Page 56 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production
P. 56
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.
Metering system
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 and 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.
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