Abstrict A high void fraction multi-phase fluid flow meter and method, wherein
a first fluid flow path including a multi-phase flow measuring device
disposed in series with a liquid flow restrictor is provided in
parallel with a second fluid flow path including a gas flow measuring
device. The presence of liquid flow in the flow meter is detected.
When liquid flow is detected, a valve in the second fluid flow path
operates to cut off fluid flow through the second fluid flow path.
Otherwise the valve in the second fluid flow path operates to divert
gas flow through the second fluid flow path. Alternatively, a negative
pressure differential is produced across the second fluid flow path
when liquid flow is present, by passing the incoming liquid flow
through a jet pump nozzle, to prevent liquid flow into the second
fluid flow path. A check valve is then disposed in the second fluid
flow path to prevent backflow from the output of the multi-flow
measuring device into the second fluid flow path. A computer outputs
an indication of the liquid flow, typically oil flow and water flow,
through the multi-phase flow meter, and the combined total amount
of gas flow through the two flow meters.
Claims What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A multi-phase fluid flow meter comprising:
a. a first fluid flow path comprising a first flow measuring device
capable of measuring gas and liquid flow, an inlet and an outlet;
b. a second fluid flow path in parallel with said first fluid flow
path, having an inlet and connected to said first fluid flow path
near the inlet of said first fluid flow path and having an outlet
connected to said first fluid flow path near the outlet of said
first fluid flow path, and further comprising a second flow measuring
device capable of measuring gas flow; and
c. a device comprising a flow chamber, said device installed at
the junctions of the inlets to said first and second flow paths,
said flow chamber comprising a flow restrictor such that the flow
of liquid through said flow restrictor creates a differential pressure
that inhibits liquid flow from entering said second fluid flow path.
2. The flow meter of claim 1 further comprising an indicator capable
of indicating the amount of liquid through said first flow measuring
device and the combined total amount of gas flow through said first
and second flow measuring devices.
3. The flow meter of claim 1 further comprising a check valve
installed in said second fluid flow path and configured to prevent
backflow when sufficient suction is created at said flow restrictor
to cause a backflow through said second fluid flow path.
4. The flow meter of claim 1 further comprising an expansion chamber
installed in said second fluid flow path upstream of said device.
5. The flow meter of claim 4 further comprising a demister contained
within said expansion chamber, said demister being capable of removing
mist from fluid flowing through said demister.
6. The flow meter of claim 1 further comprising a jet pump in
said flow chamber, said jet pump comprising said flow restrictor.
7. A multi-phase flow meter comprising:
a. a first fluid flow path comprising a first flow measuring device
capable of measuring gas and liquid flow, an inlet and an outlet;
b. a second fluid flow path in parallel with said first fluid flow
path, having an inlet and connected to said first fluid flow path
near the inlet of said first fluid flow path and having an outlet
connected to said first fluid flow path near the outlet of said
first fluid flow path, and further comprising a second flow measuring
device capable of measuring gas flow;
c. a device comprising a flow chamber, said device installed at
the junctions of the inlets to said first and second flow paths,
said flow chamber comprising a flow nozzle installed in said flow
chamber such that the flow of liquid through said flow nozzle creates
a suction that inhibits fluid flow from entering said second fluid
flow path; and
d. an indicator capable of indicating the amount of liquid through
said first flow measuring device and the combined total amount of
gas flow through said first and second flow measuring devices.
8. The flow meter of claim 7 further comprising a check valve
installed in said second fluid flow path and configured to prevent
backflow once sufficient suction is created at said flow restrictor
to cause a backflow through said second fluid flow path.
9. The flow meter of claim 7 further comprising an expansion chamber
installed in said second fluid flow path upstream of said second
flow measuring device.
10. The flow meter of claim 7 further comprising a jet pump in
said flow chamber, said jet pump comprising said flow restrictor.
11. A multi-phase fluid flow meter comprising:
a. a first fluid flow path comprising a first flow measuring device
capable of measuring gas and liquid flow, and a restrictor coupled
in series with said first flow measuring device, such that the flow
of fluid through said first flow measuring device is slowed when
said fluid includes liquid, and said first fluid flow path further
comprising an inlet and an outlet;
b. a second fluid flow path in parallel with said first fluid flow
path, having an inlet and connected to said first fluid flow path
near the inlet of said first fluid flow path and having an outlet
connected to said first fluid flow path near the outlet of said
first fluid flow path, and further comprising a second flow measuring
device capable of measuring gas flow;
c. a liquid flow detection device in said flow meter located upstream
of the inlets of said first and second fluid flow paths;
d. a device for diverting gas flow into said second fluid flow
path when liquid flow is not detected by said liquid flow detection
device and said diverting device further being capable of preventing
fluid flow from entering said second fluid flow path at its inlet
when liquid flow is detected by said liquid flow detection device;
and
e. an indicator capable of indicating the amount of liquid through
said first flow measuring device and the combined total amount of
gas flow through said first and second flow measuring devices.
12. The flow meter of claim 11 wherein said device for diverting
gas flow comprises a valve disposed in said second fluid flow path
and operable to cutoff flow in said second fluid flow path when
liquid flow is detected by said liquid flow detection device.
13. The flow meter of claim 12 wherein said liquid flow detection
device comprises process instrumentation for measuring a pressure
drop across said restrictor and closing said valve when the pressure
drop across said restrictor exceeds a predetermined threshold.
14. The flow meter of claim 12 wherein said liquid flow detection
device comprises process instrumentation for detecting the presence
of liquid flow at the inlet to said first fluid flow path and for
closing said valve to cutoff flow through said second fluid flow
path when the presence of liquid flow is detected at said inlet.
15. The flow meter of claim 14 wherein said process instrumentation
is a pressure sensor.
16. The flow meter of claim 14 wherein said liquid flow detection
device comprises a flow restrictor installed in the inlet of said
first fluid flow path, said flow restrictor being capable of producing
a pressure drop at said inlet when liquid flow is present at said
inlet.
17. The flow meter of claim 16 wherein said flow restrictor is
a jet pump.
18. The flow meter of claim 11 wherein said liquid flow detection
device comprises:
a. a flow restriction nozzle installed upstream of said inlets
to said first and second fluid flow paths;
b. a pressure sensor capable of measuring the pressure near the
outlet of said flow restriction nozzle; and
c. a valve controller coupled to said valve and capable of receiving
a signal from said pressure sensor indicative of measured pressure,
such that said valve is closed when the measured pressure exceeds
a predetermined value.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to multi-phase fluid flow meters and, in
particular, multi-phase fluid flow meters capable of accurately
measuring the flow of gas and liquid components of fluid flow over
a wide range of fluid concentrations from high gas fraction, where
the fluid to be measured is substantially void of liquid, to fluids
including a substantial liquid component. The liquid component is
further analyzed to provide flowrates of oil and water constituents.
2. Discussion of Background
In the oil industry, it is often necessary to measure the output
of oil wells under varying conditions. In particular, oil wells
typically have fluid outputs including gas and liquid components,
with the liquid components typically including water and oil. In
order to reliably measure the quantity of each component in the
oil well output, U.S. Pat. No. 5099697 discloses a multi-phase
flow meter (MPFM) for measuring multi-phase fluid flow, and particularly
three phase fluid flow including gas, water and oil. However, for
oil wells which produce large fractions of gas by volume, it is
very difficult to measure accurately the flow of each fluid component
as the MPFM must be sized for the gas volumetric flow, while the
liquid flow may be only a fraction of a percent. Under such circumstances
which demand a wide operating dynamic range and accuracy over the
entire range of operation, including the measuring of high void
fraction fluids, i.e., high gas concentration fluids, is compromised.
SUMMARY OF THE INVENTION
Accordingly, the object of this invention is to provide a novel
multi-phase fluid flow meter capable of operating with high accuracy
over a wide range of fluid concentrations, including fluids having
high void fractions.
This object and other objects are achieved according to the present
invention by providing a new and improved high void fraction multi-phase
fluid flow meter, including a first fluid flow path in which is
disposed a multi-phase flow measuring device for measuring gas and
liquid flow and a restrictor coupled in series with the multi-phase
flow measuring device thereby to slow the flow of fluid through
the measuring device when the fluid includes liquid, a second fluid
flow path provided in parallel with the first fluid flow path and
such detection in which is disposed a gas flow measuring device
for measuring gas flow, liquid flow detection device in said flow
meter, device for diverting flow of gas into the second fluid flow
path when liquid flow is not detected in the flow meter, and device
for outputting an indication of an amount of liquid flow through
the multi-phase flow measuring device and combined total gas flow
through the multi-phase flow measuring device and the gas flow measuring
device.
In one embodiment of the flow meter of the present invention, the
gas flow diversion device includes a valve disposed in series with
the gas flow measuring device. A pressure drop across the restrictor
is detected and compared with a predetermined threshold, and if
that threshold is exceeded, as occurs in the presence of liquid
flow through the restrictor, the valve operates to cut off flow
through the second fluid flow path.
In another embodiment, the presence of liquid flow at an inlet
to the first fluid path is detected and the detected presence of
liquid flow is used to actuate the valve to cut off flow through
the second fluid flow path. In this embodiment, the presence of
liquid is typically detected by passing the fluid being measured
through a flow restricting nozzle, such as a nozzle of a jet pump,
to produce a pressure drop in the presence of liquid flow, with
the pressure drop being detected by process instrumentation and
utilized to actuate the valve to cut off flow through the second
fluid flow path. Alternatively, a venturi device is provided at
the inlet to the first fluid flow path. The pressure drop across
the throat of the venturi device is utilized to detect the presence
of liquid flow and based thereon actuate the valve to cut off flow
through the second fluid flow path. Alternatively, the presence
of liquid flow may be measured by means of a densitometer which
measures the density of the fluid at the inlet to the first fluid
flow path and when the measured density indicates the presence of
liquid flow, the valve in the second fluid flow path is actuated
to cut off flow through the second fluid flow path. On the other
hand, when liquid flow is not detected in any of the above embodiments,
the valve opens to permit or divert gas flow through the second
fluid flow path so that gas flow from the second fluid flow path
is metered by the gas flow measuring device and the gas flow through
the first fluid flow path is metered by the multi-phase flow measuring
device.
In another embodiment according to the present invention, the gas
flow diversion device includes a pressure reduction device, such
as a flow restricting nozzle of a jet pump, at a point upstream
of the inlet to the second fluid flow path. In the presence of liquid
flow, a negative differential pressure is produced across the second
fluid flow path thereby preventing fluid flow from entering the
second fluid flow path at its inlet. This negative differential
pressure may create a reverse circulation flow from the multi-phase
flow measuring device to the second fluid flow path. Therefore in
a preferred embodiment, to prevent a reverse circulation flow, a
check valve is inserted in the second fluid flow path. An expansion
chamber and a de-mister, to knock off any misty droplets carried
by the gas, are preferably included in the second fluid flow path
at an inlet thereto.
Preferably, the multi-phase flow measuring device disposed in the
first fluid flow path is a three phase flow meter capable of measuring
the concentration of gas, water and oil in the fluid under measurement.
In that case, the flow meter of the present invention outputs an
indication of the total water flow and the total oil flow measured
by the three phase flow meter as well as the combined total gas
flow measured by both the three phase flow meter and the gas flow
measuring device. However, the present invention also applies where
a two-phase flow meter is used to measure flow of gas and liquid
(oil and water) in the first fluid flow path, and in that instance
the present invention outputs an indication of liquid flow through
the two-phase flow meter and combined total gas flow through the
two-phase flow meter and the gas flow measuring device.
The present invention further includes a new and improved method
of measuring multi-phase fluid flow of a fluid, including providing
a flow meter having first and second fluid flow paths in parallel
with each other, with the first fluid flow path including a multi-phase
flow measuring device for measuring both gas flow and liquid flow
and a liquid flow restrictor coupled in series with the multi-phase
flow measuring device to slow the flow of liquid through the multi-phase
flow measuring device, and the second fluid flow path including
a gas flow measuring device for measuring gas flow; detecting liquid
flow in the flow meter; controlling fluid flow through the first
and second fluid flow paths by diverting fluid flow through the
second fluid flow path when the detecting step does not detect liquid
flow in the flow meter and cutting off fluid flow through the second
fluid flow path when the detecting step detects liquid flow in the
flow meter; and outputting an indication of the amount of liquid
flow through the multi-phase flow measuring device and the combined
total amount of gas flow through the multi-phase and gas flow measuring
devices.
A first embodiment of the method invention includes detecting a
pressure drop across the flow restrictor provided in series with
the multi-phase flow measuring device and closing a valve to cut
off flow through the second fluid flow path when the pressure drop
detected exceeds a predetermined threshold.
In a second embodiment of the method of the present invention,
the presence of liquid flow is detected at an inlet to the first
fluid flow path, for example, by producing a pressure drop at the
inlet upon the presence of liquid flow, such as by passing the liquid
under measurement through either a flow restricting nozzle or a
venturi device having a throat, and detecting the resulting pressure
drop across the flow restricting nozzle or the venturi device in
the presence of liquid flow. Alternatively, the method includes
measuring one or more of predetermined properties of the fluid flowing
at the inlet, such as the fluid's density, thermal conductivity,
electrical conductivity, optical opacity, or absorption of nuclear,
electromagnetic or sound waves, or other properties such as taught
in U.S. Pat. No. 4774680 e.g., current, voltage, frequency, energy
absorption, dielecric constant, capacitance, admittance and impedance,
and closing the valve to cut off flow through the secondary flow
measuring path when the measured property indicates the presence
of liquid flow.
In another embodiment of the method of the present invention, a
negative differential pressure is created when liquid is present
by means of a jet-pump. Backflow through the second fluid flow path
is prevented by means of a check-valve in the second fluid flow
path. An expansion chamber and a de-mister in the second fluid flow
path collects carry-over liquid droplets. The expansion chamber
and de-mister can be added to each of the embodiments of the present
invention.
In a preferred embodiment of the method of the present invention,
a three phase flow meter for measuring the flow of gas, water and
oil is employed as the multi-phase flow measuring device in the
first fluid flow path. In the preferred embodiment, an indication
of the amount of water flow and the amount of oil flow through the
three phase flow meter is outputted, as well as an indication of
the combined total amount of gas flow through the three phase flow
meter and the gas flow measuring device which is located in the
second flow path. Alternatively, when a two-phase flow measuring
device is employed in the first fluid flow path, the method of the
present invention includes outputting an indication of liquid flow
through the two-phase flow meter and an indication of combined total
gas flow through the two-phase flow meter and the gas flow measuring
device.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes
better understood by reference to the following detailed description
when considered in connection with the accompanying drawings, wherein:
FIG. 1 is a schematic block diagram of a first embodiment of the
high void fraction multi-phase fluid flow meter of the present invention;
FIG. 2 is a schematic block diagram of a second embodiment of the
flow meter of the present invention;
FIG. 3 is a schematic block diagram of a third embodiment of the
flow meter of the present invention;
FIG. 4 is a schematic block diagram of a fourth embodiment of the
flow meter of the present invention;
FIGS. 5a and 5b are time charts illustrating fluid flow through
the multi-phase fluid flow measuring device in a first flow path
of the flow meter of the present invention both without and with,
respectively, the presence of a restrictor in the first fluid flow
path; and
FIG. 6 is a schematic block diagram of a fifth embodiment of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, wherein like reference numerals designate
identical or corresponding parts throughout the several views, and
more particularly to FIG. 1 thereof, a first embodiment of the flow
meter of the present invention includes a first fluid flow path
10 and a second fluid flow path 12 connected in parallel to the
path 10 both paths in communication with an inlet connecting pipe
14 at inlets 11 to said first flow path and 13 to said second flow
path and an outlet connecting pipe 16 at outlets 15 to said first
flow path and 17 to said second flow path. The first fluid flow
path 10 includes a multi-phase fluid flow meter 18 preferably implemented
as taught in commonly owned U.S. Pat. No. 5099697 coupled in
series with a restrictor 20 the output of the restrictor 20 communicating
with the output connecting pipe 16. In the embodiment disclosed
in FIG. 1 the second fluid flow path 12 includes a gas flow meter
22 coupled in series with a valve 24 which in turn is coupled to
the output of the first fluid flow path and the output connecting
pipe 16.
As shown in FIG. 1 process instrumentation in the form of pressure
sensors 26 and 28 are provided at the input and the output of the
restrictor 20. Sensors 26 and 28 have respective outputs applied
to a computer 30 which determines the difference between the pressure
sensed by the pressure sensors 26 and 28 compares the difference
to a threshold, and actuates the valve 24 when the pressure difference
exceeds a predetermined value indicative of liquid flow through
the restrictor 20. Computer 30 otherwise maintains the valve 24
open to permit gas flow through the second fluid flow path when
the detected pressure drop across the restrictor 20 is less than
the predetermined threshold, which is indicative of substantial
gas flow through the restrictor 20.
The present invention takes advantage of the recognition that in
most cases, droplets of liquid in the fluid flow tend to bunch together
(as happens in klystrons chromatographs, etc.) and would appear
at the flow meter 18 in the absence of the restrictor 20 as a
short duration spike of mainly liquid mixed with gas, as shown in
FIG. 5a. However, it is difficult to measure accurately the amount
of liquid flow in high velocity short duration spikes as shown in
FIG. 5a. This is true because of the finite response time of the
flow meter 18 and because the possibility of damaging flow meter
18. According to the present invention, the difficulty is overcome
by slowing the fluid spike to produce a liquid slurry, i.e., by
packing more liquid in the flow path 10. Therefore, according to
the present invention, the restrictor 20 is provided to "iron
out" the sharp spikes so that the fluid flow appears at the
flow meter 18 as a slug of decreased velocity and amplitude and
increased time duration. Since the input connecting pipe 14 is not
packed with incompressible fluid, i.e., is packed with a gas/liquid
mixture, restrictor 20 prolongs the duration of the liquid slug
through the flow meter 18 by packing more liquid upstream of the
flow meter 18. Thus, the restrictor 20 slows the flow of the liquid-gas
mixture through the flow meter 18 and causes it to pack-up in front
of the flow meter 18 as fluid slugs shown schematically in FIG.
5b.
Typically, the restrictor 20 is dimensioned to limit the maximum
liquid flow through the flow meter 18 to 150% of the full-scale
rating of the volumetric flow meter section of flow meter 18. For
example, if flow meter 18 is implemented according to U.S. Pat.
No. 5099097 it would include two volumetric flow meter sections
separated by a restriction. In that instant, the restrictor 20 is
dimensioned to limit the maximum liquid flow to 150% of the full
scale rating of the smaller of the two volumetric flow meter sections.
If on the other hand, a multi-phase flow meter as taught in copending
U.S. patent application Ser. No. 08/852544 incorporated by reference
herein, which includes a volumetric flow meter section in series
with a momentum flow meter section, is used, then restrictor 20
is sized to limit liquid flow to 150% of the full-scale rating of
the volumetric flow meter section. Thus, in the present invention,
the flow meter 18 is sized for the maximum liquid flow and the flow
meter 22 is sized for the maximum gas flow. At high void fraction,
e.g., 95% void fraction, the superficial gas flow is much higher
than the superficial liquid flow, i.e., approximately 20:1 higher,
and the flow meters 22 and 18 are sized accordingly.
Further elaborating on the role of the restrictor 20 as the differential
pressure p across the restrictor is proportionate to the fluid velocity
squared (V.sup.2) times the density (D), i.e., p=DV.sup.2 since
the density of the gas is quite small in comparison to the density
of the liquid, gas flow through the restrictor 20 will hardly be
affected, whereas since the density of the liquid is much greater
than the density of the gas, the pressure loss for the liquid, for
the same velocity, is much greater than the pressure loss for gas
passing through the restrictor 20. Thus, as previously indicated,
the restrictor 20 slows down liquids, and not gases, and results
in a measurable pressure drop in the presence of liquid. Since the
liquid flow peaks are "ironed out" as shown in FIG. 5b,
the flow meter 18 can thus be made much smaller than would otherwise
be necessary to accommodate the peak superficial gas flow rate,
as the restrictor 20 determines the maximum flow rate.
According to the present invention as above indicated, the pressure
sensors 26 28 and the computer 30 are utilized to detect the presence
of liquid flow through the restrictor 20 and actuate the valve 24
to cut off flow through the gas flow meter 22 so that all the fluid
from the input connecting pipe 14 passes through the multi-phase
flow meter 18 in the presence of liquid flow. In the presence of
substantial gas flow and inconsequential liquid flow, i.e., in the
presence of high void fraction fluid flow, little pressure drop
is detected across the restrictor 20 resulting in gas flow through
both the first and second fluid flow paths and gas measurement by
both the multi-phase flow meter 18 and the gas flow meter 22. In
the operation as above described, the restrictor slows down the
liquids, but not the gases. The flow meter 18 can thus be designed
to measure maximum liquid flow rates much lower than would otherwise
be required if it had to measure the maximum gas flow rate as well.
Restrictor 20 determines the maximum liquid flow rate as a result
of which flow meter 18 experiences much smaller flows. This cannot
be done in a single-phase fluid flow in which the line 14 would
have no spare space to pack more liquid in it, but does apply to
multi-phase flow including gas flow.
Valve 24 in FIG. 1 is shut when the differential pressure across
restrictor 20 exceeds a predetermined value, i.e. when a mixture
of liquid and gas is flowing through it. Flow path 12 is bigger
than flow path 10 and as the liquid is substantially incompressible,
the position of restrictor 20 is immaterial whether it is upstream
or downstream of flow meter 18. The advantage of inserting it downstream
is that absolute pressure in flow meter 18 is maintained higher,
thus it sees a smaller actual gas flow rate. The disadvantage is
that more liquid will enter by-pass flow path 12 before the valve
24 will shut. The vertical riser of by-pass path 12 requires a larger
differential pressure than the differential pressure across the
restrictor 20. Valve 24 then will shut long before the riser fills
up preventing flow of liquid through path 12.
Data on the flow rates of gas, water and oil passing through the
flow meter 18 are applied to the computer 30 along with the outputs
of the pressure sensors 26 and 28. The computer 30 controls activation
of the valve 24 to allow the high velocity gas to flow through the
second fluid flow path 12 which serves as a by-pass for the high
velocity gas. The flow of gas through the second fluid flow path
12 is metered by the gas flow meter 22 the output of which is also
applied to the computer 30. At high gas flow rate where the flow
meter 18 runs at 150% of its normal gas flow rate, the differential
pressure across the restrictor is relatively small, so the computer
30 causes the valve 24 to be maintained open and permits excess
gas to be metered by the gas flow meter 22. The computer 30 then
outputs an indication of the liquid flow, i.e., water flow and oil
flow measured by the flow meter 18 as well as a combined total
gas flow measured by the flow meters 18 and 22. Where a two-phase
flow meter is used for the flow meter 18 then the computer outputs
an indication of liquid flow through the flow meter 18 as well as
combined total gas flow through the flow meters 18 and 22.
FIG. 2 shows a second embodiment of the invention which likewise
results in the cut off of flow through the gas flow meter 22 upon
detection of the presence of liquid flow in the flow meter. In the
embodiment of FIG. 2 however, operation of the valve 24 is controlled
based on the pressure drop produced by liquid flow through the nozzle
32 of a jet pump 34 which is installed upstream of flow meter 18.
In FIG. 2 the valve 24 is controlled by the suction created by
the jet pump 34 when liquid passes through the jet pump 34. When
liquid flows through the nozzle 32 of the jet pump 34 a lower pressure
is created in the chamber 36. Valve 24 is schematically shown in
FIG. 1 and can be either a pneumatic or hydraulic diaphragm valve,
or a solenoid operated valve. As shown in FIG. 2 process instrumentation
in the form of pressure sensor 37 shown as "P.sub.3 ",
is housed in chamber 36 and is further operable to send a control
signal to valve 24. Valve controller 25 is coupled to valve 24 and
capable of receiving a signal from pressure sensor 37 indicative
of the measured pressure, such that valve 24 is actuated when the
measured pressure exceeds a predetermined value. The valve 24 is
normally open, but shuts off when liquid flow is detected. The advantage
of using a normally open valve is that if there is a power failure,
the line is not blocked to gas flow by the flow meter. The reduced
pressure produced by liquid flow is utilized to shut off the reverse
acting valve 24 causing the valve 24 to cut off flow through the
second fluid flow path 12. When gas passes through the nozzle 32
pressure in the chamber 36 is essentially the same as in the main
line 38 feeding the jet pump 34 and valve 24 opens under the action
of a reverse acting spring (not shown). Thus, liquid-gas mixture
slugs are metered by the multi-phase flow meter 18 while high flowing
gas is metered by both the flow meters 18 and 22. As in the first
embodiment, the computer 30 outputs indications of the water flow
and the oil flow through the multi-phase flow meter 18 and the combined
total gas flow through the flow meters 18 and 22 when a three-phase
flow meter 18 is employed and otherwise outputs an indication of
the liquid flow through the flow meter 18 and the combined total
gas flow through the flow meters 18 and 22 when a two-phase flow
meter 18 is employed.
In another embodiment shown in FIG. 3 the jet pump 34 is replaced
by a venturi 40 having a throat 42. In the FIG. 3 embodiment, the
low pressure created at the throat 42 of the venturi operates the
reverse acting valve 24 in a similar manner to the operation of
the jet pump of FIG. 2. The advantage of this embodiment is that
very little pressure is lost across the venturi 40 and most of the
pressure dropped at the throat 44 is recovered by the expander section
of the venturi downstream of the throat 42.
In FIG. 4 the venturi is replaced with a void fraction meter 44
which operates valve 24 in a similar manner as described with respect
to the embodiment of FIG. 1. Void fraction meter 44 can be a device
measuring the density of the fluid (nuclear, differential pressure,
etc.) or other known void fraction meter devices. There are many
types of void fraction meters: e.g. dielectric, electrical or thermal
conductivity, optical, etc. They all measure how dense the fluid
is. Use of a density meter is reserved for meters that measure the
density of the fluid in weight/volume units.
FIG. 6 illustrates another embodiment of the present invention,
and in this embodiment the differential pressure across the flow
restrictor 32 of the jet pump 34 in the chamber 36 is used to prevent
liquid flow through the second flow path 12. As shown in FIG. 6
the flow restrictor 32 is a nozzle. The pressure in line 12 is substantially
the same as at the outlet of MPFM 18. The flow restrictor 32 of
Jet Pump 34 creates a suction (lower pressure) which would suck
liquid back from the outlet of MPFM 18 if this were not prevented.
When a slug of liquid-gas mixture goes through the jet 34 it may
create a pressure drop larger than the pressure drop across the
multi-phase flow meter 18. Since differential pressure or suction
is used to inhibit liquid flow from entering the inlet of the second
fluid flow path, instead of a valve 24 as shown in FIGS. 1-4 no
separate liquid flow detection device is needed with this embodiment
of the invention. In a preferred embodiment, a check valve 24.sub.1
is employed in the by-pass fluid flow path 12. Check valve 24.sub.1
closes to prevent backflow from the output of the flow meter 18
which might otherwise result due to the pressure drop caused by
the flow of liquid through the nozzle 32. Thus, backflow through
the flow path 12 from the output of the flow meter 18 is prevented
by the check valve. The fluid mixture passes through to the multi-phase
fluid flow meter 18 and to the output connecting pipe 16. In the
presence of gas passing through the flow restrictor 32 the differential
pressure in the chamber 46 is quite small compared with that in
the by-pass path or second fluid flow path 12 and most of the flow
goes through the by-pass where it is metered by the gas flow meter
22.
In a preferred embodiment, the invention comprises an expansion
chamber 46 as shown in FIG. 6. The expansion chamber 46 provided
upstream of the flow meter 22 slows the flow of fluid to the flow
meter 22 and allows any droplets of liquid carryover to drop back
into the chamber 36. Adding a de-mister 48 enhances the effect.
The expansion chamber 46 with or without a de-mister 48 can be
used to advantage in any of the embodiments of the present invention.
Obviously, numerous modifications and variations of the present
invention are possible in light of the above teachings. It is therefore
to be understood that within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
herein.
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