CALIBRATION: TWO BECOME THREE IN EUROPEAN CALIBRATION
Europe has two major
calibration facilities for
liquid flow meters.
Both are in France: one in the
north of the country, the
other in the south. Each is
part of a pipeline
transmission system, and
although their main purpose
is to maintain the accuracy of
the systems' flow meters,
they also offer calibration to
meter manufacturers, system
integrators, and pipeline
operators.
Flow meters are supplied
with a calibration certificate
that in most cases is unit
specific. The standard
calibration is usually based at
three points along a 10 to
100 per cent flow range to
prove linearity, with each
point checked three times to
show repeatability. These are
increased to 10 and five
respectively, for meters
destined for fiscal or custody
transfer service. In many
cases, however, even this
higher level of testing is
unacceptable to the fiscal
authority.
Manufacturers calibrate
their meters on a single
product, and as Charles
Griffiths, a specialist engineer
with design consultant AC
Flow Systems, explained, 'The
use of a single fluid such as
water or Stoddards fluid for
meter performance evaluation
is acceptable for the
verification of positive
displacement (PD) meters;
but not for inferential types of
meter.' The PD meter was the
original workhorse of the
petroleum industry; but it has
been supplanted by turbine,
multipath ultrasonic, and
Coriolis meters, for the fiscal
volumetric measurement of
crude oil and refined
products.
'With the exception of the
PD and Coriolis meters, the
other types are inferential.'
Griffith stated, 'These do not
measure flow directly; but
infer it from other related
physical variables, and then
determine the flow by
computation or use of a
calibration constant
established from a reference
standard. As a consequence,
performance verification must
be undertaken by reference
to a known volumetric
standard under similar
flowing product conditions in
which it is to operate.
'Turbine meters respond to
flow by rotating to maintain a
torque equilibrium in the face
of a complex array of fluid
forces and mechanical
friction. As a result, there is
inherent in the technology a
non-linearity of K-Factor,
which is primarily Reynolds
number dependant.
'Similarly', he continued,
'the same is true for
multipath ultrasonic meters
which measure the time
difference between pairs of
ultrasound pulses directed
downstream with the flow,
and those directed upstream
against the flow.
