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How much liquid is there really in the tank?

Especially interesting for
  • the energy industry

Storage tanks such as those used for petroleum products and their derivates may have a capacity of more than 50 million litres. In such cases, temperature variations of a few tenths of a degree Celsius in the liquid can suffice to modify the volume by thousands of litres. When trading with large quantities, it is therefore very important to determine the mean temperature of the liquid inside the tank with great accuracy. Based on exhaustive experiments carried out on a real storage tank in combination with mathematical simulations, sound recommendations on how to determine the mean temperature can now be put forward for the first time.

A total of eight partners – apart from PTB and verification authorities, also manufacturers of measuring instruments, tank storage operators as well as Hamburg University of Technology (Technische Universität Hamburg-Harburg – TUHH) – dealt with the issues of reliable temperature measurement in storage tanks for petroleum products within the scope of a two-year scientific cooperation project. The investigations were aimed at elaborating recommendations to improve the relevant national and international regulations in this field, paying special attention to the aspects of legal metrology.

For the experimental investigations – which, for practical reasons, were carried out with water – a storage tank with a total capacity of 2440 m³ (14.9 m in diameter, 14 m in height) was available on the premises of the mineral oil port of Hamburg. Thirteen temperature measurement chains consisting of nine and ten, respectively, vertically arranged temperature sensors were installed with regular spacing inside the tank so that the current temperature could be measured at 123 points inside the tank. The experiments took place over a period of 16 months, and the measured data were retrieved every 10 minutes.

The exhaustive measurement data, together with sophisticated numerical simulations, make it possible to determine, for the first time, a realistic measurement uncertainty for the mean liquid temperature prevailing inside a large storage tank and to derive appropriate installation conditions for the required temperature sensors. This therefore allows a metrologically secured conversion of the liquid volumes determined under operating conditions to the reference temperature which is legally laid down and is 15 °C for the petroleum industry. The results of the measurements and of the numerical simulations also allow a transfer to other liquids, to other weather conditions and to special filling cycles with liquids of different temperatures, as encountered in practice with large storage tanks.