Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
2002-09-30
2004-05-11
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
Reexamination Certificate
active
06732595
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a high-accuracy quadrature multipath mass flow meter system and method for measuring mass flow rate.
BACKGROUND OF THE INVENTION
Quadrature multipath mass flow meter systems are used to fairly accurately determine the total mass flow rate of a fluid flowing in a conduit using quadrature integration of the product of the average fluid density &rgr;
avg
and the measured fluid velocity in each quadrature plane. See U.S. Pat. Nos. 3,564,912; 5,515,733; 6,047,602; and 4,300,401 incorporated herein by this reference.
In the prior art, the fluid velocities V
1
, V
2
, and V
3
(assuming 3 parallel quadrature planes) are calculated using ultrasonic transducers which detect the transit times of ultrasonic pulses transmitted bidirectionally through the fluid in each of the three quadrature planes. The fluid velocities in each quadrature plane are then computed as a function of the transit times of the ultrasonic energy both with and against the direction of the flow. The transit times themselves are principally a function of the paths and the speed of sound in the fluid which, in turn, is a function of the density of the fluid. Factors which influence sound speed, other than density, are dealt with below. Once the fluid velocities are calculated, quadrature integration is used to calculate the volumetric flow rate Q. if density were uniform, one could calculate the fluid's mass flow rate based on the products &rgr;
avg
V
1
, &rgr;
avg
V
2
, and &rgr;
avg
V
3
. In this case, the average density of the fluid &rgr;
avg
is measured, calculated, or assumed based on the fluid composition and conditions if it is known.
In many situations, however, the assumption that the density of the fluid in the conduit is uniform is erroneous. Temperature variations, elbows in the conduit (which act like centrifuges), and other factors can result in non-uniform density distributions in the conduit.
To illustrate the error associated with assuming that the density of the fluid in the conduit is uniform when in fact the density is different in each quadrature plane, suppose (using consistent units) that V
1
=3, V
2
=7, and V
3
=9 and the density is assumed to be uniform and of numerical value &rgr;
avg
=5.3. The average velocity is 6.33 and so the product of the averages is 33.5. But, if in reality the density is non-uniform such that the density of the fluid in each quadrature plane is different, for example, &rgr;
1
=2, &rgr;
2
=6, &rgr;
3
=8, then the average density is still 5.3, but the average of the products is now (&rgr;
1
V
1
+&rgr;
2
V
2
+&rgr;
3
V
3
)/3=40, not 33.5. Thus, using an average density and assuming the density is uniform when it is not generally results in an erroneous total mass flow rate calculation. This error occurs despite using quadrature integration or other methods which are generally perceived to yield accurate volumetric flow rate Q, and generally further assumed to yield an accurate mass flowrate when Q is multiplied by an average value for density, with density typically computed based on point measurement of temperature in the fluid and pressure measured near the wall of the conduit.
Such an error can be extremely important in many industries including the use of mass flow rate systems to determine the price to be paid for expensive commodities such as oil or even water.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a more accurate quadrature multipath mass flow meter system and method.
It is a further object of this invention to provide such a system and method which is accurate to within 0.5-1% in general industrial process control situations, and 0.25 to 0.5% in custody transfer applications.
It is a further object of this invention to provide such a system and method which reduces the error associated with the prior art wherein the density of the fluid (liquid or gas, or some multiphase mixtures) was assumed to be uniform.
It is a further object of this invention to provide such a system and method which can be used in cases where the density of the fluid varies within a conduit.
The invention results from the realization that a more accurate quadrature multipath mass flow meter system and method especially useful in connection with fluid flows having non-uniform density distributions is effected by not by taking the product of the average fluid density and the volumetric flowrate Q determined by quadrature integration of the fluid velocities in each quadrature plane, but, instead, by quadrature integration of the product of the fluid densities and the fluid velocities in each quadrature plane, to calculate the total mass flow rate of the fluid. The new method is more accurate because it a) eliminates errors associated with assuming that the density of the fluid in the conduit is uniform, or b) eliminates the errors associate with multiplying an accurate Q by an average density of an acknowledged or known density gradient and taking that “product of the averages” to be the mass flowrate.
This invention features a method of analyzing and determining the mass flow rate of a fluid flowing in a conduit, the method comprising transmitting ultrasonic energy along multiple “v paths” in multiple parallel quadrature planes through the fluid, measuring the transit time of the ultrasonic energy through the fluid with and against the flow direction of the fluid, calculating the flow velocity of the fluid in each quadrature plane based on the transit time in each quadrature plane, determining the density of the fluid in each quadrature plane, and performing quadrature integration of the product of the fluid density and fluid velocity in each quadrature plane to calculate the total mass flow rate of the fluid more accurately by eliminating errors associated with assuming that the density of the fluid in the conduit is uniform.
In one embodiment, the density is determined in each quadrature plane by calculating the speed of sound in each quadrature plane from the transit time of the ultrasonic energy through the fluid in each quadrature plane and referencing a library including speed of sound and density data for different fluids.
In other embodiments, the density of the fluid is determined by measuring the density of the fluid. In one example, the density of the fluid in each quadrature plane is measured by transmitting ultrasonic energy as a torsional wave in a waveguide sensor located in each of the quadrature planes, measuring the transit time of the ultrasonic energy in the fluid, and calculating the density of the fluid in each quadrature plane based on the transit time in each quadrature plane. In another example, the density of the fluid in a first quadrature plane is measured by transmitting ultrasonic energy in the first quadrature plane, measuring the transit time of the ultrasonic energy in the fluid in the quadrature plane, calculating the density of the fluid in the first quadrature plane based on the transit time of the ultrasonic energy in the first quadrature plane, and deriving, using Rao's rule, for example, the density of the fluid in the other quadrature planes from the measured density of the fluid in the first quadrature plane.
Typically, for the fluid velocity measurements, the ultrasonic energy is transmitted across each quadrature plane and then reflected back across each quadrature plane, and the parallel quadrature planes extend horizontally.
Further included may be the steps of measuring the temperature and pressure of the fluid and calculating the mass flow rate using the temperature and pressure measurements.
In still another example, the speed of sound is calculated based on the transit time of the ultrasonic energy through the fluid and the density of the fluid in each quadrature plane is determined using the calculated speed of sound in each quadrature plane.
A higher accuracy quadrature mass flow meter in accordance with this invention features a first set of ultrasonic transducers aligned
Iandiorio & Teska
Lefkowitz Edward
Panametrics, Inc.
Thompson Jewel
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