 ) is closed.       

     Aspect 11. The method of aspect 10 wherein during the steps of opening and subsequently closing the block valve ( 208 ) and opening and subsequently closing the bleed valve ( 214 ), the block valve ( 208 ) is opened at the same time as the bleed valve ( 214 ) is opened and the block valve ( 208 ) closed at the same time as the bleed valve ( 214 ) is closed. 
     Aspect 12. The method of any one of the preceding aspects wherein the receiving vessel is a first receiving vessel of a series of receiving vessels, the method comprising:
         connecting and disconnecting the hydrogen dispensing unit ( 1 ) to and from the first receiving vessel ( 118  or  218 ) of the series of receiving vessels ( 118  or  218 ); and   discharging the first quantity of vented hydrogen through the bleed valve ( 114  or  214 ) after having dispensed hydrogen to the first receiving vessel ( 118  or  218 ) of the series of receiving vessels ( 118  or  218 ) and before dispensing hydrogen to a subsequent second receiving vessel of the series of receiving vessels ( 118  or  218 ).       

     Aspect 13. The method of any one of the preceding aspects wherein the first quantity of hydrogen is trapped within the first one or more conduits ( 130 ,  132 ,  232 ,  233 ) between the control valve ( 104 ) and the block valve ( 108  or  208 ). 
     Aspect 14. The method of any one of the preceding aspects wherein the control valve ( 104 ) is disposed upstream of the heat exchanger ( 106 ). 
     Aspect 15. The method of any one of the preceding aspects wherein the block valve ( 108  or  208 ) is disposed downstream of the heat exchanger ( 106 ). 
     Aspect 16. The method of any one of the preceding aspects comprising limiting the pressure of the hydrogen with a pressure regulator ( 206 ) disposed in the first one or more conduits ( 232 ,  233 ) between the control valve ( 104 ) and the block valve ( 208 ). 
     Aspect 17. A method for determining leakage in a control valve ( 104 ) of a hydrogen dispensing unit ( 1 ), the method comprising:
         dispensing hydrogen from a supply vessel ( 102 ) to a receiving vessel ( 118  or  218 ) via the hydrogen dispensing unit ( 1 ), the hydrogen dispensing unit ( 1 ) comprising a heat exchanger ( 106 ) to cool the hydrogen prior to the hydrogen being dispensed into the receiving vessel ( 118  or  218 ), said dispensing continuing until a target quantity of hydrogen is dispensed and thereupon terminating said dispensing;   wherein upon terminating said dispensing, a first quantity of hydrogen is trapped within a first one or more conduits ( 130 ,  132 ,  232 ,  233 ), the first one or more conduits operatively connecting a plurality of valves, the plurality of valves including a control valve ( 104 ) and a block valve ( 108  or  208 ), said first quantity of hydrogen being trapped upon closing said plurality of valves, at least a portion of the first quantity of hydrogen having been cooled in said heat exchanger ( 106 ), the first quantity of hydrogen exerting a pressure in the first one or more conduits ( 130 ,  132 ,  232 ,  233 );   measuring the pressure of the first quantity of hydrogen in the first one or more conduits ( 130 ,  132 ) thereby determining a measured pressure increase;   comparing the measured pressure increase with an expected pressure increase (due to the temperature rise); and   determining whether the control valve ( 104 ) is leaking responsive to comparing the measured pressure increase with the expected pressure increase.       

     Aspect 18. The method of the preceding aspect wherein the receiving vessel is a first receiving vessel of a series of receiving vessels, the method comprising:
         connecting and disconnecting the hydrogen dispensing unit ( 1 ) to and from the first receiving vessel ( 118  or  218 ) of the series of receiving vessels ( 118  or  218 ); and   determining whether the control valve ( 104 ) is leaking responsive to comparing the measured pressure increase with the expected pressure increase after having dispensed hydrogen to the first receiving vessel of the series of receiving vessels ( 118  or  218 ) and before dispensing hydrogen to a subsequent second receiving vessel of the series of receiving vessels ( 118  or  218 ).       

     Aspect 19. The method of any one of the preceding aspects wherein the receiving vessel ( 118  or  218 ) is a fuel tank of a land vehicle such as a car, bus, truck, motorcycle, forklift, agricultural vehicle, construction machine, and a locomotive, or of an aircraft. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       The FIGURE shows a process flow diagram for a hydrogen dispensing unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention, it being understood that various changes may be made in the function and arrangement of elements without departing from scope of the invention as defined by the claims. 
     The articles “a” and “an” as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used. 
     The adjective “any” means one, some, or all indiscriminately of whatever quantity. 
     The term “and/or” placed between a first entity and a second entity means one of (1) the first entity, (2) the second entity, and (3) the first entity and the second entity. The term “and/or” placed between the last two entities of a list of 3 or more entities means at least one of the entities in the list including any specific combination of entities in this list. 
     The term “plurality” means “two or more than two.” 
     The phrase “at least a portion” means “a portion or all.” 
     As used herein, “first,” “second,” “third,” etc. are used to distinguish from among a plurality of steps and/or features, and is not indicative of the total number, or relative position in time and/or space unless expressly stated as such. 
     As used herein, “in fluid flow communication” means operatively connected by one or more conduits, manifolds, valves and the like, for transfer of fluid. A conduit is any pipe, tube, passageway or the like, through which a fluid may be conveyed. An intermediate device, such as a pump, compressor or vessel may be present between a first device in fluid flow communication with a second device unless explicitly stated otherwise. 
     For the purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
     As used herein, pressures are gauge pressures unless explicitly stated otherwise. 
     The sole FIGURE is a process flow diagram for describing the method. The process flow diagram includes the dispensing unit  1  and receiving tanks  118  and  218  for two respective vehicles. The FIGURE shows two dispensing legs, one having a configuration suitable for dispensing hydrogen to moderate pressure (e.g. 35 MPa) and another having a configuration suitable for dispensing hydrogen to high pressure (e.g. 70 MPa). The configuration for dispensing to moderate pressure is shown for dispensing to receiving tank  218  and the configuration for dispensing to high pressure is shown for dispensing to receiving tank  118 . The present invention is suitable for a hydrogen dispensing unit having one, two, or more dispensing legs. 
     The hydrogen dispensing unit comprises one or more hydrogen storage tanks  102 . The one or more hydrogen storage tanks can be any hydrogen storage tanks known in the art. The one or more hydrogen storage tanks may include a plurality of storage tanks suitable for cascade filling. 
     The hydrogen dispensing unit comprises a control valve  104  operatively connected to the one or more storage tanks  102  via a transfer conduit  103 . Control valve  104  may be a pressure control valve, programmable pressure regulator, or a dome loaded regulator. Control valve  104  controls the rate of transferring hydrogen from the one or more storage tanks  102  to the receiving tank  118  or receiving tank  218  depending on which dispensing leg is being used. The control valve  104  may control the rate of transferring hydrogen as a pressure ramp rate (i.e. change in pressure per unit time). The control valve  104  is operatively connected to the controller  120  and receives control signals from controller  120 . 
     The hydrogen dispensing unit comprises a heat exchanger  106  operatively connected to the control valve  104  via a transfer conduit  130 . Heat exchanger  106  cools the hydrogen as it is being dispensed from the one or more storage tanks  102  to the receiving tank  118  or receiving tank  218 . The heat exchanger  106  may include a thermal ballast (thermal capacitor) such as an aluminum block as known from US 2008/0185068. Multiple cooling blocks, as known from US 2014/0007975 may be used. A pressure relief valve  105  may be connected to conduit  130  to relieve the pressure in conduit  130  should the pressure exceed a maximum allowable pressure. The pressure leaving the heat exchanger  106  may be measured using a pressure sensor  107 . Pressure sensor  107  may be used during the controlled dispensing of the hydrogen, and to detect if the pressure in conduit  132  exceeds a desired maximum pressure. 
     For the high pressure (e.g. 70 MPa) dispensing leg, the hydrogen dispensing unit comprises a block valve  108  and a bleed valve  114  operatively connected to the control valve  104  via transfer conduits  132 ,  134 , and  136 . Block valve  108  may be constructed such that if it fails, it fails in the closed position. Bleed valve  114  may be constructed such that if it fails, it fails in the open position. The high pressure dispensing leg comprises a dispensing nozzle  110  operatively connected to the block valve  108 . The dispensing nozzle  110  can be any dispensing nozzle known in the art for hydrogen fuelling, particularly one suited for dispensing hydrogen to 70 MPa. 
     The block valve  108  in combination with the bleed valve  114  are used to reduce the pressure in the dispensing nozzle  110  prior to disconnecting the dispensing nozzle  110  from the receptacle  112  of the vehicle as is known in the art of hydrogen fueling. It may be desired to maintain the pressure in transfer conduits  134  and  136  at about 0.3 MPa during idle periods. A pressure sensor  116  may be used to measure the pressure in the transfer conduits  134  and  136 . 
     For the moderate pressure (e.g. 35 MPa) dispensing leg, the hydrogen dispensing unit may comprise a pressure regulator  206 , a block valve  208 , a bleed valve  214 , and a second block valve  226  operatively connected to the control valve  104  via transfer conduits  232 ,  233 ,  234 , and  236 . The pressure regulator  206  may limit the pressure to the moderate pressure dispensing leg to about 42 MPa. The moderate pressure dispensing leg comprises a dispensing nozzle  210  operatively connected to the block valve  226 . The dispensing nozzle  210  can be any dispensing nozzle known in the art for hydrogen fueling, particularly one suited for dispensing hydrogen to 35 MPa. 
     For the moderate pressure (35 MPa) dispensing leg, the dispensing nozzle  210  may comprise an internal block and bleed valve manifold. The pressure at the nozzle may be relieved to a desired pressure before disconnecting from receptacle  212  and the pressure in the conduits  234 ,  235 ,  236  may be maintained at a pressure ranging from 35 MPa to 42 MPa during idle periods. 
     Pressure sensors  238  and  216  may be used to measure the pressure in the respective transfer conduits. A pressure relief valve  228  and associated pressure sensor/transmitter  216  may be connected to conduit  235 . The pressure relief valve  228  may be set to relieve gas at any desired pressure for example a pressure ranging from 46 MPa to 50 MPa. 
     A block valve is any valve that is capable of blocking the flow in both directions. Any suitable block valve known in the art of hydrogen fueling may be used. 
     A bleed valve is any device that is capable of bleeding off a gas from the conduit to vent the conduit. Any suitable bleed valve known in the art of hydrogen fueling may be used. 
     The hydrogen dispensing unit comprises a controller  120  operatively connected to the control valve  104 , the various block valves, bleed valves, and pressure sensors. The controller may be a computer, process logic controller (PLC), or the like. Controllers are ubiquitous in the art of hydrogen dispensing. The controller  120  may receive signals from the pressure sensors  107 ,  116 ,  238  and  216  and send control signals to the block valves  108 ,  208 , and  226  and bleed valves  114  and  214 . 
     Hydrogen-fueled vehicles comprise a receiving tank  118  or  218 , and a respective receptacle  112  or  212 . Receiving tank  118  and receptacle  112  may be for receiving high pressure hydrogen gas (e.g. up to 70 MPa) and receiving tank  218  and receptacle  212  may be for receiving moderate pressure hydrogen gas (e.g. up to 35 MPa). Receiving tanks may have internal pressure sensors for measuring the pressure of the hydrogen contained within the respective receiving tank. The pressure sensor may communicate wirelessly with controller  120 . 
     Prior to dispensing hydrogen, the dispensing nozzle is connected to the receptacle of the vehicle, for example dispensing nozzle  110  to receptacle  112  or dispensing nozzle  210  to receptacle  212 . 
     The method is first described for the high pressure (70 MPa) dispensing leg and is applicable for any dispensing leg having a similar configuration. 
     The method comprises dispensing hydrogen from the one or more supply vessels  102  to receiving vessel  118  via the hydrogen dispensing unit  1 . The flow rate of hydrogen is controlled using control valve  104 . The hydrogen is passed through heat exchanger  106  to cool the hydrogen prior to the hydrogen being dispensed into the receiving vessel  118 . Dispensing continues until a target quantity of hydrogen is dispensed, and after the target quantity is transferred, dispensing is terminated. The target quantity may be set by the target pressure for the receiving vessel  118  such that dispensing is terminated upon reaching a target pressure. 
     After the receiving tank  118  receives the target quantity, for example by reaching the target pressure, the flow is stopped by closing block valve  108 . Then the pressure in the transfer line between block valve  108  and the dispensing nozzle  110  is reduced, for example to about 0.3 MPa by bleeding off at least a portion of the residual gas in the transfer line via bleed valve  114 . When the pressure at the dispensing nozzle is sufficiently reduced, the dispensing nozzle is disconnected from the receptacle  112 . 
     Upon terminating dispensing, a first quantity of cold hydrogen is trapped within conduits  130  and  132  between the control valve  104  and the block valve  108 . In order to prevent H 2 -containing gas losses, this residual H 2 -containing gas is not vented. The first quantity of hydrogen is trapped upon closing valves  104  and  108  and was cooled in heat exchanger  106  and therefore has a temperature of, for example, less than about −17.5° C. or less than about −33° C. The first quantity of exerts a pressure in conduits  130  and  132 . 
     As the hydrogen dispensing unit sits idle waiting to fill another vehicle fuel tank, the temperature of the first quantity hydrogen in conduits  130  and  132  will increase. As the temperature rises, so too does the pressure. At the end of dispensing, the pressure in the transfer lines of the dispensing unit could be about 76 to 80 MPa. If the dispensing terminates with hydrogen at 80 MPa in the transfer lines and the temperature of the hydrogen rises from −33° C. to +25° C., the pressure increases to about 100 MPa, which is far above the maximum allowable pressure that can be transferred to a vehicle with a maximum pressure rating of 87.5 MPa. 
     The method comprises measuring the pressure of the first quantity of hydrogen in conduits  130  and  132 , for example with pressure sensor  107 , as the temperature and pressure of the first quantity of hydrogen increases. The pressure sensor  107  is in signal communication with controller  120  and transmits a signal representative of the pressure to controller  120 . 
     When the pressure of the first quantity of hydrogen equals or exceeds a selected pressure, for example a pressure ranging from 70 MPa to 87.5 MPa, the controller provides signal instructions to block valve  108  to open and subsequently close block valve  108  while control valve  104  is kept closed thereby removing a fraction of the first quantity of hydrogen from conduits  130  and  132  and transferring the fraction of the first quantity to conduits  134  and  136 . As a result, the pressure of the gas in conduits  130  and  132  is decreased. 
     Conduits  134  and  136  are operatively connected to bleed valve  114 . The method comprises opening and subsequently closing the bleed valve  114  thereby discharging at least a portion of the fraction of the first quantity of hydrogen from conduits  134  and  136 . 
     Block valve  108  and bleed valve  114  may be opened simultaneously and closed simultaneously. Alternatively, block valve  108  may be opened and subsequently closed, followed by bleed valve  114  being opened and subsequently closed. 
     The method is now described for the moderate pressure (35 MPa) dispensing leg and is applicable for any dispensing leg having a similar configuration. 
     The method comprises dispensing hydrogen from the supply vessel  102  to receiving vessel  218  via the hydrogen dispensing unit  1 . The hydrogen is passed through heat exchanger  106  to cool the hydrogen prior to the hydrogen being dispensed into the receiving vessel  218 . Dispensing continues until a target quantity of hydrogen is dispensed, and after the target quantity is transferred, dispensing is terminated. The target quantity may be set by the target pressure for the receiving vessel  218  such that dispensing is terminated upon reaching the target pressure. 
     Upon terminating dispensing, a first quantity of cold hydrogen is trapped within conduits  130 ,  232  and  233 , a second quantity of hydrogen is trapped within conduits  234  and  236 , and a third quantity of hydrogen is trapped within conduit  235 . The first quantity of hydrogen is trapped upon closing valves  104  and  208  and was cooled in heat exchanger  106  and therefore has a temperature of, for example, less than about −17.5° C. or less than about −33° C. The first quantity of exerts a pressure in conduits  130 ,  232 , and  233 . The second quantity of hydrogen is trapped upon closing valves  208  and  226  and was also cooled in heat exchanger  106  and therefore has a temperature of, for example, less than about −17.5° C. or less than about −33° C. The second quantity of exerts a pressure in conduits  234 , and  236 . The third quantity of hydrogen is trapped upon closing block valve  226  and the dispensing nozzle  210 . Dispensing nozzle  210  has an internal valve. The third quantity of hydrogen was also cooled in heat exchanger  106  and therefore has a temperature of, for example, less than about −17.5° C. or less than about −33° C. The third quantity of hydrogen exerts a pressure in conduit  235 . 
     As the hydrogen dispensing unit sits idle waiting to fill another vehicle fuel tank, the temperature of the first quantity hydrogen in conduits  130 ,  232 , and  233  will increase, the temperature of the second quantity of hydrogen in conduits  234  and  236  will increase, and the temperature of the third quantity of hydrogen in conduit  235  will increase. As the temperature of the hydrogen rises, so too does the pressure of the hydrogen. 
     The method comprises measuring the pressure of the first quantity of hydrogen in conduits  130 ,  232 , and  233 , for example with pressure sensor  107 , as the temperature and pressure of the first quantity of hydrogen increases. The pressure sensor  107  is in signal communication with controller  120  and transmits a signal representative of the pressure to controller  120 . 
     When the pressure of the first quantity of hydrogen equals or exceeds a first selected pressure, for example a pressure ranging from 70 MPa to 87.5 MPa, the controller provides signal instructions to block valve  208  to open and subsequently close block valve  208  while control valve  104  is kept closed thereby removing a fraction of the first quantity of hydrogen from conduits  130 ,  232 , and  233  and transferring the fraction of the first quantity to conduits  234  and  236 . As a result, the pressure of the gas in conduits  130 ,  232 , and  233  is decreased. 
     In the example embodiment control valve  104  is closed upon terminating dispensing and kept closed during discharging trapped hydrogen. In alternative embodiments an additional blocking means such as a block valve may be disposed between supply vessel  102  and control valve  104  or between control valve  104  and heat exchanger  106 . In those embodiments the additional blocking means may be closed upon terminating dispensing and kept closed during discharging trapped hydrogen while control valve  104  is kept open, for example, in a minimum flow position. 
     Conduits  234  and  236  are operatively connected to a bleed valve  214 . The method comprises opening and subsequently closing the bleed valve  214  thereby discharging at least a portion of the fraction of the first quantity of hydrogen and a fraction or all of the second quantity of hydrogen from conduits  234  and  236 . 
     Bleed valve  214  may be opened before block valve  208  is opened or simultaneously with block valve  208  or after block valve  208  has been opened to remove a quantity of vented hydrogen from conduits  130  and  132 . The quantity of vented hydrogen may comprise at least a portion of the fraction of the first quantity of hydrogen in case the bleed valve  214  and block valve  208  are opened simultaneously or bleed valve  214  is opened after block valve  208 . In case bleed valve  214  is opened before block valve  208 , a portion of the hydrogen trapped in conduits  234  and  236  may be vented as the quantity of vented hydrogen thereby providing capacity for a fraction of the first quantity of hydrogen to be transferred to the second one or more conduits  234  and  236 . 
     After removal of the quantity of vented hydrogen, bleed valve  114  may be closed simultaneously with block valve  108  or after block valve  108  has been closed. For example, block valve  208  and bleed valve  214  may be opened simultaneously and closed simultaneously, or aternatively, block valve  208  may be opened and subsequently closed, followed by bleed valve  214  being opened and subsequently closed. In case block valve  208  and bleed valve  214  are operated sequentially, the pressure in conduits  234  and  236  may be lower than in  232  and  233  for the transfer of the fraction of the first quantity of hydrogen due to an earlier discharge of hydrogen from bleed valve  214 . 
     The method may further comprise measuring the pressure of the second quantity of hydrogen in conduits  234  and  236 , for example using pressure sensor  238  as the temperature of the second quantity of hydrogen increases. When the pressure of the second quantity of hydrogen equals or exceeds a second selected pressure, for example a pressure ranging from 35 to 44, the controller provides signal instructions to bleed valve  214  to open and subsequently close thereby discharging a fraction or all of the second quantity of hydrogen from conduits  234  and  236 . As a result, the pressure of the hydrogen in conduits  234  and  236  is decreased. The second selected pressure may be the same or different from the first selected pressure and, if different, may be lower than the first selected pressure. 
     The method may comprise opening and subsequently closing block valve  226  thereby removing a fraction of the third quantity of hydrogen from conduit  235  and transferring a fraction of the third quantity of hydrogen to conduits  234  and  236 . Block valve  208  and block valve  226  may be opened simultaneously and closed simultaneously. Further, bleed valve  214  may be opened simultaneously with the opening of block valve  208  and block valve  226 , and bleed valve  215  may be closed simultaneously with the closing of block valve  208  and block valve  226  to vent the trapped hydrogen. 
     The method may further comprise measuring the pressure of the third quantity of hydrogen in conduit  235 , for example using pressure sensor  216  as the temperature of the third quantity of hydrogen increases. When the pressure of the third quantity of hydrogen equals or exceeds a third selected pressure, for example a pressure ranging from 35 to 44, the controller provides signal instructions to block valve  226  to open and subsequently close thereby removing a fraction of the third quantity of hydrogen from conduit  235  and transferring the fraction of the third quantity of hydrogen to conduits  234  and  236 . As a result, the pressure of the hydrogen in conduit  235  is decreased. The third selected pressure may be the same or different than the first selected pressure and/or the second selected pressure. 
     The method may then further comprise opening a closing the bleed valve  214  thereby discharging at least a portion of the fraction of the third quantity of hydrogen from conduits  234  and  236 . 
     In another embodiment, the invention relates to a method for determining leakage in a control valve  104  of the hydrogen dispensing unit. 
     The leak detection method comprises dispensing hydrogen from the one or more supply vessels  102  to receiving vessel  118  via the hydrogen dispensing unit  1 . The flow rate of hydrogen is controlled using control valve  104 . The hydrogen is passed through heat exchanger  106  to cool the hydrogen prior to the hydrogen being dispensed into the receiving vessel  118 . Dispensing continues until a target quantity of hydrogen is dispensed, and after the target quantity is transferred, dispensing is terminated. The target quantity may be set by the target pressure for the receiving vessel  118  such that dispensing is terminated upon reaching a target pressure. 
     After the receiving tank  118  reaches a target pressure, the flow is stopped by closing block valve  108 . Then the pressure in the transfer line between block valve  108  and the dispensing nozzle  110  is reduced, for example to about 0.3 MPa by bleeding off at least a portion of the residual gas in the transfer line via bleed valve  114 . When the pressure at the dispensing nozzle is sufficiently reduced, the dispensing nozzle is disconnected from the receptacle  112 . 
     Upon terminating dispensing, a first quantity of cold hydrogen is trapped within conduits  130  and  132  between the control valve  104  and the block valve  108 . In order to prevent H 2 -containing gas losses, this residual H 2 -containing gas is not vented. The first quantity of hydrogen is trapped upon closing valves  104  and  108  and was cooled in heat exchanger  106  and therefore has a temperature of, for example, less than −17.5° C. or less than −33° C. The first quantity of exerts a pressure in conduits  130  and  132 . 
     As the hydrogen dispensing unit sits idle waiting to fill another vehicle fuel tank, the temperature of the first quantity hydrogen in conduits  130  and  132  will increase. As the temperature rises, so too does the pressure. 
     The leak detection method comprises measuring the pressure of the first quantity of hydrogen in conduits  130  and  132 , for example with pressure sensor  107 , as the temperature and pressure of the first quantity of hydrogen increases thereby determining a measured pressure increase. The pressure sensor  107  is in signal communication with controller  120  and transmits a signal representative of the pressure to controller  120 . 
     The leak detection method comprises comparing the measured pressure increase with an expected pressure increase. From the initial pressure in the conduits  130  and  132  and the initial temperature, an expected pressure increase can be calculated for an expected temperature rise of the trapped hydrogen. 
     The leak detection method comprises determining whether the control valve ( 104 ) is leaking responsive to comparing the measured pressure increase with the expected pressure increase. The comparison may be done using controller  120 .