09.10.2013
Storage tanks are, in addition to their function of storing liquids, usually used also for the measurement of the stored amount of liquid. The determination of the volume of liquid by means of level measurement and a corresponding geometric capacity table - often called a "dipping table" - is relatively well protected by numerous standards and regulations. Due to the thermal expansion caused by the function of temperature on this volume however, the re-evaluation at a corresponding reference temperature is needed for the determination of the actual amount of liquid, for which precise knowledge of the average fluid temperature in the tank is required. Temperature distribution and the behavior of liquids in large storage tanks under real conditions are not well-understood at this time. In-depth investigation results on the basis of corresponding methods for determining the average liquid temperature that can develop in a tank have been missing.
A total of eight interested partners - in addition to PTB, verification authorities, manufacturers of measuring instruments, tank farm operators and the Technical University of Hamburg-Harburg - have over a total period of two years engaged in scientific cooperation to investigate questions surrounding reliable temperature measurement in storage tanks. In addition to extensive experiments and mathematical simulations, objectives of the study included the development of recommendations for future necessary improvements to the relevant national and international regulations in this field, especially under the special aspects of legal metrology.For the experimental tests there was a tank available with a total capacity of 2440 m³ (diameter 14.9 m, height 14 m) on the site of the oil port of Hamburg, which was filled to a height of 8 m with water from the Elbe River. Two manufacturers sponsored a total of 13 temperature measuring chains that were distributed evenly in the tank. Thus, the temperature at 123 points could be measured within the tank interior in 10-minute intervals over the period from November 2011 to February 2013. Figure 1 shows an example of the temperature curves at distributed levels of a measurement chain from 00:00 to 24:00, for the period 27 to 30 January, 2012. In this time the average outdoor temperature decreased from -1.0 °C to -5.1 °C and the tank had to be heated subsequently, in order to avoid ice formation.
Figure 1: Temperature profile in the storage tank at different heights between the tank bottom (level 0 m) and 0.5 m below the liquid surface (level 7.5 m) from 27-30 January, 2012, between 00:00 and 24:00.
The measurement results were also used to verify appropriate mathematical simulations, by means of which the thermal behavior, for example of other liquids, other weather conditions, and special filling procedures, could be shown. PTB and Eichdirektion North - which contributed significantly to the project - organized an initial step to provide the results promptly to the interested public. The two-day seminar drew 70 participants from 10 countries and took place in Hamburg, Germany in September 2013.
Contact person:
Rüdiger Jost, Dept. 1.5, WG 1.51, e-mail: ruediger.jost@ptb.de