Method and apparatus for measuring mass flow

Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...

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G01F 178

Patent

active

057009570

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The invention relates to a vibratory method and apparatus for measuring mass flow.


BACKGROUND ART

In the art of measuring mass flow rates of flowing substances it is known that flowing a fluid through an oscillating flexible flow pipe produces Coriolis forces. During vibration, a cross section 1A of a pipe 1 moves along curved path 2 as shown in FIG. 1, therefore, an angular velocity .OMEGA. subject to periodic changes is associated with said cross section 1A. As a result of flow velocity v of the mass m of a fluid portion and said angular velocity .OMEGA., Coriolis force Fc distributed along the vibrating flow pipe 1 will be generated, which Coriolis force is exerted by the flowing medium to the inner wall of the pipe 1. The relationship between the said physical quantities can be expressed by the well-known formula as follows:
By indirect or direct measurement of the Coriolis force expressed above, the mass flow rate through the vibrating pipe can be determined.
The principle of the method of measurement most commonly used consists in the fact that, under the effect of Coriolis force, the state of movement of the vibrating pipes will be changed depending on the flow velocity--including e.g. zero velocity--in the pipes. Any change in the state of motion will be represented by the phase difference between the periodic time functions describing the motion of specified points of the vibrating pipes. Therefore, displacement sensors are arranged at two appropriately selected points of the flow pipes, that supply periodically changing electric signals (S1, S2) characterizing the vibrational movement at said points of the pipes:
In the practice, optical displacement sensors, electromagnetic velocity sensors or piezoelectric accelerometers are used as movement sensors. In each case, the functions f1 and f2 practically cause the electric signals S1 and S2 to be periodic functions. The information useful for the mass flow rate will be the phase difference .PHI. between these two periodic signals. This phase difference .PHI. is nearly proportional to the mass flow rate in the range of small angles.
A number of types of mass flow meters of vibration principle are known, which contain flow pipes of different form and designed on the known principles described above. Such devices are disclosed e.g. in U.S. Pat. Nos. 4,491,025 and 4,655,089, Hungarian Patent No. 198,566 and European Patent No. 0 210 308.
The most important advantage of the mass flow meters of principle also described in the patent specifications cited consists in the fact that the Coriolis force depends directly on the mass flow, essentially irrespective of the consistency (viscosity, physical state) of the flowing media.
A common feature of the solutions known so far is, that the Coriolis force generated by the forced periodic deformation (vibration) and distributed along the pipes also causes deformation which, in turn, will be superimposed on said forced deformation. The deformation caused by Coriolis force and the forced deformation are of different mode and phase, therefore, it is essentially the superposition of the two kinds of deformation that supplies the useful information, that is, the phase difference between the periodic movements measured at two suitably selected points of the pipe.
The principle of operation described above involves the following problems.
The amplitude of vibration generated by the Coriolis force and superimposed on the forced vibration (that is, the useful signal) depends, in addition to the magnitude of Coriolis force, on the rigidity and mass of the vibrating pipe as well as on the frequency and the rigidity, in order to obtain suitable useful signal. As a result of these limitations, vibrating pipes of relatively low rigidity (i.e. long and of small diameter) shall be used. It is of disadvantage in respect of practical implementation, as the most efficient way of reducing the noise sensitivity is to increase the operating frequency, on the one hand, and the pressure loss of the measuring instrumen

REFERENCES:
patent: 4127028 (1978-11-01), Cox et al.
patent: 4187721 (1980-02-01), Smith
patent: 4311054 (1982-01-01), Cox et al.
patent: 4381680 (1983-05-01), Shiota
patent: 4491025 (1985-01-01), Smith
patent: 4655089 (1987-04-01), Kapelt et al.
patent: 5357811 (1994-10-01), Hoang
patent: 5423221 (1995-06-01), Kane et al.

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