Ultrasonic Flowmeters for Liquid Hydrocarbon Custody Transfer ...

ARC INSIGHTS
By Paula Hollywood
INSIGHT# 2003-09MP
FEBRUARY 5, 2003
Ultrasonic Flowmeters for Liquid Hydrocarbon
Custody Transfer Applications
Keywords
Oil & Gas, Field Device, Flowmeter, Custody Transfer
Summary
In this Insight, custody transfer refers to a commercial accounting of an exchange
of hydrocarbon liquids between two parties. The exchange can be
internal or external in nature and the fluid itself is of little consequence to
the transaction. Most importantly, the buyer must remunerate the seller for
the fluid delivered, making accuracy of the measurement of significant concern
to both parties. For custody transfer applications,
it is essential that manufacturers select a
Now that API has approved ultrasonic
meter that meets the agreed upon measurement
flowmeters for measurement of
standard and the means for proving it. Turbine
hydrocarbon liquids, users must
determine if the process of proving the and positive displacement meters may initially appear
more cost effective; but ultrasonic flowmeters
meters exceeds the benefits of the
technology. could be the most cost effective in the long run due
to their inherent stability.
Caldon
Controlotron
Faure Herman
Krohne
API Precision UFM
Suppliers
Analysis
In October 2002, the American Petroleum Institute (API) passed Draft Standard
No. H00008, Measurement of Liquid Hydrocarbons by Ultrasonic
Flowmeters (UFMs) Using Transit-Time Technology. Similar to all
API Measurement Standards, H0008 outlines the institute’s recommended
methods for obtaining accurate measurements from UFMs
for the measurement of hydrocarbons. The standard was passed in
a record period of time, three years, considering it took 14 years to
develop a standard for Coriolis flowmeters.
Due to the limited amount of actual performance, field proving, and
calibration data currently available, the resolution passed as a “draft” indicating
it is a work in progress. API welcomes additional review and
suggestions for improving the current version. The standard applies to custody
transfer applications in particular, but also applies to other
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applications, such as allocation, check metering, and leak detection. Only
spool-type meters having two or more paths with permanently affixed
transducers are acceptable such that samples can be taken along multiple
acoustic paths. Excluded are meters having clamp-on style transducers due
to the potential ease with which the transducers could become dislodged
from their original mounting position. Naturally this approval makes suppliers
of such UFMs very happy, but what does it mean for manufacturers?
Proving Meter Certainty
The minimum requirement for fiscal measurement is a high degree of accuracy.
If a regulatory body is involved, statutory requirements generally
become much more stringent. An accepted practice of verifying meter
measurement is comparing a known prover volume to an indicated meter
volume. In addition to mandatory requirements for proving mandated by
regulatory agencies and/or standards organizations, other valid reasons
include:
CAUSE
Hydraulic distortion of flow
profile
Meter accuracy
Variations in fluid properties
i.e. viscosity
Maintenance required
EFFECT
Removes effect of installation hydraulics on
flow profile
Provides verification
Eliminates variation
Provides indication based on results
Reasons for Proving Custody Transfer Meters
Proving is the recommended method for validating meter repeatability and
proving to API standards is common throughout the world. To ensure an
accurate picture of meter performance API requires a minimum of 10,000
meter pulses be obtained. Based on the assumption of a one pulse error
each time a detector is passed, a possible error of 0.02 percent exists considering
the proving system has a start and stop detector as in a pipe prover.
The more traditional custody transfer measurement technologies such as
turbines are mechanical in nature and can be proved in a relatively short
number of proving runs, usually five, due to the inherent inertia of the rotor.
The sampling methodology of UFMs produces a greater degree of data
scatter due to the technology’s ability to measure minor variations in velocity
thus producing a wider range of repeatability. Under these
circumstances UFMs may require larger proof volumes and a greater num-
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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-
©2003 • ARC • 3 Allied Drive • Dedham, MA 02026 USA • 781-471-1000 • ARCweb.com

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lems associated with temperature stability. In order to ensure API’s recommended
10,000 pulses per run, pulse interpolation is used to compensate
for the relatively small volume of product displaced in this type of prover.
Precise time determination and pulse counting provide high accuracy with
fewer pulses. The small proof volume of compact provers limits its applicability
to proving small ultrasonic meters in the plant and is unlikely to be
used for larger custody transfer pipeline applications.
Turbine Master Meters
Master Meter
Proving with a master meter involves using an alternate technology to
prove repeatability using a meter whose calibration has been previously
determined against a standard. This approach enables proving over any
length of time. A turbine metering run, for instance, would be incorporated
into the prover and calibrated on the specific process fluid. The output of
the ultrasonic meter is then synchronized with the pulses
from the turbine using the appropriate number of pulses to
provide adequate repeatability. Master meters are seldom
used for custody transfer applications since they may have
the same inherent problems as the meters proved. The master
meter adds another dimension of uncertainty to the
system that must be accounted for however the overall size
of the prover can be reduced.
Recommendations
• Oil industry users must decide if the benefits of ultrasonic technology
for custody transfer applications outweigh any additional time and cost
required to prove it according to the API standard. A cost-benefit
analysis may be required to make this determination.
• Suppliers need to lessen the burden of proof on their customers by developing
more efficient proving methods.
Please help us improve our deliverables to you – take our survey linked to this
transmittal e-mail or at www.arcweb.com/myarc in the Client Area. For further
information, contact your account manager or the author at
phollywood@arcweb.com. Recommended circulation: All MAS-P clients. ARC
Insights are published and copyrighted by ARC Advisory Group. The information
is proprietary to ARC and no part of it may be reproduced without prior permission
from ARC.
©2003 • ARC • 3 Allied Drive • Dedham, MA 02026 USA • 781-471-1000 • ARCweb.com