LIQUEFIED GAS METERING SYSTEM

Abstract: A liquefied gas metering apparatus for use with liquefied gas dispensing equipment which conveys liquefied gas from a supply to a dispensing point and metering means (17). The metering apparatus has sensing means (50) to sense a measurable parameter of the liquefied gas related to the density of the liquefied gas particularly an electrical parameter such as the dielectric constant and to generate a sensing signal (77) indicative of the sensed parameter. The sensing means (50) includes a capacitive device (56) comprising spaced capacitor plates (71, 72) arranged to be immersed in the liquefied gas. Calculating means (76) is responsive to the sensing signal. The calculating means (76) being operative to control operation of the metering means (17) by compensating the determination by the metering means (17) of the metered liquefied gas dispensed during a dispensing operation for changes in gas composition.

Description: LIQUEFIED GAS METERING SYSTEM This invention relates to liquefied gas metering apparatus, e.g. for dispensing LPG or other liquefied gas in the filling of vehicle fuel tanks. LPG gas can vary in composition and the metering means used to meter the gas dispensed in the liquid phase, e.g. to a vehicle fuel tank, can have significant inaccuracies as a result of these differing gas compositions. In particular, the LPG gas can have varying proportions ranging, e.g., from substantially pure propane gas through mixtures of propane and butane gas up to pure butane. The flow measured during a dispensing operation can depend on the calibration of the metering apparatus that is carried out at a factory or on site upon installation. The particular gas being used during the calibration can be the most commonly encountered or the expected gas composition, however when other gas compositions are being metered, significant errors in the measuring of the gas can arise. There does not appear to have been any apparatus which has been developed so as to overcome this problem. It is an object of the present invention to provide a liquefied gas metering apparatus which enables compensation of a metered amount of liquefied gas for varying gas compositions. According to the present invention there is provided a liquefied gas metering apparatus for use with a liquefied gas dispensing system, the system having a supply of liquefied gas, liquefied gas dispensing equipment which in use conveys liquefied-gas from the supply to a dispensing point, and metering means associated with the dispensing system, the metering apparatus including sensing means for operative association with the liquefied gas dispensing system and operative to sense a measurable parameter of the liquefied gas related to the density of the liquefied gas and to generate a sensing signal indicative of the sensed parameter, the apparatus further including calculating means responsive to the sensing signal, the calculating means being operative to control operation of the metering means of the dispensing system by affecting the determination by the metering means of the metered liquefied gas dispensed during a dispensing operation so as to compensate the determination of the metered amount of liquefied gas for changes in the density of the liquefied gas and so as to thereby compensate the determination of the metered amount of liquefied gas for changes in gas composition. In one embodiment, the parameter related to the density of the liquefied gas comprises an electrical or magnetic property of the liquefied gas, the sensing means being operative to generate a sensing signal indicative of the sensed electrical or magnetic property. Preferably the property is an electrical property of the liquefied gas. It has been discovered that with changes with changes in composition of the liquefied gas, the specific gravity of the liquefied gas can vary for example, from 0.500 to 0.580. This can cause measuring or metering inaccuracies in the order of 10%. However it has been discovered that the dielectric constant of the gas in the liquid phase varies from about 1.61 to 1.8 with the change in specific gravity mentioned above. Therefore in the preferred embodiment, the sensing means is operative to sense the dielectric constant of the liquefied gas. The sensing means for sensing the electrical property may be associated in use with any convenient component of the gas dispensing system. For example, sensing means may be associated with the main supply, with the vapour eliminator, or may be located within a liquefied gas supply line. In the preferred embodiment the sensing means is located in or operatively associated with the vapour eliminator. The sensing means may include a capacitive device which in use is located so as to be immersed in the liquefied gas in the dispensing system, the capacitive device comprising spaced capacitor plates arranged to be immersed in the liquefied gas so that changes in the dielectric constant of the liquefied gas in which the capacitive plates are immersed will determine the capacitance of the capacitive device. In this embodiment, the sensing signal may comprise an electrical signal, the magnitude, frequency, phase shift or other electrical characteristics of which depend upon the capacitance of the capacitive device thereby providing an indication of the sensed dielectric constant of the liquefied gas. The sensing means in one possible embodiment may include a sensing circuit including an oscillator circuit of which the capacitive device is a component determining the frequency of the output signal of the oscillator circuit, the sensing means further including a frequency responsive circuit operative to generate output signals in response to changes of the frequency of the oscillator output signal, thereby producing output signals which depend upon the density and hence the composition of the liquefied gas. In an alternative embodiment, the sensing means may include a sensing circuit which includes a bridge circuit supplied by an AC source, the bridge circuit including the capacitive device, whereby a change in reactance of an arm of the bridge circuit within which the capacitive device is located causes a change in the output from the bridge circuit, the calculating means being responsive to the output of the bridge circuit. The calculating means may be operative to control the determination of the metered amount of liquefied gas dispensed during a dispensing operation by determining a compensating factor which is dependent upon changes in the sensed parameter related to the density of the liquefied gas. The calculating means may include a memory, the calculating means being operative to determine the compensating factor by comparing a measure of the sensed parameter related to the density of the liquefied gas with compensating factors stored in the memory to thereby obtain from the memory the compensating factor to apply to the determination by the metering means of the amount of liquefied gas dispensed. Alternatively, the calculating means may be programmable and may be operative to calculate a compensating factor by determining from a programmed formula the compensating factor to be applied to the determination of the metered amount of liquefied gas dispensed for a particular value of the sensed parameter related to the density of the liquefied gas. Instead of utilising an electrical or magnetic property, the parameter related to the density of the liquefied gas may comprise the refractive index of the liquefied gas, the sensing means being operative to generate a sensing signal indicative of the sensed refractive index. The present invention also provides a liquefied gas dispensing system for connection to a supply of liquefied gas, the system including metering means for metering and determining the amount of liquefied gas dispensed during a dispensing operation, and a liquefied gas metering apparatus as claimed in any one of the preceding claims, the metering means being responsive to the calculating means and being operative to provide a metered liquefied gas determination which is compensated for variations in the density of the liquefied gas. Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention. In the drawings: Fig. 1 shows a schematic circuit for a gas metering system incorporating gas metering apparatus according to the present invention, and Fig.2 shows schematically a possible construction of a capacitive sensing means. The drawings illustrate a liquefied gas dispensing system, particularly for dispensing LPG or other liquefied gas in filling vehicle fuel tanks. The system illustrated includes a vapour remover 10 which receives liquefied gas through line 11 from a supply tank (not shown) and an associated pump (not shown). The liquefied gas is pressurised by the pump. The vapour remover 10 comprises a tank in which gas phase can separate from the liquid phase, the gas phase being returned through the vapour return line 12 to the supply tank. The line 12 is provided with a check valve 13 and double check 14 as is known in the art. The return line 12 also includes the solenoid valve 55 which is operated by the sensing means 50 which is electrically responsive to the level of liquid in the tank 10. Supply line 15 extends from the tank 10 to a filling coupling 16, e.g. of the kind for connection to a vehicle liquefied gas fuel tank. Also provided in the supply line 15 is metering means 17 downstream of the tank 10 and operative to measure the amount of liquefied gas dispensed during a dispensing operation. The metering means may be of conventional type having a measuring chamber which has a rotary element on a shaft, the rotation of the shaft being used by the system electronics to calculate the total amount and cost of liquefied gas dispensed. This calculation is compensated or adjusted depending on the output of the sensing means 50 which varies with changes in dielectric constant of the liquefied gas. This will be described in detail below. Downstream of the metering means 17 is a dispensing control valve 20 for controlling the flow of liquefied gas in the supply line 15 between the metering means 17 and the filling coupling 16. The control valve 20 has an inlet port 21 receiving liquefied gas from the metering means 17 and an outlet port 22 for liquefied gas to be conveyed e.g. via conventional I.S.C. valve 23, sight gauge 24 and line break coupling 25 to the coupling 16. The control valve 20 is operable to open the supply line 15 if liquefied gas is pressurised upstream thereof, whereby liquefied gas dispensing flow can occur only if the liquefied gas is pressurised and whereby a significant liquefied gas pressure drop upstream of the control valve 20 will result in closing of the supply line 15. In Fig. 1 the control valve 20 is a pilot operable control valve, or differential valve, having a pilot line port 30 in selective communication via a pilot line 31 with a source of pressurised pilot fluid. The control valve 20 is operable in response to selective application of pilot fluid pressure in the pilot line 31 to open the control valve 20 to allow liquefied gas flow from the inlet port 21 to the outlet port 22. The pilot line 31 is selectively communicable with the pressurised liquefied gas in the vapour remover 10. This is achieved by providing selectively operable pilot control valve 32 in the pilot line 31. The pilot control valve 32 includes a pilot outlet 33 connected by the pilot line 31 to the control valve 20, a low pressure inlet 35 connected to a source of relatively low pressure liquefied gas, namely to the vapour return line 12, and a high pressure inlet 34 connected to the pressurised liquefied gas upstream of the valve 20. As shown the high pressure inlet 34 can be connected to the tank 10. The pilot control valve 32 is selectively operable to connect either the low pressure ir,iet 35 to the pilot outlet 33 or the high pressure inlet 34 to the pilot outlet 33. A solenoid valve 61 is provided in the pilot line 36 to allow pressurised pilot fluid to be applied to the control valve 20, the valve 61 also being controlled in response to the sensing means 50 associated with the tank 10. Preferably there is a normal fail-safe condition of the pilot control valve 32 which comprises connection of the low pressure inlet 35 to the pilot outlet 33, resulting in low pressure in the pilot line 31 and the control valve 20 being closed to liquefied gas flow therethrough. The pilot control valve 32 may be electrically operable, e.g. solenoid operated, to switch between two conditions corresponding respectively to connection of the low pressure inlet 35 to the pilot outlet 33 and connection of the high pressure inlet 34 to the pilot outlet 33. In particular the solenoid has two states: (1) not energised - corresponding to connection of inlet 35 to outlet 33 and closure of inlet 34, resulting in low pressure in pilot line 31 and control valve 20 being closed to liquefied gas flow; and (2) energised - corresponding to connection of inlet 34 to outlet 33 and closure of inlet 35, resulting in high pressure in pilot line 31 and opening of control valve 20 to liquefied gas flow therethrough. The system shown in the drawing may be operated under control of a circuit (not shown) for energising and de-energising the solenoid of pilot control valve 32 so as to cause the dispensing control valve 20 to open the supply line 15 for a short time interval following start-up of the supply pump and before dispensing through the filling coupling 16 commences so as to thereby allow pressurisation or purging of vapour in the supply line 15. This time interval may be in the order of one - two seconds. After this the control circuit causes the dispensing control valve 20 to close for a period during which the metering means 17 is reset to zero litres and zero cost. Subsequently the control circuit causes the dispensing control valve 20 to re-open for enabling metered dispensing of liquefied gas through the supply line 15 and filling coupling 16. The described operation of the pilot control valve 32 may only be possible if the valve 61 in line 36 is open following vapour elimination from tank 10. In Fig, 1 there is shown a second or duplicated series of components so that the system can enable two simultaneous dispensing operations. The repeated system components have the same reference numerals with the added suffix "a". The operation of the second series of components is exactly the same as the first series of components described above. There is a single common vapour remover tank 10 which is used for supplying liquefied gas to both the supply lines 15, 15a. This is achieved by providing a supply junction 40 between the tank 10 and the metering means 17, 17a. The supply junction 40 includes an inlet 41 and two outlets 42, 43 in communication with the inlet 41. Outlet 42 is connected to supply line 15 and outlet 43 to supply line 15a. The inlet 41 of the supply junction 40 is in fluid communication with the bottom of tank 10. This location of the inlet 41 enables duplicated components of the two dispensing lines to be closely arranged within a housing such as a standard fuel supply bowser provided at service stations. Normally with the outlet from the tank 10 in the past being provided in the side of the tank 10, generally opposite the inlet 11, there has been insufficient space within the standard bowser casing for duplication of other components, at least without having a relatively long length of line from the tank outlet to the metering means 17, 17a. This length of line from the tank 10 to the metering means 17, 17a is preferably minimised in order to minimise vapour phase arising in that length of line which might interfere with metering accuracy and for this purpose the inlet 41 of the supply junction 40 is preferably closely connected to the bottom of the tank 10 and the outlets 42, 43 are closely adjacent the respective metering means 17, 17a. In Fig. 1 this distance from the bottom of the tank 10 to metering means 17, 17a is merely illustrated schematically for describing the function of the. system whereas in practice the physical distance - would be minimised. The preferred dispenser system illustrated in Fig. 1 also enables provision of two dispensing systems within the one standard service station bowser with minimised duplication of components. The sensing means 55 has a sensitive element 56 located in the top of the tank 10 and operative to sense whether the sensitive element is located within liquid phase or within gas phase and being operative to change its electrical characteristics in response to changes in the phase of material in the tank 10 and also to changes in composition of liquid to which the sensitive element 56 is exposed. An electrical signal can be generated on line 57 in response to the change of the electrical characteristics of the sensitive element 56 and the signal can be utilised firstly to open or close the line 12 at the beginning or end of a vapour elimination operation respectively. In particular, the .valve 55 comprises a solenoid valve which, for example, may be normally open but when the sensitive element 56 becomes immersed in liquid phase, which occurs when vapour is substantially or completely eliminated from the tank 10 through the return line 12, the signal generated by the sensing means 50 may switch power to the solenoid so as to close the return line 12. The sensitive element 56 comprises a capacitive element 70 whose capacitance changes depending on whether the element 70 is immersed in gas or in liquid and depending on the density of the liquid when fully immersed in liquid. In Fig. 2, the capacitive element 70 comprises two conductive plates 71, 72 which are arranged generally parallel and spaced apart, the plates being arranged within the tank 10 at the top so that the fluid, whether it be gas or liquid, within the tank 10 flows between the plates 71, 72, the capacitance of the element 70 changing depending upon the size and spacing of the plates 71, 72 and the dielectric properties of the gas phase and liquid phase in which the plates are immersed. The capacitive element 70 is connected within a sensing circuit 65 of the sensing means 50, In Fig. 2, the sensing circuit components are mounted on a circuit board 74 which also supports one of the plates 72, the circuit components being encapsulated in housing 75. The sensing circuit 65 in Fig. 1 comprises an oscillator circuit 58 of which the capacitive element 70 is a component determining the frequency of the oscillator circuit. The sensing means 50 further includes a frequency responsive circuit 59 operative to produce an output in response to a predetermined change of frequency sensed by that circuit 59. With this arrangement, the frequency of the oscillator 58 changes as a result of the capacitive element 70 being immersed in liquid phase after initially being located in gas phase and dependent upon the density of the liquid phase, and output signals on lines 57 and 77 can be produced. The output signal on line 57 is used to switch a solid state relay 60 in response to sensed phase changes, the relay 60 in turn switching power to and from the solenoid valve 55 located in a return line 12. The output signal on line 57 is also coupled via switching relay 62 to the solenoid valve 61 located in the pilot supply line 36 extending to the pilot operated dispensing control valve 32. The location and function of the dispensing control valve 32 has been described earlier. By providing the further solenoid valve 61 in the pilot line 36 to the dispensing control valve 32, it is possible to prevent the opening of the dispensing control valve 32 for as long as elimination of vapour is progressing. Effectively this provides a further control to enable prevention of premature dispensing operations leading to incorrect metering of dispensed liquefied gas. During operation of the liquefied gas dispensing system, after initial start-up of the system for initiating a dispensing operation, the supply line 15 which may be a flexible hose may be temporarily opened and the pump started to ensure that the line 15 is filled with liquid phase. This temporary opening may be carried out particularly if the system has not been used for some time, e.g. 15 minutes, or if vapour was detected during the previous dispensing operation. After this preliminary procedure, the system may check for the presence of vapour in the vapour eliminator. If vapour is detected, the system may prevent dispensing flow until vapour is no longer detected. During the dispensing operation, the presence of vapour can be continually monitored so that, if vapour is detected, the dispensing operation can be terminated and, at the same time, a flag can be set to initiate the hose 'filling operation described above upon initiation of the next dispensing operation. Returning to Fig. 1, the apparatus further includes calculating means 76 responsive to the sensing signal on line 77. When the sensing element 56 is immersed in liquefied gas, the calculating means 76 is operative to affect the determination by the metering means 17 of the metered gas dispensed during a dispensing operation so as to compensate the determination of the metered amount of gas for changes in the dielectric constant of the gas and thereby compensate the determination of the metered amount of gas for changes in gas composition. The sensed capacitance change of element 56 can be converted by the sensing means 50 to a digital output on line 77 that will in turn provide for example a measure of the dielectric constant. From calibration or predetermination of the relationship between the dielectric constant and the errors in the measuring operation of the metering means 17 the compensation required can be determined. In this way, the sensing signal on line 77 from the sensing means 50 can be fed to the calculating means 76 which in turn can control determination of the metered amount of liquefied gas dispensed by compensating for changes in the sensed dielectric constant. The determination of the compensation factor can be by means of a predetermined table of compensating factors stored in a memory 78 so that the specific gravity or electrical parameter dependent on the specific gravity determined can be compared by the calculating means to values in a look-up table in the memory 78 to obtain from the table a compensating factor to apply in the metering operation so as to ensure an accurate measuring of gas dispensed. Alternatively the calculating means 76 may be programmable and be operated to calculate or determine from a programmed formula a factor by which the measuring operation is adjusted or compensated. For example, the calculating means 76 may comprise a programmable calculating means which in use is programmed so as to compute from an appropriate formula or algorithm the compensating factor to be applied to the determination of the amount of liquefied gas dispensed. It will be appreciated that the sensing element 56 need not be located within an oscillator circuit 58 which in turn is monitored by a frequency responsive circuit 59. For example, the capacitive element 70 may be located within a bridge circuit supplied by an AC source so that the change in reactance of an arm of the bridge within which the capacitive element is located causes a change in the output which can be sensed and used to compensate the metered amount of gas. The determination of the compensation to be applied in the measurement of gas dispensed during a dispensing operation may be carried out on a "once off" basis. For example, the dielectric constant may be sensed at the beginning of a dispensing operation and a compensating factor determined which is thereafter applied to the operation of the metering means 17 for the remainder of the liquefied gas dispensing operation. Alternatively, the determination of the specific gravity may be carried out continuously or at intervals during a dispensing operation so that the compensation of the metered amount of gas can be continuously or continually carried out during the dispensing operation. It will be seen that the apparatus according to the preferred embodiment of the present invention as herein described and illustrated can enable more accurate determination of liquefied gas dispensed during a dispensing operation by compensating for the specific gravity changes that occur with variations in composition of the liquefied gas being supplied. This increase in accuracy of the metering operation can substantially reduce unfair transactions in which either the purchaser of the liquefied gas or the supplier of the liquefied gas is being disadvantaged by metering inaccuracies occurring in the past. The specific gravity is a property related to the density of the liquefied gas. In the past, temperature changes have been sensed to enable a compensation factor to be applied during metering operations. Use of the present invention to determine and apply a compensating factor based on a density related sensing enables elimination of the temperature sensing since the temperature variations affecting density and hence the metering accuracy will automatically be compensated by the present invention. It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiment(s) of the invention as herein described without departing from the scope of the invention as defined in the claims.

Claims: CLAIMS 1. A liquefied gas metering apparatus for use with a liquefied gas dispensing system, the system having a supply of liquefied gas, liquefied gas dispensing equipment which in use conveys liquefied gas from the supply to a dispensing point, and metering means (17) associated with the dispensing system, the metering apparatus being characterised by sensing means (50) for operative association with the liquefied gas dispensing system and operative to sense a measurable parameter of the liquefied gas related to the density of the liquefied gas and to generate a sensing signal (17) indicative of the sensed parameter, the apparatus being further characterised by calculating means (76) responsive to the sensing signal, the calculating means (76) being operative to control operation of the metering means (17) of the dispensing system by affecting the determination by the metering means of the metered liquefied gas dispensed during a dispensing operation so as to compensate the determination of the metered amount of liquefied gas for changes in the density of the liquefied gas and so as to thereby compensate the determination of the metered amount of liquefied gas for changes in gas composition. 2. Apparatus as claimed in Claim 1 characterised in that the parameter related to the density of the liquefied gas comprises an electrical or magnetic property of the liquefied gas, the sensing means (50) being operative to generate a sensing signal (77) indicative of the sensed electrical or magnetic property. 3. Apparatus as claimed in Claim 2 characterised in that the sensing means (50) is operative to sense an electrical property of the liquefied gas. 4. Apparatus as claimed in Claim 3 characterised in that the sensing means (50) is operative to sense the dielectric constant of the liquefied gas. 5. Apparatus as claimed in Claim 4 characterised in that the sensing means (50) includes a capacitive device (56) which in use is located so as to be immersed in the liquefied gas in the dispensing system, the capacitive device (56) comprising spaced capacitor plates (71, 72) arranged to be immersed in the liquefied gas so that changes in the dielectric constant of the liquefied gas in which the capacitor plates are immersed will determine the capacitance of the capacitive device. 6. Apparatus as claimed in Claim 5 characterised in that the sensing signal (77) comprises an electrical signal, the magnitude, frequency, phase shift or other electrical characteristics of which depend upon the capacitance of the capacitive device (56) thereby providing an indication of the sensed dielectric constant of the liquefied gas. 7. Apparatus as claimed in Claim 5 or 6 characterised in that the sensing means (50) includes a sensing circuit (65) which includes an oscillator circuit (58) of which the capacitive device (56) is a component determining the frequency of an output signal of the oscillator circuit (56), the sensing means (50) further including a frequency responsive circuit (59) operative to generate output signals in response to changes of the frequency of the oscillator output signal, thereby producing output signals (77) which depend upon the density and hence the composition of the liquefied gas. 8. Apparatus as claimed in Claim 5 or 6 characterised in that the sensing means (50) includes a sensing circuit (65) which includes a bridge circuit supplied by an AC source, the bridge circuit including the capacitive device (56), whereby a change in reactance of an arm of the bridge circuit within which the capacitive device is located causes a change in the output from the bridge circuit, the calculating means (76) being responsive to the output of the bridge circuit. 9. Apparatus as claimed in any one of the preceding claims characterised in that the calculating means (76) is operative to control the determination of the metered amount of liquefied gas dispensed during a dispensing operation by determining a compensating factor which is dependent upon changes in the sensed parameter related to the density of the liquefied gas. 10. Apparatus as claimed in Claim 9 characterised in that the calculating means (76) includes a memory (78), the calculating means (76) being operative to determine the compensating factor by comparing a measure of the sensed parameter related to the density of the liquefied gas with compensating factors stored in the memory (78) to thereby obtain from the memory the compensating factor to apply to the determination by the metering means (17) of the amount of liquefied gas dispensed. 11. Apparatus as claimed in Claim 9 wherein the calculating means (76) is programmable and is operative to calculate a compensating factor by determining from a programmed formula the compensating factor to be applied to the determination of the metered amount of liquefied gas dispensed for a particular value of the sensed parameter related to the density of the liquefied gas. 12. Apparatus as claimed in Claim 1 characterised in that the parameter related to the density of the liquefied gas comprises the refractive index of the liquefied gas, the sensing means (50) being operative to generate a sensing signal (77) indicative of the sensed refractive index. 13. A liquefied gas dispensing system for connection to a supply of liquefied gas, the system including metering means (17, 17a) for metering and determining the amount of liquefied gas dispensed during a dispensing operation, and a liquefied gas metering apparatus as claimed in any one of the preceding claims, the metering means (17, 17a) being responsive to the calculating means (76) and being operative to provide a metered liquefied gas determination which is compensated for variations in the density of the liquefied gas.

Inventors: CLARK JOHN KEITH, PAHL RONALD EDWIN, CLARK, JOHN, KEITH, PAHL, RONALD, EDWIN
Applicants: LPG ENG PTY LTD, LPG ENGINEERING PTY. LTD.
Application year: 1991
Main classification: G01F15
IPC codes: B67D5, F17C13, G01F1, G01F15, G01N9, G01N27, G01N33