Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-08-29
2004-03-09
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C073S861355
Reexamination Certificate
active
06704666
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to Coriolis flowmeters, and in particular, to methods and systems for measuring properties of a flow tube and of a material flowing through the flow tube.
STATEMENT OF THE PROBLEM
Coriolis flowmeters measure mass flow and other information for fluids flowing through a flow tube in the flowmeter. Coriolis flowmeters are comprised of a Coriolis sensor and associated meter electronics. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more flow tubes of a straight or a curved configuration. Each flow tube configuration in a Coriolis flowmeter has a set of natural modes of vibration, which may be of a simple bending, twisting, torsional, or coupled type. Each flow tube is driven to oscillate at a resonance in one of these natural modes of vibration. Fluid flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the flow tube or flow tubes, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, fluid-filled system are defined in part by the combined mass of the flow tubes and the fluid flowing through the flow tubes.
When there is no flow through the flowmeter, all points along the flow tube oscillate, due to an applied driver force, with substantially identical phase or small initial fixed phase offset which can be corrected. As fluid begins to flow, Coriolis forces cause points along the flow tube to have a different phase. The phase on the inlet side of the flow tube commonly lags the driver, while the phase on the outlet side of the flow tube leads the driver. Pickoffs are affixed to the flow tube to measure the motion of the flow tube and to produce sinusoidal pickoff signals representative of the motion of the flow tube. The meter electronics processes pickoff signals to determine the phase difference between the pickoff signals. The phase difference between two pickoff signals is proportional to the mass flow rate of fluid through the flow tube.
An important component of Coriolis flowmeters, and of vibrating tube densitometers, is the drive or excitation system. The drive system operates to apply a periodic physical force to the flow tube which causes the flow tube to oscillate. The drive system includes a driver mechanism mounted to the flow tube of the flowmeter and a drive circuit for generating a drive signal to operate the driver mechanism. The driver mechanism typically contains one of many well known arrangements, such as a magnet mounted to one flow tube and a wire coil mounted to the another flow tube or brace bar in an opposing relationship to the magnet.
A drive circuit continuously applies a periodic drive voltage to the driver mechanism. The drive voltage is typically sinusoidally or square shaped. In a typical magnetic-coil drive mechanism, the periodic drive voltage causes the coil to produce a continuous alternating magnetic field. The alternating magnetic field of the coil and the constant magnetic field produced by the magnet force the flow tube to vibrate in a sinusoidal pattern. Those skilled in the art will recognize that any device capable of converting an electrical signal to mechanical force is suitable for application as a driver. (See, U.S. Pat. No. 4,777,833 issued to Carpenter and assigned on its face to Micro Motion, Inc.) Also, one need not use a sinusoidal signal but rather any periodic signal may be appropriate as the driver signal (See, U.S. Pat. No. 5,009,109 issued to Kalotay et. al. and assigned on its face to Micro Motion, Inc.).
For a dual tube flowmeter, a typical mode in which Coriolis flowmeters are typically driven to vibrate is a first out-of-phase bending mode. The first out-of-phase bending mode is the fundamental bending mode at which the two flow tubes of a dual tube Coriolis flowmeter vibrate in opposition to one another. However, this is not the only mode of vibration present in the vibrating structure of a Coriolis flowmeter driven in the first out-of-phase bending mode. Higher modes of vibration may also be excited in the flow tubes. For example, a first out-of-phase twist mode may be excited as a result of a fluid flowing through the vibrating flow tube and the Coriolis forces caused by the flowing fluid. Other higher modes of vibration that may be excited include in-phase and lateral modes of vibration. There may be hundreds of vibration modes actually excited in a Coriolis flowmeter that is driven to oscillate in the first out-of-phase bending mode. Even within a relatively narrow range of frequencies near the first out-of-phase bending mode, there are at least several additional modes of vibration that are excited by the vibration of the flow tube by the drive system. In addition to multiple modes being excited by the driver, additional undesired modes of vibration can also be excited due to vibrations external to the flowmeter. For example, a pump located elsewhere in a process line might generate a vibration along a pipeline that excites a mode of vibration in a Coriolis flowmeter.
As discussed above, the driver vibrates the flow tube at a resonant frequency. As the density of the fluid inside the flow tube changes, the resonant frequency changes. The change in resonant frequency squared is inversely proportional to the change in density, as described in the following equation:
Δ
f
2
=
K
Δρ
where f represents the resonant frequency, K represents a proportionality constant, and &rgr; represents density. The period (&tgr;) of the resonant frequency can also be used, as described in the following equation:
&Dgr;&rgr;=K&Dgr;&tgr;
2
where &tgr; represents the period of the resonant frequency. Users of Coriolis flowmeters may want to measure the absolute density rather than the relative change in fluid density. A calibration of the Coriolis flowmeter may be required to determine a proportionality constant K and a reference fluid density. Calibration of the Coriolis flowmeter is accomplished by measuring the frequency/period of resonance with two known fluids. The absolute fluid density can be calculated using the following equation:
ρ
m
e
a
s
u
r
e
d
=
[
ρ
2
-
ρ
1
τ
2
2
-
τ
1
2
]
(
τ
m
e
a
s
u
r
e
d
2
C
(
T
)
-
τ
1
2
)
+
ρ
1
where &tgr;
1
and &tgr;
2
represent the tube period using two known fluids and &rgr;
1
and &rgr;
2
represent the densities of the two known fluids. C(T) is a temperature compensation for changes in the material of the Coriolis flowmeter due to temperature.
Unfortunately, the temperature of the fluid is often different than the ambient temperature around the flowmeter. The flow tube of the Coriolis flowmeter may grow or shrink due to thermal expansion. For a curved tube flowmeter, the thermal expansion may not be a problem because the flow tube is free to expand or shrink. For a straight tube flowmeter, the thermal expansion of the flow tube may be a problem because the flow tube is constrained from expansion along its axis by a case, a brace bar, or other means. The thermal expansion can result in a change in the resonant frequency due to temperature even though the fluid density may be unchanged. The meter electronics can compensate for the thermal expansion using a temperature correction, but the meter electronics have not been effectively adapted to handle thermal expansion by a more reliable means. This temperature correction is an indirect estimate of the tension/compression because it assumes a coefficient of thermal expansion.
Straight tube flowmeters are generally more sensitive to changes in boundary conditions than are curved tube flowmeters. Boundary con
Duft Setter Ollila & Bornsen LLC
Hoff Marc S.
Micro Motion Inc.
Raymond Edward
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