Experimental and Simulation Investigation on Failure Mechanism of a Polyethylene Elbow Liner Used in an Oilfield Environment

Internal collapse of a polyethylene (PE) liner in the steel elbow between furnace and manifold of a second-stage separator occurred during the operation of the crude oil treatment system in a certain factory. To analyze the causes and prevent such cases from happening again, macroscopic and microscopic observation, FTIR, density and hardness measurements, thermogravimetric analysis (TGA), tensile test, and thermal stress simulation were conducted. The results show that the PE liner was swelled by the conveying medium, resulting in a decrease in yield strength and module. Moreover, high operating temperature leads to softening and expansion of the swelled liner pipe. Finally, the stress caused by the PE liner’s inability to expand freely due to the external constraints of steel elbow increased dramatically and induced the liner pipe collapse inward, resulting in pipe blockage. Failure mechanism is further verified by computer simulation.