WO1991014130A1

WO1991014130A1 - Improvements in liquefied gas dispensing

Info

Publication number: WO1991014130A1
Application number: PCT/AU1991/000075
Authority: WO
Inventors: John Keith Clark; Ronald Edwin Pahl
Original assignee: Lpg Engineering Pty. Ltd.
Priority date: 1990-03-05
Filing date: 1991-03-05
Publication date: 1991-09-19
Also published as: CA2077688A1; NZ237322A

Abstract

A vapour eliminator and liquified gas dispensing system comprising a vessel (10) which is connected to receive liquified gas from a supply. The vessel (10) has a vapour discharge line (12) connected to the top (51) of the vessel so that vapour can be discharged. Sensing means (50) has a capacitive element (70) located in the top (51) of the vessel and which changes its capacitance in response to change in the phase of material in the vessel (10) to which the capacitive element (70) is exposed. The sensing means (50) generates phase signals in response to changes of the capacitance of the element (70) whereby the phase signals can be utilised to open or close the vapour discharge line (12) at the beginning or end of a vapour elimination operation respectively. The sensing circuit (65) may comprise an oscillator circuit (58) of which the capacitive element (70) is a component determining the frequency of the oscillator circuit, and a frequency responsive circuit (59) operative to produce an output signal in response to a predetermined change of the oscillator frequency. Alternatively, the sensing circuit (65) may comprise a bridge circuit supplied by an AC source, the capacitive element (70) being located in an arm of the bridge circuit whereby a change in reactance of the arm of the bridge within which the capacitive of element is located causes a change in the output of the bridge circuit thereby causing generation of the phase signal. Alternatively, the sensing circuit (65) may include a pulse generator (71), the characterisation of the pulse train depending on the capacitance of the element (70).

Description

IMPROVEMENTS IN LIQUEFIED GAS DISPENSING This invention relates to the dispensing of liquefied gas, and particularly, but^" not exclusively, to the dispensing of LPG, e.g. for automotive fuel supply. A currently used system for dispensing LPG from a supply tank utilises a vapour eliminator located in the supply line upstream of the metering means which measures the amount of liquefied gas being dispensed. The purpose of the vapour eliminator is to remove all gas phase or vapour from the line through which the dispensing flow takes place since the presence of the gas or vapour can lead to inaccuracies in the measured fuel dispensed. The gas or vapour can build up in the system upstream of the metering means particularly if there is a relatively long delay between successive operations of the system. Existing types of vapour eliminator are as follows:

1. Float types in which a float valve in the top of a chamber of the vapour eliminator rises when the chamber fills with liquid and shuts off a return line to the supply tank. When vapour enters or forms in the chamber, the float valve falls and allows the vapour in the top of the chamber to return to the supply tank.

2. Constant bleed types in which the vapour eliminator has a small opening in the top that allows a small amount of liquid (and/or vapour) to return to the tank at all times. A very sensitive differential valve is used in this type of system to be able to sense when vapour is being eliminated to shut down the meter.

The float type vapour eliminators rely on some mechanical components which can be difficult or costly to manufacture, assemble and service and can lead to mechanical faults and unreliability. The constant bleed type vapour eliminators, by relying on very sensitive differential valves, can be relatively complex and expensive and can be prone to development of faults because of the sensitivity of the valves.

It is an object of the present invention to provide a vapour eliminator and a liquefied gas dispensing system incorporating such a vapour eliminator and in which the above described disadvantages of the prior art can be overcome or alleviated. According to the first aspect of the present invention there i provided a vapour eliminator for a liquified gas dispensing system the vapour eliminator comprising a vessel which in use in th dispensing system is connected to receive liquified gas from supply, the vessel in use having a vapour discharge line connected t the top of the vessel so that vapour can be discharged from th vessel through the discharge line, the vapour eliminator furthe including sensing means having a sensitive element located in use i the top of the vessel, the sensitive element being operative t change its electrical characteristics in response to changes in th phase of material in the vessel to which the sensitive element i exposed, the sensing means being operative to generate phase signal in response to predetermined changes of the electrica characteristics of the sensitive element whereby the phase signal can be utilised to open or close the vapour discharge line at th beginning or end of a vapour elimination operation respectively.

Preferably the sensitive element of the sensing means comprise a capacitive element whose capacitance changes depending upon whethe the element is immersed in gas or in liquid. The capacitive elemen may comprise two conductive plates which are arranged generall parallel and spaced apart, the plates being arranged to be located i use within the vessel at the top thereof so that the fluid, whethe it be gas or liquid, within the vessel enters the space between th plates, the capacitance of the element changing depending upon th dielectric properties of the gas phase and liquid phase in which th plates are immersed, the sensing means including a sensing circuit i which the capacitive element is connected.

In one possible embodiment, the sensing circuit comprises a oscillator circuit of which the capacitive element is a componen determining the frequency of the oscillator circuit, the sensin means further including a frequency responsive circuit operative t produce an output signal in response to a predetermined change o frequency sensed by the frequency responsive circuit, whereby th change in the frequency of the oscillator resulting from th capacitive element being immersed in liquid phase after initiall being located in gas phase, enables the output signal to b produced. In an alternative possible embodiment, the sensing circui comprises a bridge circuit supplied by an AC source, the capacitiv element being located in an arm of the bridge circuit whereby a change in reactance of the arm of the bridge within which the capacitive of element is located causes a change in the output of the bridge circuit thereby causing generation of the phase signal. In a further possible embodiment the sensing circuit may comprise a pulse generator, the output pulses therefrom being modified by the capacitive element 70, the output of the pulse generator being supplied to one input of a comparator, the comparator having a second input supplied with a constant voltage, the sensing circuit being constructed so that the output of the comparator switches between states in response to predetermined variations in capacitance of the capacitive element depending on whether that element is immersed in liquid or gas phase.

The sensing means may include liquid stabilising or calming means associated with the sensitive element and operative to subdue or eliminate ripples or waves on the surface of the liquid in the vessel as the liquid in the vessel reaches and commences to immerse the sensitive element whereby instability or uncertainty in the operation of the sensing means and generation of the phase signal is reduced. Also to reduce instability at the transition point of sensing a change in phase, the sensing means may include circuitry providing hysteresis in its operation so that the liquid level in the vessel must reach a high level in which the sensitive element is substantially completely immersed in liquid before the phase signal is generated and so that, when the liquid level in the vessel is falling due to a build up of gas phase within the vessel, a liquid level significantly lower than said high level must be reached before the phase signal indicating presence of the gas phase is generated. In a further or alternative possible instability reducing embodiment the sensing means may include a delay circuit, the delay circuit being operative to delay generation of the phase signal during a period of rising liquid level or a period of falling liquid level so as to enable liquid levels where some instability in generation of the phase signal may occur can be passed before the phase signal can be generated.

According to the second aspect of the invention there is provided a liquified gas dispensing system including a vapour eliminator according to the first aspect of the invention, the vessel being connected to receive liquified gas from a supply, the system further including a liquified gas supply line extending from the vessel to metering means for measuring the flow of liquified gas and thence to a discharge outlet, the vapour discharge line including a selectively operable valve responsive to the phase signal from the sensing means so as to open and close the discharge line to the flow of vapour in response to the phase signal.

The valve in the vapour discharge line preferably comprises a solenoid valve which is normally open but which is closed when the sensitive element of the sensing means becomes immersed in liquid phase in the vessel which occurs when vapour is substantially or completely eliminated from the vessel through the vapour discharge line, the closure of the solenoid valve occurring in response to generation of the phase signal by the sensing means which initiates switching of power to the solenoid so as to close the vapour discharge line. When the solenoid valve closes the vapour discharge line, a small bleed flow of fluid may be possible through the vapour discharge line.

The vessel preferably comprises a pressure vessel having a domed top, the sensitive element of the sensing means being located at the highest point of the domed top, the sensitive element being closely adjacent the vapour discharge line which taps into the top of the vessel at substantially the same highest point.

The supply line may include a pilot operated dispensing control valve to control flow of liquified gas through the supply line, the control valve being operable by means of a supply of pressurised pilot fluid connected to the control valve through a pilot supply line, the pilot supply line including a solenoid valve operable to control supply of pressurised pilot fluid to the control valve, the solenoid valve located in the pilot supply line being operable in response to the phase signal from the sensing means to prevent opening of the control valve for as long as the sensing means is sensing gas phase in the vessel and elimination of gas from the vessel through the vapour discharge line is progressing, thereby preventing premature dispensing of liquified gas through the supply line leading to incorrect metering of dispensed liquified gas.

Possible and preferred features of the present invention are illustrated in and 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 is a schematic view of a liquefied gas dispenser system according to a possible and preferred embodiment of the present invention,

Fig. 2 illustrates a possible capacitive element for sensing the presence of vapour in the tank, and Fig. 3 is a circuit diagram for a possible sensing circuit.

Fig. 1 illustrates 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 tank 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. In the tank 10, 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 according to the preferred embodiment of the present invention which is operated by the sensing means 50 which is electrically responsive to the level of liquid in the tank 10. This will be more fully described later. 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 of liquid dispensed and the cost of that dispensed gas.

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 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 tank 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 inlet 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, that solenoid valve 61 also being controlled in response to the sensing means 50 associated with the tank 10 as described later. 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 Fig. 1 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 to 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 reopen for enabling metered dispensing of liquefied gas through the supply line 15 and filling coupling 16.

The described operation of the pilot control valve may only be possible if the valve 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 vapour remover 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 liquid 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 vapour eliminator 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 liquid tank 10 and the outlets 42, 43 are closely adjacent the respective metering means 17, 17a. In the drawing 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 vapour eliminator according to the present invention includes the tank 10 which has domed ends, particularly the upper end 51 with the vapour discharge or return line 12 being connected to the upper end of the tank 10 generally centrally of the domed shape.

The vapour eliminator further includes sensing means 55 having 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 to which the sensitive element 56 is exposed. An electrical phase signal can be generated on line 57 in response to the change of the electrical characteristics of the sensitive element 56 and the phase signal can be utilised to open or close the line 12 at the beginning or end of a vapour elimination operation respectively. The liquefied gas dispensing system in Fig. 1 further includes a selectively operable valve 55 located within the vapour discharge line 12. The valve 55 is responsive to the sensing means 50 so as to open and close the return line 12 to the flow of vapour in response to the phase signal on line 57. The selectively operable 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 substantiallyor completely eliminated from the tank 10 through the return line 12, the phase signal generated by the sensing means 50 may switch power to the solenoid so^" as to close the return line 12,

In the preferred embodiment when the solenoid valve 55 closes the return line 12, there is preferably a very small or bleed flow possible through the return line 12. The provision for a bleed flow will not affect significantly the operation of the dispensing system but may enable the elimination of a hydrostatic valve which may be needed if a complete closure of the return line 12 is otherwise effected. 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. 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 dielectric properties of the gas phase and liquid phase in which the plates are immersed. With this arrangement, the capacitive element 70 may be coupled 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 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 an output phase signal on line 57 can be produced. The output signal is used to switch a solid state relay 60 which in turn can switch power to and from the solenoid valve 55 located in a return line 12.

It will be appreciated that the sensitive element 56 need not be a capacitive element but may be some other sensitive element or transducer which changes electrical properties or produces a signal upon a change in the phase of the medium within which it is located. Similarly, if it is found that a capacitive element is to be preferred, the capacitive element 70 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 (not illustrated) supplied by an AC source so that the change in reactance of an arm of the bridge within which the capacitive element 70 is located causes a change in the bridge output which can be sensed and used to open or close the solenoid valve 55 in the return line 12.

In Fig. 3 the sensing circuit comprises a pulse generator 71, e.g. CMOS circuit type 555, for generating pulses. The duration of each pulse may be set by resistor 72 and capacitor 73 and the duration between pulses controlled by capacitive element 70. When the output of the pulse generator 71 on line 75 goes high, the capacitive element 70 charges rapidly through diode 76. When the output line 75 goes low, the capacitive element 70 discharges slowly through resistor 77. Capacitor 78 functions to average or to smooth the output which is supplied to input 79 of voltage comparator 80. The other input 81 on the voltage comparator 80 is connected across the voltage divider formed by resistors 82, 83 which are connected between the supply rail 85 and earth rail 86. Transistor 87 comprises the load between supply rail 85 and earth 86 so that the switching of the voltage comparator in response to variations in the capacitance of the capacitive element 70, corresponding to immersion of the capacitive element 70 in liquid and gas phase, can be detected by switching of the transistor 87.

The sensitive element 56 is located at the highest point within the tank 10. In the case where the tank 10 is a pressure vessel having a domed top 51, the sensitive element 56 is located at the highest point of the domed top 51 and is also closely adjacent the return line 12 which taps into the top of the tank 10 at substantially the same highest point. There is a possibility that the change in electrical properties of the sensitive element 56 may be progressive as the liquid level within the tank 10 either rises as the vapour is returned to the supply or falls upon the formation or build up of vapour phase in the tank. In this case, there may some difficulty in reliable switching of the sensing means 50 between its respective states in response to the detection of gas or liquid phase. This, in turn, may cause some instability or uncertainty in the switching of the solenoid valve 55 in the return line 12 for example. To alleviate this possible problem, there may be provided baffles or other such liquid stabilising or calming means surrounding the sensitive element 56 so that, for example, any ripples or waves within the surface of the liquid as the liquid reaches and commences to immerse the sensitive element 56 will not result in repeated switching of the solenoid valve 55 on and off. The baffles may serve the purpose of damping or substantially eliminating any waves in the liquid surface. If desired also the sensing means may include circuitry having some hysteresis within its operation so that, for example, the sensitive element 56 must be substantially completely immersed in liquid phase before the sensing means 50 generates the electrical phase signal on line 57 triggering switching of the solenoid valve 55. Conversely, when the liquid level is falling due to the build up of gas phase within the vapour eliminator tank 10, a liquid level substantially below the first higher level may be required to open the solenoid valve 55, Alternatively, there may be some delay in switching of the relay 60 which in turn controls the power to the solenoid so that the rising or falling liquid level has time to pass any levels where valve switching instability may occur.

The output signal of the sensing means 50 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 utilising the vapour eliminator of the present invention, 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.

It will be seen that the vapour eliminator and the liquefied gas dispensing system utilising the vapour eliminator according to the preferred embodiment of the present invention substantially eliminates the mechanical sensing of vapour phase at the top of the vapour eliminator vessel, thereby enabling the problems of the prior art outlined above to be alleviated or substantially overcome. In particular, the vapour eliminator and system utilising the vapour eliminator according to the preferred embodiment of the present invention can be manufactured readily and relatively simply and the operation can be particularly reliable.

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

13• CLAI MS

1. A vapour eliminator for a liquified gas dispensing system, the vapour eliminator comprising a vessel (10) which in use in the dispensing system is connected to receive liquified gas from a supply, the vessel (10) in use having a vapour discharge line (12) connected to the top (51) of the vessel so that vapour can be discharged from the vessel (10) through the discharge line (12), the vapour eliminator further including sensing means (50) having a sensitive element (56) located in use in the top (51) of the vessel, the sensitive element (56) being operative to change its electrical characteristics in response to changes in the phase of material in the vessel (10) to which the sensitive element (56) is exposed, the sensing means (50) being operative to generate phase signals in response to predetermined changes of the electrical characteristics of the sensitive element (56) whereby the phase signals can be utilised to open or close the vapour discharge line (12) at the beginning or end of a vapour elimination operation respectively.

2. A vapour eliminator as claimed in Claim 1 characterised in that the sensitive element (56) of the sensing means comprises a capacitive element (70) whose capacitance changes depending upon whether the element (70) is immersed in gas or in liquid.

3. A vapour eliminator as claimed in Claim 2 characterised in that the capacitive element (70) comprises two conductive plates (71, 72) which are arranged generally parallel and spaced apart, the plates (71, 72) being arranged to be located in use within the vessel (10) at the top (51) thereof so that the fluid, whether it be gas or liquid, within the vessel enters the space between the plates (71, 72), the capacitanc- of the element (70) changing depending upon the dielectric properties of the gas phase and liquid phase in which the plates (71, 72) are immersed, the sensing means (50) including a sensing circuit (65) in which the capacitive element is connected.

4. A vapour eliminator as claimed in Claim 3 characterised in that the sensing circuit (65) comprises an oscillator circuit (58) of which the capacitive element (70) is a component determining the frequency of the oscillator circuit, the sensing circuit (65) further including a frequency responsive circuit (59) operative to produce an output signal in response to a predetermined change of frequency sensed by the frequency responsive circuit (59), whereby the change in the frequency of the oscillator (58) resulting from the capacitive element (70) being immersed in liquid phase after initially being located in gas phase, enables the output signal to-be produced.

5. A vapour eliminator as claimed in Claim 3 characterised in that the sensing circuit (65) comprises a bridge circuit supplied by an AC source, the capacitive element being located in an arm of the bridge circuit whereby a change in reactance of the arm of the bridge within which the capacitive of element is located causes a change in the output of the bridge circuit thereby causing generation of the phase signal.

6. A vapour eliminator as claimed in Claim 3 characterised in that the sensing circuit (65) comprises a pulse generator (71), the output pulses therefrom being modified by the capacitive element (70), the output of the pulse generator (71) being supplied to one input (79) of a comparator (80), the comparator 80 having a second input (81) supplied with a constant voltage, the sensing circuit 65 being constructed so that the output of the comparator (80) switches between states in response to predetermined variations in capacitance of the capacitive element (70) depending on whether that element (70) is immersed in liquid or gas phase.

7. A vapour eliminator as claimed in any one of the preceding claims characterised in that the sensing means (50) includes liquid stabilising or calming means associated with the sensitive element (56) and operative to subdue or eliminate ripples or waves on the surface of the liquid in the vessel (10) as the liquid in the vessel reaches and commences to immerse the sensitive element (56) whereby instability or uncertainty in the operation of the sensing means (50) and generation of the phase signal is reduced.

8. A vapour eliminator as claimed in any one of the preceding claims characterised in that the sensing means (50) includes circuitry providing hysteresis in its operation so that the liquid level in the vessel (10) must reach a high level in which the sensitive element (56) is substantially completely immersed in liquid before the phase signal is generated and so that, when the liquid level in the vessel (10) is falling due to a build up of gas phase within the vessel (10), a liquid level significantly lower than said high level must be reached before the phase signal indicating presence of the gas phase is generated.

9. A vapour eliminator as claimed in any one of Claims 1 to 7 characterised in that 'the sensing means (50) includes a delay circuit, the delay circuit being operative to delay generation of the phase signal during a period of rising liquid level or a period of falling liquid level so as to enable liquid levels where some instability in generation of the phase signal may occur can be passed before the phase signal can be generated.

10. A liquified gas dispensing system characterised by a vapour eliminator as claimed in any one of the preceding claims, the vessel

(10) being connected to receive liquified gas from a supply, the system further including a liquified gas supply line (15) extending from the vessel (10) to metering means (17) for measuring the flow of liquified gas and thence to a discharge outlet (16), the vapour discharge line (12) including a selectively operable valve (55) responsive to the phase signal from the sensing means (50) so as to open and close the discharge line to the flow of vapour in response to the phase signal.

11. A system as claimed in Claim 10 characterised in that the valve (55) in the vapour discharge line (12) comprises a solenoid valve which is normally open but which is closed when the sensitive element (56) of the sensing means (50) becomes immersed in liquid phase in the vessel (10) which occurs when vapour is substantially or completely eliminated from the vessel through the vapour discharge line (12), the closure of the solenoid valve (55) occurring in response to generation of the phase signal by the sensing means (50) which initiates switching of power to the solenoid so as to close the vapour discharge line (12).

12. A system as claimed in Claim 11 characterised in that, when the solenoid valve (5) closes the vapour discharge line (12), a small bleed flow of fluid is possible through the vapour discharge line (12).

13. A system as claimed in any one of Claims 10 to 12 characterised in that the vessel (10) comprises a pressure vessel having a domed top (51), the sensitive element (56) of the sensing means (50) being located at the highest point of the domed top (51), the sensitive element (56) being closely adjacent the vapour discharge line (12) which taps into the top of the vessel (10) at substantially the same highest point.

14. A system as claimed in any one of Claims 10 to 13 characterised in that the supply line (15) includes a pilot operated dispensing control valve (20) to control flow of liquified gas through the supply line (15), the control valve (20) being operable by means of a supply of pressurised pilot fluid connected to the control valve (20) through a pilot supply line (31), the pilot supply line including a solenoid valve (61) operable to control supply of pressurised pilot fluid to the control valve (20), the solenoid valve (61) located in the pilot supply line (31) being operable in response to the phase signal from the sensing means (50) to prevent opening of the control valve (20) for as long as the sensing means (50) is sensing gas phase in the vessel (10) and elimination of gas from the vessel through the vapour discharge line (12) is progressing, thereby preventing premature dispensing of liquified gas through the supply line (15) leading to incorrect metering of dispensed liquified gas.