36 Flow Level Pressure
Accurately Measuring LNG Tanker Boil-Off Gas for Process Effi ciency & Environmental Compliance
Steven Craig, Engineer, Fluid Components International 1755 La Costa Meadows Drive, San Marcos, California 92078 USA Tel:
760-744-6950
With the recent growth in demand and production of liquefi ed natural gas (LNG), there has been an increase in trade and the need to transport it globally via LNG tanker ships. This increase in LNG transport has driven the need for expansion of the tanker fl eets with increased capacity, wherein the LNG ships are typically carrying up to 266,000 m³ around the world. The LNG tanker fl eets are unique because LNG cargo generates waste gas, commonly known as boil-off gas (BOG). This waste gas can be used as fuel for the ships boilers and must be accounted for in order to comply with current marine propulsion regulations for energy effi ciency. Accurate fl ow measurement of the BOG to the ships boilers is essential to tanker operators.
Ship handling of LNG cargo and boil-off gas (BOG) Fig 1. LNG Tanker
In order to transport natural gas, it is economical to convert it to LNG. The conversion process involves cryogenically cooling the natural gas to -163°C at atmospheric pressure, at which point the gas condenses to a liquid and is ready for transport. The refrigerated tanks on board the ship ensure the liquid gas remains cryogenic during storage. BOG results when the LNG within the ships storage tanks vapourises due
to subsequent ambient heat input to the tanks during transport. In the past LNG tanker ships used an onboard re-liquefaction plant to recycle the LNG vapor back to liquid and into the storage tanks.
BOG problems
On January 1, 2013 the International Maritime Organisation (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 effi ciency for ships in order to reduce the CO2
emissions. Though
many ships have already implemented energy effi ciency 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, odourised, compressed and then distributed to the boilers which power the steam turbines used for the ships propulsion. The fl ow measurement
of the BOG to the ships boilers is a critical component because the IMO regulation requires low CO2 emissions, which means they must accurately measure the amount of BOG they are using in their boilers for energy effi ciency.
BOG measurement solution
The ST110 Series Flow Meter from Fluid Components International (FCI) meets the accuracy requirements for measuring BOG aboard tankers, at land terminals, storage facilities and points of distribution. This advanced air/gas fl ow meter combines powerful electronic features and an advanced fl ow sensor design to provide precision measurement, reliability and economy.
The ST110 Flow Meter (Fig 2) is designed with FCI’s unique VeriCal In-Situ Calibration Verifi cation System. Flow meters equipped with the VeriCal System (Fig 3) can perform periodic fi eld functional testing and calibration verifi cation of the fl ow meter’s measurement
performance without extracting the fl ow meter from the pipe or process to avoid shutting down the process for a lengthy period.
For BOG measurement, the ST110 Flow Meter with VeriCal features an internal purge tube that runs the length of the probe to the sensor and allows the operator to generate a known fl ow across the sensor element. The resultant signal output can then be compared to the factory baseline test certifi cate.
Fig 2. ST110 Air/Gas Flow Meter
The ST110 Series’ electronics can meet both current and future need for BOG measurement outputs, process information and communications. Whether the need is for 4-20 mA analog, frequency/pulse, alarm relays or digital bus communications such as HART, Fieldbus, Profi bus or Modbus, it provides a solution.
For LNG operators, the ST110 Flow Meter features a graphical, multivariable, backlit LCD display/ readout. 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 Flow Meter stores up to fi ve calibration groups to support a broad fl ow 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 and other process gases. The insertion style ST110 Flow Meter features a thermal fl ow sensing element that measures fl ow 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, the ST110 Flow Meter operates at up to 850ºF (454ºC) and is available with both integral and remote (up to 1000 feet [300 meters]) electronics versions. The ST110 includes system wide agency approvals for hazardous environments, and a rugged, NEMA 4X/IP67 rated 316 stainless steel enclosure. Approvals include SIL-1, ATEX, IECEx, FM and FMc.
Fig 3. VeriCal In-Situ Calibration System
AUGUST / SEPTEMBER •
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