HYDROCARBON
ENGINEERING
74
measure the pressure and temperature of the product as it
passes through the metering system for the purpo
ses of
determining a net volume at standard conditions (14.7 psia
and 60˚F). A metering system also typically includes
strainers and air eliminators, which help remove entrained
gas and solids from the product before it is measured.
These accessories and others all work together to increase
the integrity and accuracy of the metering system’s
measurement in conjunction with protecting downstream
equipment. In many
crude oil
applications, the
metering system
also includes a
product sampling
system to measure
sediment and water
content for
integration into the
net standard volume
correction
calculation and to
determine crude oil
quality.
Finally, metering
systems also usually
have a dedicated
volumetric prover,
or the connection to
a means of proving.
A prover contains a
known, precise
volume, that is
certified by
calibration using
nationally and
internationally
traceable standards.
During operation of
the metering system,
the product that is
measured by the
meter is also
measured by the
prover; the results
are compared and
developed into a
meter factor that is
used in calculating
product volume and
verifies the meter’s
accuracy and
repeatability at
operating
conditions.
The prover can
be used at any time
during the operation
of the metering
system, in which the
conditions are constant with regards to liquid product,
temperature, pressure, density, API gravity and viscosity.
The meter correction factor is then calculated by the flow
computer and applied to the meter reading dynamically
during a batch transfer or unilaterally to the volume
transferred. That ability to prove as the transaction of
product occurs and when operating conditions change
gives assurance to both the buyer and seller that what is
being measured is accurate.
Figure 1.
Various types of custody transfer accuracy metering applications and
equipment.
