Measuring and testing – Instrument proving or calibrating – Volume of flow – speed of flow – volume rate of flow – or mass...
Patent
1983-10-13
1986-12-16
Chin, Peter
Measuring and testing
Instrument proving or calibrating
Volume of flow, speed of flow, volume rate of flow, or mass...
G01F 2500
Patent
active
046287248
DESCRIPTION:
BRIEF SUMMARY
This invention relates to meter provers for use, for example, in proving rotary flowmeters of the type producing an electrical output pulse per increment of rotation. The number of pulses per unit volume is a characteristic of the meter which is defined as the k factor and it is the purpose of the prover to enable calibration of the meter k factor. A principal application is the measurement of oil flow rates and it is here a requirement for the meter k factor to be determined to an accuracy of at least 0.02%.
The conventional form of meter prover utilizes the passage of a sphere along an accurately dimensioned pipe between fixed detectors to displace a known volume of fluid. The volume of fluid displaced by the prover passes in series through the meter to be proved and the number of pulses generated in the meter during the passage of the sphere between the detectors is counted to enable determination of the k factor. To achieve the necessary high accuracy, a large displaced volume is essential and provers of 20 meters in length are not uncommon.
Proposals have been made to reduce the size of meter provers by using piston and cylinder arrangements and by reducing inherent inaccuracies in the prover to the extent that large displaced fluid volumes are no longer necessary. Significant improvements in inherent accuracy have been achieved through the use of electronic pulse handling techniques. Thus, in one example, pulse interpolation techniques are used to "inject" a train of uniformally spaced pulses between each pulse generated in the rotary meter undergoing proving. In this way, it is possible to compare the known volume displaced by the meter prover not merely with the number of whole pulses provided by the rotary meter but also with the number of interpolated pulses representing "fractions" of meter pulses.
A second known technique for improving accuracy is concerned to deal with the fact that through irregularities in the meter turbine, equal increments of rotation may not necessarily be associated with equal displaced fluid volumes. To remove this possible source of inaccuracy, the proving run is defined not by fixed points in the cylinder but by the receipt of meter pulses. In this way it can be arranged that the proving run covers an integral number of rotations of the meter turbine. With this technique, the piston displacement becomes a variable and must be measured with a suitable length encoder.
To enable advantage to be taken of the theoretical improvements in accuracy offered by such techniques, great care must be taken to ensure that the volume measured as displaced by the prover (whether a constant volume or variable) is identical with that flowing through the meter being proved. It is essential that there should be no leakage past the piston in the proving run and also that the proving run should be initiated under steady flow conditions and in such a manner as admits of complete reproduceability of results, within of course the constraints of unavoidable random errors.
Because of the need to have steady flow conditions, it is desirable for flow to be established through the cylinder of the prover before the piston is launched on a proving run. Various proposals have been made to this end and attention is drawn in particular to the following prior art.
U.S. Pat. No. 3,492,856
French published application No. 2471590
British patent specification No. 2023295B
French published application No. 2481449
British published application No. 2088566
The above documents evince the improvements in theoretical accuracy achievable through pulse techniques as described above but are of primary relevance here in connection with their disclosure of different methods for launching a prover piston.
In U.S. Pat. No. 3,492,856, the piston is provided with a through aperture on the axis of the piston. A valve is provided in this aperture which is normally open to enable a flow to be established through the piston when the piston is at rest; the valve being closed to initiate a proving run. A similar approach is ado
REFERENCES:
patent: 3492856 (1970-02-01), Francisco
patent: 3998089 (1976-12-01), Grove et al.
patent: 4152922 (1979-05-01), Francisco
patent: 4372147 (1983-02-01), Waugh et al.
patent: 4481805 (1984-11-01), Dobesh
Chin Peter
Skeltonhall Limited
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