Abstract:
A liquid natural gas tank (LNG) pressure management system is provided. The system includes an LNG bulk tank, an LNG fuel tank operably connected to the LNG bulk tank, a compressed natural gas (CNG) accumulator operably connected to the LNG fuel tank and a liquid natural gas cryogenic pump operably connected to the liquid natural gas fuel tank and the liquid natural gas bulk tank. The pressure management system is configured to direct boil off gas (BOG) to the compressed natural gas accumulator upon a predetermined condition.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to liquid natural gas fuel tanks and more particularly to a system and method for nun aging boil-off gas in a liquid natural gas fuel tank using a compressed natural gas accumulator. 
       BACKGROUND 
       [0002]    Natural gas is one of the cleanest burning fuels presently known. Moreover, the current cost of natural gas makes it a cost effective alternative to other fuels due to its easy access and long term availability. Natural gas is commonly stored and/or delivered in two different forms, compressed natural gas (CNG) and liquefied natural gas (LNG). 
         [0003]    The use of CNG as a fuel for motor vehicles has been known for many years and has become increasingly popular in many areas of the world especially as gasoline prices rise. One obstacle to the use of the compressed natural gas vehicles is that compressed natural gas vehicles require a greater amount of space for fuel storage than conventional gasoline powered vehicles. Since it is a compressed gas, rather than a liquid like gasoline, CNG takes up more space for each unit of gasoline equivalence. Therefore, the tanks used to store the CNG usually take up greater space in a vehicle, which runs on CNG compared to a conventional gasoline tank. 
         [0004]    Natural gas, however, can be liquefied as LNG and stored in a specially designed insulated cryogenic tank, The LNG can then he heated to CNG on hoard the vehicle and the CNG can be used to fuel the engine. The primary advantage of LNG compared to CNG is that it can be stored at a relatively low pressure (20 to 150 psi) at about one-third the volume and one-third the weight of an equivalent CNG storage tank system. The major disadvantage is the need to deal with the storage and handling of a cryogenic (−160° C., −260° F.) fluid through the entire process of bulk transport and transfer to fleet storage. 
         [0005]    For example, one challenge associated with a natural gas or a diesel/natural gas dual fuel powered systems is the amount of time required to fill the LNG fuel tank and empty the LNG fuel tank at the LNG fuel station. Boil off of LNG causes excessive pressure buildup in LNG fuel tanks. Gaseous natural gas or boil off gas (BOG) accumulates at the top of the LNG fuel tank during fuel transfer, which increases filling time and reduces the volume in the LNG fuel tank available to hold LNG used as fuel for the system. A similar problem occurs when the LNG fuel tank is emptied for service reasons. 
         [0006]    Boil off of LNG is caused by heat addition to the LNG fuel during storage and fueling. Heat can transfer through the shell of the fuel tank and into the LNG fuel during operation. For example, heat may be transferred through the hose and dispensers used to fuel an LNG fuel tank to the LNG fuel and generate BOG in the LNG fuel tank. 
         [0007]    The amount of time it takes to refuel can have significant economic consequences for a natural gas powered system because the system is not in service when it is being refueled and cannot return to service until refueling is complete. Therefore, it would he advantageous to reduce the amount of time required for refueling and discharging the LNG fuel tank such that the natural gas powered engine could return to service more quickly and any downtime for the system is minimized. 
         [0008]    Different approaches have been taken to address this problem of BOG accumulating in the LNG fuel tank during LNG tilling and emptying procedures. One approach has been to vent the BOG from the LNG fuel tank into the atmosphere, but environmental concerns related to potentially negative effects of releasing methane gas into the atmosphere do not make this approach desirable. Furthermore, a considerable amount of natural gas could be wasted if the BOG is simply vented to the atmosphere. 
         [0009]    Another approach has been to vent the BOG from the LNG fuel tank to the bulk LNG supply tank that is used to supply LNG to the fuel tank. While this approach, immediately relieves pressure in the fuel tank caused by BOG, it introduces heat from the BOG into the LNG supply tank, which potentially could create an undesirably high pressure in the LNG supply tank. This heat would then ultimately be re-introduced to the LNG fuel tank. Other approaches include transferring the BOG to a pipeline system, using the BOG as fuel for a local primer mover, or returning the BOG to a liquid state through the use of a re-liquefying system. 
         [0010]    The presently disclosed system and method for managing boil-off gas in the fuel tank is directed at overcoming one or more of these disadvantages in currently available LNG fuel systems. 
       SUMMARY 
       [0011]    In accordance with an embodiment of the present disclosure, a liquid natural gas tank pressure management (LNG) system is provided. The system includes an LNG bulk tank, a LNG fuel tank operably connected to the LNG bulk tank, a compressed natural gas (CNG) accumulator operably connected to the LNG fuel tank, and an LNG cryogenic pump operably connected to the LNG fuel tank and the LNG bulk tank, wherein the pressure management system is configured to direct boil off gas to the compressed natural was accumulator upon a predetermined condition. 
         [0012]    In accordance with an embodiment of the present disclosure, a method of assembling a liquid natural gas pressure management system is provided. The method includes providing an LNG bulk tank, connecting an fuel tank to the LNG bulk tank, connecting a CNG accumulator to the LNG fuel tank, connecting an LNG cryogenic pump to the LNG bulk tank and the LNG fuel tank; and configuring, the pressure management system to direct boil off gas to the CNG accumulator when a predetermined condition occurs. 
         [0013]    In accordance with an embodiment of the present disclosure, a liquid natural gas tank pressure management system is provided. The system includes means for storing bulk LNG, means for storing LNG fuel operably connected to the means for storing bulk LNG, means for accumulating compressed natural gas (CNG) operably connected to the means for storing LNG fuel and means for pumping liquid natural gas operably connected to the means for storing bulk liquid natural gas and the means for storing liquid natural gas fuel. BOG is released from the LNG fuel tank and stored in the CNG accumulator upon a predetermined condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  presents a process diagram of a liquid natural gas tank vapor management system according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present disclosure provides an LNG pressure management system  10  as shown in  FIG. 1 . The system  10  includes an LNG bulk tank  20 , an LNG fuel tank  30  that may be operably connected to the LNG bulk tank  20  and a CNG accumulator  40  that is operably connected to the LNG fuel tank  30 . BOG is released from the LNG fuel tank  30  and stored in the CNG accumulator  40  when certain conditions are met as described in detail below. 
         [0016]    Natural gas is maintained in a liquid state in the LNG fuel tank  30 . The LNG fuel tank  30  is an insulated cryogenic tank designed to hold liquefied natural gas at a very low temperature to maintain its liquefied form. For example, LNG is usually stored in the LNG fuel tank  30  at −260° F. and at 40 psig. 
         [0017]    The LNG fuel tank  30  may be used to store LNG for a natural gas fuel system (not shown), e.g., to supply an engine that is powered by natural gas. The LNG fuel tank  30  defines a capacity to hold a certain volume of LNG and at a maximum operating pressure. The maximum operating pressure is the maximum pressure that the LNG fuel tank  30  can safely withstand without damage. The maximum operating pressure of the LNG fuel tank  30  may be a function of the capacity of the LNG fuel tank  30  as well as the operating conditions of the LNG fuel tank  30 . The LNG fuel tank  30  may be configured to operate at pressures that are below the maximum operating pressure and may be operably connected to the LNG bulk tank  20  and operably connected to the CNG accumulator  40 . 
         [0018]    The LNG bulk tank  20  may be configured to store a large volume of LNG. The LNG bulk tank  20  may be an insulated cryogenic tank designed to hold. liquefied natural gas at a. very .  low temperature to maintain its liquefied form (i.e., −260° F. and at 40 psig). The LNG bulk tank  20  may be placed in ground, above ground or in LNG carriers. The LNG bulk tank  20  may be used to transfer LNG to and from the LNG fuel tank  30 . Therefore, during refueling, the LNG bulk tank  20  supplies the LNG fuel tank  30  with LNG. During emptying, the LNG in the LNG fuel tank  30  is transferred to the LNG bulk tank  20 . 
         [0019]    The CNG accumulator  40  may be configured as a vessel to store BOG. The CNG accumulator  40  may be operated, at a higher pressure than the LNG fuel tank  30 . The CNG accumulator  40  has the capacity to hold a certain volume of CNG at a maximum operating pressure. The maximum operating pressure is the maximum pressure that the CNG accumulator  40  can safely withstand without damage. The maximum operating pressure. of the CNG accumulator  40  may be a function of the capacity of the CNG accumulator  40  as well as the operating conditions of the LNG pressure management system  10 . The CNG accumulator  40  may be configured to operate at pressures that are below the maximum operating pressure and may be operably connected to the LNG fuel tank  30 . In some embodiments according to the present disclosure, the CNG accumulator  40  is operably connected to the LNG fuel tank  30  and the LNG bulk tank  20 . 
         [0020]    The LNG fuel tank  30  requires periodic refueling during normal operation. The LNG fuel tank  30  may also be discharged or emptied of LNG on occasion for service. The LNG hulk tank  20  contains LNG and is used to supply the LNG fuel tank  30  with LNG. An LNG cryogenic pump  50  may be used to pump the LNG from the LNG bulk tank  20  to the LNG fuel tank  30 . An LNG transfer valve  60  may be operably connected between the LNG hulk tank  20  and the LNG fuel tank  30 . During refueling, LNG flows from the LNG bulk tank  20  through the LNG cryogenic pump  50  through the LNG transfer valve  60  and into the LNG fuel tank  30 . 
         [0021]    The LNG fuel tank  30 , however, inherently experiences some heat transfer caused by conduction to the LNG fuel tank  30  during filling and emptying procedures. The heat source may be from ambient temperature or it may be heat generated by various pieces of process equipment, such as the pumps and hoses used to transfer LNG from the LNG fuel tank  30  to the LNG bulk tank  20 . 
         [0022]    As heat is added to the LNG fuel tank  30 , the temperature of the LNG rises and a portion of the LNG will evaporate, increasing pressure in the LNG fuel tank  30 . The LNG fuel tank  30  may be configured to withstand some of these pressure increases from the evaporated liquid. However, when the pressure in the LNG fuel tank  30  becomes too high as a result of the evaporated LNG or vapor, the pressure may be reduced by releasing BOG. 
         [0023]    A fuel vapor valve  70  may be operably connected between the LNG fuel tank  30  and the CNG accumulator  40 . A CNG compressor  80  may also be used to transfer the BOG from the LNG fuel tank  30  to CNG accumulator  40 . The CNG accumulator  40  and the GNU compressor  80  may also be part of the LNG refueling system (not shown). 
         [0024]    When the pressure in the LNG fuel tank  30  exceeds the maximum operating pressure of the LNG fuel tank  30 , the BOG from the LNG fuel tank  30  is released and stored in the CNG accumulator  40 . BOG may be released from the LNG fuel tank  30  by flowing through the fuel vapor valve  70  and the CNG compressor  80  and pumped to the CNG accumulator  40 . 
         [0025]    In an embodiment according to the present disclosure, the fuel vapor valve  70  may be controlled using a controller  110 . For example, pressure sensor(s) may be used. to measure the pressure within the LNG fuel tank  30 . The controller  110  may be configured to receive a signal from the pressure sensor(s) and monitor the pressure of the LNG fuel tank  30 . The controller  110  may be configured to operate the fuel vapor valve  70  to an open position to release BOG from the LNG fuel tank  30  when the pressure of the LNG fuel tank  30  exceeds the maximum operating pressure of the LNG fuel tank  30 . 
         [0026]    During, the refueling process, the LNG bulk tank  20  may be connected to the LNG fuel tank  30  through the LNG cryogenic pump  50  by way of the LNG transfer valve  60 . BOG management may occur during refueling as the CNG accumulator  40  is connected to the vapor section of the LNG fuel tank  30  by way of the fuel vapor valve  70  and the CNG compressor  80  based on electrical commands received from the controller  110  during the refueling process. The CNG compressor  80  is employed to maximize the quantity of BOG stored in the CNG accumulator  40  and the LNG fuel tank  30 . The controller  110  manages the LNG fuel tank  30  pressure as well as the CNG accumulator  40  pressure within predetermined pressure ranges for each throughout the refueling process. 
         [0027]    When service procedures necessitate emptying the LNG fuel tank  30 , LNG bulk tank  20  may be connected to the LNG fuel tank  30  by way of the LNG transfer valve  60  and the LNG cryogenic pump  50  is bypassed. BOG management may occur during the empting procedures as the CNG accumulator  40  is directly connected to the vapor section of the LNG fuel tank  30  by way of shut of valve  90  and fuel vapor valve  70 . The CNG accumulator  40  pressure is applied to the vapor section of the LNG fuel tank  30  to push LNG from the liquid section of the LNG fuel tank  30  to the liquid section of the LNG bulk tank  20 . The controller  110  manages LNG fuel tank  30  pressure within a predetermined pressure range throughout the emptying process. 
         [0028]    During both the LNG refueling and emptying processes, the LNG valve  100  is used to isolate the CNG accumulator  40 , shut off valve  90 , CNG compressor  80  and the fuel vapor valve  70  from the LNG bulk tank  20 . If emptying the LNG bulk tank  20  becomes necessary for service or maintenance reasons, the CNG accumulator  40  may be connected to the vapor section of the LNG bulk, tank  20  by way of the shut-off valve  90  and the LNG valve  100 . In this case, the fuel vapor valve  70  would seal off the CNG portion of the LNG pressure management system  10  and the LNG transfer valve  60  would be used to transfer LNG front the LNG bulk tank  20  to another LNG storage system (not shown). 
         [0029]    A similar process may be followed when the LNG bulk tank  20  is being filled. When filling the LNG bulk tank  20  the CNG accumulator  40  may be connected to the vapor section of the LNG bulk tank  20  by way of the CNG compressor  80  and the LNG valve  100 . The LNG bulk tank  20  pressure is controlled based on electrical commands received by the LNG valve  100  from the controller  110  during refueling. The CNG compressor  80  is employed to maximize the amount of BOG stored in the CNG accumulator  40 . In this case, the shut off valve  90  would prevent direct connection between the CNG accumulator  40  and the LNG bulk tank  20  and the fuel vapor valve  70  would seal off the CNG portion of the LNG pressure management system  10 . The LNG transfer valve  60  would be used to transfer LNG from the LNG source (not shown) to the LNG bulk tank  20 . 
         [0030]    In an embodiment of the present disclosure, the CNG accumulator  40  may be removed from the pressure management system  10  and the BOG stored in the CNG accumulator  40  to be used in other natural gas applications. For example, the BOG may be re-liquefied and used in a natural gas powered engine. If design considerations permit, a re-liquefier may be included as part of the system  10 . The CNG accumulator  40  may also be removed from the system  10  in certain circumstances and replaced with a new CNG accumulator  40 . 
         [0031]    During the refueling operation, the pressure within the CNG accumulator  40  may exceed the maximum operating pressure of the CNG accumulator  40 . In this instance, it may no longer be feasible to continue to release BOG from the LNG fuel tank  30  and store the BOG in the CNG accumulator  40 . 
         [0032]    A shut-off valve  90  may be configured to close off the CNG accumulator  40  to prevent BOG flow to the CNG accumulator  40  and direct the BOG to the LNG bulk tank  20 . Although there are disadvantages to introducing BOG to the LNG bulk tank  20 , the ability to route BOG to the LNG bulk tank  20  provides an additional location on the LNG pressure management system  10  to transfer the BOG. 
         [0033]    During emptying procedures, the LNG may be transferred from the LNG fuel tank  30  to the LNG bulk tank  20 . The LNG flows from the LNG fuel tank  30  and through the LNG transfer valve  50  and into the LNG bulk tank  20 . The presence of BOG in the LNG fuel tank  30  provides pressure to discharge the LNG from the LNG fuel tank  30  to the LNG bulk tank  20 . Therefore, the more BOG in the LNG fuel tank  30 , the greater the pressure in the LNG fuel tank  30  and the faster LNG will discharge from the LNG fuel tank  30 . 
         [0034]    BOG may also be present in the CNG accumulator  40  during emptying. Again, the presence of BOG will provide additional pressure to assist with the discharge of the LNG form the LNG fuel tank  30  to the LNG bulk tank  20 . In this case, the shut-off valve  90  may be opened to release the BOG from the CNG accumulator  40  and allow the BOG to flow through the fuel vapor valve  70  and into the LNG fuel tank  30 . LNG valve  100  may be closed, to prevent BOG from entering the LNG bulk tank  20 . As the BOG flows into the LNG fuel tank  30 , the pressure quickly discharges LNG from the LNG fuel tank  30 . Once the LNG fuel tank  30  is emptied of LNG then the LNG fuel tank  30  and the natural gas fuel system (not shown) may be serviced. 
       INDUSTRIAL APPLICABILITY 
       [0035]    The LNG pressure management system  10  of the present disclosure may be applied to any engine that uses natural gas a fuel, including diesel/natural gas dual fuel powered engines. For example, machines that can be powered using natural gas, such as a truck, car, bus, or any mobile machine. The LNG pressure management system  10  of the present disclosure may also be used in off-highway machines such as mining trucks, locomotives and marine applications. 
         [0036]    The LNG pressure management system  10  may be part of an LNG fuel station that is used to fill and empty an LNG fuel tank  30 . The LNG pressure management system  10  of the present disclosure allows BOG that has accumulated in the LNG fuel tank  30  to be released from the LNG fuel tank  30  and stored in the CNG accumulator  40  when the LNG fuel tank  30  is being refueled. Using a CNG accumulator  40  to store the accumulated BOG from the LNG fuel tank  30  significantly reduces the time required to fill and empty the LNG fuel tank  30 . The faster filling and emptying times allow the natural gas fuel system to return to operation faster and therefore greatly reduces downtime. Additionally, the LNG fuel tank  30  will also be able to hold a greater volume of LNG because the BOG that accumulates in the LNG fuel tank  30  during emptying and filling procedures is transferred to the CNG accumulator  40 . 
         [0037]    When the LNG fuel tank  30  is emptied, the BOG in the CNG accumulator  40  provides additional pressure that is used to discharge the LNG from the LNG fuel tank  30  and into the LNG bulk tank  20 . This reduces the time required to discharge the LNG fuel tank  30  when the LNG fuel tank  30  needs to be serviced. 
         [0038]    The many features and advantages of the disclosure are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the disclosure which fall within its true spirit and scope. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the disclosure to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the disclosure.