Technical Publication
Accurately Measuring LNG Tanker Boil-Off Gas
For Process Efficiency and Environmental Compliance
Steven Craig, Engineer
Fluid Components International LLC
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Accurately Measuring LNG Tanker Boil-Off Gas
For Process Efficiency and Environmental Compliance
Steven Craig, Engineer, Fluid Components International LLC
BOG Problems
On January 1, 2013 the International Maritime Organization (IMO)
implemented an amendment to the International Convention for
the Prevention of Pollution from Ships whereby they added a new
regulation to increase the energy effciency for ships in order
to reduce the CO2 emissions. Though many ships have already
implemented energy effciency measures, it’s now mandatory
that they meet the new IMO regulation. LNG tanker ships are
powered by steam turbines with boilers that are fueled by either
methane, oil, or a combination of both. In recent years the ships
are now using the LNG BOG that is produced during transport as
fuel for the ships boilers.
The BOG is collected, heated to ambient temperature,
Figure 1. LNG Tanker odorized, compressed and then distributed to the boilers
which power the steam turbines used for the ships propulsion.
The fow measurement of the BOG to the ships boilers is a
critical component because the IMO regulation requires low
W ith the recent growth in demand and production
of liquefed natural gas (LNG), there has been an
increase in trade and the need to transport it globally
CO2 emissions, which means they must accurately measure
the amount of BOG they are using in their boilers for energy
effciency.
via LNG tanker ships. This increase in LNG transport has
driven the need for expansion of the tanker feets with
increased capacity, wherein the LNG ships are typically BOG Measurement Solution
carrying up to 266,000 m³ around the world. The LNG tanker
The ST110 Series fow meter from Fluid Components International
feets are unique because LNG cargo generates waste
(FCI) meets the accuracy requirements for measuring BOG
gas, commonly known as boil-off gas (BOG). This waste
aboard tankers, at land terminals, storage facilities and points
gas can be used as fuel for the ships boilers and must
of distribution. This advanced air/gas fow meter combines
be accounted for in order to comply with current marine
powerful electronic features and an advanced fow sensor design
propulsion regulations for energy effciency. Accurate
to provide precision measurement, reliability and economy.
fow measurement of the BOG to the ships boilers is
The ST110 fow meter (Figure 2) is designed with FCI’s
essential to tanker operators.
unique VeriCal In-Situ Calibration Verifcation System. Flow
meters equipped with the VeriCal System (Figure 3) can perform
periodic feld functional testing and calibration verifcation of the
Ship Handling of LNG Cargo fow meter’s measurement performance without extracting the
and Boil-off Gas (BOG) fow meter from the pipe or process to avoid shutting down the
In order to transport natural gas, it is economical to convert it to process for a lengthy period.
LNG. The conversion process involves cryogenically cooling the For BOG measurement, the ST110 fow meter with VeriCal
natural gas to -163 °C at atmospheric pressure, at which point features an internal purge tube that runs the length of the probe
the gas condenses to a liquid and is ready for transport. The to the sensor and allows the operator to generate a known fow
refrigerated tanks on board the ship ensure the liquid gas remains across the sensor element. The resultant signal output can then
cryogenic during storage. BOG results when the LNG within the be compared to the factory baseline test certifcate.
ships storage tanks vaporizes due to subsequent ambient heat The ST110 Series’ electronics can meet both current and
input to the tanks during transport. In the past LNG tanker ships future need for BOG measurement outputs, process information
used an onboard re-liquefaction plant to recycle the LNG vapor and communications. Whether the need is for 4-20 mA analog,
back to liquid and into the storage tanks. frequency/pulse, alarm relays or digital bus communications
Fluid Components International LLC g www.fluidcomponents.com g Page 2
Accurately Measuring LNG Tanker Boil-Off Gas
For Process Efficiency and Environmental Compliance
Steven Craig, Engineer, Fluid Components International LLC
such as HART, FOUNDATION™ feldbus, PROFIBUS or Modbus, it
provides a solution.
For LNG operators, the ST110 fow meter features a
graphical, multivariable, backlit LCD display/readout that can
be read at locational times. It provides local information with
a continuous display of all process measurements and alarm
status, as well as service diagnostics.
Designed for complex gas measurements such as LNG and
methane, the ST110 fow meter stores up to fve calibration
groups to support a broad fow range, differing gas mixtures,
multiple gases, and obtains up over a 500:1 turndown. An on-
board data logger with a removable 2-GB micro-SD memory card
that stores 21 million readings is also included.
The ST110 can be calibrated to measure LNG, methane
Figure 2. ST110 Air/Gas Flow Meter
and other process gases. The insertion style ST110 fow meter
features a thermal fow sensing element that measures fow from
0.25 to 1000 SFPS (0.07 NMPS to 305 NMPS) with accuracy of
± 0.75 percent of reading, ± 0.5 percent of full scale.
Designed for demanding oil/gas industry applications, Thermal Dispersion Technology
the ST110 fow meter operates at up to 850 ºF (454 ºC) and is The ST110 fow meter’s precision fow sensors are designed with
available with both integral and remote (up to 1000 feet [300 thermal dispersion constant current technology, which places
meters]) electronics versions. The ST110 includes system wide two thermowell protected platinum RTD temperature sensors
agency approvals for hazardous environments, and a rugged, in the process stream (Figure 4). One RTD is heated while the
NEMA 4X/IP67 rated 316 stainless steel enclosure. Approvals other senses the actual process temperature. The difference in
include SIL-1, ATEX, IECEx, FM and FMc. temperature between the two sensors is measured and is directly
Figure 3. VeriCal In-Situ Calibration System
Fluid Components International LLC g www.fluidcomponents.com g Page 3
Accurately Measuring LNG Tanker Boil-Off Gas
For Process Efficiency and Environmental Compliance
Steven Craig, Engineer, Fluid Components International LLC
proportional to the mass fow rate of the fuid. This technique
supports direct mass fow measurement and does not require
the addition of temperature and pressure sensors for mass fow
computations.
Conclusions
At this time, FCI fow meters are operating successfully in LNG
BOG applications aboard tankers and at land terminals. The ST110
fow meter’s accuracy, reliability, rugged design and certifcation
to global industry standards makes it ideal for the LNG industry.
In addition, thermal mass fow meters feature a unique no moving
parts design that requires virtually no maintenance with low
operating and lifecycle cost over a long life.
The combination of accurate performance, low maintenance
and long life offers distinct advantages over other fow
measurement technology solutions. With 50 years’ of experience
in the oil/gas industry, FCI’s thermal fow meters are designed
with this industry in mind and are especially well suited for its
rugged operational environment.
With the demand and production growing for natural gas
Fig 4. Thermal Dispersion Technology Principle of Operation
worldwide, FCI’s thermal mass fow meters are helping the oil/
gas industry and the shipping industry provide cleaner energy
solutions by helping to reduce the greenhouse gasses that are
a cause of global warming. FCI is committed to helping industry
protect the environment through more reliable monitoring of air/
gas quality for sustainable best industrial process practices. g
Fluid Components International LLC g www.fluidcomponents.com g Page 4
