Managing boil off gas

Louise Smyth

As the world looks to tackle poor air quality and react to climate change, the growth in global demand for natural gas is set to continue. Offering up to 45% less CO2 per unit of energy than oil or coal and with plentiful reserves, natural gas is increasingly seen as an efficient, cost-effective energy source that helps meet the global energy challenge.

However, production of natural gas is often far from consumer markets and transferring over long distances by pipeline is not practical. Fortunately, natural gas is easily converted to liquefied natural gas (LNG) by cooling it to -160˚C where it reduces to 1/600th of its gas volume, allowing it to be easily stored and shipped. On arrival it is regasified with 1m³ of LNG vaporising to produce 600m³ of natural gas.

Located in the port of Bilbao, Spain, the Bahía de Bizkaia Gas regasification plant receives LNG from around the globe via bulk carriers and has a storage capacity of 450,000m³. The regasification plant features four vaporisers with a total send-out rate of 800,000Nm3/h following which natural gas is supplied for domestic, commercial and industrial consumption and the generation of electric power. LNG is also transloaded to trucks for delivery to satellite plants by road.

To measure LNG at -160˚C at up to 186t/h with CT accuracy and account for the resulting boil-off gas (BOG), BBG partnered with Krohne to deliver a complete cryogenic custody transfer system certified under the EU Measuring Instruments Directive 2014/32/EU to MID-005 (measuring systems for liquids other than water) with full ATEX hazardous area approval.

The system is based on Krohne’s twin bent tube 316L stainless steel Optimass 6400 high capacity Coriolis flowmeters with a minimum operating temperature of -200˚C. To ensure accuracy, two DN150 flowmeters are installed in series on the LNG line serving as the process measurement flowmeter and master meter.

The advanced Entrained Gas Management function of the flowmeters is used to control the transfer process by identifying the presence of BOG during start-up. The two-phase flow diagnostics signal puts the system into recirculation mode to reduce the temperature until only LNG is flowing. BOG is accurately measured by an Optimass 6400 DN100 flowmeter certified to MI-002 (gas meters) on a bypass line routed to a reliquefaction unit and then returned into the system as LNG.

Krohne also delivered a Summit 8800 digital CT flow computer to transfer all LNG and BOG flow measurement, pressure, temperature and other process-critical data to the DCS using Modbus RS-485.

From initial consultation to final system certification, Krohne worked closely with Bahía de Bizkaia Gas, acting as the project manager and main instrument provider. The MI-005-certified CT system was designed and commissioned by the company’s expert custody transfer team.

Juan Carlos Ballesta is with Krohne

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