Measuring and testing – Volume or rate of flow – By measuring tracer concentration
Patent
1995-03-01
1996-10-22
Chilcot, Richard
Measuring and testing
Volume or rate of flow
By measuring tracer concentration
73 3, G01F 1704
Patent
active
055678858
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to measuring the flow rate of a fluid stream using a dilution technique.
In particular, the invention relates to a feedback flow meter system using a dilution technique which can be used in the calibration of a fluid stream flowmeter.
Flowmeters require calibrating for use in a particular application and this requires obtaining independent measurements of the fluid stream flow rates through the flowmeter.
Several methods are known for calibrating flowmeters, one example of which utilizes the dilution technique. This technique involves introducing a tracer stock gas into the process fluid stream and measuring its resulting dilution or concentration. The amount of dilution is equal to the ratio of the flow rate of the tracer stock gas before introduction into the fluid stream to the flow rate of the process fluid stream. Thus, if the tracer stock gas flow rate and the dilution amount are known, then the flow rate of the process fluid stream can be determined. The tracer stock gas flow rate can be measured, for example, by a mass flow controller and one method of measuring the dilution is by means of a gas chromatograph equipped with an appropriate detector. A problem with this technique is that it is necessary to calibrate the gas chromatograph. Calibration is carried out by the provision of a set of gas bottles containing a range of known standard dilutions for passing through the gas chromatograph.
The preparation of the standard dilutions involves an intricate procedure comprising evacuating a set of gas bottles, carefully introducing into the bottles known volumes of tracer stock gas, forcing varying volumes of air into each of the bottles to produce the required range of dilutions, and then weighing the bottles. This procedure is very time consuming and requires careful attention since one mistake can render the whole set of bottles useless.
For accurate results calibration of the gas chromatograph is preferably carried out with the gas chromatograph connected to the process pipe. This has the disadvantage that it is necessary to transport the heavy gas bottles to the gas chromatograph. Furthermore, the tracer stock gas is rapidly used up and when a bottle has been emptied the set of standard dilutions prepared from it are no longer of any use.
It is an object of this invention to provide a method of measuring the flow rate of a fluid stream which is suitable for in-situ calibration of a flowmeter and which overcomes the aforementioned disadvantages.
It is also an object of this invention to avoid the need to measure the absolute concentrations of the tracer stock gas.
According to the invention we provide a method of measuring the flow rate of a fluid stream, the method comprising the steps of injecting a tracer gas into a gas stream having predetermined substantially constant flow rate during a period of measurement, introducing the resulting tracer stock gas stream into the fluid stream at said substantially constant flow rate, obtaining a sample of said fluid stream at a location downstream of the point at which the tracer stock gas stream is introduced and measuring the relative concentration of the tracer stock gas in the fluid stream sample, the method further including the steps of extracting a sample of said tracer stock gas stream at a location upstream of the point at which the tracer stock gas stream is introduced into the fluid stream, injecting said extracted tracer stock gas stream sample at a flow rate which can be varied into the gas stream to form a tracer stock gas sample-in-gas stream, obtaining a sample of said tracer stock gas sample-in-gas stream at a location downstream of the point at which the tracer stock gas stream sample is injected, measuring the relative concentration of the tracer stock gas stream sample in the tracer stock gas sample -in-gas stream and adjusting the flow rate of the extracted tracer stock gas stream sample so as to equalize the value of the two measured concentrations, whereby the conditions necessary to equalize the concent
REFERENCES:
patent: 3036229 (1962-05-01), Kemp et al.
patent: 3435660 (1969-04-01), Sternberg
patent: 4167870 (1979-09-01), Haas
patent: 5062291 (1991-11-01), Asay
Artis Jewel V.
British Nuclear Fuels PLC
Chilcot Richard
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