Abstract:
A cryogenic fluid delivery system includes a tank adapted to contain a supply of cryogenic liquid, with the tank including a head space adapted to contain a vapor above the cryogenic liquid stored in the tank. A liquid withdrawal line is adapted to communicate with cryogenic liquid stored in the tank. A vaporizer has an inlet that is in communication with the liquid withdrawal line and an outlet that is in communication with a vapor delivery line. A pressure building circuit is in communication with the vapor delivery line and the head space of the tank. The pressure building circuit includes a flow inducing device and a control system for activating the flow inducing device when a pressure within the head space of the tank drops below a predetermined minimum pressure and/or when other conditions exist.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/708,749, filed Oct. 2, 2012, the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to cryogenic delivery systems and methods and, in particular, to a cryogenic liquid delivery system and method with active pressure building capabilities. 
       BACKGROUND 
       [0003]    This invention relates to a delivery system and method of a cryogenic fluid, such as liquefied natural gas (LNG), from a storage tank to a use device, such as a natural gas powered vehicle engine. An embodiment of the system of the invention is particularly suited for markets in which pre-saturation of the LNG fuel is not performed, though it may also function as a source of “trim heat” if the tank pressure falls below a pre-defined level. 
         [0004]    Many heavy-duty vehicle engines require that the intake pressure of natural gas be around 100 psig. In most markets, LNG is saturated, or heat is added, to a point at which its vapor pressure is roughly equal to the pressure required b the use device. This process of building saturation pressure is typically performed at LNG fueling stations. However, there exist some markets in which this saturation of the fuel before transferring it to the vehicle storage tank is not performed or is not performed to an extent great enough to achieve 100 psig saturated liquid in the vehicle tank after fueling. Thus, the storage tank may end up being filled with LNG well below the desired pressure. 
         [0005]    One proposed method for building tank pressure is to utilize a pressure building circuit that is common on many stationary cryogenic cylinders. These circuits function by utilizing gravity to feed liquid cryogen into a vaporizer. Upon vaporization of the liquid, its volume expands and the evolved gas is routed to the vapor space above the cryogen, building a head of vapor pressure above the liquid phase in the tank. However, there are three distinct problems with this type of circuit for LNG vehicle tanks. First, as most LNG vehicle tanks are mounted horizontally, there is small liquid head pressure compared to a vertical tank to force liquid into the vaporizer. Second, since LNG vehicle tanks are used M mobile applications, any vapor pressure that is built above the liquid phase will quickly collapse as soon as the vehicle is in motion and the liquid and vapor phases mix. It may take several hours or more to add enough heat in this fashion to fully saturate the bulk of LNG in the tank. Third, because pressure building coils are gravity feed systems, the feed line is directly connected to the liquid space. In a vehicle accident, this open liquid line can be damaged, creating a fire hazard due to the large volumes of gas generated from a liquid leak. 
         [0006]    Another proposed solution is referenced in U.S. Pat. No. 5,163,409 to Gustafson et at whereby compressed natural gas (CNG) is used to add vapor pressure above LNG to deliver the fuel at an elevated pressure. However, this solution requires a second tank for CNG be mounted on the vehicle, which would add weight and occupy space on the vehicle chassis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view of a prior art ENG delivery system; 
           [0008]      FIG. 2  is a schematic view of an embodiment of the delivery system of the invention; 
           [0009]      FIG. 3  is an enlarged schematic view of an embodiment of the flow inducing device of the delivery system of  FIG. 2 ; 
           [0010]      FIG. 4  is a schematic view of an embodiment of a controller for activating the flow inducing device of  FIGS. 2 and 3 ; 
           [0011]      FIG. 5  is a schematic view of a multiple cryogenic tank system in an embodiment of the delivery system of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0012]    While the invention is described below in terms of liquid natural gas (LNG) as the cryogenic liquid, it is to be understood that the invention is not so limited and may be used with other types of cryogenic liquids in other applications. 
         [0013]    The LNG delivery system described below overcomes the aforementioned shortcomings of the prior art by including a compressor or pump situated on a parallel path downstream of the vaporizer to actively force natural gas vapor hack into the vehicle fuel tank, adding heat to the tank at a rate that far exceeds that which could be accomplished by passive systems. Compressor operation is controlled by a control system that monitors the system pressure, turning the compressor on when system pressure is low and of when the system pressure reaches a predefined point. Once the liquid is saturated, the LNG delivery system functions as the system described in commonly owned U.S. Pat. No. 5,421.161 to Gustafson, the contents of which are hereby incorporated by reference. 
         [0014]      FIG. 1  depicts the fuel delivery system in the &#39;161 patent and a brief description is included here for clarity. A cryogenic tank  22  contains cryogenic product such as LNG consisting of liquid cryogen  26  with a vapor filling the tank vapor space or head space  36  above it. Liquid line  24  communicates with the bottom of tank  22  where liquid  26  is contained, Product withdrawal line  28  connects liquid line  24  to the gas use device such as a vehicle engine. A heat exchanger or vaporizer  32  is located in withdrawal line  28  to vaporize the cryogen before it is delivered to the use device. The withdrawal line also contains a tank mounted excess flow valve  48 , which protects the downstream piping in case of as line break. Valve  10  in withdrawal line  28  may be taken to represent the throttle of a vehicle with the idea that demand for product is constantly changing. Economizer circuit  34  includes vapor tube  40 , which communicates with head space  36 , and includes economizer regulator  38 , which is set at a predetermined pressure threshold. In this mariner, when the pressure in tank  22  exceeds the set point of regulator  38 , the vapor in head space  36  may be withdrawn through vapor line  40  and to the use device through withdrawal line  28 , which lowers the pressure in tank  22 . However, due to the horizontal nature of LNG vehicle fuel tanks, there is often sufficient hydrostatic pressure to cause liquid  26  to be withdrawn even when regulator  38  is open. Therefore, a biasing relief valve  42  is included in liquid line  24  to cause economizer circuit  34  to be the path of least resistance when regulator  38  is open. A small orifice  44  is located in parallel with relief valve  42  to allow back flow to the tank during transient periods of high to low use. 
         [0015]    Referring now to  FIG. 2 , a fuel system with the components previously described plus an additional active pressure: building circuit  55  is shown, Inlet line  51  branches of withdrawal line  28  downstream of vaporizer  32 . Flow inducer  52  causes vaporized gas to flow from inlet line  51  to outlet line  53  which returns the gas to vapor line  40  through cheek valve  54 . 
         [0016]      FIG. 3  shows one possible embodiment of the flow inducing device  52 . Pressure vessel  60  operates at the same pressure as outlet line  53 . Compressor  61  has inlet  63  piped directly to inlet line  51  and has outlet  65  open to the interior of vessel  60 . 
         [0017]    It should be noted that flow inducing device  52  is not limited to a compressor housed inside a pressure vessel. but may take on other forms of actively moving a fluid against a pressure gradient such as a positive displacement pump or other type of motor. Additionally, the process piping of the flow inducing device may be configured in other manners, such as by piping the compressor outlet to the tank and leaving the compressor inlet open to the interior of the vessel. 
         [0018]      FIG. 4  shows one possible embodiment of a controller or control system circuit to activate or deactivate the flow inducing device. A power source, such as a battery  75 . supplies a voltage to device  60  via electrical circuit  76 . The voltage is controlled by several relays and switches (where the terms “relay” and “switch” are used interchangeably herein), which dictate logical events that must occur simultaneously to in order to supply power to device  60 . In order for flow inducing device  60  to operate, the vehicle&#39;s ignition switch or key must be turned on to close ignition relay  70 , and the system pressure must be below a predetermined threshold to close pressure relay  71 . Additionally, it is desirable that the engine is operating, in order for device  60  to operate for two reasons: first, to avoid excessive vehicle battery drain and second, to ensure an adequate amount of heat is supplied to vaporizer  32 . A signal indicating that the engine is in operation will close engine operating relay  72 , A manual bypass switch  73 , connected in parallel with engine operating relay  72 , is provided for rare instances when a user may desire to operate the compressor when the engine is not operating (for example, when the tank pressure is too low to even support the engine to start). 
         [0019]    A further description of the logical events for the controller or control system circuit are as follows. The signal to close ignition relay  70  can he simply taken from the vehicles ignition switch  80  ( FIG. 4 ). The signal to close pressure relay  71  requires that the pressure in the system is below a predefined limit. Therefore, a pressure switch or sensor should be included in the system of  FIG. 2  to sense a system pressure in one of several locations such as the head space  36  of the tank, as illustrated by sensor  82  in  FIGS. 2 and 4 , or somewhere in pressure building circuit  55 , as illustrated by sensor  83  in  FIGS. 2 and 4 , and can be used to close relay  71  when the sensed pressure is below the pressure threshold or predetermined minimum pressure. A signal to close the engine operating relay  72  may come from a variety of sources that may serve as an engine operating sensor. The most direct source would be a signal from the on vehicle electrical system,  84  in  FIG. 4 , that senses if the engine is operating or not via the engine&#39;s electronic control circuitry. Alternatively, an indirect method of detecting the engine operating may be used by including a temperature switch or sensor in inlet line  51 , as illustrated by sensor  86  in  FIGS. 2 and 4 , or in the heat exchange space surrounding vaporizer  32 , as illustrated by sensor  88  in  FIGS. 2 and 4 , such that relay  72  closes if the temperature is above a predetermined threshold. 
         [0020]    It should be appreciated that the controller may take on other forms not limited to the above description. In any case, the primary goals of the control system are 1) to prevent over pressurization of the cryogenic tank; 2) to prevent excessive discharge of the vehicle&#39;s battery when the engine is not operating; and 3) to avoid damage to both vaporizer  32  and flow inducing device  52  due to low temperatures when the engine is not operating. In an alternative embodiment, the controller could be omitted completely and the control system could consist of simply a manually controlled “on” and “off” switch or other manual control switch or device. 
         [0021]    A typical setup and operation of the described system in accordance with an embodiment of the method of the invention is as follows. The minimum allowable inlet pressure to the engine is 70 psig. To allow an adequate buffer in addition to the largest expected pressure drop from the tank to the engine, one might conclude that the normal operating pressure of the tank should be around 100 psig. Therefore, the economizer regulator is set to open at 100 psig, which will work to lower tank pressure to this level when tank pressure exceeds that value. With an economizer set at 100 psig, it would be logical to have the set point on the flow inducing device around 95 psig. Though technically feasible to have flow inducing device active at 100 psig or higher, it is not best practice because there would then be two active competing devices operating at the same time causing unnecessary energy consumption and wear on the components. In this example, suppose the vehicle fuel tank is filled with LNG saturated at 80 psig. When the engine is restarted after fueling, the compressor will immediately turn on and begin to build a false head pressure in the vapor space. In this example, suppose the compressor moves fluid at a rate of 100 L/min. In about one minute, the pressure may rise to 95 psig at which point the compressor will turn off. However, when the vehicle starts driving and the liquid and gas phases slosh together inside the tank, much of that false vapor head pressure will recondense back to liquid phase, and the tank pressure will drop back to a pressure near its starting pressure. The lower pressure will trigger the compressor to turn on again. While the vehicle is in motion and the liquid and gas phases are in thermodynamic equilibrium, the rate of pressure rise will be much slower, and the saturation of the LNG may increase to 95 psig in several minutes. With fuel saturated at the desired level, the compressor may not need to function again until the tank is again fueled with LNG that is not properly saturated to the required level. 
         [0022]    In an alternative embodiment of the delivery system of the invention illustrated in  FIG. 5 , a single pressure building circuit  55  may be used in a system consisting of multiple tanks  80   a,    80   b,  etc. (while two tanks are shown, an alternative number may be used) configured in parallel. As in the embodiment described previously with respect to  FIG. 2 , inlet line  51  branches off of the withdrawal line downstream of vaporizer  32 , and a flow inducer  52  causes vaporized gas to flow from inlet line  51  to outlet line  53 . Outlet line  53  returns the gas to the vapor line of the. economizer circuit and then to the tank head space via check valves  54   a  and  54   h  for tanks  80   a  and  80   b,  respectively. 
         [0023]    The controller or control system circuit illustrated in  FIG. 4  may also he used to activate or deactivate the flow inducing device  52  of  FIG. 5 , Of course, alternative controllers or control system circuits could be used. Returning to the embodiment of  FIGS. 4 and 5 , the signal to close pressure relay  71  requires that the pressure in the system is below a predefined limit. Therefore, a pressure switch or sensor should be included in the system of  FIG. 5  to sense a system pressure in several locations, such as the bead spaces of the tanks  80   a  and  80   b  or somewhere in pressure building circuit  55 , and can be used to close relay  71  of  FIG. 4  when the sensed pressure is below the pressure threshold or predetermined minimum pressure. If the pressure in one of the tanks is below the pressure threshold or predetermined minimum pressure (for example, tank  80   a ) and the other pressure of the other tank is not (for example, tank  80   b ), the chock valve corresponding to the tank that is not below the pressure threshold or minimum pressure (check valve  54   b  in the present example) will prevent gas from the pressure building circuit  55  from entering that tank (tank  80   b  in the present example). 
         [0024]    While the preferred embodiments of the invention have been shown and described, it will he apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.