Data processing: measuring – calibrating – or testing – Measurement system – Time duration or rate
Reexamination Certificate
2002-01-03
2004-11-09
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Time duration or rate
Reexamination Certificate
active
06816808
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
A disclosed embodiment of the invention relates generally to the detection of errors in ultrasonic transit time measurements. Even more particularly, a disclosed embodiment of the invention relates to the identification of mistakes in peak selection for the ultrasonic waveforms.
2. Description of the Related Art
After a hydrocarbon such as natural gas has been removed from the ground, the gas stream is commonly transported from place to place via pipelines. As is appreciated by those of skill in the art, it is desirable to know with accuracy the amount of gas in the gas stream. Particular accuracy for gas flow measurements is demanded when gas (and any accompanying liquid) is changing hands, or “custody.” Even where custody transfer is not taking place, however, measurement accuracy is desirable.
Gas flow meters have been developed to determine how much gas is flowing through the pipeline. An orifice meter is one established meter to measure the amount of gas flow. Certain drawbacks with this meter existed, however. More recently, another type of meter to measure gas flow was developed. This more recently developed meter is called an ultrasonic flow meter.
FIG. 1A
shows an ultrasonic meter suitable for measuring gas flow. Spoolpiece
100
, suitable for placement between sections of gas pipeline, has a predetermined size and thus defines a measurement section. Alternately, a meter may be designed to attach to a pipeline section by, for example, hot tapping. As used herein, the term “pipeline” when used in reference to an ultrasonic meter may be referring also to the spoolpiece or other appropriate housing across which ultrasonic signals are being sent. A pair of transducers
120
and
130
, and their respective housings
125
and
135
, are located along the length of spoolpiece
100
. A path
110
, sometimes referred to as a “chord” exists between transducers
120
and
130
at an angle &thgr; to a centerline
105
. The position of transducers
120
and
130
may be defined by this angle, or may be defined by a first length L measured between transducers
120
and
130
, a second length X corresponding to the axial distance between points
140
and
145
, and a third length D corresponding to the pipe diameter. Distances D, X and L are precisely determined during meter fabrication. Points
140
and
145
define the locations where acoustic signals generated by transducers
120
and
130
enter and leave gas flowing through the spoolpiece
100
(i.e. the entrance to the spoolpiece bore). In most instances, meter transducers such as
120
and
130
are placed a specific distance from points
140
and
145
, respectively, regardless of meter size (i.e. spoolpiece size). A fluid, typically natural gas, flows in a direction
150
with a velocity profile
152
. Velocity vectors
153
-
158
indicate that the gas velocity through spool piece
100
increases as centerline
105
of spoolpiece
100
is approached.
Transducers
120
and
130
are ultrasonic transceivers, meaning that they both generate and receive ultrasonic signals. “Ultrasonic” in this context refers to frequencies above about 20 kilohertz. Typically, these signals are generated and received by a piezoelectric element in each transducer. To generate an ultrasonic signal, the piezoelectric element is stimulated electrically, and it responds by vibrating. This vibration of the piezoelectric element generates an ultrasonic signal that travels across the spoolpiece to the corresponding transducer of the transducer pair. Similarly, upon being struck by an ultrasonic signal, the receiving piezoelectric element vibrates and generates an electrical signal that is detected, digitized, and analyzed by electronics associated with the meter.
Initially, D (“downstream”) transducer
120
generates an ultrasonic signal that is then received at, and detected by, U (“upstream”) transducer
130
. Some time later, U transducer
130
generates a return ultrasonic signal that is subsequently received at and detected by D transducer
120
. Thus, U and D transducers
130
and
120
play “pitch and catch” with ultrasonic signals
115
along chordal path
110
. During operation, this sequence may occur thousands of times per minute.
The transit time of the ultrasonic wave
115
between transducers U
130
and D
120
depends in part upon whether the ultrasonic signal
115
is traveling upstream or downstream with respect to the flowing gas. The transit time for an ultrasonic signal traveling downstream (i.e. in the same direction as the flow) is less than its transit time when traveling upstream (i.e. against the flow). In particular, the transit time t
1
, of an ultrasonic signal traveling against the fluid flow and the transit time t
2
of an ultrasonic signal travelling with the fluid flow may be defined:
t
1
=
L
c
-
V
x
L
(
1
)
t
2
=
L
c
+
V
x
L
(
2
)
where,
c=speed of sound in the fluid flow;
V=average axial velocity of the fluid flow over the chordal path in the axial direction;
L=acoustic path length;
x=axial component of L within the meter bore;
t
1
=transmit time of the ultrasonic signal against the fluid flow; and
t
2
=transit time of the ultrasonic signal with the fluid flow.
The upstream and downstream transit times can be used to calculate the average velocity along the signal path by the equation:
V
=
L
2
2
x
t
1
-
t
2
t
1
t
2
(
3
)
with the variables being defined as above.
The upstream and downstream travel times may also be used to calculate the speed of sound in the fluid flow according to the equation:
c
=
L
2
t
1
+
t
2
t
1
t
2
(
4
)
To a close approximation, equation (4) may be restated as:
V
=
c
2
Δ
t
2
x
(
5
)
where,
&Dgr;t=t
1
−t
2
(6)
So to a close approximation at low velocities, the velocity v is directly proportional to &Dgr;t.
Given the cross-section measurements of the meter carrying the gas, the average velocity over the area of the meter bore may be used to find the volume of gas flowing through the meter or pipeline
100
.
In addition, ultrasonic gas flow meters can have one or more paths. Single-path meters typically include a pair of transducers that projects ultrasonic waves over a single path across the axis (i.e. center) of spoolpiece
100
. In addition to the advantages provided by single-path ultrasonic meters, ultrasonic meters having more than one path have other advantages. These advantages make multi-path ultrasonic meters desirable for custody transfer applications where accuracy and reliability are crucial.
Referring now to
FIG. 1B
, a multi-path ultrasonic meter is shown. Spool piece
100
includes four chordal paths A, B, C, and D at varying levels through the gas flow. Each chordal path A-D corresponds to two transceivers behaving alternately as a transmitter and receiver. Also shown is an electronics module
160
, which acquires and processes the data from the four chordal paths A-D. This arrangement is described in U.S. Pat. No. 4,646,575, the teachings of which are hereby incorporated by reference. Hidden from view in
FIG. 1B
are the four pairs of transducers that correspond to chordal paths A-D.
The precise arrangement of the four pairs of transducers may be more easily understood by reference to FIG.
1
C. Four pairs of transducer ports are mounted on spool piece
100
. Each of these pairs of transducer ports corresponds to a single chordal path of
FIG. 1B. A
first pair of transducer ports
125
and
135
includes transducers
120
and
130
recessed slightly from the spool piece
100
. The transducers are mounted at a non-perpendicular angle &thgr; to centerline
105
of spool piece
100
. Another pair of transducer ports
165
and
175
including associated transducers is mounted so that its chordal path loosely forms an “X” with respect to the chor
Freund, Jr. William R.
Murray Gail Paulin
Zanker Klaus Joachim
Conley & Rose, P.C.
Daniel Industries Inc.
Lau Tung S
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