Hydrocarbon Processing - April 2022 - 82

Process Controls, Instrumentation and Automation
reduce adverse effects due to sun radiation
or extreme cold, pressure transmitters may
the biggest uncertainty sources in the
temperature measurement.
An uncertainty estimation may reveal major, and often unexpected,
contributors to the estimated uncertainty. A system designer may
use these revelations to take appropriate mitigative actions to
reduce uncertainty in the overall measurement.
be provided with sunshades, or installed in
an enclosure with vortex coolers or enclosures
with space heaters, as applicable and
suitable for each particular installation.
Effect of RTD accuracy in temperature
measurement. Accurate measurement
of operating temperature is
required to calculate the volume of fluid,
particularly liquid, at standard or base
condition. The analysis of the temperature
measurement uncertainty budget
table may reveal an accuracy tolerance of
a resistance temperature detector (RTD)
as one of the biggest contributing factors
in temperature measurement uncertainty.
The RTD Type B normally used in the
industry has an accuracy of tolerance of
0.3°C; the RTD Type A has an accuracy
tolerance of 0.15°C.
To reduce the effect of RTD element
uncertainty in the overall temperature
measurement uncertainty, each RTD element
may be calibrated and the specific
values for the Callendar-Van Dusen constants
for that RTD sensor may be used in
the temperature transmitter.
Effect of loop validation acceptance
tolerance. Primary and secondary measurement
devices used in custody measurement
are regularly validated against
reference standards of the lowest possible
uncertainty. Company standards normally
specify the acceptance difference
between the reference standard and field
transmitter before an adjustment of the
field transmitter is performed. The field
transmitter is adjusted if the output is outside
the acceptance tolerance.
Operating companies typically allow
0.5°F of tolerance between the field
transmitter and the certified reference
thermometer before the field transmitter
is adjusted. However, an uncertainty
budget for the temperature measurement
may reveal that the acceptance tolerance
in transmitter calibration can be one of
82 APRIL 2022 | HydrocarbonProcessing.com
To mitigate this source of uncertainty,
the reference instruments of least uncertainty
and least possible loop acceptance
tolerance may be used. Also, to avoid the
creeping in uncertainty due to analogue
signal conversions and loop acceptance
tolerance, a digital signal may be employed
for secondary instruments.
Effect of
uncertainty in density
measurement. Product density requires
a calculation of the volume of the liquid
from operating condition to the volume
of liquid at standard condition of 15°C
(60°F) at atmospheric pressure. Though
the value of density is an important parameter
in the calculation, the accuracy
of the density measurement can be (unexpectedly)
not as important in the overall
uncertainty in the standard volume. For a
typical crude oil custody metering application,
a ±0.5% uncertainty in the operating
density measurement (±0.5 kg/m3
)
may result in a ±0.04% uncertainty in
CTL. The normal process density meter
may be sufficient for the application need,
as uncertainty in CTL would be further
eclipsed by uncertainty in gross volume.
Effect of meter runs in parallel. Custody
metering systems normally consist of
multiple flowmeters in parallel meter runs.
The common notion is that the higher the
number of meters runs, the higher the uncertainty.
However, due to the non-correlations
effect of various measurements in
parallel meter runs, the overall measurement
uncertainty in a multiple meter run
system tends to be lower than a single meter
run measurement.
Effect of dimension measurement.
The meter tube internal diameter (ID)
and orifice plate bore diameter are important
parameters to calculate flowrate using
an orifice meter. An uncertainty of 1% in
meter tube ID measurement can result in
a 0.5% uncertainty in flowrate. However,
2
the same level of uncertainty (1%) in an
orifice bore ID measurement can result in
a significant 2.5% uncertainty
in the flowrate. Uncertainty in
orifice bore ID has a higher impact
on the uncertainty in the
flowrate, as the sensitivity coefficient
is higher compared to
low sensitivity coefficient for
meter tube ID measurement.
Takeaway. The evaluation of uncertainty
depends on detailed knowledge of the
measurement application. The quality
of the uncertainty estimation therefore
depends on the assessor's knowledge,
understanding and critical analysis abilities.
Any uncertainty estimation process
provides insight into the impact of each
input on the output, and provides tools to
identify its importance. This helps users
to take appropriate mitigative measures to
reduce overall uncertainty in the output.
Reductions in uncertainty depend on the
practical feasibility and commercial viability
on implementation.
NOTES
The suggestions and guidelines provided in this
article should be considered general in nature and not
authoritative and final.
REFERENCES
1
International Organization for Standardization (ISO)
5168, " Measurement of fluid flow-Procedures for
the evaluation of uncertainties, " 2005.
Joint Committee for Guides in Metrology (JCGM)
200, " International vocabulary of metrology-Basic
and general concepts and associated terms, " 2012.
CHANDULAL N. BHATASANA holds a BS degree
in engineering and has more than 25 yr of experience
in custody/fiscal metering and instrumentation.
He is the Chairman of the Custody Measurement
Standards Committee for Saudi Aramco. Mr.
Bhatasana has authored numerous papers and
has received patents on innovative techniques
related to custody metering. The author can be
reached at chandulal.bhatasana@aramco.com
SALEM A. ALSHAHRANI is a Hydrocarbon
Measurement Engineer with more than 6 yr of
experience with Saudi Aramco, mainly in the field
of custody/royalty measurement systems. He leads
the technology program of process automation
systems domain. Mr. Alshahrani holds a BS degree in
mechanical engineering from King Fahad University
of Petroleum and Minerals.
ALI A. AL-QAHTANI is a Hydrocarbon Measurement
Engineer with more than 6 yr of experience with
Saudi Aramco, mainly in the field of custody/royalty
measurement systems. He has developed multiple
uncertainty calculation models related to fiscal
measurement systems and earned a BS degree in
mechanical engineering from King Fahad University
of Petroleum and Minerals.
http://www.HydrocarbonProcessing.com
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