## Abstrict A flow meter for a multiphase flow which comprises a cross-correlation
flow meter (26) for measuring basic values to calculate component
ratios of respective fluids constituting a multiphase fluid (2)
comprising a gas and a plurality of liquids in a pipe (1) through
which the multiphase fluid flows; and an arithmetic circuit (22)
for calculating flow rates of the respective fluids, which acquires
information concerning ratios of liquid phase components of the
multiphase fluid (2) on the basis of both measured values obtained
by the cross-correlation flow meter (26) at an instance when the
pipe (1) is filled with liquid alone during the passage of the multiphase
fluid (2) through the pipe (1) provided with the cross-correlation
flow meter (26) and characteristic values of the respective fluids
of the multiphase fluid (2), acquires information concerning ratios
of the respective fluids from a time average of the measurements
obtained by the cross-correlation flow meter (26) and the respective
characteristic values, then obtains the component ratios of the
respective fluids by utilizing the fact that a sum of the component
ratios of the respective fluids becomes (1), calculates an average
flow velocity of the multiphase fluid (2) from time between fluctuations
of the measured values obtained by the cross-correlation flow meter
(26), and calculates flow rates of the respective fluids by utilizing
the respective component ratios and the average flow velocity.
## Claims What is claimed is:
1. A method of measuring flow rates of respective fluids in a multiphase
fluid, the multiphase fluid consisting of a gas phase constituted
by a gas and a liquid phase constituted by a plurality of liquids,
the method comprising the steps of: arranging a cross-correlation
flow meter in a pipe through which the multiphase fluid is flowing,
the flow meter comprising two constituent meters and a sensor, the
constituent meters each measuring a first basic value of the multiphase
fluid at a plurality of time intervals to obtain a plurality of
first basic value measurements and the sensor measuring a second
basic value of the multiphase fluid; determining a maximum value
of the plurality of first basic value measurements, the maximum
value corresponding to the first basic value at an instant when
the fluid flowing through the pipe and passing the corresponding
constituent meter consists only of the liquid phase; calculating
a mean value of the plurality of first basic value measurements;
detecting a pattern of fluctuations of the first basic value measured
by each constituent meter; comparing the pattern of fluctuations
of the first basic value measured by one of the constituent meters
with the pattern of fluctuations of the basic value measured by
the other constituent meter to determine a time lag between the
fluctuations; calculating component ratios of respective liquids
in the liquid phase from the maximum value and characteristic values
of the respective liquids; calculating component ratios of the respective
fluids in the multiphase fluid from the mean value, a characteristic
value of the gas in the gas phase, and the component ratios of the
respective liquids; determining a flow velocity of the gas phase
from the time lag between the fluctuations; determining a flow velocity
of the liquid phase from the second basic value; and calculating
flow rates: of the respective fluids in the multiphase fluid from
the component ratio of the fluid in the gas phase, the flow velocity
of the gas phase, the component ratios of the liquids in the liquid
phase, and the flow velocity of the liquid phase.
2. The method of claim 1 wherein the first basic value is a variable
property selected from electrostatic capacitance and radiation transmittance
and the second basic value is a differential pressure.
3. The method of claim 2 wherein each of the constituent meters
is a capacitance water cut meter and the first basic value is electrostatic
capacitance.
4. The method of claim 2 wherein each of the constituent meters
is a radiation densitometer and the first basic value is radiation
transmittance.
5. The method of claim 2 wherein the differential pressure is
obtained from a differential pressure type flow meter.
6. A flow meter for determining flow rates of respective fluids
in a multiphase fluid, the multiphase fluid consisting of a gas
phase constituted by a gas and a liquid phase constituted by a plurality
of liquids, the flow meter comprising: a cross-correlation flow
meter and a sensor arranged in a pipe through which the multiphase
fluid is flowing, the cross-correlation flow meter consisting of
two constituent meters each measuring a first basic value of the
multiphase fluid at a plurality of time intervals to obtain a plurality
of first basic value measurements, and the sensor measuring a second
basic value of the multiphase fluid; means for determining a maximum
value of the plurality of first basic value measurements, the maximum
value corresponding to the first basic value at an instant when
the fluid flowing through the pipe and passing the respective constituent
meter consists only of the liquid phase; means for calculating a
mean value of the plurality of first basic value measurements; means
for detecting a pattern of fluctuations of the first basic value
measured by each of the constituent meters; means for comparing
the pattern of fluctuations of the first basic value measured by
one of the constituent meters with the pattern of fluctuations of
the basic value measured by the other constituent meter to determine
a time lag between the fluctuations; means for calculating component
ratios of respective liquids in the liquid phase from the maximum
value and characteristic values of the respective liquids; means
for calculating component ratios of the respective fluids in the
multiphase fluid from the mean value, a characteristic value of
the gas in the gas phase, and the component ratios of the respective
liquids; means for determining a flow velocity of the gas phase
from the time lag between the fluctuations; means for determining
a flow velocity of the liquid phase from the second basic value;
and means for calculating flow rates of the respective fluids in
the multiphase fluid from the component ratio of the fluid in the
gas phase, the flow velocity of the gas phase, the component ratios
of the liquids in the liquid phase, and the flow velocity of the
liquid phase.
7. The flow meter of claim 6 wherein the first basic value is
a variable property selected from electrostatic capacitance and
radiation transmittance and the second basic value is a differential
pressure.
8. The flow meter of claim 7 wherein each of the constituent meters
is a radiation densitometer, the first basic value is radiation
transmittance, and the sensor is a differential pressure type flow
meter.
9. The flow meter of claim 6 wherein each of the constituent meters
is a capacitance water cut meter, the first basic value is electrostatic
capacitance and the sensor is a differential pressure type flow
meter.
10. The flow meter of claim 6 wherein each of the two constituent
meters comprises a cylindrical driving electrode for applying a
voltage signal of predetermined amplitude and frequency to the multiphase
fluid, a cylindrical measurement electrode virtually grounded for
detecting a current flowing through the multiphase fluid, and a
cylindrical dummy electrode having a potential identical to that
of the cylindrical measurement electrode, the driving electrode
being arranged in parallel with the measurement electrode within
the pipe for obtaining a water phase ratio in the multiphase fluid
by measuring an electrostatic capacitance between the cylindrical
driving electrode and the cylindrical measuring electrode, and the
dummy electrode being placed between the driving electrode and the
measurement electrode for reducing a part of an electric line force
toward the measurement electrode through the vicinity of a wall
of the pipe.
11. The flow meter of claim 10 wherein an inner diameter of the
pipe is expressed by D, inner diameters of the respective cylindrical
electrodes are substantially equal to D, the width of the cylindrical
driving electrode is expressed by l.sub.s, the width of the cylindrical
measurement electrode is expressed by l.sub.m. the width of the
cylindrical dummy electrode is expressed by l.sub.d, a distance
between the cylindrical driving electrode and the cylindrical measurement
electrode is expressed by L, and a distance between the cylindrical
driving electrode and the cylindrical dummy electrode is expressed
by x, and: l.sub.s divided by D equals 0.3 to 1.0; l.sub.m divided
by D equals 0.3 to 1.0; l.sub.d divided by D equals 0.1 to 0.5;
L divided by D equals 1.0 to 2.0; and x divided by D equals 0.4
to 1.2.
12. A method of measuring flow rates of respective fluids in a
multiphase fluid, the multiphase fluid consisting of a gas phase
constituted by a gas and a liquid phase constituted by a plurality
of liquids, the method comprising the steps of: arranging a cross-correlation
flow meter in a pipe through which the multiphase fluid is flowing,
the flow meter comprising two constituent meters each measuring
a basic value of the multiphase fluid at a plurality of time intervals
to obtain a plurality of basic value measurements; determining a
maximum value of the plurality of basic value measurements, the
maximum value corresponding to the basic value at an instant when
the fluid flowing through the pipe and passing the corresponding
constituent meter consists only of the liquid phase; calculating
a mean value of the plurality of basic value measurements; detecting
a pattern of fluctuations of the basic value measured by each constituent
meter; comparing the pattern of fluctuations of the basic value
measured by one of the constituent meters with the pattern of fluctuations
of the basic value measured by the other constituent meter to determine
a time lag between the fluctuations; calculating component ratios
of the phases in the multiphase fluid from the maximum value, a
characteristic value of each of the liquids in the liquid phase,
a characteristic value of the gas in the gas phase, and the mean
value; determining the inner diameter of the pipe; and calculating
flow rates of the respective phases in the multiphase fluid from
the component ratios of the respective phases, the average flow
velocity of the multiphase fluid, and the inner diameter of the
pipe.
13. The method of claim 12 wherein the basic value is a variable
property selected from electrostatic capacitance and radiation transmittance.
14. The method of claim 13 wherein each of the constituent meters
is a capacitance water cut meter and the basic value is electrostatic
capacitance.
15. The method of claim 13 wherein each of the constituent meters
is a radiation densitometer and the basic value is radiation transmittance.
16. A flow meter for determining flow rates of respective fluids
in a multiphase fluid, the multiphase fluid consisting of a gas
phase constituted by a gas and a liquid phase constituted by a plurality
of liquids, the flow meter comprising: a cross-correlation flow
meter in a pipe through which the multiphase fluid is flowing, the
flow meter comprising two constituent meters each measuring a basic
value of the multiphase fluid at a plurality of time intervals to
obtain a plurality of basic value measurements; means for determining
a maximum value of the plurality of basic value measurements, the
maximum value corresponding to the basic value at an instant when
the fluid flowing through the pipe and passing the corresponding
constituent meter consists only of the liquid phase; means for calculating
a mean value of the plurality of basic value measurements; means
for detecting a pattern of fluctuations of the basic value measured
by each constituent meter; means for comparing the pattern of fluctuations
of the basic value measured by one of the constituent meters with
the pattern of fluctuations of the basic value measured by the other
constituent meter to determine a time lag between the fluctuations;
means for calculating component ratios of the phases in the multiphase
fluid from the maximum value, a characteristic value of each of
the liquids in the liquid phase, a characteristic value of the gas
in the gas phase, and the mean value; means for determining the
inner diameter of the pipe; and means for calculating flow rates
of the respective phases in the multiphase fluid from the component
ratios of the respective phases, the average flow velocity of the
multiphase fluid, and the inner diameter of the pipe.
17. The flow meter of claim 16 wherein the basic value is a variable
property selected from electrostatic capacitance and radiation transmittance.
18. The flow meter of claim 17 wherein each of the constituent
meters is a capacitance water cut meter and the basic value is electrostatic
capacitance.
19. The flow meter of claim 17 wherein each of the constituent
meters is a radiation densitometer and the first basic value is
radiation transmittance.
## Description BACKGROUND
The present invention relates to an apparatus for on-line measurement
of flow rates of respective fluids of a multiphase fluid comprising
oil, water, gas, etc. and flowing in a pipe, without separating
the respective fluids.
Conventionally, a flow meter for a multiphase flow composed of
three or more sensors such as a water cut meter utilizing a difference
in electrical properties among fluids, a density meter utilizing
differences in density among fluids, and a flow meter measuring
a total flow rate or a flow velocity of a multiphase fluid has been
employed for measuring flow rates of respective fluids constituting
a multiphase fluid.
FIG. 7 is a block diagram of such a conventional flow meter for
a multiphase flow. This flow meter for a multiphase flow is constituted
by a total of three sensors, that is, a cross-correlation flow meter
6 composed of two capacitance water cut meters 3 and 3' and a gamma-ray
densitometer 7 for measuring an average density of a multiphase
fluid 2.
These capacitance water cut meters 3 and 3' are composed of electrodes
4 and 4' and impedance measurement circuits 5 and 5' provided in
a pipe 1 and the gamma-ray densitometer 7 is composed of a source
of gamma rays 8 and a detector 9.
An absolute pressure meter 10 and a thermometer 11 are used for
temperature correction of parameters such as density and dielectric
constant of respective fluids and a (volume) flow rate of gas.
Now, a principle of measurement for the conventional apparatus
is described.
An electrostatic capacity C of a multiphase fluid 2 which consists
of oil, water, and gas and flows in a pipe 1 is measured with a
capacitance water cut meter 3 and a transmittance .lambda. for gamma
rays of the multiphase fluid 2 is measured with a gamma-ray densitometer
7. Equation (1)
wherein H.sub.P represents an oil phase ratio, H.sub.W represents
a water phase ratio, and H.sub.G represents a gas phase ratio for
the multiphase fluid 2 is established.
When known relative dielectric constants of oil, water, and gas
are expressed by .epsilon..sub.P, .epsilon..sub.W, and .epsilon..sub.G,
Equation (2):
is established for the relationship between the known relative
dielectric constants and the electrostatic capacity C measured.
When known densities of oil, water, and gas are expressed by .rho..sub.P,
.rho..sub.W, and .rho..sub.G, Equation 3:
is established for the relationship between the known densities
and the gamma-ray transmittance .lambda. measured.
Then, f.sub..epsilon. (C) and f.sub..rho. (.lambda.) are intrinsic
functions of the capacitance water cut meter 3 and the gamma-ray
densitometer 7 and provide an average dielectric constant of the
multiphase fluid 2 from an electrostatic capacity C and an average
density of the multiphase fluid 2 from the transmittance .lambda.,
respectively.
On the other hand, a cross-correlation flow meter 6 composed of
two capacitance water cut meters 3 and 3' measures a travel speed
of fluctuations of electrostatic capacity C, that is, an average
flow velocity u of the multiphase fluid 2.
An arithmetic circuit 12 to calculate flow rates of the respective
phases calculates an oil phase ratio H.sub.P, a water phase ratio
H.sub.W, and a gas phase ratio H.sub.G for the multiphase fluid
2 from the simultaneous equations (1)-(3) and then calculates flow
rates of the respective fluids Q.sub.P, Q.sub.W, and Q.sub.G from
Equations (4-1), (4-2), and (4-3) using a cross section A of the
pipe 1 and the average flow velocity u.
A method of obtaining a flow velocity from fluctuations of a multiphase
fluid is described in detail in Japanese Patent Application No.
8-128389 etc.
However, such a conventional apparatus requires a combination of
at least three sensors such as two capacitance water cut meters
and a gamma-ray densitometer to obtain component ratios and average
flow velocities of respective fluids constituting a multiphase fluid
and thus interferes with simplification and size-reduction of a
structure of a flow meter for a multiphase flow.
In addition, since an average flow velocity u is solely used to
calculate flow rates, there has been such a problem that errors
in flow rates of respective fluids measured become larger when a
velocity slip (difference in velocity) exists between gas and liquid.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a flow meter
for a multiphase flow composed of a smaller number of sensors than
the conventional apparatuses and to provide a flow meter for a multiphase
flow enabling highly accurate measurement in spite of velocity slip
between gas and liquid.
According to the method of the present invention, a cross-correlation
flow meter for measuring basic values to calculate component ratios
of respective fluids constituting a multiphase fluid comprising
a gas and a plurality of liquids is provided to obtain the component
ratios of the respective fluids on the basis of the measured values
of the cross-correlation flow meter; when there is no velocity slip
between a gas phase and a liquid phase in the multiphase fluid,
an average flow velocity of the multiphase fluid is obtained on
the basis of time between fluctuations of the measured values of
the cross-correlation flow meter and then flow rates of the respective
fluids are obtained by utilizing the respective component ratios
and the average flow velocity; and when there is a velocity slip
between the gas and liquid phases, a flow velocity of the gas phase
of the multiphase fluid is obtained on the basis of time between
fluctuations of the measured values of the cross-correlation flow
meter, and a sensor for measuring basic values to calculate a flow
velocity of the liquid phase of the multiphase fluid is provided
to obtain the flow velocity of the liquid phase on the basis of
the measured values of the sensor, and then the flow rates of the
respective fluids are calculated by utilizing the component ratio
of a fluid in gas phase, the flow velocity of the gas phase, the
component ratios of fluids in the liquid phase and the flow velocity
of the liquid phase.
The cross-correlation flow meter comprises two component ratio
meters for measuring predetermined electrical values in a pipe through
which the multiphase fluid flows, and the component ratios of the
respective fluids are obtained by acquiring information concerning
component ratios of the fluids in the liquid phase components from
both measured values obtained by the component ratio meters at an
instance when the pipe is filled with liquid alone during the passage
of the multiphase fluid through the pipe and electrical characteristic
values of the respective fluids, acquiring information concerning
the component ratios of the respective fluids from both a time average
of the measured values obtained by the component ratio meters and
the respective electrical characteristic values, and utilizing the
fact that a sum of the component ratios of the respective fluids
becomes 1.
In addition, the cross-correlation flow meter comprises two radiation
densitometers for measuring radiation transmittance in a pipe through
which the multiphase fluid flows, and the component ratios of the
respective fluids are obtained by acquiring information concerning
component ratios of the fluids in the liquid phase from both measured
values obtained by the radiation densitometers at an instance when
the pipe is filled with liquid alone during the passage of the multiphase
fluid through the pipe and densities of the respective fluids, acquiring
information concerning component ratios of the respective fluids
from both a time average of the measured values obtained by the
radiation densitometers and the respective densities, and utilizing
the fact that a sum of the component ratios of the respective fluids
becomes 1.
A differential pressure of the multiphase fluid is measured with
a differential pressure type flow meter and a flow velocity of the
liquid phase is obtained on the basis of the measured differential
pressure, an average density of the multiphase fluid, and an intrinsic
coefficient for the differential pressure type flow meter.
The apparatus of the present invention comprises a cross-correlation
flow meter provided in a pipe through which a multiphase fluids
comprising a gas and plurality of liquids, for measuring basic values
to calculate component ratios of respective fluids constituting
the multiphase fluid; and an arithmetic circuit for calculating
flow rates of the respective fluids, by acquiring information concerning
component ratios of fluids in a liquid phase of the multiphase fluid
from both measured values obtained by the cross-correlation flow
meter at an instance when the pipe is filled with liquid alone during
the passage of the multiphase fluid through the pipe provided with
the cross-correlation flow meter and characteristic values of the
respective fluids of the multiphase fluid, acquiring information
concerning the component ratios of the respective fluids from both
a time average of the measured values obtained by the cross-correlation
flow meter and the respective characteristic values, obtaining the
component ratios of the respective fluids by utilizing the fact
that a sum of the component ratios of the respective fluids becomes
1 calculating an average flow velocity of the multiphase fluid
on the basis of time between fluctuations of the measured values
obtained by the cross-correlation flow meter, and utilizing the
respective component ratios and the average flow velocity for the
calculation of the flow rates.
The apparatus of the present invention comprises a sensor provided
in a pipe, for measuring basic values to calculate a flow velocity
of a liquid phase of a multiphase fluid; and the arithmetic circuit
for calculating flow rates of the respective fluids with an additional
function to calculate a flow velocity of a gas phase of the multiphase
fluid on the basis of time between fluctuations of the measured
values of the cross-correlation flow meter when there is a velocity
slip between a gas phase and a liquid phase of the multiphase fluids,
to calculate a flow velocity of the liquid phase on the basis of
the measured values obtained by the sensor, and to calculate flow
rates of the respective fluids by utilizing the component ratio
and the flow velocity of the gas phase and the respective component
and the flow velocity of the liquid phase.
In addition, the cross-correlation flow meter comprises two component
ratio meters for measuring an electrostatic capacity of the multiphase
fluid and the characteristic values are in relative dielectric constant.
in addition, the cross-correlation flow meter comprises two radiation
densitometers for measuring radiation transmittance of a multiphase
fluid and characteristic values are in density.
Each of the component ratio meters comprises a cylindrical driving
electrode for applying a voltage signal of predetermined amplitude
and frequency to the multiphase fluid and a cylindrical measurement
electrode virtually grounded for detecting a current flowing in
through the multiphase fluid, both electrodes being arranged in
parallel with the pipe through which the multiphase fluid flows,
so as to measure a water phase ratio in the multiphase fluid by
measuring an electrostatic capacity between the cylindrical driving
electrode and the cylindrical measurement electrode, wherein a cylindrical
dummy electrode with a potential identical to that of the cylindrical
measurement electrode is placed between the cylindrical driving
electrode and the cylindrical measurement electrode so as to reduce
a part of an electric line of force toward the cylindrical measurement
electrode through the vicinity of the wall of the pipe.
When an inner diameter of the pipe and inner diameters of the respective
cylindrical electrodes are expressed by D, the widths of the cylindrical
driving electrode, the cylindrical measurement electrode, and the
cylindrical dummy electrode are expressed by l.sub.s, l.sub.m, and
l.sub.d, respectively, a distance between the cylindrical driving
electrode and the cylindrical measurement electrode is expressed
by L, and a distance between the cylindrical driving electrode and
the cylindrical dummy electrode is expressed by x, l.sub.s /D=0.3-1.0
l.sub.m /D=0.3-1.0 l.sub.d /D=0.1-0.5 L/D=1.0-2.0 x/D=0.4-1.2.
The constitution of the present invention enables to constitute
a flow meter for a multiphase flow with a smaller number of sensors
than conventional ones and thus enables simplification and size-reduction
of apparatuses. In addition, even when there is a velocity slip
between gas and liquid, highly accurate measurement of flow rates
of the respective phases can be achieved by only adding a sensor.
Furthermore, the accuracy of measurement of flow rate is enhanced
through improvement of a component fraction meter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a flow meter for a multiphase flow
according to Embodiment 1;
FIG. 2 is a flow pattern of a multiphase fluid in a pipe;
FIG. 3 shows a time-series waveform showing the output values of
a capacitance water cut meter;
FIG. 4 is a block diagram of a flow meter for a multiphase flow
according to Embodiment 2;
FIG. 5 is a block diagram of a capacitance water cut meter utilizing
parallel cylindrical electrodes;
FIG. 6 shows the relationship between a water phase ratio and an
electrostatic capacity value measured by the capacitance water cut
meter of FIG. 5; and
FIG. 7 is a block diagram of a conventional flow meter for a multiphase
flow.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIG. 1 is a block diagram of a flow meter for a multiphase flow
according to the present invention. A cross-correlation flow meter
26 is composed of two capacitance water cut meters 23 and 23', which
comprise electrodes 24 and 24' provided in a pipe 1 and impedance
measurement circuits 25 and 25' receiving the outputs from the electrodes
24 and 24' to measure an electrostatic capacity C of a fluid flowing
through the pipe 1. The impedance measurement circuits 25 and 25'
may be those measuring impedance at one frequency, that is, those
possessing a source of voltage signal of one frequency.
A multiphase fluid comprising a gas and a plurality of liquids
flows, in most cases, in a flow pattern with large bubbles 33 such
as a slug flow and a plug flow through the pipe 1 as shown in FIG.
2. In FIG. 2 32 denotes a liquid phase consisting of oil and water.
FIG. 3 shows a time-series waveform of electrostatic capacity C
measured by the capacitance water cut meter 23 at the time when
the multiphase fluid flows in this pattern. The time-series waveform
of electrostatic capacity C exhibits a vibrational waveform. The
electrostatic capacity C becomes smaller when a large bubble 33
is passing through the electrode 24 and it becomes the maximum 37
when the large bubble 33 has passed the electrode 24 and only a
liquid phase 32 exists. A ratio between oil and water is not changed
before, while, or after a large bubble 33 passes through.
When an oil phase: ratio, a water phase ratio, and a gas phase
ratio of a multiphase fluid 2 consisting of gas, water, and oil
are expressed by H.sub.P, H.sub.W, and H.sub.G, respectively, Equation
(5) is established:
When known relative dielectric constants of the oil, water, and
gas are expressed by .epsilon..sub.P, .epsilon..sub.W, and .epsilon..sub.G,
Equation (6):
is established for the relationship between relative dielectric
constant and a mean C.sub.mean of the electrostatic capacities C
measured by the capacitance water cut meter 23.
For the maximum C.sub.max of electrostatic capacity C, on the other
hand, since relative dielectric constants of oil and water only
should be considered, Equation 7:
is established.
Then, f.sub..epsilon. (C.sub.mean) and f.sub..epsilon. (C.sub.max)
are intrinsic functions of the capacitance water cut meter 23 and
provide an average dielectric constant of the multiphase fluid 2
from the electrostatic capacity C.sub.mean and the maximum dielectric
constant of the multiphase fluid 2 from the electrostatic capacity
C.sub.max.
An average flow velocity u of the multiphase fluid 2 is measured
from time between fluctuations of electrostatic capacity C which
can be detected as a result of constituting the cross-correlation
flow meter 26 with two capacitance water cut meters 23 and 23'.
An arithmetic circuit 22 to calculate flow rates of the respective
phases calculates an oil phase ratio H.sub.P, a water phase ratio
H.sub.W, and a gas phase ratio H.sub.G for a multiphase fluid 2
from the simultaneous equations (5)-(7) and then calculates flow
rates of the respective fluids Q.sub.P, Q.sub.W, and Q.sub.G from
Equations (4-1), (4-2), and (4-3) using a cross section A of the
pipe 1 and the average flow velocity u.
The arithmetic circuit 22 to calculate flow rates of the respective
phases can incorporate measurements from an absolute pressure meter
10 and a thermometer 11 as conventionally, to perform temperature
correction of parameters such as density and dielectric constant
of the respective fluids and (volume) flow rate of gas.
Embodiment 2
Now, a flow meter for a multiphase flow, which is effective when
there is a velocity slip (difference in flow velocity) between gas
and liquid of a multiphase fluid, is described. FIG. 4 shows a block
diagram of the flow meter in which a differential pressure the flow
meter 27 is added to the configuration of FIG. 1. Capacitance water
cut meters 23 and 23' may have a voltage signal source at a frequency,
as in Embodiment 1. The differential pressure type flow meter 27
is provided with a differential pressure gauge 28.
Since a multiphase fluid flows, in most cases, in a flow pattern
with large bubbles called a slug flow or a plug flow, regardless
of the presence of a velocity slip between gas and liquid, component
fractions of the respective fluids contained can be calculated according
to a principle similar to that in Embodiment 1. That is, when a
multiphase fluid 2 consists of gas, water, and oil, an oil phase
ratio H.sub.P, a water phase ratio H.sub.W, and a gas phase ratio
H.sub.G can be measured using only a capacitance water cut meter
23. In addition, since fluctuations of electrostatic capacity C
detected due to the fact that a cross-correlation flow meter is
constituted by two capacitance water cut meters 23 and 23' are caused
by travel of large bubbles, a travel speed of large bubbles, that
is, a gas phase velocity u.sub.G of the multiphase fluid 2 can be
measured from time between fluctuations.
On the other hand, when a velocity slip s between gas and liquid
(=gas phase velocity u.sub.G /liquid phase flow velocity u.sub.L)
is considered, a relationship between known densities of oil, water,
and gas, .rho..sub.P, .rho..sub.W, and .rho..sub.G, and an average
density of the multiphase fluid 2 .rho..sub.M, is expressed by
Equation (8):
wherein, f.sub.P, f.sub.W, and f.sub.G are weighted coefficients
of density of the respective fluids and provided by a function of
H.sub.P, H.sub.W, H.sub.G, and s.
A relationship between a differential pressure .DELTA.p.sub.v detected
with a differential pressure type flow meter 27 and a liquid phase
flow velocity u.sub.L of a multiphase fluid 2 is expressed by Equation
(9): ##EQU1##
wherein C.sub.v is an intrinsic flow rate coefficient of a differential
pressure type flow meter 27.
Then, an arithmetic circuit 29 to calculate flow rates of the respective
phases calculates the respective component fractions, H.sub.P, H.sub.W,
and H.sub.G, using Equations (5)-(7) in Embodiment 1 and a liquid
phase flow velocity u.sub.L by solving the simultaneous equations
(8) and (9).
In addition, the arithmetic circuit 29 to calculate flow rates
of the respective phases calculates flow rates of the respective
fluids of the multiphase fluid 2 Q.sub.P, Q.sub.W, and Q.sub.G,
by Equations (10-1), (10-2), and (10-3) using the component ratios
H.sub.P, H.sub.W, and H.sub.G calculated based on the measurements
obtained by the capacitance water cut meter 23 the gas phase velocity
u.sub.G calculated based on the measurements obtained by the cross-correlation
flow meter 26 and the liquid phase velocity u.sub.L calculated
based on the measurement by the differential pressure type flow
meter 27.
When a cross-correlation flow meter is constituted by two microwave
water cut meters or two gamma-ray densitometers, instead of the
capacitance water cut meters 23 and 23', component fractions of
the respective fluids constituting a multiphase fluid may be obtained
in a similar way of thinking.
Component ratios of the respective fluids constituting a multiphase
fluid may be obtained, in a similar way of thinking, by using electrical
characteristic values such as conductivity and magnetic permeability
instead of relative dielectric constant.
As shown by the above respective embodiments, component ratios
of a multiphase fluid, for example, a multiphase fluid consisting
of oil-water-gas can be measured by using only conventional sensors
for measuring component fractions of a two-phase fluid consisting
of gas and water, such as a water cut. meter and a gamma-ray densitometer
so that a structure of a component ratio meter can be downsized.
In addition, since it is sufficient to provide only one measurement
condition, measurement can be simplified. For example, when an electrostatic
capacity is used, it is sufficient to conduct measurement at one
frequency, and when microwave and gamma rays are used, it is sufficient
to perform measurement at one wavelength.
In addition, even when there is a velocity slip between gas and
liquid, an addition of one sensor enables measurement of flow rates
of respective phases with high accuracy.
Finally, a constitution of a capacitance water cut meter which
further increases accuracy of the flow meter for a multiphase flow
of the present invention is described.
FIG. 5 is a block diagram of the capacitance water cut meter. In
the capacitance water cut meter, a cylindrical dummy electrode 46
with a width of l.sub.d is additionally placed between a cylindrical
driving electrode 41 and a cylindrical measurement electrode 42
both electrodes being arranged apart from each other in the pipe
43 through which a multiphase fluid flows, with the intention of
measuring an electrostatic capacity of a multiphase flow by an electrostatic
capacity measurement circuit or an impedance measurement circuit
45 based on all output values of these electrodes 41 42 and 46
to obtain a water phase ratio based on the electrostatic capacity.
In other words, between a cylindrical driving electrode 41 to apply
a voltage signal at a certain amplitude and a certain frequency
and a cylindrical measurement electrode 42 virtually grounded (in
a condition where grounded potential is obtained actually even if
it is not actually grounded) to detect a current flowing in through
a multiphase fluid, a dummy electrode 46 with a potential identical
to that of the cylindrical measurement electrode 42 is arranged-to
measure an electrostatic capacity between the cylindrical driving
electrode 41 and the cylindrical measurement electrode 42 while
absorbing and reducing a part of an electric line of force toward
the cylindrical measurement: electrode 42 though the vicinity of
the wall of the pipe 43.
When an inner diameter of a pipe 43 and inner diameters of the
respective cylindrical electrodes are. expressed by D, widths of
a cylindrical driving electrode 41 a cylindrical measurement electrode
42 and a cylindrical dummy electrode 46 are expressed by l.sub.s,
l.sub.m, and l.sub.d, respectively, and a distance between the cylindrical
driving electrode 41 and the cylindrical measurement electrode 42
is expressed by L, and a distance between the cylindrical driving
electrode 41 and the cylindrical dummy electrode 46 is expressed
by x, by setting l.sub.s /D=0.3-1.0 l.sub.m /D=0.3-1.0 l.sub.d /D=0.1-0.5
L/D=1.0-2.0 x/D=0.4-1.2
a capacitance water cut meter is optimized. Especially preferably,
l.sub.d /D=0.1-0.2 and x/D=0.4-0.8.
Using the capacitance water cut meter shown in FIG. 5 a phantom
experiment using water and a plastic bar simulating several representative
flow patterns was conducted. The results of measurement of the distribution
of an electrostatic capacity against a water phase ratio of the
fluid are shown in FIG. 6 wherein
The results show that the dispersion of electrostatic capacity
values due to a difference in flow pattern of a multiphase fluid
was greatly improved as compared with those in conventional flow
meters at the same water phase ratio.
In other words, the water cut meter of the present invention can
reduce dispersions of measured electrostatic capacity values due
to a difference in flow pattern and can thereby provide a water
phase ratio of a multiphase fluid accurately without specifying
a flow pattern. |