LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIAL REPORT [611279]

LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIAL REPORT
Perfecting liquefied natural gas
analysis techniques and methods
Following these procedures will improve
measurement accuracy and reliability
S. HALE, Emerson Process Management, Houston, Texas
The nature of liquefied natural gas (LNG) raises many chal –
lenges when it comes to the measurement and reporting of
its composition for the purpose of ship loading and unload –
ing. LNG’s extreme temperature, and the difficulty with keeping
it in liquid form, introduce unique sample-handling issues, while
the batch-handling operation makes reporting difficult. At the
same time, the accuracy and reliability of the LNG measurement
is uniquely critical since the loading and unloading operations are
highly time-sensitive with no second chances. Delays in loading or
unloading because of measurement issues are not tolerable when
the cost of keeping a ship in port is considered. For example, one
LNG operator had two measurement systems fail right before the
docking of a large tanker—a problem that could have resulted in
significant penalties had the failures not been remedied heroically.
Additionally, disparities in the measurement are often not known
until after the unloading is complete at the destination and com –
parisons to the load report are made.
Sample handling. The problem that nearly brought the afore –
mentioned LNG operator to its knees was a failure in the sample-
handling system that resulted in damage to the measurement tech –
nology. The sample-handling challenge is frequently underrated
and can be one of the weakest links in an LNG system. Many
designers attempt to develop their own sample-handling system
without regard for the lessons learned by existing operators and ven –
dors or the standards and guidelines developed by the industry.
For a sample-handling system to measure a truly representative
sample, the LNG must be maintained in the liquid phase right
up to the point where it is vaporized. At that point, it must be
vaporized uniformly into a single-phase vapor state. As illustrated
in Fig. 1, there is a significant two-phase region that the sample-
handling system (Fig. 2) must move the sample through before
it is a single-phase vapor. If the sample is transported while in
the two-phase region, the different velocities of the liquid and
gas phases will result in a composition change of the sample as a
whole once it reaches the point at which it is in the single-phase
vapor state. In other words, if the LNG begins to vaporize in the
sample lines, the nitrogen and methane will boil off first, produc -ing pockets of gas in the liquid stream that reach the vaporizer
at different times, resulting in varying compositions. The result
is that the LNG sample reaching the vaporizer will consist of an
unrepresentative liquid sample rich in the heavier components
plus slugs of methane-rich vapor. The vapor sample leaving the
vaporizer will be inconsistent, with wild swings in composition.
This is then fed into the gas chromatograph and sample cylinders
resulting in unrepresentative analysis.
To overcome some of the problems associated with LNG
vaporization, an accumulator is used immediately after the vapor –
izer to reconstitute the sample. This step, however, can only cor –
rect for small fluctuations. If there is significant vaporization in
the sample lines, then large slugs of gas can actually insulate the
vaporizer from the LNG, once again resulting in the sampled gas
being unrepresentative of the actual flowing stream.
Icing on the sample lines is an indication that the sample is
vaporizing in the lines. Modern installations utilize vacuum-
jacketed tubing from the sample probe to the vaporizer that is
located within 7 ft to ensure that the liquid phase is maintained
right up to the vaporizer.
1,200
1,000
800600400Psia
200
0
–260 –210 –160 –110
°FLiquid
regionTwo–phase
region
LNGVapor
sampleVapor
region
–60 –10 40
LNG phase diagram. FIG. 1Originally appeared in:
July 2010, pgs 47-49.
Used with permission.
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LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIAL REPORT
Even if the LNG sample does reach the vaporizer in the pure
liquid phase, the vaporizer must be designed to add sufficient
heat to the liquid sample and to allow for over 600-to-1 expan –
sion from the liquid phase to the gas phase without causing
sample fractionation.
Vaporizer design has also undergone significant changes.
“Vaporizing regulators,” common in the process industry, attempt
to perform sample vaporizing and pressure regulation, but do not
have the heating capacity or volume expansion allowances to do
either job well when it comes to LNG.
The best performing vaporizers are designed specifically for
LNG and perform the vaporization separate from the gas pres –
sure regulation. These vaporizers take a small volume of liquid
sample and flash the sample off quickly, providing very little
restriction and allowing the sample to expand over 600 times
in volume. To ensure correct operation, the outlet vaporizer’s
temperature is monitored to guard against liquid carryover. After
the sample is vaporized, it enters the accumulator and is then
pressure-controlled and sent to the gas chromatograph for ana –
lyzing. Regardless of what approach the LNG operator takes to
sample handling, it’s critical that basic standards are met. These
include ISO8943 and the requirements listed in GIIGNL LNG
Custody Transfer Handbook .
Gas chromatograph operation. While LNG composition
is similar to natural gas, it has unique properties due to the pro -cess requirements of chilling the feed gas to the low temperatures
required to produce LNG. Carbon dioxide (CO 2) levels must be
as low as possible. They typically are less than 50 ppm in LNG
streams to prevent possible solid formation in the liquefication
process. CO 2 has a relatively high freezing point of –70.6°F , as do
some other hydrocarbons higher than butane.
Typical LNG has a high energy value because of the lack of
non-hydrocarbon components, rather than high concentrations
of heavy hydrocarbon components typically seen in high heat –
ing value “pipeline-quality” gas. As there is virtually no CO 2,
the specific gravity of the gas is also lower than the typical natu –
ral gas that may have up to 2 mole % CO 2. This has a major
effect on the Wobbe Index of the gas, which will be much higher
from LNG sources. Table 1 shows the typical LNG composition
imported into the US.
Hardware considerations. Due to the unique characteris –
tics of LNG and the high value placed on measurement accuracy,
gas chromatographs used for analysis must be designed for the
application and have an analysis repeatability of +/- 0.25 BTU per
1,000 BTU (0.025% of Energy Value). This repeatability must be
achieved over the full ambient temperature range of the instru –
ment, which in the case of an LNG gas chromatograph, should
be at least 0°F to 130°F .
The ambient temperature range of the gas chromatograph is
just one of the ruggedized characteristics demanded by the LNG
application. The gas chromatograph instrument is placed on
the docks and the closer to the actual loading area the better for
the measurement. Therefore, the analyzer must be designed to
withstand true maritime conditions. And it must do so without
requiring much dock space. Gas chromatographs are frequently
mounted directly on the docks where the swing-arm moves across
for loading. While LNG gas chromatographs will generally be
enclosed in a three-sided shelter for operator comfort, the smaller
the footprint, the more desirable for the application.
Operators should also pay close attention to utility requirements.
Gas chromatographs that require an air-conditioned environment
in an analyzer house are not appropriate for LNG applications.
They significantly increase utility cost and enlarge the footprint and
may increase installation and operational costs by over 50%.
Overall cost-of-ownership should, in fact,
be a critical factor in selecting a gas chromato –
graph for LNG use. The hardware should be
easily maintained, but, at the same time, not
too expensive to maintain. Frequently, LNG
systems may be built by contractors who have
little stake in the ongoing operational costs of
their systems. It’s unwise for the LNG opera –
tor to trade a low-cost gas chromatograph
for long-term elevated costs of maintenance.
Since 80% of faults that arise in gas chro –
matographs require the overhaul of analy –
sis valves, this is an area to consider closely.
If replacement is too costly, the operating
expenses can mount significantly to several
multiples of the original cost of the system.
Software considerations. As impor –
tant as the hardware criteria in system
selection, software also plays a huge role in
LNG analysis. It’s essential that the analysis,
PIPIAccumulatorPITI
LNG vaporizing panelFilterVaporizer
SCS GC SHSLiquid
filter/
shutoffVacuum-jacked
tubingLNGVapor return/boil-off gas Gas
chromatograph
Typical LNG sample-handling system. FIG. 2TABLE 1. Typical compositions of LNG imports into the US
1075 High 1130 High 1130 Low
1075 ethane ethane ethane 1160 1100
Methane 94.73 92.3 86.53 89.94 88.33 91.8
Ethane 3.8 7.5 12 6 6 6
Propane 1.17 0.2 1.33 3 4.3 1.4
iso-Butane 0.3 – 0.06 0.53 0.5 0.4
n-Butane – – 0.08 0.53 0.5 0.4
iso-Pentane – – – – 0.37 –
BTU 1,063 1,070 1,124 1,125 1,154 1,095
Wobbe Index 1,387 1,391 1,420 1,420 1,436 1,404
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LIQUEFIED NATURAL GAS DEVELOPMENTS SPECIAL REPORT
reporting and communication software be purpose-built to the
application. Gas chromatographs with laboratory-style software
are too complex for LNG and are unsuitable for its operational
and diagnostic requirements.
Unlike pipeline or process gas chromatographs which run
24/7, gas chromatographs used for LNG ship loading and unload –
ing will only be run while the ship is in the dock. On a pipeline,
an operator can calibrate and start the system locally since it will
run continuously for years. For LNG, the gas chromatograph
should be able to start, stop and calibrate remotely from the con –
trol room to save operator time and make the operation efficient.
LNG gas chromatographs need to be able to communicate
to host devices such as flow computers, SCADA systems and
distributed control systems (DCSs). Due to the high number of
values reported by the gas chromatograph, the MODBus serial
communication protocol, over either serial or Ethernet commu –
nication links, is used. A recent development is using Foundation
fieldbus to connect to the DCS, which may suit the project design
philosophy better and tie into plant-wide diagnostic monitoring
software with virtually no customization. Whatever the protocol
for host system communication, additional provision should be
made to allow remote diagnostics. Such capability allows highly-
trained personnel to be able to analyze, diagnose and maintain
multiple devices from a central location, often off site.
Reporting software is also very specific to LNG require –
ments. In most custody transfer agreements, there is a reporting
requirement for:
•  Average  composition  over the entire load
•  Spot sample after stable flow is achieved (typically  after 
one hour)
•  Spot sample at the 25%, 50% and 75% ship tank level.
Gas chromatograph systems designed for LNG have dedicated
control and reporting software that allow the operator to:•  Run the calibration  sequence remotely before the load/
unload operation is started
•  Start the analysis and log each analysis as part of the load
• Allow operator-initiated  storing of spot sample compositions 
at the 1 hr, 25%, 50% and 75% tank level times
• Allow for suspension  of logging due to process disturbances 
such as pump failure
•  Stop the analysis
• Allow for the removal of inconsistent  analysis due to process 
upsets based on the agreement of all parties
• Generate  a load report with an average composition,  the spot 
sample analysis, and calculated energy and physical properties for
each reported composition.
While some system designers have custom-programmed all of
these functions into the host system, gas chromatograph vendors
who regularly supply to the LNG market have off-the-shelf pack –
ages that do all of these functions already and are used in multiple
locations. Using such an off-the-shelf application reduces pro –
gramming costs, saves time, performs all of the functions correctly,
and gains instant acceptance from third-party auditors.
Meeting the challenge. The significant value of an LNG
ship load justifies the most accurate and reliable LNG analysis
system. Fortunately, today’s LNG analysis technology allows for
reliability and accuracy while maintaining a low ownership cost.
System design must assure that the sample is handled appropriately,
analyzed accurately and that results are reported correctly in facing
the challenges of LNG temperature and batch reporting. There’s
no room for compromise when you don’t get a second chance. HP
Shane Hale has been in the oil and gas industry for over 15 years, of which the
last 12 have been with Emerson Process Management. Mr. Hale is the natural gas
product marketing manager and a member of the AGA Transmission Measurement
Committee and the ISO TC193 Committee.
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