Ultrasonic 2-phase flow apparatus and stratified level detector

Details Ultrasonic 2-phase flow apparatus and stratified level detector Ultrasonic 2-phase flow apparatus and stratified level detector

1999-09-01

2002-05-14

Larkin, Daniel S. (Department: 2956)

Measuring and testing

Liquid analysis or analysis of the suspension of solids in a...

Content or effect of a constituent of a liquid mixture

C073S861040

Reexamination Certificate

active

06386018

ABSTRACT:

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 two-phase flow. More particularly, a disclosed embodiment of the invention relates to the detection of liquid in a gas stream by an ultrasonic meter. Even more particularly, a disclosed embodiment of the invention relates to the measurement by an ultrasonic meter of the amount and form of flow for a liquid travelling through a pipeline and the properties of the accompanying gas stream.
2. Description of the Related Art
After a hydrocarbon)such as natural gas has been removed from the ground, it is commonly transported from place to place via pipelines. Often this gas stream also contains a certain amount, or percent fraction, of liquid. 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. It is also extremely desirable to know how much liquid is being transported along with the gas stream. For example, if the gas contains “natural gas liquids” or condensates, a seller of gas wants extra compensation for this energy-rich liquid. Thus, particular accuracy for gas flow and liquid fraction measurements is demanded when gas (and any accompanying liquid) is changing hands, or “custody.”
Gas flow meters have been developed to determine how much gas is flowing through the pipeline. One type of meter to measure gas flow is called an ultrasonic flow meter. Ultrasonic flow meters, also named sonic or acoustic flow meters, are revolutionizing the gas industry because of their many advantages.
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. 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 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 sppooliece
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 twenty kilohertz. Typically, these signals are generated and received by a piezoelectric element in each transducer. Initially, D transducer
120
generates an ultrasonic signal that is then received at, and detected by, U transducer
130
. Some time later, U transducer
130
generates a reciprocal ultrasonic signal that is subsequently received at and detected by D transducer
120
. Thus, U and D transducers
120
and
130
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). The upstream and downstream transit times can be used to calculate the average velocity along the signal path. Given the cross-section measurements of the meter carrying the gas, the average velocity over the area of the gas may be used to find the quantity of gas flowing through spoolpiece
100
. Alternately, a meter may be designed to attach to a pipeline section by, for example, hot tapping, so that the pipeline dimensions instead of spoolpiece dimensions are used to determine the average velocity of the flowing gas.
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
including transducers
120
and
130
is 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 chordal path of transducer ports
125
and
135
. Similarly, transducer ports
185
and
195
are placed parallel to transducer ports
165
and
175
but at a different “level” (i.e. a different radial position in the pipe or meter spoolpiece). Not explicitly shown in
FIG. 1C
is a fourth pair of transducers and transducer ports. Taking
FIGS. 1B and 1C
together, the pairs of transducers are arranged such that the upper two pairs of transducers corresponding to chords A and B form an X and the lower two pairs of transducers corresponding to chords C and D also form an X.
Referring now to
FIG. 1B
, the flow velocity of the gas may be determined at each chord A-D to obtain chordal flow velocities. To obtain an average flow velocity over the entire pipe, the chordal flow velocities are multiplied by a set of predetermined constants. Such constants are well known and were determined theoretically.
This four-path configuration has been found to be highly accurate and cost effective. Nonetheless, other ultrasonic meter designs are known. For example, other ultrasonic meters employ reflective chordal paths, also known as “bounce” paths. Referring now to
FIG. 18
, a spoolpiece or pipeline
1800
includes ultrasonic paths
1810
and
1820
representing two travel paths of generated ultrasonic signals. A first ultrasonic signal originates at a first transducer along the circumference of the spoolpiece and follows path
1810
. After generation, this first ultrasonic signal travels through the center
1830
of the spoolpiece before refle

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