Note: Descriptions are shown in the official language in which they were submitted.
WO 92/02788 2 ~ ~ 8 7 8 ~ PCI`/AlJ9t/00341
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
~eter 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.~., from substantially pure propane
~as 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 tha~ is c~rried 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 c~mpositions.
Accordin~ to the, present invention there is provided a
liquefied gas meterin~ 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~as
from the supply to a dispensing point, and metering means associated
with the dispensing system, the metering apparatus including sensin~
means for operative association with the lique~i~d gas dispensing
system and operative to sense a measurable parameter of the liquefied
~as 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,
~he calculating means being operative ~o control operation of the
metering means of the dispensing system by affec:ing the
determination by the metering means of the metered liquefied ~as
dispensed during a dispensing opera~ion so as to compensate the
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W O 92/02788 PCT/AU91/00341
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determination of the metered amount of liquefied gas for changes in
the density of the liquefied gas and so as to thereby compensate the
deter~ination of the metered amount of liquefied gas for changes in
gas composition.
S In one embodiment, the parameter related to the density of the
liquefied gas comprises an electrical or magne~ic 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 ~.580. This can
cause measuring or metering inaccuracies in ~he 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 se~se the dielectric
constant of the liquefied gas.
The sensing means for sensing the electrical property may be
2a associated in use with any convenient component of the gas dispensi~g
system. For example, sensin~ 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 sensin~
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
dlspensing 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, phas~ shift
nr 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 ~ component de~ermining the f requency of the
W O 92/02788 2 0 ~ ~ 7 PCT/AU91/0~341
output signal of th~ oscillator cir~uit, the sensing means further
including a frequency responsive circuit operative to generate output
signals in response to chan~es of the frequency of ~he oscillator
output signal, thereby producing ou~put signals which depend upon the
S 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 uf the
bridge circuit.
rhe calculating means may be operative to c~ntrol the
determination cf the metered amount ~f liquefied gas dispensed during
a dispensin~ 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 ~o calculate a compensating factor
2S by determining from a programmed formula the compensating factor to
be applied to the determination of the metered amount of liguefied
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 ~o generate a sensing signal indicative of the sensed
refractive index.
The present invention also pro~ides a liquefied gas dispensing
3S system for conneetion to a supply of liguefied gas, the system
including metering means for meterin~ and detenmining the amount of
liquefied gas dispensed during a dispensing operation, and
liquefied gas metering apparatus as claimed in any one of the
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preceding claims, the metering means bein~ responsive to the
calcula~ing 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 thP 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. l shows a schematic circuit for a ~as metering system
incorporating gas metering apparatus according to the presen~
invention, and
Fi~.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
vchicle fuel tanks.
The system illustrated includes a vapour remover 10 which
receives liquefied gas throu~h line ll from a supply tank (not sho~n)
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 ~he supply tank. The
line 12 is pro~ided with a check valve 13 and double check 14 as is
~nown 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 lS 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 lO and operative to measure the amount of
liquefied gas dispensed during a dispensing operation. The metering
means may be of conventional type having a measurin~ chamber which
has a rotary element on a shaft, the rotation of the shaft bein~ used
by the system electronics to calculate the total amount and cost of
liquefied gas dispensed. This calculation is co~pensated or adjusted
dependin~ on the output of the sensing means 50 which varies with
W O 92/02788 PCT/AU91/00341
2~87~ ~5
changes in dielectric constant of the lique~ied gas. This will be
described in detail below.
Downstream of the metering means 17 is a dispensing control
valve 20 for con~rolling the flow of liquefied gas in the supply line
15 between the metering means 17 and the filling coupling 16. The
control valvP 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 gau~e 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 ups~ream 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 ~0 in
selective communication via a pilot line 3I 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 lique~ied gas
flow from the inlet port 21 to the outlet por~ 22. The pilot line 31
is selectively communicable with the pressurised liquefied gas in the
vapaur remover lO. This is achleved by providin~ 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 3l 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
hi~h pressure inlet 34 can be connected to the tank 10. The pilot
control valve 32 is selectively operable to cormect 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, the valve 61 also being controlled in response to
the sensing means 50 associated with the ~ank lO.
Preferably there is a normal fail-safe condition of the pilot
control valve 32 which comprises connection of ~he low pressure inlet
WO 9:;!/1)2788 PCI/AU91/00341
2(188 7~t~
35 to the pilot o et 3~, resulting in low pressure in the pilot
line 31 and the control ~alve ~0 being closed to liquefied gas flow
therethrough.
The pilot control valve 32 may be electrically operable, e.~.
solenoid operated, to switch between ~wo conditions corresponding
respectively to connection of the low pressure inlet 35 to the pilot
outlet 33 and connection of the high pressure inlet 34 ~o the pilot
outlet 33. In particular the solenoid has two states: (1) not
ener~ised - corresponding to connection of inlet 35 to outlet 33 and
closure of inlet 34, resulting in lo~ 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 ~as 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 dispensin~
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
con~rol circuit causes the dispensing control valve 20 to clase for a
period during which the metering means 17 is reset to zero litres and
~5 zero cos~. Subsequently the control circuit causes the dispensin~
control valve 20 to re-open for enabling metered dispensing of
liquefied gas through the supply line 15 and filling coupling 16,
The described opera~ion of the pilot control valve 32 may only
be possible if the valve 61 in line 36 is open following vapour
~0 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 dispensi~g
operations. The repeated system components have the same reference
numerals with the added suffix t'a". The opera~ion 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 g~s to both the supply
lines 15, l5a. This is achieved by providing a supply junction 40
W O 92/02788 2 0 8 8 7 ~ 1 PCT/AU91/00341
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 ~ith the inlet 41. Outlet 42 is connected to supply
l me 15 and outlet 43 to supply line l5a. 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
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length of line from the tank 10 to the metering means 17, 17a is
preferably minimised in order io minimise vapour phase arising in
that length of line which might interfere with meterin~ accuracy and
for this purpose ~he 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 o~ two dispensin~ 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 alsn to changes
in composition of liquid to which the sensitive element SS is
exposed. An electrical signal can be ~enerated on line 57 in
response to the change of the electrical characteristics of the
sensitive element ~6 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
W O 92/02788 PCTiAU91/00341
~0887~ -
sensitive element 56 becomes immersed in liquid phase, which occurs
when vapour is substantially or completely eliminated from the tank
throu~h the return line 12, the signal generàted 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 capaci~ance chan~es depending on whether the element 70 is
immersed in gas or in liquid and dependin~ on the density of the
liquid when fully immersed in liquid, In Fig. 2, the capacitive
ele~ent 70 comprises ~wo conductive plates 71, 72 which are arranged
~enerally parallel and spaced apart, the pla~es bein~ arranged within
~he tank 10 at the top so that the fluid, whether it be gas or
liquid, within ~he tank 10 flows between the plates 71, 72, the
capacitance of the element 70 chan~ing depending upon the size and
spacing of the plates 71, 72 and the dielectric properties nf the gas
phase and liquid phase in which the plates are immersed. The
capacitive element 70 is connec~ed 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
~2, the circuit components being encapsulated in housing 75.
The sensing circuit 65 in Fig. 1 comprises an oscillator
circuit 58 of ~hich the capacitive elemen~ 70 is a component
determining the frequency of the oscillator circuit. The sensin~
means S0 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 oscilla~or 58 changes as a result of the capaciti~e
el~ment 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
outpuS signal on line 57 is used to switch a solid state relay 60 in
response to sensed phase changes, the relay 60 in turn switchin~
power to and from the solenoid valve 55 located in a return line 12.
~he output signal on line 57 is also coupled via switching
relay 62 to the solenoid valve 61 loca~ed in the pilot supply line 36
extendin~ 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
W O 92/02788 2 ~ 8 8 7 8 ~ PCT/AU91/00341
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 ~apour is progressing. Effectively this provides a
further control to enable prevention of premature dispensing
operations leading ~o incorrect metering of dispensed liquefied ~as,
Durin~ 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 ~ilied wi~h
liquid phase. This temporary opening may be carried out particularly
if the system has no~ 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 presenre of
vapour in the vapour eliminator. If vapour is detected, the system
lS may prevent dispensin~ 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 ter~inated and, at the same time, a flag can be set
to initiate the hose YillinO operation described above upon
initiation of the next dispensing operation.
Returni~g 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 calculatin~
means 76 is operative to a~ect the determination by the metering
means 17 of the metered 6as dispensed during a dispensing operation
so as to compensate ~he determination of the metered amoun~ of gas
for changes in the dielectric constant of the ~as and thereby
compensate the determination of the metered amount of gas for chan~es
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 rela~ionship between Ihe
dielectric constant and the errors in the measuri~g 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
determina~ion of ~he metered amount of liquefied gas dispensed by
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W O 9~/02788 2 0 ~ ~ 7 8 ~1 PCT/AU91/00341
1 0
compensating for chan~es in the sensed dielectric constant.
The determination of the compensation factor can be by means of
a predetermined eable of compensa~ing fac~ors 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
~able a compensating factor to apply in the metleri11g operation so as
to ensure an accurate measuring of gas dispensed. Al~erna~ively the
calculating means 76 may be programmable and be operated to calculate
or determine from a programmed formula a ~actor 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 pro~rammed so as to compute from an appropriate
formula or algorithm the compensatin~ factor to be applied to the
determination of the amount of liquefied gas dispensed,
It will be appreciated that the sensing element 56 need no~ 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 c~an~e in reactance of an arm of the brid~e within
which the capacitive element is located causes a change in the output
~hich can be sensed and used to compensate the metered amount o~ ~as.
The determination of the compensation to be applied in the
measurement of ~as 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 ~ravity 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 durin~
the dispensi~g 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 lique~ied gas
dispensed durin~ a dispensing operation by ~ompensa~ing for the
specific ,ravity chan~es tha~ occur ~ith variations in composition of
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W O 92/0278B PCT/AU91/00341
2~87~
the liquefied gas bein~ supplied. This increase in accuracy of the
metering operation can substantially reduce unfair transactions i~
which either the purchaser of the liquefied gas or ~he supplier of
the liquefied gas is being disadvantaged by ~netering inaccuracies
occurring in the past.
The specific gravity is a property relaCed ~o thP density Df
the liquefied gas. In the past, temperature changes have heen sensed
to enable a compensation factor to be applied during metcrin~
operations. Use of the present invention to determine and apply a
compensating factor based on a density related sensing enables
elimination of ~he temperature sensing since ~he temperature
variations affecting density and hence the metering accuracy will
au~omatically be compensated by the.present invention.
It is to be understood that various alterations, modifications
andlor 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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