Chemical Engineering May 2020 - 62

Gulf Coast 2020 Special Advertising Section
Gas Composition Matters for Flow Meter Accuracy
Case study shows flow meters calibrated to measure " natural gas " were programmed on
incorrect gas compositions leading to accuracy errors of ±10%.
Gas
N
atural gas is colorless and odorless, so
natural gas processors and transporters
add odorant to the gas to identify leaks by
smell. Odorization systems typically require
independent measurements of the amount of
natural gas odorized and the amount of odorant
consumed. Maintaining the proper ratio of
odorant to gas is essential for quality control.
The North American Energy Standards
Board (NAESB) suggests composition ranges
of various components in natural gas, with a
typical analysis contains approximately 94.9
percent methane, 2.5 percent ethane, 1.6
percent nitrogen, and 1.0 percent other components.
However, it is reported that over 10
percent ethane can be present in natural gas
obtained from horizontal drilling. In contrast,
significant amounts of heavier hydrocarbons
may be present in natural gas from other sources.
These types of differences in composition
can adversely affect flow meter performance
leading to accuracy errors of ±10%.
In a recent case study of a transporter in
Europe, the existing flow meters were configured
for the NAESB's typical composition and
calibrated using the factory-standard surrogate
natural gas. The composition of the actual natural
gas was significantly different, it varied by
site, and was likely to change over time.
The transporter wanted the ability to configure
the actual composition of its natural gas
in the field without returning the flow meters
to the factory for calibration, all without loss of
accuracy.
The manufacturer used the actual natural
gas compositions to perform calculations that
confirmed the flow measurement errors were
like those experienced by the transporter. The
traditional approach to resolve this problem
would be to return the flow meters to the factory
for recalibration with various surrogate
gases representative of the various natural gas
streams. This was not acceptable to the transporter
in the short term and would have made
compensating for future composition cumbersome
in the long term.
Consequently, the existing flow meters
were equipped with a gas selection library
programmed into the flow meters' electronics.
Using this technology, each component in the
Methane
Ethane
Nitrogen
Carbon Dioxide
Propane
Iso-Butane
Normal-Butane
Oxygen
Iso-Pentane
87.0 - 96.0
1.8 - 5.1
1.3 - 5.6
0.1 - 1.0
0.1 - 1.5
0.01 - 0.3
0.01 - 0.3
0.01 - 0.1
86.5
7.2
0.1
0.7
3.5
0.8
85.8
4.8
4.6
2.2
2.3
0.1
Trace - 0.14
Normal-Pentane Trace - 0.14
Hexanes Plus
H2
% Error
Trace - 0.06
Trace - 0.02
N/A
0.24
0.00
0.12
0.08
0.24
0.00
+4.0
0.67
0.00
0.23
0.15
0.11
0.00
+7.9
0.15
0.00
0.01
0.01
0.01
0.00
+9.8
NAESB Range Texas Nigeria Canada Italy
93.1
3.0
0.1
2.1
0.8
0.2
85.6
7.7
3.1
1.3
1.7
0.2
0.30
0.00
0.12
0.06
0.05
0.00
+10.1
Chemical Compositions of
Natural Gas (in mole percent)
natural gas could be configured in 0.1-percent
increments in the field to measure the flowing
natural gas with no loss of accuracy.
As a result, the transporter was able to
create a schedule to test for changes in gas
composition. When changes were detected,
the flow meters' gas selection technology was
used to program the new gas composition into
the electronics to maintain the highest gas flow
measurement accuracy.
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62
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Chemical Engineering May 2020

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