Flow metering

Details Flow metering Flow metering

1996-12-06

2001-10-23

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Volume or rate of flow

With indirect temperature or density compensation

Reexamination Certificate

active

06305231

ABSTRACT:

The invention relates to flow measurement, and especially to the “relative” measurement of mass flow rate of multiphase flows in particular to the flow of gas-solids mixtures. In some applications an absolute measurement of mass flow rate is not required. For example, in fossil burning electrical power generation, coal is heated, pulverised and then injected into a furnace through a number of pipes, each fed from a manifold or “flow splitter” which in turn is fed from a larger diameter pipe. Accurate control of the “split” of the fuel is important in helping to increase the combustion efficiency. It is therefore only necessary to measure the relative quantity of solids passing along each pipeline.
When solids are conveyed in a pneumatic transport system, electrical charging of the particles occurs due primarily to frictional contact between particles and between particles and the pipe wall. The total charge cannot be predicted since it depends upon the variable conditions of solids transportation such as the solids velocity, the type of material, the particle size and shape, moisture content, the material comprising the conveying pipe walls, the roughness of the inner surface of the conveying pipe, the diameter of the pipe and the distance travelled by the particle. The charge polarity is also dependent on those conditions.
The mass flow rate of the solids can be determined inferentially by separately measuring the solids velocity and the solids concentration. The mass flow rate is obtained from the product of the two parameters. The velocity of the particles can be readily measured by cross correlation of induced signals derived from annular electrostatic sensors fitted into the pipe wall. A signal related to the solids concentration is obtained from the root mean square value of the signals detected by annular electrodes.
If we have the same flow conditions in several pipes (same solids material, equal transport distances, equal velocity and pipe conditions), it is reasonable to assume the signal amplitude to be related to the quantity of solids transported through that pipe. It has been shown from earlier experimental tests that under “lean phase” flow conditions this assumption is valid. Therefore, a relative measurement of the solids loading can, in principle, be achieved.
In practice electrostatic sensors are used mounted flush with the pipe wall but the spatial sensitivity is determined by an inverse square law leading to erroneous results if a non-uniform solids distribution occurs, as is quite likely to happen in practice.
At least in its preferred embodiments, the present invention is directed to providing a solution to this problem.
In one aspect the invention provides a method of measuring a characteristic of a flowing fluid comprising measuring said characteristic by means of two or more sensors which have differing responses to a variable affecting said measurement, and combining the outputs (as herein defined) of the sensors, so that the differences in the responses thereof cancel each other and the combined output is substantially independent of said variable.
Included in the term “output” are signals derived from the output, for example an RMS value thereof.
The characteristic may be a concentration of charged particles in said flowing fluid.
The characteristic thus may be a concentration of solid particles in the flowing fluid. Alternatively, the characteristic may be another phase concentration, for example liquid droplets in a gas flow, or droplets of one liquid in another immiscible carrier liquid.
The variable may be a distance of the particle or droplet from the sensor when it is sensed thereby.
The method may also include scaling or otherwise modifying the output of at least one of the sensors before the sensor outputs are combined.
In another aspect the invention provides apparatus for measuring a characteristic of a flowing fluid comprising at least two sensors for measuring said characteristic, said sensors having differing responses to a variable affecting said measurement, and means for combining the outputs of the sensors so that the differences in the responses thereof cancel each other out and the combined output is substantially independent of said variable.
Thus when the characteristic is a concentration of charged particles the sensors may be electrostatic electrodes.
The electrodes may be of different widths, measured in the direction of flow.
The apparatus may have a flow passage for the flowing fluid, the electrodes being annular and spaced from each other in the flow direction.


REFERENCES:
patent: 4266188 (1981-05-01), Thompson
patent: 4509366 (1985-04-01), Matsushita et al.
patent: 4751842 (1988-06-01), Ekrann et al.

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