SICK Automation, a leading worldwide provider of industrial sensors, assisted Spain’s largest LNG provider, Enagas, detect and solve the measurement error in high boil-off gas (BOG) losses at its Huelva plant.

Significant turbulence, caused by an unfavourable piping layout, influenced the FLOWSIC600 ultrasonic gas flow meter, generating error measurements. Neither recalibration nor expansion were physically and economically viable, leading SICK to implement a low-cost correction function based on CFD (Computational Fluid Dynamics) analysis. Accurate BOG measurements of -120 °C have since been recorded by the FLOWSIC600

The challenges of supplying Liquified Natural Gas (LNG)

Liquefied natural gas (LNG) is considered one of the most interesting methods of supplying countries or industries with low-CO2 energy, and requires no politically sensitive cross-country pipelines. LNG is produced by cooling natural gas to below -161° C ( -258 °F), reducing it to one six-hundredth of its original volume. This makes LNG transferrable in tankers, anywhere, if cooling is kept constant. However, as cooling cannot always be kept consistent, evaporated or boil-off gas occasionally forms in tanks on land and on ships. This BOG is generally used as reserve energy or is burned off to prevent overpressure.

Spain relies on natural gas to provide 75% of its energy requirements, with local energy provider Enagas operating six LNG terminals in-country. LNG is delivered and stored in these terminals, transported by tankers, or re-gasified and fed into the company’s 11,000-kilometre-long network of pipes. If BOG occurs while in transit, the evaporated gas in tanks can be re-cooled economically, while largely avoiding burn off.

The SICK 2-path ultrasonic meter, FLOWSIC600, was first installed in the 40-inch boil-off-gas pipeline in the LNG terminal at Huelva, Spain. Such installation would 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 can operate at very low temperatures (down to -194 °C) without causing any pressure losses. After a few months’ use, gas losses (unaccounted-for-gas) of 0.18% were discovered in the plant’s overall balance. While this figure seems minimal, it is equivalent to a monetary loss of EUR 2 million, annually.

A solution based on computer simulation (CFD)

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

After identifying the cause of measurement error, SICK went on to develop potential solutions. The first option considered the device concept and, with the large pipe nominal size, incorporation of perturbations via calibration was made possible. Another option was to add further measurement paths, which required mechanical modifications. In the end, Enagas and SICK agreed on a CFD solution.

A computer simulation of the plant was produced to calculate the flow profile in the measurement section. Based on these calculations and analyses, a flowrate measurement error of up to 7.4% was revealed. The CFD analysis additionally helped reduce the influence of unintended installation effects.

SICK’s final step was the calculation of a correction function as based on the computer simulation. With a successful transition of the simulated correction function into the real device, SICK saved Enagas 0.1 % of LNG (equivalent to one million Euros per annum.

The FLOWSIC600 ultrasonic gas flow meter has been measuring the boil-off gas reliably at -120 °C for almost 10 years. Additionally, since 2017, BOG is no longer burnt but rather is compressed and sent into the gas transmission network.