Note: Descriptions are shown in the official language in which they were submitted.
W 0 91/14130 2 ~ 7 7 ~ 8 8 PCTtAU91tO0075
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
automo~ive 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 ob~ect of the present invention to provide a vapoureliminator 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.
W O 91/14130 ~ 8:8 P ~ /AU91/000~
According to the first aspect of the present invention there is
provided a vapour eliminator for a liquified gas dispensing system,
the vapour eliminator comprising a vessel which in use in the
dispensing system is connected to receive liquified gas from a
supply, the vessel in use having a vapour discharge line connected to
the top of the vessel so that vapour can be discharged from the
vessel through the discharge line, the vapour eliminator further
including sensing means having a sensitive element located in use in
the top of the vessel, the sensitive element being operative to
change its electrical characteristics in response to changes in the
phase of material in the vessel to which the sensitive element is
exposed, the sensing means being operative to generate phase signals
in response to predetermined changes of the electrical
characteristics of the sensitive element whereby the phase signals
can be utilised to open or close the vapour discharge line at the
beginning or end of a vapour elimination operation respectively.
Preferably the sensitive element of the sensing means comprises
a capacitive element whose capacitance changes depending upon whether
the element is immersed in gas or in liquid. The capacitive element
may comprise two conductive plates which are arranged generally
parallel and spaced apart, the plates being arranged to be located in
use within the vessel at the top thereof so that the fluid, whether
it be gas or liquid, within the vessel enters the space between the
plates, the capacitance of the element changing depending upon the
dielectric properties of the gas phase and liquid phase in which the
plates are immersed, the sensing means including a sensing circuit in
which the capacitive element is connected.
In one possible embodiment, the sensing circuit comprises an
oscillator circuit of which the capacitive element is a component
determining the frequency of the oscillator circuit, the sensing
means further including a frequency responsive circuit operative to
produce an output signal in response to a predetermined change of
~frequency sensed by~ the frequency responsive circuit, whereby the
change in the frequency of the oscillator resulting from the
capacitive element being immersed in liquid phase after initially
being located in gas phase, enables the output signal to be
produced. In an alternative possible embodiment, the sensing circuit
comprises a bridge circuit supplied by an AC source, the capacitive
~W O 91/14130 2 0 7 7 6 8 8 PCT/AU91/00075
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 si8nificantly lower than said high level must be reached before
the phase signal indicating presence of the gas phase is generated.
In a further or aiternative 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
W O 91/14130 PCT/AU91/000~
bei ~ ~o~ ~ c8t8d 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 meaos 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. Uhen the solenoid valve closes the vapour dischar~e
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
W O 91/14130 2 a 7 7 6 8 8 PCT/AU91/00075
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 limitin~ 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 dispensin~ 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 t2
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. T~1e 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
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 ~o be
W O 91/14130 , PCT/AU91/00075
2 ~ 7`~ 6 ~ 8 " 6
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. I 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. - ;- - -
35 ` 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
~ 0 91/14130 2 0 7 7 6 8 8 PCT/AU91/00075
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. I 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
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. I 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
- 30 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 ~unction 40 includes an inlet 41 and two outlets 42, 43 in
communication with the inlet 41. Outlet 42 is connected to supply
35 - line 15 and outlet 43 to supply line 15a. The inlet 41 of the supply
~unction 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
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W O 91/14130 PCT/AU91/OOOZ~
20776~8 8
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
W O 91/14130 2 ~ 7 7 6 8 8 PCT/AU91/00075
.. . .
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 substantially or 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 dependin~ on whether the element 70 is
immersed in gas or in liquid. In Fig. 2, the capacitive element 70
comprises ewo conductive plates 71, 72 which are arran&ed generally
parallel and spaced apart, the plates being arranged within the tank
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. I 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 S9 operative
~ to produce an output `in response to a predetermined change of
frequency sensed ;by that circuit 59. ~ith 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
:
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W O 91/14130 2 0 ~ 8 I r 10 PCT/AU91/000 ~
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 lQ 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.
W O 91/14130 2 0 7 7 6 8 8 PCT/AU91/00075
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
hys~eresis 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.
W O 91/14130 ~ ~ q q ~ ~ ~ 12 PCT/AU91/000 ~
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 chec~ 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.
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