Ultrasonic Flowmeters for Liquid Hydrocarbon Custody Transfer ...

ARC Insights, Page 3
ber of proving runs to achieve the same uncertainty as a turbine or positive
displacement meter. In actuality there is no difference in a repeatability
range of 0.05% in 5 runs versus a range of 0.14% in 12 runs, meaning the
operator can choose to increase the number of runs to account for the inherent
proving variability of different suppliers. The standard is practical
without affecting performance allowing operators to select up to 20 runs to
meet repeatability standards.
Volumetric Proving Methods
There are three commonly accepted proving methods, the pipe or ball
prover, the small volume or compact prover, and the master meter. All
three methods have their own idiosyncrasies and it is up to the discretion of
the user which method to use. Regardless of the selected method, good
industry practice recommends proving be performed with the flowmeter in
place to account for any hydraulic effects on the installation.
Stationery Pipe Prover
Pipe Provers
A pipe prover is a closed system generally consisting of a section of pipe of
constant size in which a piston is moved along by the liquid to
be measured. Meter pulses are counted as the piston moves between
the start and stop detectors embedded in the pipe run.
Because the volume between the detectors is known, the pulses
per liter of product can be easily deduced. The advantage of
pipe provers is continuous flow through the meter being calibrated.
Bi-directional provers offer long term stability while unidirectional
provers offer minimal pressure loss. Despite their
large size, pipe provers will likely be the system of choice although
their larger proof volume is necessary for reliable
proving of ultrasonic flowmeters.
Trailer Mounted SVP
Small Volume Prover
As the name implies, small volume provers (SVPs) are small in
size and are sometimes referred to as compact or piston provers.
To facilitate field calibrations, they can be made portable by
mounting on a truck or trailer. In this fashion the prover is
more useful across the plant and it becomes more economical
than requiring a separate prover for each meter. In an SVP the
displacer does not move freely as it is a piston connected to a
piston rod. SVPs use a faster test cycle which minimizes prob-
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