Refrigerated tanks base plate-heating hazards – A case study of Ethylene tanks

Abstract

Cryogenic storage tanks are commonly used for the storage of refrigerated liquids at sub-zero temperatures. Such tanks are often of double-wall construction with thermal insulating material in the annular space between the two tanks. These tanks commonly employ foundation heating (electrical or steam) to prevent freezing of the soil beneath the tank and the associated risk of frost heave. Frost heave occurs when water beneath the tank freezes and unevenly expands leading to structural damage. A case study presented in this paper is for a storage tank where liquid ethylene is contained at -104°C , in which the foundation heating system conduits were found to be severely corroded. This resulted in Class 1, Division 2 violation of Hazardous Area Classification Code of the company’s engineering standards and also potential for frost upheave and structural damage of ethylene tank. Thermal finite element analysis was used to estimate the temperature distribution beneath the tank and the potential for frost heave. In the analysis, the effect of parameters like soil conductivity, soil temperature, environmental temperature and insulation conductivity on soil temperature was evaluated. Sensitivity analysis was used to determine the critical parameters and field temperature measurements were used to quantify these parameters through best fit with analysis results.. Ground water table heights were measured at wells in the area and local historical data. Detailed analysis indicated that the heat transfer from ethylene tank to the soil did not result in sub-zero temperatures beneath the tank and would not cause frost heave. As a result, the foundation heating was discontinued utilizing Management of Change processes. In addition to compliance with Hazardous Area Classification, substantial heating costs were also saved as an outcome of this study. Details are reported in this paper.