Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
2001-01-03
2002-10-08
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
Reexamination Certificate
active
06460419
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is related to an ultrasonic flow measuring technology, and particularly, to an ultrasonic flow measuring method for measuring flow velocities on a plurality of fluid flowing sections and then computing a flow or flowrate, if ultrasonic transducers are mounted on a pipe that had been already arranged on a place.
2. Description of the Background
A general ultrasonic flow measuring method is based on the fundamental technical background as follows: an ultrasonic one channel flowmeter is designed to measure a flow velocity V
D
on a part of a fluid flow section, for example the inner diameter of a pipe, using an ultrasonic wave and multiply the flow velocity V
D
by a flow coefficient K along with a fluid section area S to calculate a flow. An ultrasonic multi-channel flow measuring method includes steps of measuring a flow velocity V
D
and flow velocities on chords divided into a plurality of sections, using an ultrasonic wave, to calculating an average flow velocity V
S
of a fluid flow section and multiplying V
S
by a section area to calculate a flow. Another method is known to measure an average horizontal flow velocity at a plurality of water depths in an open sluice in order to compute a flow.
Typical ultrasonic flow measuring methods and apparatuses there for are disclosed as follows:
U.S. Pat. No. 5,531,124 granted on Jul. 2, 1996
U.S. Pat. No. 4,646,575 granted on Jul. 25, 1987
U.S. Pat. No. 4,860,593 granted on Aug. 29, 1989
U.S. Pat. No. 5,780,747 granted on Jul. 14, 1998
U.S. Pat. No. 4,676,321 granted on Jul. 25, 1996
Russian Pat. No. 2,138,782 granted on Sep. 27, 1999
The ultrasonic flow measuring methods already known have common properties as follows:
1) A flow measuring section is selected to be a section S in a right angle to a direction of a fluid flow. In case of a pipe, a section rectangular to a centerline is selected.
2) Therefore, a flow velocity in a right angle direction to a section to be firstly measured by an ultrasonic wave is calculated. At that time, it is assumed that the direction of the flow velocity is corresponded to a fluid flow direction.
3) An ultrasonic flow velocity measuring method includes a frequency difference method and a phase difference method, but these methods are based on transit time difference method, which has been broadly used.
A typical transit time difference flow velocity measuring expression is as follows:
V
=
L
2
2
d
t
2
-
t
1
t
1
t
2
=
L
2
2
d
Δ
t
t
1
t
2
(
1
)
Wherein, L is an interval distance between paired transducers
1
and
2
, d is a projection distance of L in which d=Lcos&PHgr;, t
1
is a transit time in a flow velocity direction from the paired transducer
1
to the paired transducer
2
and t
2
is a transit time in a direction contrary to a flow velocity from the paired transducer
2
to the paired transducer
1
(referring to FIG.
1
).
A flow computing expression of an ultrasonic one-channel flow computing method is as follows:
Q=K·V
D
·S
(2)
Wherein, K is a flow coefficient, V
D
is a flow velocity on a diametric line to be measured by the expression (1) and S is a section area of fluid as defined above, for example an inner section area of a pipe.
One of flow calculation expressions for an ultrasonic multi-channel flow measuring method is as follows:
Q=V
S
·S
(3)
Wherein, V
S
is a total average flow velocity on a plurality of chords to be measured by the expression (1).
An ultrasonic flowmeter has most characteristics as follows: unlike another flowmeter, mounting transducers on a pipe that had been already arranged in a place can perform a flow measurement. Even under the condition that fluid is transported through the pipe, the transducers can be mounted on the pipe through the drilling work thanks to the technology progress. For the characteristics, the ultrasonic flowmeter is very often used.
Particularly, the ultrasonic multi-channel flow measuring method can measure a flow, exactly, even if a condition that K=constant, for example a distance of a straight portion of a pipe becomes at least 25D and Re>10
4
, is not secured and a flow velocity distribution is not a normal state, or if the inner diameter of the pipe is relatively larger. Therefore, the characteristics enable the ultrasonic flowmeter to be used as a flowmeter for a larger pipe.
FIG. 2
shows five chords for measuring a flow velocity, but the number of chord can be increased as requested. As shown in
FIG. 2
, in order that d=L
i
·cos&PHgr;
i
=const, mounting angles &PHgr;
i
of paired transducers
1
i
and
2
i
are not equal to each another.
As represented in the expressions (2) and (3), a flow measuring error &dgr;
Q
is considered as a sum of a flow velocity measuring error &dgr;
V
and a section area measuring error &dgr;
S
. The flow measuring error &dgr;
Q
in the ultrasonic one-channel flow measuring method is as follows:
&dgr;
Q
=&dgr;
K
+&dgr;
VD
+&dgr;
S
(4)
The flow measuring error &dgr;
Q
in the ultrasonic multi-channel flow measuring method is as follows:
&dgr;
Q
=&dgr;
Vi
+&dgr;
M
+&dgr;
S
(5)
Wherein, &dgr;
K
is a flow coefficient error, &dgr;
M
is an error followed by calculating an average flow velocity of a section using a flow velocity V
i
measured on a plurality of chords, for example an approximate integral error of an expression that
V
S
=
1
2
R
∫
-
R
+
R
V
(
r
)
ⅆ
r
In the expressions (4) and (5), the flow measuring error &dgr;
Q
is determined by the flow velocity measuring error &dgr;
V
and the section area measuring error &dgr;
S
. Therefore, in order to enhance the accuracy of the flow measuring, the flow velocity measuring error &dgr;
V
and the section area measuring error &dgr;
S
are significantly reduced. In the flow velocity measuring expression (1), assuming that the transit time measuring errors includes an accidental error component, a flow velocity measuring error is as follows:
&dgr;=(2&dgr;
L
+&dgr;
d
)+{square root over (&dgr;
2
t1
+&dgr;
2
t2
+&dgr;
2
&Dgr;t
)}=(2&dgr;
L
+&dgr;
d
)+
A
(6)
A={square root over (&dgr;
2
t1
+&dgr;
2
t2
+&dgr;
2
&Dgr;t
)}
Wherein, &dgr;
L
is a measuring error of an interval distance L, and &dgr;
d
is a measuring error of d, in which L and d are a constant to be inputted into an arithmetic logic processor or microprocessor after being measured. Therefore, the symbols of the &dgr;
L
and &dgr;
d
are not changed. In other words, these errors are a fixing error. &dgr;
t1
, &dgr;
t2
and &dgr;
&Dgr;t
are errors of each of transit times t
1
and t
2
, and the error &Dgr;t=t
2
−t
2
.
As represented in the expression (6), even through t
1
and t
2
are precisely measured under the condition that A is reduced enough to be ignored, if &dgr;
L
and &dgr;
d
are relatively larger, the flow velocity measuring error &dgr;
V
becomes larger. Herein, what the measuring error &dgr;
L
of L is multiplied by 2 is because of L
2
. In case of the pipe, the section area S is calculated by measuring ab inner diameter D as follows:
S
=
π
D
2
4
The calculation error of the section area is as follows:
&dgr;
S
=2&dgr;
D
(7)
Wherein, &dgr;
D
is a measuring error of an inner diameter D.
Therefore, the measuring errors of geometrical integers or constants L, d, D appear as a flow measuring error as follows:
&dgr;
Q
=(2&dgr;
L
+&dgr;
d
+2&dgr;
D
)+
A
(8)
These errors are a fixing error represented as an arithmetical sum with their symbols being known.
In case of a flowmeter of a flange type, the inner diameter D is measured several times to obtain its average value {overscore (D)}, so &dgr;
S
=2&dgr;
D
is secured to become smaller. But, measuring the interval distance L
i
between the transducers is not simple. T
Fuller Benjamin R.
International Hydrosonic Co., Ltd.
Thompson Jewel
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