Development of an accelerated test method for prediction of long term behaviour of thermal conductivity for preinsulated district heating pipes

The method is describe below. Method - 1. Celluar gas composition is measured in 4 pre-insulated pipes d/D 60.3/125. 2. The pipes are sealed at the ends so that there is no diffusion. 3. The pipes are aged at 90 deg. C. 4. Thermal conductivity is measured after 0, 7, 14, 28, 56, 112, 225, 440 and 550 days. 5. Cellular gas composition is measured. Result - 1. Thermal conductivity is plotted in a diagram as a function of time. 2. A line is drawn parallel to the abscissa cutting the graph in such a way that the area below the line and above the graph is equal to the area above the line and below the graph. 3. At the point where the line cuts the ordinate, the long-term insulating property is read. 4. The result is stated as the average of 4 measurements. 5. Gas composition before and after the measurement is reported. Preconditions - 1. Only CO2, O2 and N2 diffuse through the casing pipe. 2. The partial pressure of cyclopentane in the foam is not changed with time. 3. The activation energy for the permeability of O2 through HDPE with a density of 944 kg/m3 is 38J/mole. This results in an ageing factor of 20. 4. The activation energy for the permeability of CO2 and N2 corresponds to that of oxygen. 5. The activation energy is constant in the range 25 - 90 deg. C