A multi-phase fluid flow meter based on passive and non-intrusive
acoustics is described for use in field applications on pipes. The
present design uses the pipe's characteristic acoustic frequency
and its amplitude variation in conjunction with a differential pressure
measurement to obtain the total mass flow rates and mass flow rates
of each phase. In addition to the mass flow rates the void/liquid
fraction, fluid velocities and densities become viable estimates.
Instrument construction is simple and very robust, allowing for
use in extreme environments. This includes down hole as well as
1. A completely passive and non-intrusive method of determining
the mass flow rate within a pipe having two phase fluid flow therein,
comprising the steps of:
providing vibration sensing means in intimate physical contact
with the pipe to be measured;
determining the characteristic vibrational frequency of said pipe;
creating known data by measuring the amplitude of the characteristic
vibrational frequency while varying the mass flow rates of fluids
through said pipe;
measuring the differential pressure in said pipe across two points
thereon on either side of the portion thereof contacted by said
vibration sensing means; and
determining the mass flow rate of an unknown fluid flow in said
pipe by comparing the characteristic vibrational frequency amplitude
from the unknown fluid flow with the known data and said differential
2. The method according to claim 1 wherein the flow rate of each
phase of said two phase flow is varied while creating said known
3. A non-invasive passive acoustic system for determining mass
flow rate of fluid flowing in two phase flow in a pipe comprising:
accelerometer vibration sensing means in intimate physical contact
with the pipe;
means for determining the characteristic vibration frequency of
means for measuring changes in amplitude of said characteristic
vibrational frequency of said pipe;
differential pressure sensing means mounted on said pipe and spanning
said vibration sensing means and connected to provide an output;
means responsive to said differential pressure sensing means output
and said amplitude of said characteristic vibrational frequency
for determining mass flow characteristics of fluid flow in said
BACKGROUND OF THE INVENTION
1. The Field Of the Invention
The present invention pertains to a method and apparatus to determine
the flow regime within a pipe using passive acoustic techniques.
2. The Prior Art
The ideal multi-phase flow meter would determine composition of
flowing fluids and the flow rates of each phase, without impeding
the flow and/or reacting with the fluids contained within a pipe.
This meter would be capable of use in extreme temperatures, pressures
and hostile chemical environments while providing accurate results.
The construction of this meter would be such that it would be simple
and suitable for field applications as well as usage in the laboratory.
Historically, there are devices that perform some of these measurements
and a few can operate in environments approaching extreme hostility.
For example, nuclear densitometer techniques (see U.S. Pat. No.
4683759) are a reliable and robust means of obtaining an average
fluid density in pipes containing flowing fluids. Since the instrument
is externally mounted on the pipe, it would not interfere with or
react with the flowing fluids. Instrument construction typically
allows for usage in hostile conditions. The shortcomings of this
approach to fluid characterizations are: the statistical nature
of the measurements; the necessity of long lived and often high
energy radioactive sources; and potential interpretational difficulties
when the pipe under investigation contains gases, liquids and fluctuations
in chemical composition.
An additional example of the prior art and applications to fluid
characterization involves the use of ultrasonic techniques. These
types of measurement systems can be intrusive or non-intrusive,
depending on the application. Composition of two-phase fluids can
be investigated using an intrusive transit time method, as described
in U.S. Pat. No. 5115670. This method contemplates the measurement
of the transit time of a sound wave between an ultrasonic source
and a detector located diagonally across a pipe. In principle this
transit time can be used to calculate the speed of sound in two-phase
flow. This allows the calculation of the mixture's linear velocity
and composition. These quantities allow the calculation of mass
flow rates or the energy flow rates. The calculated results and
their accuracy, for example steam quality, may depend on separate
fluid property correlations. The fluid's chemical composition may
effect the sensor's longevity.
Fluid velocities can be obtained using Doppler flow meters (see
U.S. Pat. No. 5115670). These ultrasonic devices can be non-intrusive
(externally mounted in the pipe) and protected from the environment.
The idea behind these devices is that an ultrasonic signal is continuously
transmitted into a pipe containing fluids where scattering occurs
from suspended solids, air bubbles, discontinuities or disturbances
in the flowing stream. The scattered signal is detected and its
frequency is compared to the transmitted frequency. The difference
in these frequencies is proportional to the fluid's velocity. These
measurements are considered most accurate when evaluating fluids
with Newtonian flow profiles and containing suspended particles
or air bubbles.
Generally, the designs of existing flow measurement systems using
nuclear, acoustic or electromagnetic methods only address a few
of the idealized capabilities and concentrate on measuring a restricted
set of parameters while actively probing the medium of interest.
These measurement systems can be intrusive or non-intrusive and
some may require a side stream sample to obtain the required data.
Examples of some of the active acoustic flow measurement systems
can be found in U.S. Pat. Nos. 4080837; 4236406; and 4391149.
Passive types of measurement techniques in pipes, specifically
simple detection of acoustic emissions or "listening,"
are available, but are limited in scope and applications. For instance,
acoustic emissions can be used to detect: slug flow and the presence
of sand in multi-phase pipelines (see U.S. Pat. No. 5148405);
leaks in natural gas pipelines (see U.S. Pat. No. 5117676); and
steam quality when the acoustic emissions are obtained from a calibrated
steam jet produced by an orifice (see U.S. Pat. No. 4193290).
The use of acoustic emissions as a passive and non-intrusive method
in quantitative characterization of multi-phase flow in pipes appears
to be novel.
SUMMARY OF THE INVENTION
The present invention estimates the fluid physical properties of
fluid flowing within a pipe by measuring existing noise to obtain
several physical parameters that traditionally require separate
measurements and instrumentation. The present invention is completely
passive, non-intrusive and does not use radioactive materials. It
can be applied to liquid-liquid systems as well as gas-liquid systems.
BRIEF DESCRIPTION OF THE DRAWING
The present invention will now be described, by way of example,
with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic representation of the present invention;
FIG. 2 is a frequency characteristic for one-inch pipe;
FIG. 3 is a frequency characteristic dependent upon pipe diameter;
FIG. 4 is a correlation of mass rate, differential pressure and
characteristic frequency amplitude.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
It is commonly known that multi-phase fluids flowing through a
pipe generate noise within the pipe. In principle, this naturally
occurring phenomenon should be able to provide information about
the fluids flowing in the pipe.
FIG. 1 diagrammatically illustrates an apparatus that has been
used to quantitatively characterize a variable mixture of gas and
liquid flowing under pressure through a pipe 10. A differential
pressure measurement means 12 makes a differential pressure measurement
across the point where the acoustic data is to be obtained. An accelerometer
14 or other vibration sensitive sensor, is attached to the pipe
10 at the point where the measurement is desired. The electrical
signals from this sensor 14 if necessary, are fed into a pre-amplifier
16 for amplification and transmission to signal processing instrumentation
18. This instrumentation 18 preferably would include: analyzer means
20 capable of performing a Fast Fourier Transform (FFT) on the incoming
time domain signals; frequency and amplitude tracking means 22;
display means 24 for visual monitoring of the spectra; data storage
means 26 and data entry and processing means 28.
Several measurements were performed to identify the relevant parameters
required to quantitatively describe the flowing gas-liquid mixtures.
These experiments were conducted at low pressures and ambient temperatures
using 0.75 inch, and 1 inch in diameter 8 feet in length steel pipes
(schedule 40 pipes were used in all cases). Air and water were used
as gas-liquid mixtures. The flow rates in these measurements ranged
from 0.25 GPM to 4 GPM for water and 3 SCFM to 60 SCFM for air.
These values correspond to total mass rates ranging from 2.3 lbm/min
to 36 lbm/min.
The measurements have shown that the identification of the pipe's
characteristic vibrational frequency and its amplitude variation
are important to quantifying the fluid flow. As an example the acoustic
spectrum observed for a 1 inch in diameter pipe that contained a
flowing mixture of 4 GPM water and 30 SCFM air, is shown in FIG.
2. While the entire spectrum can be used to identify the flow regime,
the same spectra also defines the pipe's characteristic vibrational
frequency. That is, the most distinguishable single frequency common
to all flow regimes. For the 1 inch diameter pipe, the peak at 8.4
kHz identifies the pipe's characteristic frequency. This high frequency
infers that the pipe's vibrations are primarily radial vibrational
modes rather than transverse or longitudinal vibrational modes.
These measurements also suggested that the characteristic frequency
should be strongly dependant on the pipe's internal diameter. This
dependance has been measured in pipes ranging from 0.5 inches in
diameter to 2 inches in diameter and is shown in FIG. 3.
Knowing the characteristic frequency of a pipe, either through
a spectral measurement or extrapolation from a relationship such
as shown in FIG. 3 it is then possible to track the signal amplitude
at this frequency while changing the flow rates of the fluids contained
in the pipe. This evaluation was performed by establishing a constant
liquid flow rate then incrementally increasing the gas flow rate.
At the highest obtainable gas rate, the gas rate was reduced to
its initial value, then the liquid flow rate was incrementally increased
to a new constant value. The gas rate was again incrementally increased
to its highest value.
A small fraction of the data obtained from the 0.75 inch in diameter
pipe is illustrated in FIG. 4. Here the volumetric flow rates have
been converted to total mass flow rates and displayed as a function
of the measured characteristic frequency amplitudes. The data produce
a family of curves. The curves shown correspond to initial liquid
mass flow rates, starting with the uppermost curve of 33.7 lbm/min,
16.9 lbm/min, 8.7 lbm/min, and 4.4 lbm/min, respectively. The solid
curves represent linear curve fits to the data. For a measured characteristic
frequency amplitude it is possible to assign several values for
the total mass rate. This difficulty is eliminated with the knowledge
of the differential pressure or pressure loss measured across the
data point by sensor 12. The dashed curve in FIG. 4 represents a
differential pressure isobar of approximately 2.2 psi, measured
across the 8 feet in length pipe used in the experiments.
Knowledge of the characteristic frequency amplitude and differential
pressure uniquely defines the total mass rate and associated curve
for further calculations. Extrapolation of this curve to zero signal
amplitude defines the liquid mass rate. The difference between the
total mass rate and liquid mass rate determines the gas mass rate.
The ratio of the gas mass rate to the total mass rate defines the
void fraction or alternatively the liquid fraction. Superficial
gas and liquid velocities may be calculated from the knowledge of
the internal diameter and gas/liquid mass flow rates. Knowing the
mass rates of each of the fluids and the void fraction, allows estimates
of average fluid velocities and fluid densities. Finally, the results
are independent of the flow regime within the pipe during the observation.
The present invention may be subject to many changes and modifications
which would occur to one skilled in the art. The present specification
is therefor intended in all respects to be illustrative and not
restrictive of the scope of the present invention as defined by
the appended claims.