LNG: SICK helps Enagas to reduce error in boil-off gas measuring

Spain’s largest LNG provider Enagas experienced unexpectedly high BOG boil-off gas losses (unaccounted for gas in energy balance) at its Huelva plant. This was due to significant turbulence caused by an unfavourable piping layout that influenced the FLOWSIC600 ultrasonic gas flow meter. Neither recalibration nor expansion were physically and economically viable. SICK found a low-cost alternative solution to the problem by implementing a correction function based on a CFD analysis. Since then, the device has been measuring the BOG accurately at  -120 °C.

Liquefied natural gas (LNG) is considered one of the most interesting ways of supplying countries or industries with low-CO2 energy – without requiring politically-sensitive cross-country pipelines. Liquefied natural gas is produced by cooling natural gas to below  -161 °C ( -258 °F). It is then reduced to one six-hundredth of its original volume and can be transported anywhere in tankers. However, as cooling cannot always be kept constant, evaporated or boil-off gas (BOG) occasionally forms in tanks on land and on ships. It is generally used as reserve energy or burned off to prevent overpressure.

Spain has relied on natural gas to provide 75 percent of its energy requirements for years. Spanish energy provider Enagas operates six LNG terminals in Spain alone. The LNG delivered is stored in these terminals and transported on by HGV, or regasified and fed into the company’s 11,000 kilometre-long network of pipes. Evaporated gas in tanks can be recooled economically, largely avoiding burn off.

The 2 path ultrasonic meter FLOWSIC600 by SICK was installed in the 40" boil-off-gas pipeline in the LNG terminal at Huelva, Spain, to determine the BOG volume as precisely as possible upstream of the flare burner. In principle, this was an ideal choice, as the device’s titanium transducer makes it perfect for use even at very low temperatures down to  -194 °C without causing any pressure losses. After a few months’ use, 0.18 % gas losses (unaccounted-for-gas) were found in the plant’s overall balance. While this figure sounds low, it is equivalent to a monetary loss of EUR 2 million – annually.

A solution based on computer simulation (CFD)

Enagas asked manufacturer SICK to help find the root cause. After an in-depth plant inspection and analysis of the installation conditions, significant turbulence was found in the flow where the FLOWSIC600 was installed. This was caused by two 90° pipe bends upstream of the inlet section. SICK was not aware of the application and piping design when selecting the flow meter. The significant turbulence influenced the measurement results of the 2 path measurement device, with a 40 inches nominal diameter.

After identifying the cause, it was time to develop potential solutions. The device concept and the large pipe nominal size made it impossible to incorporate the perturbations via calibration. Another option was to add further measurement paths, which would have required mechanical modifications. Ultimately, Enagas and SICK agreed on a solution based on CFD (Computational Fluid Dynamics). A computer simulation of the plant was produced to calculate the flow profile in the measurement section. Based on these calculations, the measurement error was deduced. Result: The analysis revealed a flowrate measurement error of up to 7.4%.

As the final step, a correction function was calculated based on the computer simulation. Transitioning the correction function to the real device was a total success. The correction reduced the balance loss by 0.1 % or an equivalent of one million Euros per annum. Enagas considered this level acceptable.

The FLOWSIC600 ultrasonic gas flow meter has now been measuring the boil-off gas reliably at  -120 °C for almost 10 years (since 2017, BOG is no longer burnt, and after being compressed is sent out to the gas transmission network). The CFD analysis helped reduce the influence of unintended installation effects.

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