Pipelines

Measurement of Yield Stress for Crude Oils and Petroleum Products

The Yield Stress for mineral oils to be pumped in dependence on temperature is an important parameter for pipeline operation. Due to the extensive measurement procedure for this parameter, there are no systematic data available up to now.
Therefore, in a first step, a compact measuring system, based on an model pipeline, was developed. Measurements with tubes of different length and diameter showed no influence of the geometry of the measuring system on the yield stress and proved the results to be transferable.
The influence of thermal and mechanical sample preparation was studied. A measuring procedure was established, which guarantees, that the yield stress data are not influenced by sample alteration of sample history.
The temperature dependence of the yield stress was measured for 14 crude oils and crude oil blends. A significant variance was observed for the yield stress data and for the temperature dependence. For the pure crude oils studied, a correlation with the n-paraffin concentration was found. Oils with high n-paraffin concentraions showed a big impact of temperature variation on yield stress. For samples with lower nparaffin concentrations this effect is less distinct.
A comparison of yield stress data optained with. the model pipeline to data from a rotation viscosimeter showed comparable results for pure crude oils with low n-paraffin concentrations. Crude oils with high nparaffin concentraions showed significant differences between the two sets of data. No systematic behaviour was observed for the crude oil blends studied.