INDIRECT METHODS FOR DETERMINING THE MASS OF LIQUEFIED NATURAL GAS IN CRYOGENIC TRANSPORTATION TANKS

Objective. Flow metering and the evaluation of cryogenic product losses at production and consumption facilities involve determining the mass of the vapor-liquid medium in the tank equipment.

Methods. The calculation of the mass of the cryogenic vapor-liquid medium is associated with the determination of the density of the vapor and liquid phases. The densities of media, in turn, depend on their component  composition, pressure, and temperature, which, in general, cannot be  determined reliably without direct measurements. However, for the estimation of mass in tanks, the state of the medium in which is accepted as equilibrium, the problem can be significantly simplified, and the missing information recovered based on indirect methods. This type of system includes cryogenic onboard fuel systems and, in particular, cryogenic fuel tanks, since the movement of the vehicle leads to mixing of the stored product, and an assumption can be made about the equilibrium of the  vapor-liquid medium. Similarly, the methodology can be extended to  transport tanks and multimodal reservoirs. 

Results. A review and comparison of the applied methods for calculating the density of the vapor-liquid medium and equilibrium compositions for hydrocarbon liquefied natural gas type mixtures in the cryogenic temperature range are presented. The existing methods for calculating the  density and phase composition of cryogenic multicomponent media are  complex in practical engineering and cannot be recommended for estimating the consumption of liquefied natural gas in the production tasks of fuel flow  metering and quantity control. An easy-to-use simplified calculation device  for determining the state of a vapor-liquid medium in cryogenic tanks based on approximation dependencies is proposed.

Conclusion. The simplified method is approximate in nature but is based on strict physical dependencies, and therefore does not lead to a significant increase in the error when varying the initial conditions. The liquefied natural gas composition, which can be obtained from the product data sheet, and the pressure of the vapor-liquid medium are transmitted to  the methodology as initial data. 

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