Abstrict A mass flow meter for a gas dispenser includes apparatus and method
for determining specific heat ratio of the gas as a function of
two pressure measurements in relation to a nozzle and for determining
mass flow rate as a function of the specific heat ratio, the two
pressure measurements in relation to the nozzle, and a density measurement
of the gas.
Claims The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. Apparatus for measuring specific heat ratio k of a gas, comprising:
a sonic nozzle having a flow passage that converges from an entrance
to a throat that has a cross-sectional area A.sub.t and diverges
after the throat to an exit;
a staging plenum adjacent said entrance that is large enough to
have stagnant pressure conditions of a gas that flows through said
staging plenum, into said nozzle, and through said throat at sonic
choked or velocity;
stagnant pressure sensing means connected to said staging plenum
for measuring stagnant pressure P.sub.o of a gas in said staging
plenum;
flow pressure transducer means tapped into said flow passage at
a location having a cross-sectional area A.sub.i adjacent said throat
for measuring pressure P.sub.i of a gas flowing through said nozzle;
and
microprocessor means connected to said stagnant pressure sensing
means and to said flow pressure transducer means for determining
specific heat k of a gas flowing through said sonic nozzle in choked
or sonic flow condition according to relationships that can be expressed
by the formula: ##EQU11##
2. The apparatus of claim 1 wherein said flow pressure transducer
means is tapped into said flow passage at a location upstream of
said throat.
3. The apparatus of claim 1 including densitometer means positioned
for determining density .rho..sub.o of said gas at stagnant pressure
P.sub.o, said microprocessor means also being connected to said
densitometer means for determining mass flow rate m of said gas
according to relationships that can be expressed by the formula:
##EQU12##
4. The apparatus of claim 3 wherein said microprocessor means
is programmed to determine specific heat k and mass flow rate m
on a real-time basis.
5. The apparatus of claim 3 wherein said microprocessor means
is programmed to determine specific heat k and mass flow rate m
at short sequential time intervals.
6. The apparatus of claim 5 wherein said microprocessor means
is programmed to calculate mass flow over one of said time intervals
by multiplying mass flow rate m by one of said time intervals.
7. The apparatus of claim 6 wherein said microprocessor means
is programmed to calculate sequential mass flows over a plurality
of said sequential time intervals and to add said sequential mass
flows together to determine cumulative mass flow over the duration
of said plurality of said time intervals.
8. A method of determining specific heat k of a gas, comprising
the steps of:
flowing said gas through a sonic nozzle that has a flow passage
which converges from an entrance to a throat having a cross-sectional
area A.sub.t and diverges from the throat to an exit under sufficient
pressure differential between said entrance and said exit to cause
the gas flow to choke at sonic velocity in said throat;
measuring stagnant pressure P.sub.o of said gas at said entrance;
measuring pressure P.sub.i of said gas flowing at a location in
said flow passage having a cross-sectional area A.sub.i ; and
determining specific heat k of said gas according to relationships
that can be expressed by the formula: ##EQU13##
9. The method of claim 8 wherein said step of measuring pressure
P.sub.i is done at a location that is in the converging portion
of said flow passage slightly upstream of said throat.
10. A method of measuring mass flow of a gas, comprising the steps
of:
flowing said gas through a nozzle that has a flow passage which
converges from an entrance to a throat that has a cross-sectional
area A.sub.t and diverges from the throat to an exit under sufficient
pressure differential between said entrance and said exit to cause
the gas flow to choke at sonic velocity in said throat;
measuring stagnant pressure P.sub.o of said gas at said entrance;
measuring flow passage pressure P.sub.i of said gas at a location
in said flow passage having a cross-sectional area A.sub.i ;
determining specific heat k of said gas according to relationships
that can be expressed by the formula: ##EQU14## measuring density
of said gas at said stagnant pressure P.sub.o ; and determining
mass flow rate m of said flow of gas according to relationships
that can be expressed by the formula: ##EQU15##
11. The method of claim 10 wherein said step of measuring flow
passage pressure P.sub.i is done at a location in the converging
portion of said flow passage that is upstream from said throat.
12. The method of claim 11 including the steps of measuring said
stagnant pressure P.sub.o and said flow passage pressure P.sub.i
and determining specific heat k on a real time basis.
13. The method of claim 12 including the steps of measuring density
.rho..sub.o of said gas at said stagnant pressure P.sub.o and determining
mass flow rate m on a real time basis.
14. The method of claim 11 wherein said step of measuring flow
passage pressure P.sub.i is done at a location in the converging
portion of said flow passage that is closer to said throat than
to said entrance.
15. The method of claim 10 including the steps of measuring said
stagnant pressure P.sub.o, flow passage pressure P.sub.i, and density
.rho..sub.o at sequential time intervals while said gas is flowing
and using said P.sub.o, P.sub.i, and .rho..sub.o measurements to
determine specific heat k and mass flow rate m at sequential time
intervals.
16. The method of claim 15 including the steps of determining
mass m of gas that flows in each of the intervals by multiplying
mass flow rate m determined for each time interval by the time elapsed
in the time interval.
17. The method of claim 16 including the step of determining accumulated
mass of gas that flows over an extended time period by adding the
incremental mass determinations for each time interval in the extended
time period.
18. A method of measuring mass flow rate of a flowing gas that
varies in molecular composition, comprising the steps of:
determining specific heat ratio k of the gas;
determining mass flow rate m of the flowing gas as a function of
the specific heat ratio k of the gas;
redetermining specific heat ratio k of the gas as molecular composition
of the gas varies; and
redetermining mass flow rate m of the flowing gas as a function
of the redetermined specific heat ratio k of the gas.
19. The method of claim 18 including the steps of determining
the specific heat ratio k by flowing the gas through a conduit that
includes a nozzle which converges from a wider cross-section of
said conduit to a narrower throat section such that there is a pressure
differential between the gas in said conduit upstream of said nozzle
and the gas in said nozzle resulting in a pressure upstream of said
nozzle and a pressure in said nozzle, measuring the pressure upstream
of said nozzle and the pressure in said nozzle, and determining
said specific heat ratio of said gas as a function of the ratio
of said pressure in said nozzle to the pressure upstream of the
nozzle.
20. The method of claim 19 including the steps of flowing said
gas through said nozzle under conditions of sonic velocity choked
flow in said nozzle.
21. The method of claim 20 including the step of measuring said
pressure in said nozzle upstream of said throat.
22. The method of claim 21 including the steps of measuring said
pressure upstream of said nozzle under substantially stagnant condition.
23. The method of claim 22 including the step of measuring density
of said gas at the stagnant pressure condition and determining the
mass flow rate m as a function of said density, said ratio of pressure
in the nozzle to pressure upstream of the nozzle, and said specific
heat ratio k.
24. A method of determining specific heat ratio k of a gas, comprising
the steps of flowing the gas through a conduit that includes a nozzle
which converges from a wider cross-section of said conduit to a
narrower throat section such that there is a pressure differential
between the gas in said conduit upstream of said nozzle and the
gas in said nozzle resulting in a pressure upstream of said nozzle
and a pressure downstream of said nozzle; measuring the pressure
upstream of said nozzle and the pressure in said nozzle and determining
said specific heat ratio of said gas as a function of the ratio
of said pressure in said nozzle to the pressure upstream of the
nozzle.
25. The method of claim 24 including the steps of flowing said
gas through said nozzle under conditions of sonic velocity choked
flow in said nozzle.
26. The method of claim 25 including the step of measuring said
pressure in said nozzle upstream of said throat.
27. The method of claim 26 including the steps of measuring said
pressure upstream of said nozzle under substantially stagnant condition.
28. The method of claim 27 including the step of determining said
specific heat ratio k of said gas according to relationships that
can be expressed by the formula: ##EQU16## where P.sub.i is said
pressure in said nozzle, P.sub.o is said pressure upstream of said
nozzle, A.sub.t is the cross-sectional area of said throat section,
and A.sub.i is the cross-sectional area of the nozzle at the location
where said pressure P.sub.i in said nozzle is measured.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to gas flow meters, and more particularly
to a method and apparatus for dispensing and measuring natural gas
into vehicle tanks.
2. State of the Prior Art
Dispensing gaseous fuels, such as compressed natural gas, into
motor vehicle tanks at the consumer retail point of sale presents
problems that are either not encountered or not significant in the
more conventional liquid fuel distribution and retail sales apparatus
and systems. For example, natural gas is transferred and stored
under high pressure, which, according to the ideal gas law, varies
as a function of temperature, and which requires special fittings,
tanks, and safety considerations that are more complex than liquid
fuel handling equipment and components. Also, measuring the quantity
of a gas being dispensed at the point of sale into a customer's
motor vehicle tank is not as easy as measuring a liquid fuel.
Tanks for storing compressed natural gas aboard vehicles are manufactured
to withstand pressures within normal ranges, such as 3000 to 4000
p.s.i.g., with an extra safety margin. Because gas pressure varies
as a function of temperature, and because the temperature of a fuel
tank on a motor vehicle cannot be controlled easily or reliably
the tank manufacturers rate their tanks for a maximum safe operating
pressure at a convenient and normal "standard temperature",
usually 70.degree. F. (19.degree. C.). For example, a tank rated
at 3000 p.s.i.g. at 70.degree. F. (19.degree. C.) is designed to
withstand safely whatever pressure that amount of gas will exert
at whatever higher temperatures it would likely encounter in extreme
environments. According to the ideal gas law, PV=nRT, where the
volume V of the tank, the moles n of the gas, and the universal
gas constant R, are all constant for a given tank containing a given
amount of gas, the pressure P varies in a direct, one-to-one relation
to the absolute temperature T. Therefore, a tank filled to the maximum
rated pressure of 3000 p.s.i.g. at the standard temperature of
70.degree. F. (19.degree. C.) will have a pressure of about 3455
p.s.i.g. at 150.degree. F. (63.3.degree. C.). Consequently, the
tank is built with a margin of safety to withstand such higher pressures
at reasonably foreseeable higher temperatures. However, it would
be unsafe to fill a tank at the standard 70.degree. F. (19.degree.
C.) to a pressure higher than its maximum rated 3000 p.s.i.g. pressure,
because, if the tank is then exposed to a higher extreme but foreseeable
temperature, the resulting higher pressure could exceed the safety
margin and possibly rupture the tank. For the same reason, filling
the tank to the rated 3000 p.s.i.g. pressure at a lower temperature,
such as 32.degree. F. (0.degree. C.) would result in a potentially
dangerous overfill, if the vehicle is exposed to a substantially
higher temperature shortly thereafter, which could increase the
pressure to unsafe levels.
At the same time, it is inefficient and undesirable to underfill
a customer's vehicle tank, because underfilling reduces driving
range for the customer and prematurely aborts a sale for a vendor.
Consequently, it is desirable to provide a complete, but proper
fill to a maximum pressure that is compensated for the then current
temperature in a manner that would result in the proper 3000 p.s.i.g.
maximum pressure if the actual temperature was raised to the standard
70.degree. F. (19.degree. C.) rating temperature.
Of course, it is also desirable to get a proper measure of the
gas delivered to a customer's vehicle tank as a basis for calculating
the point of sale price to be charged to the customer or for other
record purposes.
A significant problem, both for determining the maximum tank fill
and for measuring the amount of fuel delivered in a gas rather than
a liquid setting is that because of its compressibility, the quantity
of gas is difficult to measure independent of its temperature and
pressure, which are usually different in the customer's vehicle
tank than in the vendor's dispenser. However, it is preferred, at
least from the vendor's standpoint, if not also for purposes of
governmental regulation of weights and measures, to not have to
rely on instruments or transducers in the customer's tank to measure
pressure and temperature.
Our U.S. Pat. No. 5238030 which is incorporated herein by reference,
was directed to a method and apparatus for determining a temperature
compensated maximum tank pressure and automatic fill cut-off when
such maximum pressure is reached and for measuring the mass of the
natural gas fuel delivered, all from instruments or transducers
in the dispenser, rather than in a customer's tank. However, restrictions
and pressure losses through the delivery hose, tank valves, and
lines made accurate cut off pressure still difficult to measure
with that invention. Therefore, we developed the invention of our
U.S. Pat. No. 5259424 which is also incorporated herein by reference,
to provide an improved and more accurate method and apparatus for
measuring the size of the customer's tank and then determining the
additional mass of gas needed to get a complete fill of natural
gas in the customer's tank corrected for standard pressure and temperature,
all for use in combination with a mass flow measuring method and
apparatus for determining maximum fill cut off and the amount (mass)
of the gas actually dispensed.
While the sonic nozzle flow meter system used in our U.S. Pat.
Nos. 5238030 and 5259424 described above is reasonably accurate
and probably more accurate than most other flow meters used in natural
gas dispensers, it assumes a constant molecular composition gas
mass at (standard) pressure and temperature. Therefore, it loses
accuracy when the molecular composition of the natural gas varies.
It also maintains accuracy only in a restricted flow range, loses
accuracy in subsonic flow, and is a function of back pressure. Therefore,
there is still a need for more improvement in metering accuracy
for measuring the amount of gas dispensed into a vehicle tank without
having to rely on pressure and temperature transducers in the vehicle
tank itself.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of this invention to provide
improved gas flow measurement in a natural gas dispenser.
A more specific object of this invention is to provide a method
and apparatus for measuring gas flow that is accurate even when
the molecular composition or mass of the gas varies.
It is also an object of this invention to provide a mass flow metering
system for natural gas dispensers that is accurate over a wider
range of flow rates, and is not so sensitive to back pressures and
periods of subsonic flow during a vehicle tank fill operation.
Additional objects, advantages, and novel features of the invention
shall be set forth in part in the description that follows, and
in part will become apparent to those skilled in the art upon examination
of the foregoing or may be learned by the practice of the invention.
The objects and advantages of the invention may be realized and
attained by means of the instrumentalities and in combinations particularly
pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with
the purpose of the present invention, as embodied and broadly described
herein, the method of this invention for providing more accurate
mass flow measurements of a flowing gas that varies in molecular
composition generally includes determining the specific heat ratio
of the gas and determining mass flow rate as a function of the specific
heat ratio. The method also includes flowing the gas through a nozzle
and determining specific heat ratio as a function of pressure measurements
upstream from and in the nozzle and determining the specific heat
ratio as a function of the ratio of the pressure in the nozzle to
the pressure upstream of the nozzle. Mass flow rate can then be
determined by measuring density of the gas under the pressure measured
upstream of the nozzle and determining mass flow rate as a function
of the density, pressure ratio, and specific heat ratio. Gas flowing
through the nozzle is choked at sonic velocity in the throat.
The apparatus of this invention generally comprises a flow channel
that includes a nozzle, a first pressure transducer upstream of
the nozzle entrance, a second pressure transducer tapped into the
nozzle, and a densitometer positioned to measure gas density at
the pressure measured by the first pressure transducer.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated herein and form
a part of this specification illustrate the preferred embodiment
of the present invention, and together with the description, serve
to explain the principles of the invention.
In the Drawings
FIG. 1 is a schematic diagram of a natural gas dispensing system
utilizing the improved flow metering method and apparatus of this
invention;
FIG. 2 is a perspective view of a gas metering apparatus constructed
according to the present invention;
FIG. 3 is a top plan view of the gas metering apparatus shown in
FIG. 2;
FIG. 4 is a right (downstream) side elevation view of the gas metering
apparatus shown in FIG. 2;
FIG. 5 is a rear elevation view of the gas metering apparatus shown
in FIG. 2; and
FIG. 6 is a cross-sectional view of the gas metering apparatus
of this invention taken along lines 6--6 of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in the schematic diagram of FIG. 1 a gas dispenser 100
connected to a high pressure gas supply, represented by the large
storage tank 110 via a high pressure conduit or pipe 114 is adapted
for dispensing gas, such as compressed natural gas (CNG), into a
receiver or vehicle tank 300. The dispenser 100 measures mass flow
of the gas being dispensed into the vehicle tank 300 on a real time
basis based on a real time determination of specific heat of the
gas being dispensed, as will be described in more detail below.
The gas is delivered via the high pressure conduit 114 from the
supply tank 110 to an inlet plenum 56 in dispenser 100 where a
densitometer 600 connected to the inlet plenum 56 measures the density
.rho..sub.o of the natural gas before it flows through a sonic or
critical flow nozzle 52 and into the receiver or vehicle tank 300
via dispenser hose 28. In addition to gas density .rho..sub.o measurements
from densitometer 600 stagnant pressure P.sub.o measurements by
a pressure transducer 96 just upstream of the sonic nozzle 52 and
dynamic pressure P.sub.i measurements in the nozzle 52 by a pressure
transducer 310 are also obtained. These parameters of gas density
.rho..sub.o, stagnant gas pressure P.sub.o, and a flowing gas pressure
P.sub.i in the sonic nozzle 52 can be used according to this invention
to determine specific heat of the gas and then to determine the
mass flow rate of the gas flowing though the nozzle 52. The mass
flow rate can be used on a real time basis to measure and record
the mass of gas dispensed into the vehicle tank 300. According to
the principles of the invention in our U.S. Pat. No. 5238030
the size of the tank and the additional mass of gas required to
fill the tank to a temperature compensated maximum pressure fill
can be determined, and an automatic shut-off can be implemented,
based on the mass flow rate acquired according to this invention.
Of course, where sales price is based on mass, a simple additional
calculation multiplying price per unit of mass times the total mass
of the gas dispensed can also produce the total sales price of the
gas dispensed into the vehicle tank 300. On the other hand, if sales
price is based on a standard cubic foot, the total mass dispensed
can be convened to standard cubic feet by wellknown mole, molecular
weight, pressure, temperature, and volume relationships.
The measurement of mass flow rate of the gas according to this
invention makes beneficial use of relationships of specific heat
to mass of the gas and the fact that flow through a sonic nozzle
is isotropic (no energy loss) to obtain accurate mass flow rate
measurements of the gas, regardless of molecular composition or
non-ideal gas characteristics of the gas being measured. To implement
this invention, a pressure tap 304 is positioned at, or preferably
slightly (a few thousandths of an inch) upstream of, the throat
446 of the nozzle 52 to get a pressure P.sub.i measurement of the
fluid flowing through a cross-section i of the nozzle 52. The mass
flow rate m.sub.i of the gas through section i can be expressed
by the equation:
where:
.rho..sub.1 = density of the gas at the cross-section i;
A.sub.i = cross-sectional area of the nozzle at cross-section i;
and
V.sub.i = velocity of the gas flowing through the cross-section
i.
Of course the cross-sectional area A.sub.i of the nozzle 52 at
section i is fixed by its dimensions and can be determined by scalar
measurement. The gas density .rho..sub.i at the cross-section i
can be determined by the relationship: ##EQU1## where: .rho..sub.o
= density of the gas at stagnant pressure P.sub.o ;
P.sub.o = stagnant pressure of the gas just upstream of the sonic
nozzle;
P.sub.i = pressure of the gas flowing through the cross-section
i of the nozzle; and
k = specific heat ratio of the gas (heat capacity per unit of mass).
The velocity v.sub.i at the cross-section i can also be expressed
in terms of .rho..sub.o, P.sub.o, P.sub.i, and k by applying the
energy equation to get ##EQU2## Therefore, substituting equation
(2) and (3) into equation (1) yields mass flow rate m, as follows:
##EQU3## Likewise, mass flow rate at the cross-section t of the
smallest area or throat 446 can be expressed as
where:
.rho..sub.t = gas density at the throat t;
A.sub.t = cross-sectional area at the throat t; and
V.sub.t = velocity of gas flow at the throat cross-section t.
When the sonic nozzle is choked, the mass flow m.sub.t, at the
throat 446 is fixed for a particular gas temperature T and pressure
P.sub.o upstream of the nozzle 52 as explained in more detail in
our U.S. Pat. Nos. 5238030 and 5259424. The density .rho..sub.t
of the gas at the throat 466 is ##EQU4## and the velocity V.sub.t
of the gas at the throat 466 is ##EQU5## where: R = the universal
gas constant; and
T.sub.o = temperature of the gas just upstream of the nozzle.
Therefore, substituting equations (6) and (7) into equation (5)
yields mass flow rate m.sub.t at the throat in terms of the formula:
##EQU6## Since: ##EQU7## (state equation), equation (8) reduces
to: ##EQU8##
Since the mass flow rate m.sub.t at the throat cross-section t
under choked condition is constant, the mass flow rate m.sub.i at
cross-section i upstream of the throat is also constant and is the
same as the mass flow rate m.sub.t at the throat. Therefore, equations
(4) and (9) can be equated to yield: ##EQU9## Simplifying provides
a relationship among P.sub.i to P.sub.o ratio, A.sub.t to A.sub.i
ratio, and specific heat ratio k, as follows: ##EQU10## Therefore,
since the cross-sectional areas A.sub.t and A.sub.i are fixed by
the geometry of the nozzle 52 and by the placement of the pressure
tap 304 in the nozzle 52 obtaining pressure measurements for P.sub.i
and P.sub.o enables a solution for specific heat ratio k from equation
(11). Then, using the calculated specific heat ratio k from equation
(11) along with the already measured pressures P.sub.i and P.sub.o
and also obtaining a measurement of the gas density P.sub.o at the
pressure P.sub.o, an accurate mass flow rate m.sub.i can be calculated
from equation (4), regardless of the molecular composition of the
gas being dispensed.
As mentioned above, the mass flow rate m.sub.i at the cross-section
i is the same as the mass flow rate m.sub.t at the throat cross-section
t or, for that matter, is the same as the mass flow rate m through
any section of the nozzle 52. Therefore, m.sub.i calculated for
a given pressure P.sub.i, a given pressure P.sub.o, and a given
density .rho..sub.o is a measure of the instantaneous mass flow
rate m delivered to the vehicle tank 300. When the P.sub.i, P.sub.o,
and .rho..sub.o values are updated frequently on a real time basis,
fed to a microprocessor 306 and recalculated rapidly, the resulting
sequentially recalculated values of m.sub.i provide effectively
real time mass flow rate m measurements of the gas dispensed to
the vehicle tank 300. Multiplying mass flow rate m by time, which
can also be done rapidly at many short time intervals with the microprocessor
306 provides a cumulative or running total of the mass of gas dispensed
into the vehicle tank 300 on a real time basis.
The flowing pressure P.sub.i and stagnant pressure P.sub.o can
be obtained with respective state- of-the-art pressure transducers
310 and 96 which can be, for example, a 5000 p.s.i.a pressure
transducer Engineering Measurements Company of Longmont, Colo.,
EG&G Chandler Enginering of P.O. Box 470710 Tulsa, Okla. The
gas density .rho..sub.o can be obtained with a state-of-the-art
densitometer 600 for example, a Model UGC 278 densitometer manufactured
by EG&G Chandler Engineering of P.O. Box 470710 Tulsa, Okla.,
or a Model FD 700 manufactured by Sarasota Automation, Inc., of
10335 Landsbury, Suite 300 Houston, Tex., or a Model 7812 manufactured
by Schlumberger Industries, Transducer Dvision, 11321 Richmond Avenue,
M-102 Houston Tex., or a densitometer manufactered by Engineering
Measurements Company of Longmont, Colo.
In operation, the main valve 312 at the supply tank 110 is opened
to allow gas to flow through supply pipe 114 to the intake plenum
56 of the dispenser 100 of this invention. The densitometer 600
can be connected to the intake plenum 56 and density readings can
be fed through a connection 318 to the microprocessor 306. From
the intake plenum 56 the gas flows through an intermediate conduit
308 to the staging plenum 314 where stagnation pressure P.sub.o
is obtained through a pressure tap 316 by the P.sub.o pressure transducer
96. A shut-off valve 320 is positioned between the supply tank 110
and the staging plenum 314 for turning the gas flow "on"
and "off" in response to signals from the microprocessor
306. The pressure signals can be fed from transducers 310 and 96
via respective connections 307 and 309 to the microprocessor 306.
In the preferred embodiment dispenser 100 the shut-off valve 320
is positioned between the conduit 308 and staging plenum 314 in
axial alignment with the sonic nozzle 52 as shown in FIG. 1 and
6 and it is pilot operated with gas pressure from the intermediate
conduit 308. The pilot gas pressure is directed by pilot supply
line 340 through a solenoid-operated pilot valve 342 and then through
either pilot operating line 343 or 344 to close or open the shut-off
valve 320. Signals from the microprocessor 306 via a connection
345 actuate the solenoid pilot valve 342 to apply gas pressure to
a pilot actuator (not shown in FIG. 1) on shut-off valve 320 to
close the shut-off valve 320 or to release gas pressure to open
shut-off valve 320 as will be described in more detail below.
Gas is delivered from the dispenser 100 to the vehicle tank 300
by a delivery or dispenser hose 28. The dispenser hose 28 and intake
pipe 346 of vehicle tank 300 have respective mating couplers 348
and 349 for connecting the dispenser hose 28 to vehicle tank 300.
A valve 350 usually manually operated, is provided in the intake
pipe 346 of tank 300 and a delivery valve 44 also usually manually
operated, is provided on the distal end of dispenser hose 28. The
proximal end of dispenser hose 28 is connected to an outlet plenum
62 positioned at the downstream end of the nozzle 52.
With couplers 348 349 connected together and the valve 350 on
vehicle tank 300 opened, the delivery valve 44 can also be opened.
A signal can be sent automatically via connection 352 or manually
via a switch (not shown) to start the microprocessor and let it
know that the couplers 348 349 and valves 350 44 are set for a
fill operation.
The microprocessor 306 is preferably programmed to go through an
initialization procedure similar to that described in our U.S. Pat.
Nos. 5238030 and 5259424 to calibrate the P.sub.i and P.sub.o
pressure transducers 310 and 96 to each other. In other words, with
the shut-off valve 320 still closed and with the delivery valve
44 and tank valve 350 opened, the stagnant pressures throughout
the system downstream of shut-off valve 320 all the way to and including
the vehicle tank 300 should equalize, so the pressure transducers
310 and 96 should read the same values. If not, the microprocessor
306 is programmed to add a correction value equal to the difference
in the two transducers 310 96 to the readings of one of them as
explained in more detail in our U.S. Pat. Nos. 5238030 and 5259424.
As also mentioned in our U.S. Pat. Nos. 5238030 and 5259424
it might be necessary to first briefly open and then close valve
320 to pop open a one way check valve (not shown) that is usually
positioned in the vehicle tank 300 in order to get a read on the
initial pressure in tank 300 with pressure transducers 310 or 96.
The fill operation can be initiated with a start switch 301 or
any other device that sends a start signal to the microprocessor
306 which in turn initializes time and other functions, as described
above. To start the flow of gas, the microprocessor 306 sends a
signal via connection 345 to solenoid 341 to open pilot valve 342
which applies gas pressure via conduit 344 to open the shut-off
valve 320 as will be described in more detail below. When the shut-off
valve 320 is opened, gas flows from the supply tank 110 assuming
main tank valve 312 is already open, as is the usual case after
initial set up and assembly. The gas flows through inlet plenum
56 where densitometer 600 takes density .rho..sub.o measurements
of the gas and through conduit 308 to staging plenum 314. The plenums
56 and 314 should be large enough to have substantially stagnant
pressure conditions, in spite of gas flowing through them, so that
pressure P.sub.o and density .rho..sub.o measurements are at stagnant
conditions. The plenums 56 and 314 should also be in close proximity
to each other and to the sonic nozzle 52 to ensure that the gas
for which density .rho..sub.o measurements are taken is substantially
the same in composition as the gas being measured in the sonic nozzle
52. With the valve 320 open, the pressure in the inlet plenum 56
should be substantially the same as the pressure P.sub.o in staging
plenum 314 so the density .rho..sub.o measurement is made essentially
at P.sub.o pressure. The P.sub.o pressure is measured by the pressure
transducer 96 tapped into staging plenum 314 as described above.
Actually, the density .rho..sub.o reading could be taken in the
staging plenum 314 and the inlet plenum 56 could be eliminated
in alternately structured dispensers.
From the staging plenum 314 the gas flows into the converging
section 442 of the sonic nozzle 52 through the throat 446 and diverging
section 444 of the sonic nozzle 52 and into the outlet plenum 62.
As long as the ratio of pressure P.sub.2 downstream of the nozzle
52 to the pressure P.sub.o upstream of the nozzle 52 is less than
critical (approximately 0.85), the flow of gas through the throat
446 is choked at sonic velocity and cannot increase. Consequently,
the flow where pressure P.sub.i is measured, which is preferably
slightly upstream of the throat 446 is always subsonic. While it
is not necessary to the determination of mass flow according to
this invention, when flow through the nozzle 52 is choked at sonic
velocity, a pressure transducer 92 for measuring the pressure P.sub.2
at the outlet of the diffuser section 444 is tapped into outlet
plenum 62 and connected to microprocessor 306 to monitor the P.sub.2
/P.sub.o ratio. If the P.sub.2 /P.sub.o rises above the critical
level of about 0.85 such as near the end of a dispensing cycle
when the vehicle tank 300 is almost filled, then Equation (4) above
is used to calculate mass flow rate m, assuming a constant specific
heat k equal to the specific heat k that was previously measured
when the flow was critical, i.e., when the P.sub.2 /P.sub.o was
less than about 0.85. The sensitivity of Equation (4) to specific
heat k changes is very small for P.sub.2 /P.sub.o ratios greater
than critical, so any changes in specific heat k during subsonic
flow have a negligible effect on overall metering accuracy. Of course,
the mass flow rate m determinations according to this invention
with the formula of Equation (4) above can also be made with specific
heat ratio k measurements or values obtained with other, more conventional
methods or from look-up tables after determining molecular composition
of the gas, such as with a chromatograph. However, the method and
apparatus for determining specific heat ratios k according to this
invention, as described above, have the advantage of real-time application,
which is significant for natural gas, which varies in composition
in different locations and even in the same locations from well
to well or from time to time.
From the outlet plenum 62 the gas continues flowing through dispenser
hose 28 to the vehicle tank 300 as described above. At the same
time, the P.sub.o and P.sub.i information from transducers 96 and
310 are fed to the microprocessor 306 which uses that information
along with preprogrammed area A.sub.t of the throat cross-section
t and area A.sub.i of the cross-section i in converging section
442 of sonic nozzle 52 to determine the specific heat ratio k of
the gas according to equation (11), as described above. The microprocessor
306 then uses the determined value of specific heat ratio k along
with the measured gas density .rho..sub.o, stagnant pressure P.sub.o,
flowing pressure P.sub.i, and area A.sub.i of the cross-section
i to determine mass flow rate m.sub.i according to equation (4),
as described above. The microprocessor 306 can be programmed to
use the specific heat k or some average of recent specific heat
k values for calculating mass flow rate m with Equation (4) when
the P.sub.2 /P.sub.o ratio reaches or climbs above critical (about
0.85), as described above. Of course, multiplying mass flow rate
m.sub.i times the elapsed time from any time interval yields the
mass of gas flowed through the nozzle 52 during that interval. Real
time in terms of minutes or seconds, or even fractions of seconds
can be input to the microprocessor 306 by an appropriate timing
device 302. The shorter the time intervals used, i.e., the more
sequential recalculations of specific heat k and mass flow measurements
made during a vehicle tank fill, the more accurate will be the mass
flow measurements, because the values of the parameters k, .rho..sub.o,
P.sub.o, and P.sub.i will be more specific and accurate for such
shorter time intervals. The microprocessor 306 can also tabulate
sequential mass determinations of sequential time intervals throughout
the gas fill process to keep a real time running total of mass delivered
to the vehicle tank 300. For purposes of this invention, sequential
recalculations of specific heat k and mass flow measurements at
time intervals on the order of milliseconds yields very accurate
mass flow values. The method and apparatus of this invention can
measure mass flow of natural gas being dispensed significantly more
accurately than the mass flow determinations obtained by the inventions
described in our U.S. Pat. Nos. 5238030 and 5259424.
While this invention is directed primarily to improved mass flow
measuring in gas dispensers, it is appropriate to note that the
microprocessor 306 with an additional ambient temperature or gas
temperature input (not shown), can be programmed to determine the
volume of the vehicle tank 300 a temperature compensated maximum
pressure for the vehicle tank 300 the additional mass of gas required
to fill the vehicle tank 300 to that temperature compensated maximum
pressure, the mass of gas actually dispensed, and to output a signal
to the solenoid 341 to operate the pilot valve 342 to shut-off valve
320 when the mass of gas actually dispensed equals the mass required
for maximum temperature compensated fill, all as described in our
U.S. Pat. No. 5238030. When the fill is complete and the shut-off
valve 320 is closed, the hose valve 44 and vehicle tank valve 350
can be closed and the coupler 348 349 can be disconnected. When
disconnecting couplers 348 349 it may be desirable to open bleed
valve 390 to bleed residual gas trapped between valves 44 and 350
to a vent recovery system 391 which is well known in this art and
not a part of this invention.
The electrical power supplies to the density and pressure transducers,
amplifiers, filters, analog-to-digital converters, and other electric
circuit components to the microprocessor connections that implement
this invention are well-known and within the capabilities of persons
skilled in that art, once the principles of this invention are understood,
so they are not described in any greater detail. A number of safety
features, such as those to detect broken or disconnected hoses,
as described in our U.S. Pat. Nos. 5238030 and 5259424 as well
as other safety features may be used with this invention. However,
such safety features are not a part of this invention, so are not
described in anymore detail herein.
The structures and mechanical components of the valve and fluid
flow dispenser measuring apparatus according to this invention might
not be as obvious, so an exemplary embodiment is shown in FIGS.
2-6. A housing 303 contains the inlet plenum 56 shut-off valve
320 staging plenum 314 sonic nozzle 52 and outlet plenum 62
as best seen in FIG. 6. A pipe fitting 305 sealed with an o-ring
seal 307 connects the high pressure supply line 114 to the inlet
plenum 56 which is bored into the upstream face 354 of housing
303. A similar pipe fitting 309 and o-ring seal 311 connects the
threaded end of the dispenser hose 28 to the outlet plenum 62 which
is bored into the downstream face 355 of housing 303. Therefore,
the gas flows into the housing 303 from the source of supply 110
(not shown in FIGS. 2-6), through fitting 305 as indicated by arrow
313 and it flows out of housing 303 through fitting 309 as indicated
by arrow 315 to vehicle tank 300 (not shown in FIGS. 2-6).
The densitometer 600 is connected via a duct 317 bored into the
housing 303 from the top surface 353 to the inlet plenum. An annular
filter 319 in plenum 56 protects the densitometer 600 from solid
or liquid impurities that might be in the gas. The intermediate
conduit 308 extends through the housing 303 to a valve chamber 321
that is bored into the housing 303 from the top surface 353 substantially
perpendicular to the conduit 308. The valve chamber 321 is bored
in four different diameter sections 322 323 324 325 with the
smallest section 322 extending into the outlet plenum 62.
A nozzle plug 326 with the sonic nozzle 52 extending axially therethrough
is positioned in the bottom of the smaller intermediate section
323 of the valve chamber 321 and is sealed by o-ring seals 327
328. An annular P.sub.i pressure chamber 330 is recessed into the
peripheral surface of nozzle plug 326 between the o-ring seals 327
328. The P.sub.i pressure tap 304 is bored laterally from the annular
P.sub.i pressure chamber 330 through the nozzle plug 326 to intersect
perpendicularly with the sonic nozzle 52 slightly above the throat
446. The P.sub.i pressure transducer 310 (shown only in FIG. 1 )
is connected by a tube 332 to a P.sub.i pressure port 331 that extends
from the left side 357 of housing 303 into an intersection with
the smaller intermediate section 323 of valve chamber 321 in alignment
with the annular P.sub.i chamber 330. An axial recess 332 in the
top of the nozzle plug 326 forms the lower part of staging plenum
314.
A cylindrical sleeve 333 with an internal axial bore 334 is positioned
in the top of the smaller intermediate section of valve chamber
321 and forms the upper part of staging plenum 314. An o-ring seal
329 encircles the periphery of sleeve 333 and a tapered upper rim
forms a valve seat 335. A space between the top of nozzle plug 326
and the bottom of sleeve 333 forms an annular extension 336 of staging
plenum 314. The P.sub.o pressure tap 316 is bored laterally through
housing 303 from a P.sub.o pressure port 337 in downstream face
355 into the valve chamber 321 in alignment with the annular extension
336. The P.sub.o pressure transducer 96 (shown only in FIG. 1) is
connected to port 337 via a tube 338 so the P.sub.o pressure transducer
96 can measure the P.sub.o pressure in staging plenum 314.
A valve plunger 360 is positioned slidably in the larger intermediate
section 324 of valve chamber 321 for opening and closing the staging
chamber 314 to the intermediate conduit 308. A soft valve closure
361 preferably fabricated of a soft neoprene or rubber material,
is adhered to a bolt 362 that is screwed into the lower end of plunger
360 for seating in the valve seat 335. The lower end of plunger
360 is a narrowed neck 363 that leaves an annular space 364 around
the plunger 360 that is connected to intermediate conduit 308. A
port 365 in the downstream side 355 of housing 303 connects to the
annular space 364 thus also to intermediate conduit 308 which
is open to the supply gas pressure line 114. Therefore, the pilot
supply tube 340 attached to port 365 always has high pressure gas,
regardless of the position of valve plunger 360 in valve chamber
321. This gas pressure in pilot supply tube 340 can be supplied
selectively by the 3-way pilot valve 342 (FIG. 1) to either the
top of plunger 360 via pilot operating tube 344 and port 366 to
move the plunger 360 downwardly into seat 335 or to the underside
of flange 367 of plunger 360 via pilot operating tube 343 and port
368 to move the plunger 360 upwardly away from seat 335.
The pilot operated shut-off valve 320 is comprised primarily of
the valve seat 335 the slidable plunger 360 and the soft closure
361 which assembly opens or closes the staging plenum 314 to intermediate
conduit 308. The enlarged flange 367 on the upper end of plunger
360 fits in the largest diameter section 325 of valve chamber 321.
The annular space 369 under the flange 367 is connected to port
368 by a tap 370 bored laterally into housing 303 to the lowest
part of the largest diameter section 325 of valve chamber 321. Connecting
this annular space 369 to gas pressure via pilot tube 343 while
bleeding pressure off the top of plunger 360 will cause the plunger
360 to move upwardly, thus opening the value 320.
The port 366 to which the other pilot tube 344 is connected is
bored into a bonnet 371 which, along with seal 372 closes the
top of valve chamber 321. A tap 373 connects the port 366 to the
space 374 in valve chamber 321 between the top of plunger 360 and
the bottom of bonnet 371. Applying gas pressure to this space 374
while bleeding pressure off the annular space 369 causes the plunger
360 to move downwardly and seat in the valve seat 335 thus closing
valve 320. Seals 375 376 around the plunger 360 on either side
of annular space 369 prevent leakage around the sides of the plunger
360.
Of course, the valve 320 could be actuated by air pressure, by
a solenoid, or by any other well-known mechanism, and does not have
to be pilot gas operated for purposes of this invention. The P.sub.2
pressure transducer 92 (FIG. 1 ) is connected by a tube 378 to a
port 377 extending from the left side 357 into the outlet plenum
62.
The foregoing is considered as illustrative only of the principles
of this invention. Further, since numerous modifications and changes
will readily occur to those skilled in the art, it is not desired
to limit the invention to the exact construction and operation shown
and described, and accordingly, all suitable modifications and equivalents
may be considered as falling within the scope of the invention as
defined by the claims which follow. |