Measuring and testing – Volume or rate of flow – By measuring transit time of tracer or tag
Patent
1995-08-29
1998-02-17
Dombroske, George M.
Measuring and testing
Volume or rate of flow
By measuring transit time of tracer or tag
G01F 168
Patent
active
057193410
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to methods of measuring the velocity of flow of a liquid or gas (fluid), along a pipeline for example, and to apparatus for carrying out such measurement.
Various methods for measuring flow rates of liquids and gases have been proposed hithero. For gases, these include rotameters and turbinemeters, measuring the kinetic energy of the gas or measurement based on thermal losses.
Some of the aforementioned techniques have the disadvantage that they have to be calibrated for the specific properties of the gas whose flow rate is being measured. In addition many standard techniques involve a substantial pressure drop along the path of fluid flow. The present invention is particularly suited for use in a low mass flow application, typically 0-10 m.sup.3 hr.sup.-1.
Examples of other types of fluid velocity measurement are described below.
German Patent Application DE-A-3301855 describes a method of labelling a fluid using acoustic or vibrational pulses. These are converted into electric pulses and combined with characteristic data of the medium to provide information on velocity of the fluid. The method is well developed. The technique is influenced by the fluid being measured. It is a "time of transit" method.
French Patent Application FR-A-2518257 describes a thermal labelled time of transit method of measuring fluid flow and a related device. Fluid enters tangentially into a vortex chamber and fluid velocity is measured within the chamber.
French Patent Application FR-A-2585465 describes an apparatus for measuring flow velocity which removes a volume of fluid from a flow path and replaces it with a substantially equal volume of thermally labelled fluid. The apparatus then detects the "time of flight" of the thermally labelled fluid and calculates the flow velocity.
WO-A-9309404 describes an apparatus for measuring flow velocity, using a time-of-flight measurement.
A flowmeter for measuring the velocity of water along boreholes has been proposed in a technical report of the Water Research Centre entitled "Heat-Pulse Flowmeter for Boreholes" by C. R. Dudgeon et al. dated March 1975. The flowmeter operates by forming a thermal pulse in water, by passing the water over a heating grid, and detecting the passing of the thermal pulse downstream of the formation point. This method has worked satisfactorily for water in which the kinetic energy of the molecules is relatively low. The method is known as thermal labelling and a thermal pulse was used for this purpose.
U.S. Pat. No. 4,228,683 describes a method for thermal labelling of a transient fluid. Fluid is deflected through one pipe and then returned to a second where it is sensed. A variety of sensors may be used. However, the basic method may not be intrinsically safe for use in explosive environments. Use is made of a differential output from thermocouples to operate a flip-flop. This requires a threshold sensing device and a comparator in order to calibrate the system.
A first aspect of the present invention is based on the surprising discovery that thermal labelling also works for gases, in which the molecules are moving relatively quickly. It might be expected that any thermal pulse produced in a gas would dissipate in too short a time for its velocity to be measured but experiments, to be described below, have demonstrated that this is not the case.
In addition, the invention overcomes problems experienced with existing thermal labelling techniques, in that such techniques are not capable of being used safely in explosive and/or corrosive environments. An example of an explosive and/or corrosive environment is a land-fill site, where refuse is tipped and covered. Increasingly gases, usually methane, are collected from these land-fill sites and used to generate electricity.
It has been a problem to safely and accurately measure and monitor the amount of gases produced. The present invention overcomes this problem and removes the need for prior calibration of equipment.
According to a further aspect of the present invention there i
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C.R. Dudgeon et al, "Heat-Pulse Flowmeter For Boreholes" Water Research Centre, Technical Report, Mar. 1975 pp. 1-69.
Bartington John Keith
Reynolds Patrick John
British Technology Group Limited
Dombroske George M.
Patel Harshad
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