LNG Measurement Challenges

by | Aug 13, 2013 | Industry, Measurement Instrumentation, Oil & Gas | 0 comments

In an earlier post, Measuring Liquefied Natural Gas, we looked at the move to liquefied natural gas (LNG) as a fuel for marine vessels.

Emerson's Arjan Van GinkelEmerson’s Arjan van Ginkel gave me a peek at an unpublished whitepaper, Best practices for LNG measurement using Micro Motion Coriolis flow meters, which he is submitting and presenting at the Metrology for LNG Conference, which will take place October 17-18 in De Lindenhof in Delft, The Netherlands.

European regulations around minimizing sulphur hexafluoride in the marine industry is driving the need for cleaner-burning fuel sources. LNG is a prime candidate, since:

Natural gas is made free of carbon dioxide, sulphur hexafluoride and particles to enable easy liquefaction. In liquid form it is compressed 600 times in volume, which allows for easy transport. Cleaning of these greenhouse gases brings down the energy intensity, although due to its high compressibility, volume wise more energy can be transported.

LNG as a fuel source makes sense from an economic and technical standpoint:

With the abundance of LNG, the drive away from oil, and the clean status of the fuel, it is already proven as a suitable fuel for trucks and now the marine industry sees its clean future in LNG as well. Low sulphur fuels are too expensive (65% of the operating cost is fuel related), LNG is better for the engine, and scrubbers are costly and have not yet proven effective.

Measurement is the challenge since volume is a function of pressure and temperature. For marine vessels:

…pressure requirements may differ (tanks on trucks have a variety of tank pressures; 8-11 or 18 bar-a), temperatures shift as well and density with it.

Mass measurement provides a standard way to measure LNG. Another issue is that no global quality standard for LNG exists, and energy contents may differ. Arjan recommends that:

…it may be worth checking the energy content, the Gross Calorific Value (GCV). This can be measured from a sample of the natural gas obtained using a sample taker. A heat exchanger and then a gas density meter can supply this measurement.

Knowing the energy content is important for engine performance and longevity:

LNG typically contains propane, which helps ignite the fuel in the engine. If the methane number is too low, the propane may ignite early, causing the engine to knock. This will result in damage to an engine and knowing the methane number will help to avoid this.

Knowing the LNG gas composition from the purchaser or having a gas chromatograph to measure the components helps in knowing the optimum combustion control strategy.

Arjan highlights the requirements for the European Union [hyperlinks added]:

LNG-Custody-Transfer-Metering-SystemWithin the EU there are regulations for the transfer of fluids. These are captured within the Measuring Instruments Directive (MID). Companies in the EU and other joining countries are obliged to follow these regulations for installations and custody transfer points. Annex MI-005 applies for LNG – a liquid other than water. Within this annex there are five categories, each having its own accuracy requirements based on difficulty of measurement. LNG, and all other cryogenic liquids, fall into category 2.5. This means that the complete system must measure the fiscal transfer within 2.5% accuracy over the complete batch. The accuracy of the flow meter is required to be within 1.5%. Both the system components and the system itself need to comply with MID regulations.

Arjan explains the role of Coriolis flow measurement technology [hyperlinks added]:

Bunkering LNG can be performed very accurately when using Micro Motion Coriolis flow meters. Volumetric meters will need compensation for density and most current technologies are unable to get close to the same accuracy. Moreover, most volumetric meters need stable flows, making it difficult to provide the compact installation preferred when building a bunker facility.

He sums up how Coriolis measurement technology addresses the MID standards [hyperlink added]:

The Micro Motion ELITE series, thanks to its unique design and geometry, and the patented Young-modulus (for temperature correction), meets the requirements described within the MID. It also allows the best measuring accuracy on LNG in mass, density and volume, even to the lowest densities of 300kg/m3 and within the regulations of MID and OIML.

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