LNG density measurement by gravimetric method

Abstract

Density is a critical parameter in the liquefied natural gas (LNG) industry, which necessitates the study of measurement methods. Standard methods for calculating LNG density are employed in downstream LNG custody transfer, but it still faces challenges in density measurement in the upstream, such as in LNG plants and LNG refueling station. In this paper, a gravimetric density measurement method was established for LNG plants, which is SI-traceable as it relies on volume and mass. A home-made sampling device was developed and employed to obtain the LNG volume and mass. The principle involves trapping LNG in a known-volume quantitative tube within the sampling device. The LNG is then transferred to a vacuum sampling cylinder and weighed using a mass compactor. This approach not only provides representative samples but also enables compositional analysis while performing density calculations according to ISO 6578. Feasibility and practicality tests of this novel method were conducted in an LNG plant. The test results indicate that the reproducibility is 0.6 %(RSD), and the relative expanded uncertainty is 2.0 % (k = 2). Compared to the revised Klosek and McKinley method (RKM), it shows a relative difference of −0.3 % to 1.5 %, demonstrating the applicability to LNG plants.