Oil and gas production handbook An introduction to oil ... - ABB Group

For liquids, turbine meters with dual pulse outputs are most common.
Alternatives are positive displacement meters (pass a fixed volume per
rotation or stroke) and coriolis mass flow meters. These instruments cannot
cover the full range with sufficient accuracy. Therefore, the metering is split
into several runs, and the number of runs depends on the flow. Each run
employs one meter and several instruments to provide temperature and
pressure correction. Open/close valves allow runs to be selected and control
valves can balance the flow between runs. The instruments and actuators
are monitored and controlled by a flow computer. If the interface is not
digital, dual pulse trains are used to allow direction sensing and fault finding.
To obtain the required accuracy, the meters are calibrated. The most
common method is a prover loop. A prover ball moves though the loop, and
a calibrated volume is provided between the two detectors (Z). When a
meter is to be calibrated, the four-way valve opens to allow oil to flow behind
the ball. The number of pulses from it passes one detector Z to the other and
is counted. After one loop, the four-way valve turns to reverse flow direction
and the ball moves back, providing the same volume in reverse, again
counting the pulses. From the known reference volume, number of pulses,
pressure and temperature the flow computer can calculate the meter factor
and provide accurate flow measurements using formulas from industry
standard organizations such as API MPMS and ISO 5024. The accuracy is
typically ± 0.3% of standard volume.
Gas metering is similar, but instead,
analyzers will measure hydrocarbon
content and energy value (MJ/scm or
BTU, Kcal/scf) as well as pressure
and temperature. The meters are
normally orifice meters or ultrasonic
meters. Orifice plates with a diameter
less than the pipe are mounted in
cassettes. The pressure differential
over the orifice plate as well as
pressure and temperature, is used in
standard formulas (such as AGA 3
and ISO 5024/5167) to calculate normalized flow. Different ranges are
accommodated with different size restrictions.
Orifice plates are sensitive to a buildup of residue and affect the edges of the
hole. Larger new installations therefore prefer ultrasonic gas meters that
work by sending multiple ultrasonic beams across the path and measure the
Doppler effect.
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