Oil and gas production handbook - Process Control and ...

interface is not digital, dual pulse trains are used to allow direction sensing and fault
finding.
To obtain 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 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 form 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 build up of residue and
wear on 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.
Gas metering is less accurate
than liquid, typically ±1.0% of
mass. There is usually not a
prover loop, instead the
instruments and orifice plates
are calibrated in separate
equipment.
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