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A comparison of static and dynamic mass measurement methodsof LNG utilizing weighbridge and coriolis mass flow meters

There is a lack of universal accepted reference methods for traceable dynamic metering of LNG quantity. The static method of weighing is well established and gives good results in controlled conditions. By comparing static (scale/weighbridge) and dynamic (mass flow meter) methods for the measurement of LNG mass this work gives an approach to this challenge.

The main elements utilized for the comparison consist of: LNG, a road tanker, a weighbridge and a coriolis flow meter (three meters used simultaneously for this test). In addition there is a production facility for LNG and a receiving terminal for unloading LNG from road tanker.

Coriolis meters:CMF1: Installed on road tankerCMF2: Installed at terminalCMF3: Installed at terminal

1. Gross weight of full LNG road tanker is measured on weighbridge (Mgross )2. The LNG is transferred from the road tanker to the LNG terminal and quantity is dynamically measured with three

independent coriolis mass flow meters (MCMF1, MCMF2, MCMF3 )3. Tare weight of the empty LNG road tanker is measured on weighbridge (Mtare )4. The net weight of LNG cargo (Mnet = Mgross – Mtare = full and empty weight difference of step 1. and 3.) is compared to the

coriolis mass flow meters indicated mass.

5. The relative measurement error of coriolis mass flow meter compared to weighbridge, ECMF , is calculated according

to the formula:

where n denotes coriolis meter number.

Introduction: Test procedure and calculation of result:

Haukås terminal:• Terminal is a LCNG (liquified-compressed natural gas) refuelling station for city buses in the Bergen area

• Has a 80 m3 LNG storage tank at terminal

• Three independent coriolis mass flow meters utilized for

the testing

Road tanker: • Gross weight 50 tonne, net weight 22 tonne (45 m3 LNG)

• Prominent isolated tanks

• Closed system, no flaring or boil off

• All tests with same road tanker and driver

• Tractor disconnected from semi-trailer during weighing

• “Dummy test” without LNG transfer to explore the effect

of potential buildup of road debris between weighings

Results:

A total of 5 reproduced comparisons of the weighbridge and each of the 3 coriolis mass flow meters were carried out.

LNG road tanker route in the Bergen area showing locations of production facility, weighbridge and recieving terminal.

Weighbridge:• 60 tonne capacity with 2 kg resolution

• 2 x 9 meter cast sections resting on 6 weight cells

• Calibrated and traceable to national primary standards

• Verified stability during test period (checked before and after testing)

• Good shield against the environment

• 21 km on road from transfer terminal Haukås

Conclusion:

Date: 20120905 20120910 20120918 20121008 20121015 Average Mean Absolute Deviation

ECMF1 1,83 % -0,27 % 0,74 % 0,26 % 0,79 % 0,67 % 0,54 %ECMF2 -0,18 % -0,1 1 % -0,23 % -0,08 % -0,19 % -0,16 % 0,05 %ECMF3 -0,12 % -0,08 % -0,1 9 % -0,03 % -0,13 % -0,1 1 % 0,05 %

ECMFn= MCMFn – Mnet

Mnet

x 100%

In this work it is shown that by doing careful measurements it is possible to achieve excellent results from the comparison of static and dynamic measurements methods for the quantity of LNG. The low value of mean absolute deviation of CMF2 and CMF3 indicates that the proposed method of testing has good reproducibility and that the testing has been performed in a metrological stable environment. CMF1 should be further investigated for erroneous operation. The method can be performed on-site with minimum to no interference with running operation.

Tore Mortensen and Henning Kolbjørnsen, Justervesenet, Fetveien 99, N-2007 Kjeller, NORWAY

This work is carried out as part of a so-called Joint Research Project (JRP) under the European Metrology Research Program (EMRP) that is jointly supported by the European Commission and the participating countries within the European Association of National Metrology Institutes (EURAMET e.V).