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ingapore continued to remain the world’s top bunkering port in 2020. The bunker sales reached 49.8 million t, an increase of 5% year-on-year.1 Introduction of Coriolis mass flow meters (MFMs) in the bunker transfer process has acted as a catalyst for this growth. Today there are more than 200 MFMs installed on board the bunker tankers that securely oversee close to US$19 billion worth of very low sulfur fuel oil bunker fuel transactions in Singapore.2 The margin of error here is razor thin; even a 0.5% error would amount to close to US$93 million in losses. With so much at stake and the bar raised with high standards set using MFM for technology excellence, something special was needed to maintain and safeguard this standard in a sustainable way. Although the mass flow metering system (MFM system) has demonstrated reliable performance over the years, local regulations and custody transfer requirements warranted that MFM be periodically re-calibrated to maintain its capability for bunker fuel transactions. In the absence of a local traceable calibration facility which covers the flow/fluid characteristics for the bunkering application, the MFM must be sent overseas for a re-calibration. This process is expensive and time consuming as it requires a MFM to be removed from the installation site and sent to the suitable calibration laboratory. To avoid financial burdens on the tanker operators and to ensure availability of bunker tankers, the local authority recommended meter verification, a metrologically acceptable pragmatic approach.3 It recommended periodic zero verification (ZV), starting with
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quarterly ZV in the first year followed by half yearly in the following year. After three years of use, the MFM is required to be calibrated by a recognised and traceable flow facility. Although this approach does not verify the meter’s performance in-situ at actual operating conditions, it ensures that the health of MFM is validated. However, the industry demanded a more holistic approach of validating the meter's performance to ensure continuity of bunker custody transfer operations. Given the challenges the application presented, meter verification using a master meter (MM) was the only viable option.
General requirements Generally, a custody transfer application demands stringent accuracy requirements. The MFM system for bunkering has to fulfil the overall measurement uncertainty (MU) of 0.5%, in accordance with SS 648. The MFM from this system must be water calibrated to +/- 0.1% permissible error, and not exceeding 0.2% MU of uncertainty claim for conversion from water to hydrocarbons fuel measurement. Comparatively for the MM, the requirements prescribed for use as the reference standard were more stringent and complex. These included requirements for bidirectional flow, linearity, stability, reproducibility, repeatability error, etc. The MM together with its associated devices are required to be calibrated in an IEC/ISO 17025 accredited calibration laboratory with water and oil of similar properties as bunker fuel. As a rule of thumb, the MM needs to be at least three times better than the MFM under test.
