Measurement: Flow meters - technical background: Inferential meters
Inferential meters are designed to deduce the volume of oil passing through a pipeline by measuring some physical characteristic such as speed of flow. There are several basic types of inferential meters, including turbine meters, orifice plates, vortex-velocity and vortex-shedding meters.
Turbine meters are commonly used for raising revenue accounts. They are used mainly in long distance pipelines (including receipts of crude oil from North Sea platforms) and the loading of ships.
Turbine meters measure the rate of flow of oil in a pipeline and relate it to the pipeline diameter to give a read out in terms of the desired units of volume.
They are virtually unaffected by friction and inertia, having no mechanical parts other than a free-moving turbine with magnetic-tipped vanes. The meter should, in theory, be accurate over a wide range of flow rates. The turbine is rotated by the oil as it passes through a metering chamber made of non-magnetic material inserted in the pipeline and is mounted on special bearings so that its axis of rotation is coincident with the axis of the pipeline. It is constructed so that the pressure of the oil against it counteracts the thrust on its bearings, thus reducing friction and inertia to negligible proportions.
Rotation of the turbine is detected by one or more sensing heads fitted to the metering chamber. Meters may also be encountered on which sensing is by means of a photo-electric cell mounted over a window in the metering chamber and activated by light reflected from a tiny mirror incorporated in the turbine. In either case the sensing head generates and sends pulse to an electronic computer which counts them and applies a pre-set factor (determined during proving of the meter). A digital display registers the volume of oil which has passed through the metering chamber.
Orifice plate meters
An orifice plate meter measures the pressure differential caused by the presence in a pipeline of a plate with a hole in it which restricts the diameter of the pipeline. It is sufficiently accurate for revenue accounting.
In the vortex-velocity meter, a vortex develops in a cylindrical chamber off set in the side of the meter tube carrying the main flow stream. There is a direct relationship between the number of revolutions of the vortex and the volume of the main flow. The volume flowing through the meter is recorded by counting the revolutions of a rotor mounted within the vortex chamber. Connection between the rotor and the revolution counter is by magnetic or electronic means.
A vortex-shedding meter measures the number of vortices created and shed per second around a specially designed obstruction in the pipeline. Pulse signals are generated at a frequency proportional to the flow rate.
For further information on the most common types of meters and what affect them see HCOTEG174280
For information on other types of flow meters see HCOTEG174290