Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
2001-11-16
2003-11-25
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
Reexamination Certificate
active
06651514
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for monitoring characteristics of a flow stream in a pipeline. More precisely, the present invention relates to the novel application of using a flow conditioner in conjunction with a flowmeter to measure and compare several properties of a flow stream, including volumetric flow rate, speed of sound, and density of the flow stream.
Pipelines transport a large percentage of the liquid and gaseous fossil fuel products used in the world today. It is critical to both industry operations and fiscal accountability to accurately monitor and meter these products as they are transported through pipeline systems. Therefore, pipeline monitoring and metering operations must be accurate, reliable, and cost effective over a wide range of conditions.
One of the most common flowmeters in use today is the orifice meter.
FIG. 1
depicts an orifice meter
50
comprising a fitting
52
having ends
54
,
56
for installing the meter
50
directly into the piping section
70
, typically by bolting or welding. Housed internally of the fitting
52
is a thin plate
58
that extends.across the diameter of the piping section
70
, oriented perpendicular to the direction of the flow stream
75
, as indicated by the flow arrows. The thin plate
58
includes a bore or opening (orifice
55
), that is typically concentric, but may also be eccentric.
In operation, when the flow stream
75
reaches the orifice plate
58
, the flow is forced through the orifice
55
, thereby constricting the cross-sectional flow area. Due to the principal of continuity, the mass flow rate entering the orifice
55
must equal-the mass flow rate exiting the orifice
55
. Therefore, because the cross-sectional area of flow is reduced at the orifice
55
, the flow velocity through the orifice
55
increases to maintain the mass flow rate. Further, due to the principle of the conservation of energy, because the velocity of the flow increases through the orifice
55
, the corresponding pressure must decrease.
Thus, the volumetric flow rate (Q
&Dgr;p
) through an orifice
55
having a small diameter (d) within a piping section having a larger diameter (D) is given by:
Q
Δ
p
=
CEA
o
2
Δ
p
o
ρ
k
where
(
1
)
E
=
1
1
-
β
4
=
velocity
of
approach
factor
(
2
)
A
o
=
π
d
2
4
,
and
(
3
)
β
=
d
D
,
and
where
(
4
)
C is the discharge coefficient, which is a function of &bgr; and the Reynolds number, and C≈0.6. When an orifice meter
50
is used to measure volumetric flow rate (Q
&Dgr;p
), the differential pressure (&Dgr;p
1
o
) across the orifice plate
58
is measured utilizing a differential pressure transducer
60
. The transducer
60
is connected across the orifice plate
58
via upstream pressure tap
62
and downstream pressure tap
64
to measure the differential pressure (&Dgr;p
o
). Further, a known value of density (&rgr;
k
) of the flow stream is provided, which may be determined using the pressure and temperature of the flow stream and compressibility data compiled and published by a standards-producing agency such as the American Gas Association (AGA) or the American Petroleum Institute (API). Alternatively, the density (&rgr;
k
) value may be measured online using a device, such as a densitometer (not shown). Then, from the above equations (1) through (4), using a known value of C, and calculated values for E and A
o
, the volumetric flow rate through the orifice (Q&Dgr;
p
) can be calculated.
Another type of flowmeter commonly utilized today is the ultrasonic flowmeter. Ultrasonic flowmeters determine flow stream properties by transmitting ultrasonic waves across a known path length through the flow stream, receiving the ultrasonic waves, and measuring the transit time for those waves to travel across the known path length. The transit time of the ultrasonic waves are then used to determine the velocity of the fluid. As shown in
FIG. 2
, a typical ultrasonic flowmeter has at least two opposing transducers
20
,
30
that are oriented at an angle (&agr;) to the direction of the flow stream
25
, as indicated by the flow arrow. Ultrasonic waves are transmitted from transducer
20
toward transducer
30
along flow path
22
and from transducer
30
toward transducer
20
along flow path
32
. The transit time of the ultrasonic waves in each direction is recorded by a processor (not shown). The two transit times, t
1
and t
2
, are represented by the following equations:
t
1
=
L
c
us
+
V
us
(
X
L
)
and
(
5
)
t
2
=
L
c
us
-
V
us
(
X
L
)
,
(
6
)
where c
us
is the speed of sound in the flow medium, X is the distance between the transducers parallel to the flow direction, as shown in
FIG. 2
, L is the straight line distance between the two transducers, as shown in
FIG. 2
, and V
us
is the average velocity of the flow. Equation (5) for transit time t
1
includes a positive velocity term due to the flow path
22
of the ultrasonic wave being generally in the same direction as the direction of flow
25
. In contrast, equation (6) for transit time t
2
includes a negative velocity term due to the flow path
32
of the ultrasonic wave being generally opposed to the direction of flow
25
. Solving equations (5) and (6) simultaneously for the two unknowns (V
us
and c
us
) yields:
V
us
=
L
2
2
X
(
t
2
-
t
1
t
1
×
t
2
)
and
(
7
)
c
us
=
L
2
(
t
1
+
t
2
t
1
×
t
2
)
(
8
)
Therefore, for a single pair of ultrasonic transducers
20
,
30
, the average velocity of the flow stream (V
us
) and the speed of sound in the flow stream (c
us
) can be determined by knowing the geometric configuration of the transducers relative to the piping (X, L) and measuring the transit times (t
1
, t
2
) of the ultrasonic waves. Ultrasonic meters have the advantage of providing flow stream data without obstructing the flow through the pipeline. Examples of ultrasonic flowmeters are shown and described in U.S. Pat. No. 4,646,575 and U.S. Pat. No. 5,546,812, both of which are hereby incorporated herein by reference for all purposes.
Velocity of a flow stream as it moves through a pipeline can be determined by an ultrasonic meter, as described above, or by other types of velocity meters, such as turbine, vortex, or electro-magnetic velocity meters. For any such velocity meter, once the velocity of the flow stream (V
vm
) is determined, and the cross-sectional area of the pipe (A
p
) is calculated, the volumetric flow rate (Q
vm
) can be determined from the following equation:
Q
vm
=V
vm
×A
p
(9)
The reliability of any flowmeter depends upon the quality of the flow stream being measured. To provide the most accurate and reliable measurements, the flow stream should be fully developed with a symmetric velocity profile. The flow stream should also be free of swirls and other flow anomalies. An ideal, fully developed flow stream is only achievable in closely controlled laboratory situations, but such conditions can be approximated in industrial applications using a few known methods, either alone or in combination. All of the methods used to approximate a fully developed flow involve isolating the flowmeter from any disturbances caused by pipeline features such as bends, variations in piping diameter, or other meters.
One method used to approximate a fully developed flow stream is to provide a long, straight length (run) of pipe upstream of the flowmeter. Any pipeline-created anomalies will dissipate as the flow stream travels through this long run of pipe. However, the lengths of straight pipe required to sufficiently develop the flow can be in excess of one hundred times the diameter of the pipe. Therefore, flow conditioners have been develop
Conley & Rose, P.C.
Daniel Industries Inc.
Lefkowitz Edward
Thompson Jewel V.
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