Vessels, columns and storage tanks in the process industry are often filled with hazardous media under atmospheric conditions, e.g. fuels, naphtha or fertilizer. Due to filling, emptying and environmental conditions the tank content is exposed to thermal changes. An extraordinary problem occurs, when the tank is cooled down with an immense system temperature drop. During this process the pressure decreases and condensate is formed that produces vacuum inside the tank. To prevent vacuum in the tank, tank breathing valves must be sized appropriately. In the ARTEM project started in 2017, a new tank breathing model is being developed which takes into account the major physical influencing factors such as condensation on the tank wall, fog formation in the gas phase and internal flow phenomena.
In April and June 2019, unique measurements were carried out on a 200 m3 storage tank on the PROTEGO® Headquarter in Braunschweig to investigate vacuum inside of storage tanks more closely and for validation purpose of the model. These measurements are so exceptional as they provide an unprecedented wealth of information about the breathing process in storage tanks. A total of approximately 80 sensors have been installed in order to be able to measure variables in the tank, on the outside of the tank and in the environment of the tank. In the measurement, variables have also been recorded which have not yet been considered in detail in any previous publication, such as the rain film thickness when cooling the tank with water on the outside of the tank or the humidity in the tank.
In the tests, the approx. 20 m long and approx. 4 m high air-filled storage tank was heated by solar radiation or the overflow with hot water and then cooled down with cold water at a volume flow of 8 m3/h. Beside the temperatures in the gas, the wall and the environment also basic quantities were seized, like the volume flow of the breathed in air, the differential pressure in the tank and the water mass flow rate. A further special feature of the measurement was the recording of the test with a high-resolution thermal imaging camera, as well as with several common thermal imaging cameras, in order to obtain a comprehensive temperature profile over the entire outer tank surface.
The aim of the measurement was to obtain comprehensive information about the breathing process in order to create a validation basis for the ARTEM model. To obtain a deep understanding of the processes inside of storage tanks, a new venting model is currently under development at the CSE-Institute. With the new numerical model ARTEM (advanced reactor and storage tank emission model) all the complex location-dependent physical phenomena influencing tank breathing will be considered for transient tank breathing. With the ARTEM model the breathing volume flow for sizing of breathing valves on storage tank roof tops will be calculated and the model will be derived from a CFD model by a parameter study and sensitivity analysis. The CFD model and the ARTEM model will be validated against these unique measurements.
Currently, the measurement data are being analyzed at the CSE and prepared for publication. In addition, further measurement campaigns on a smaller scale but in a climatic chamber are planned in which measurements with condensable storage media will be performed.