Patent Publication Number: US-9885447-B2

Title: Metering system and method for cryogenic liquids

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Provisional Patent Application No. 61/731,287, filed Nov. 29, 2012, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to dispensing systems for cryogenic fluids and, in particular, to a metering system and method for cryogenic liquids. 
     BACKGROUND 
     The use of liquid natural gas (LNG) as an alternative energy source for powering vehicles and the like is becoming more and more common as it is domestically available, environmentally safe and plentiful (as compared to oil). As a result, the need for dispensing systems and methods that accurately meter cryogenic liquids, such as LNG, has grown. 
     An example of an effective prior art cryogenic liquid metering system is provided in commonly assigned U.S. Pat. No. 5,616,838 to Preston et al., the contents of which are hereby incorporated by reference. The &#39;838 patent discloses mounting a cryogenic liquid meter within an insulated cryogenic metering container so that the meter is submerged in cryogenic liquid that is provided from a storage tank and dispensed. This avoids two-phase flow through the meter and permits accurate metering without the need to initially circulate the cryogenic liquid through the meter to pre-cool the meter (prior to each dispensing session). 
     In addition, the &#39;838 patent discloses that a volumetric flow rate of the cryogenic liquid being dispensed is read by the meter, and that this data is provided to a microprocessor. Temperature data from a temperature sensor positioned in the cryogenic metering container, or differential pressure data from a pair of vertically spaced pressure sensors positioned in the cryogenic metering container, is provided to the microprocessor so that the density of the cryogenic liquid being dispensed may also be determined. The microprocessor is then able to calculate and display the metered amount of cryogenic liquid/LNG dispensed to the use device. 
     While the system of the &#39;838 patent performs well, the unknown composition of most LNG makes use of temperature to determine density (i.e. “temperature compensation”) unacceptable. LNG is made up of mostly methane, but includes different levels of hydrocarbons, such as carbon dioxide and nitrogen. 
     With regard to use of pressure differential data to determine density, the dynamic nature of the cryogenic liquid as it flows into and out of the cryogenic metering container creates issues such as “noise” in the taps of the pressure sensors. 
     A need therefore exists for a metering system and method for cryogenic liquids that addresses at least some of the above issues. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a storage tank and a cryogenic metering chamber and related piping and pump in an embodiment of the metering system and method for cryogenic liquids of the present invention; 
         FIG. 2  is an enlarged schematic view of the cryogenic metering chamber of  FIG. 1  and the related components; 
         FIG. 3  is an enlarged schematic view of the metering element of  FIG. 2 ; 
         FIG. 4  is a top plan view of the stabilizing column of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     While the present invention will be described below in terms of a system and method for dispensing LNG, it is to be understood that they may be used to dispense alternative types of cryogenic liquids. 
     In accordance with an embodiment of the system and method of the present invention, as illustrated in  FIG. 1 , an insulated storage tank  10  contains a supply of cryogenic liquid, such as LNG  12 . As will be explained in greater detail below, the LNG is provided to an insulated cryogenic metering chamber  14  via liquid inlet line  16 . It should be noted that the insulation may optionally be omitted for cryogenic metering chamber  14 . The transfer of LNG from the storage tank  10  to the cryogenic metering chamber  14  may be accomplished by pressure differential, by a pump  18  or other cryogenic liquid transfer systems and methods known in the art. A recirculation line  20  also is connected between the storage tank and the cryogenic metering chamber, the use of which will also be explained below. 
     With reference to  FIG. 2 , liquid inlet line  16  connects with a spray fill line  22  that is vertically positioned within cryogenic metering chamber  14 . The spray fill line  22  features spray openings  24  in the top end. A recirculation column  26  features a recirculation inlet  28  and is connected to recirculation line  20 , which is provided with recirculation valve  30 . A meter run, indicated in general at  32 , includes a meter line  34 , having an inlet near the bottom of the cryogenic metering chamber. Meter line  34  is connected to dispensing line  36 , which features a dispensing valve  38 . A metering element  40  is positioned within the meter line, and communicates with a flow differential pressure transmitter  42 . 
     In accordance with the illustrated embodiment of the invention, a stabilizing column  46  is positioned within the cryogenic metering chamber and features a low pressure sensor or tap  48 , a middle pressure sensor or tap  50  and a high pressure sensor or tap  52 . Each pressure tap communicates with the stabilizing column  46  and a stabilizing column differential pressure transmitter  54 . It should be noted that only two of the pressure taps are required, the third tap is optional. 
     Flow differential pressure transmitter  42  and stabilizing column differential pressure transmitter  54  each communicate with a controller  60 , such as a microprocessor, via wireless or wire connections. Recirculation valve  30  and dispensing valve  38  may be automated and also connected to microprocessor  60  for operation. 
     In operation, LNG is initially transferred from storage tank  10  ( FIG. 1 ) to an empty cryogenic metering chamber  14  with the recirculation valve  30  open, the dispensing valve  38  closed and pump  18  on. As a result, LNG flows into the cryogenic metering chamber via liquid inlet line  16 , as indicated by arrow  61  in  FIG. 2 , spray fill line  22  and spray openings  24 . The LNG flowing through spray openings  24  collapses any pressure head in the cryogenic metering chamber  14 . When the LNG in the cryogenic metering chamber, illustrated at  62 , reaches the level of the recirculation inlet  28  of the recirculation column, the LNG flows through recirculation column  26  and line  20  and back to the storage tank, as indicated by arrow  65 . After a period of time that is sufficient to ensure that the metering chamber is filled with LNG, the pump  18  is shut off (automatically or manually). As a result, the flow of LNG into the cryogenic metering chamber stops, and the metering element  40  is submerged in LNG. The recirculation valve  30  remains to its normal, open position. 
     When it is desired to dispense LNG, with reference to  FIG. 2 , a hose attached to the dispensing line  36  (see also  FIG. 1 ) is connected to a vehicle or other use device, and the system is activated, such as by the user pushing a “Dispense” button in communication with the controller or microprocessor  60 . When this occurs, pump  18  starts, while recirculation valve  30  remains open. LNG then flows into the cryogenic metering chamber through spray fill line  22  (and spray openings  24 ) and eventually rises to the level of recirculation inlet  28 . The LNG then flows through recirculation column  26  and line  20  and back to the storage tank, as indicated by arrow  65 . As a result, LNG flows through the recirculation valve  30  until discharge pressure, as measured by any or all of pressure taps  48 ,  50  and/or  52 , and proper flowing conditions in the metering chamber  14  are achieved. This typically may take, as an example only, approximately fifteen seconds or less. Microprocessor  60  then opens dispensing valve  38  so that dispensing of LNG to the use device through the meter run  32  commences. 
     With reference to  FIG. 3 , metering element  40  uses the Bernoulli principle that relates flow to pressure drop across an orifice and features a flow restriction or orifice  70  having an inlet side  72  and an outlet side  74 . An inlet pressure sensor or tap  76  communicates with the inlet side  72  and an outlet pressure sensor or tap  78  communicates with the outlet side  74 . As a result, a differential pressure is transmitted to the microprocessor  60  by the differential pressure transmitter  42 , and the microprocessor determines the volumetric flow rate and mass flow rate through the metering element using the following equations: 
     
       
         
           
             
               
                 
                   
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                     C 
                     × 
                     
                       A 
                       2 
                     
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                           M 
                         
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                           ρ 
                           liq 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
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                   Q 
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                     C 
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                       A 
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                           2 
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                             DP 
                             M 
                           
                         
                         
                           ρ 
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                   Equation 
                   ⁢ 
                   
                       
                   
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                     2 
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     Where:
         Q=volumetric flow rate [m^3/s]   m{dot over ( )}=mass flow rate [kg/s]   C=orifice flow coefficient   A 2 =cross-sectional area of the orifice [m^2]   DP M =Meter Diff. press. across orifice [kg/(m×s 2 )]   ρ liq =fluid density [kg/m 3 ]
 
As will be explained in greater detail below, ρ liq  is determined using the differential pressure transmitter  54 
       

     With reference to  FIG. 2 , stabilizing column  46  features a continuous sidewall that defines an interior of the stabilizing column. The sidewall has a number of openings  80  that are spaced along its vertical length. The top and bottom of the stabilizing column may be open. This permits LNG  62  from the cryogenic metering chamber to travel into the interior of the stabilizing column and maintain the same temperature as the LNG in the cryogenic metering chamber. As an example only, the stabilizing column may be a one inch (1″) diameter, thin wall tube with holes spaced two inches apart (from the edges). Preferably, the stabilizing column is made of steel, or another metal material, and is mounted within one-eighths of an inch (⅛″) from the interior surface of the cryogenic metering chamber side wall, with the holes facing towards the interior surface. As illustrated in  FIG. 4  for low pressure tap  48 , each pressure tap ( 48 ,  50  and  52 ) preferably extends into or near the longitudinal axis or horizontal center of the interior of the stabilizing column  46 . 
     In addition, as illustrated in  FIG. 2 , low pressure tap  48  features an associated sensing line  48   a  running to differential pressure transmitter  54 , middle pressure tap  50  features an associated sensing line  50   a  running to the differential pressure transmitter, and high pressure tap  52  features an associated sensing line  52   a  running to the differential pressure transmitter. The sensing lines  48   a ,  50   a  and  52   a  preferably feature equal inner diameters and lengths with each inner diameter being uniform through the length of the sensing line. This aids in minimizing surging in the sensing lines during pressure swings in the metering chamber. 
     The stabilizing column  46  takes out “noise” at the pressure taps that otherwise would be caused by LNG flow within the cryogenic metering chamber. In addition, the positioning of the pressure taps near the center of the stabilizing column minimizes the effects of bubbles that form during temperature swings on each pressure tap opening. The holes  80  of the stabilizing column also minimize the effects of pressure drop during flow. 
     The low pressure tap  48  and high pressure tap  52  are used to measure a differential or column pressure, which is sent to the microprocessor via stabilizing column differential pressure transmitter  54 . As a result, the microprocessor  60  may calculate the density of the LNG in the cryogenic metering chamber using the following equation:
 
ρ liq =[(DP Ctrans )/( g   c   ×H   C )]ρ gas   Equation (3)
 
     Where:
         ρ liq =fluid density [kg/m 3 ]   DP Ctrans =Measured Differential Pressure across Column [kg/(m×s) 2 ]   g c =gravitational acceleration=9.80665 m/s 2      H c =Tap distance or height of density column [m]   ρ gas =gas density (in sensing lines) [kg/m 3 ]       

     The addition of ρ gas  in Equation (3) compensates for the density of the gas in the sensing lines when determining the density of the LNG. 
     The microprocessor combines the density calculated using Equation (3) above with the data from metering element  40 , and calculates the mass flow rate and volumetric flow rate using Equation (1) and Equation (2) above. As a result, the metered amount of LNG delivered to the use device may be displayed via a display  82 . The middle pressure tap  50  can be swapped for the low pressure tap  48  to increase the resolution of the density reading. 
     Returning to the dispensing operation, a few seconds after the dispensing valve  38  is opened, microprocessor  60  closes recirculation valve  30 . This delay helps “soften” the dispensing and metering start by preventing extreme pressure swings within the cryogenic metering chamber. This is desirable because pressure swings can cause a pressure pulse that the high and low pressure taps (or high and middle pressure taps) see at slightly different times, and thus could corrupt the pressure differential data transmitted by stabilizing column differential pressure transmitter  54  to microprocessor  60 . 
     Furthermore, as illustrated in  FIG. 2 , the spray openings  24  are positioned near the top of the cryogenic metering chamber, while the inlet to the meter run  32  is positioned near the bottom of the cryogenic metering chamber. This minimizes stratification in the cryogenic metering chamber, which could otherwise effect the accuracy of the density determination by the pressure taps. 
     When dispensing is completed, the user may press a “Stop” button or the like so that the microprocessor  60  closes dispensing valve  38  and opens recirculation valve  30 . The user then disconnects the filling hose from the use device and LNG therein travels back to the cryogenic metering chamber through check valve  84  ( FIG. 2 ). 
     Pump  18  keeps running after the “Stop” button is pressed, and LNG circulates between the metering chamber and the storage tank, and LNG will continue to enter the cryogenic metering chamber via the spray holes  24  and exit via recirculation inlet  28 . After a period of time that is sufficient to ensure that the metering chamber is filled with LNG, the pump automatically stops running. As a result, the flow of LNG into the cryogenic metering chamber stops, and the metering element  40  remains submerged in LNG. 
     If the quantity of LNG to be metered is large, the meter run  32  may be placed external to the cryogenic metering chamber  14 . More specifically, small transfers need to be metered extremely accurate from the start of flow. Warm meters will have errors in the order of 5 lbs. (100 lbs. transfer would be a 5% error). With transfers that deliver large quantities, this error is not significant. 
     As illustrated in  FIG. 2 , a temperature probe  90  may optionally be positioned within the cryogenic metering chamber and placed in communication with microprocessor  60 . In addition, the microprocessor may be programmed with a lookup table listing densities of methane through the range of temperatures typically detected by temperature probe  90  when the system is in operation. The lookup table may also, or alternatively, list densities for other elements typically present in LNG through the range of temperatures typically detected by the temperature probe. As a result, the microprocessor may calculate the percent (%) methane or percent (%) heavies of the LNG within the cryogenic metering chamber using the temperature from the temperature probe  90  and the density from the stabilizing column differential pressure transmitter  54 . 
     In view of the above, the invention provides a system and method for metering cryogenic liquids that includes a dynamic densitometer that measures the density of flowing liquid. The design is extremely robust, with no moving parts, and the meter can be calibrated to meter any liquid ranging in density including, for example, from LNG to nitrogen. 
     While the preferred embodiments of the invention have been shown and described, it will be 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.