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 high void fraction measuring multi-phase fluid flow meter
for measuring flow of a multi-phase fluid, comprising:
a first fluid flow path including a first flow measuring device
for measuring gas and liquid flow and a restrictor coupled in series
with said first flow measuring device thereby to slow the flow of
fluid through said first flow measuring device when said fluid includes
liquid;
a second fluid flow path in parallel with said first fluid path
and including a second flow measuring device for measuring gas flow;
means for detecting liquid flow in said flow meter;
means for diverting flow of gas into said second fluid flow path
liquid flow when liquid flow is not detected by said detecting means;
means for outputting an indication of the amount of liquid flow
through said first flow measuring device and combined total amount
of gas flow through said first and second flow measuring devices.
2. The flow meter of claim 1 wherein said diverting means comprises:
valve means disposed in said second fluid flow path and operable
to cut off gas flow in said second fluid flow path when liquid fluid
flow is detected by said detecting means.
3. The flow meter of claim 2 wherein said detecting means comprises:
means for measuring a pressure drop across said restrictor and
actuating said valve means when the pressure drop across said restrictor
exceeds a predetermined threshold.
4. The flow meter of claim 2 wherein said detecting means comprises:
means for detecting a presence of liquid flow at an inlet to said
first fluid flow path and for actuating said valve means to cut
off liquid gas flow through said second fluid flow path when the
presence of liquid flow is detected at said inlet.
5. The flow meter of claim 4 wherein said means for detecting
comprises:
means for producing a pressure drop at said inlet when liquid flow
is present at said inlet and actuating said valve means to cut off
liquid flow through said second fluid flow path when the detected
pressure drop at said inlet exceeds a predetermined value.
6. The flow meter of claim 5 wherein said means for producing
a pressure drop at said inlet when liquid flow is present comprises:
a flow restricting nozzle.
7. The flow meter of claim 4 wherein said means for detecting
comprises:
a venturi device having a throat provided at said inlet; and
means for detecting a pressure drop across said throat and actuating
said valve means to cut off flow through said second fluid flow
path when the pressure drop across said throat exceeds a predetermined
amount.
8. The flow meter of claim 2 wherein said means for detecting
comprises:
means for measuring a predetermined property of the fluid at an
inlet to said first fluid path and actuating said valve means to
cut off liquid flow through said second fluid flow path when the
measured property of said fluid indicates the presence of liquid
flow.
9. The flow meter of claim 1 wherein said diverting means comprises:
a jet to create a negative differential pressure in said second
fluid flow path; and
a check valve disposed in said second fluid flow path.
10. The flow meter of claim 9 wherein said jet comprises:
a flow restricting nozzle.
11. The flow meter of claims 1 2 3 4 5 6 7 8 or 10 comprising:
at least one of an expansion chamber and a de-mister provided in
the second fluid flow path.
12. The flow meter of claims 1 2 3 4 5 6 7 8 9 or 10 wherein:
said first flow measuring device comprises a multi-phase fluid
flow measuring device for measuring flow of gas, water and oil through
said first fluid flow path; and
said outputting means includes means for outputting an indication
of the amount of water flow and the amount of oil flow measured
by said multi-phase fluid flow measuring device and the combined
total amount of gas flow measured by said multi-phase fluid flow
measuring device and said second flow measuring device.
13. The flow meter of claim 12 comprising:
at least one of an expansion chamber and a de-mister provided in
said second fluid flow path.
14. A method of measuring multi-phase fluid flow of a multi-phase
fluid, comprising:
providing a flow meter having first and second fluid flow paths
in parallel with each other, said first fluid flow path including
a first flow measuring device for measuring both gas and liquid
flow and a liquid flow restrictor coupled in series with said first
fluid flow measuring device to slow the flow of liquid though said
first flow measuring device, said second fluid flow path including
a second flow measuring device for measuring gas flow;
detecting liquid flow in said flow meter;
controlling fluid flow through said first and second fluid flow
paths by diverting fluid flow through said second fluid flow path
when said detecting step does not detect liquid flow in said flow
meter and cutting off fluid flow through said second fluid flow
path when said detecting step detects liquid flow in said flow meter;
and, outputting an indication of the amount of liquid flow through
said first flow measuring device and the combined total amount of
gas flow through said first and second flow measuring devices.
15. The method of claim 14 wherein:
said detecting step comprises detecting a pressure drop across
said flow restrictor; and,
said controlling step comprises diverting fluid flow through said
second fluid flow path when the detected pressure drop across the
restrictor is less than a predetermined threshold and cutting off
gas flow through said second flow measuring device when the detected
pressure drop exceeds said predetermined threshold.
16. The method of claim 14 wherein said detecting step comprises:
detecting the presence of liquid flow at an inlet to said first
fluid path.
17. The method of claim 16 wherein said detecting step comprises:
producing a pressure drop at said inlet when liquid flow is present;
and
detecting when said pressure drop exceeds a predetermined threshold
indicative of liquid flow.
18. The method of claim 16 wherein said detecting step comprises:
passing the fluid at said inlet through a flow restricting nozzle
to produce a pressure drop across said nozzle when liquid flow is
present.
19. The method of claim 16 wherein said detecting step comprises:
providing at said inlet a venturi device having a throat; and
detecting when a pressure drop across said throat exceeds a predetermined
threshold.
20. The method of claim 16 wherein said detecting step comprises:
measuring at least one predetermined property indicative of liquid
flow at said inlet.
21. The method of claim 14 wherein:
said detecting step comprises producing a negative pressure differential
across said second fluid flow path when liquid flow is present;
and
said controlling step comprises,
disposing a check valve in series with said second flow measuring
device in said second fluid flow path, said check valve operating
to prevent backflow of liquid from an outlet of said first flow
measuring device through said second fluid flow path in the presence
of said negative pressure differential when said negative pressure
differential is produced in the presence of liquid flow.
22. The method of claim 21 wherein said detecting comprises:
passing the fluid through a flow restricting nozzle at said point
upstream of said second fluid flow path to produce said negative
pressure differential.
23. The method of claims 14 15 16 17 18 19 20 21 or 22
comprising:
disposing at least one of an expansion chamber and a de-mister
in said second fluid flow path to separate liquid from fluid flow
in said second fluid flow path.
24. The method of claims 14 15 16 17 18 19 20 21 or 22
comprising:
using a multi-phase fluid flow measuring device as said first flow
measuring device to measure flow of gas, water and oil through said
first fluid flow path; and
said outputting step comprising outputting an indication of the
amount of water flow and the amount of oil flow measured by said
multi-phase fluid flow measuring device and an indication of the
combined total amount of gas flow measured by said three phase fluid
flow measuring device and said second flow measuring device.
25. The method of claim 24 comprising:
disposing at least one of an expansion chamber and a de-mister
in said second fluid flow path to separate liquid from fluid flow
in said second fluid flow path.
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 liquid voids, where
the fluid to be measured is substantially void of liquid, to fluids
including a substantial liquid component.
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 more than 97% 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, 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
in which is disposed a gas flow measuring device for measuring gas
flow, means for detecting liquid flow in said flow meter, means
for diverting flow of gas into the second fluid flow path when liquid
flow is not detected in the flow meter, and means 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
diverting means 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 gas 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 of gas 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 and utilized to actuate the valve
to cut off gas 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 gas flow through the second fluid flow
path. Alternatively, the presence of liquid flow is measured by
means of a densiometer 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 gas 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 operates to divert gas flow
through the second fluid flow path so that gas flow is metered by
the gas flow measuring device and the multi-phase flow measuring
device.
In another embodiment according to the present invention, the diverting
means 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 to
prevent fluid flow through the second fluid flow path. This negative
differential pressure may create a reverse circulation flow from
the multi-phase flow measuring device to the second fluid flow path.
Therefore, 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, is 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 actuating a valve to cut
off gas 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, dielectric constant, capacitance, admittance and impedance,
and actuating the valve to cut off gas flow through the gas flow
measuring device 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. Circulating flow of liquid is prevented
by means of a check-valve in the second fluid flow path. An expansion
chamber and a demister in bypassing second fluid flow path helps
to collect carry-over liquid droplets. The expansion chamber and
demister 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 flowing through the three
phase flow meter and the gas flow measuring device. 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 input connecting pipe
14 at inputs thereto and an output connecting pipe 16 at outputs
thereto. 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. 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 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 proportional 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 gas 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 gas 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. 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 gas 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 nozzle
32 of the jet pump 34 in the chamber 36 is used to prevent fluid
flow through the flow path 12. The pressure in line 12 is substantially
the same as at the outlet of MPFM 18. The Jet Pump 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 creates a pressure drop larger than
the pressure drop across the multi-phase flow meter 18. In this
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 nozzle 32 the differential pressure in the chamber 46 is quite
small compared with that in the by-pass path 12 and most of the
flow goes through the by-pass where it is metered by the gas flow
meter 22.
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. |