Common Damage Factors Of Polyurethane Directly Buried Insulation Pipes

The circumferential stress generated by internal pressure on the pipe wall is the main stress. When the circumferential stress is too high, there will be infinite plastic flow on the polyurethane pipe wall, leading to pipe burst. For plastic flow, stress analysis should be performed. Due to the fact that the circumferential stress under internal pressure is the stress of polyurethane insulation film, the circumferential stress under internal pressure should not exceed the basic allowable stress. However, under the conditions of urban heat supply network, the circumferential stress under internal pressure should not exceed the basic allowable stress. Due to the fact that the circumferential stress under internal pressure is much smaller than its larger value, this failure mode generally does not occur.
2. Cyclic plastic deformation in pipelines is caused by displacement and force, while in directly buried heat pipes, temperature plays a decisive role. Polyurethane directly buried insulation pipes should have low thermal conductivity; When the temperature changes greatly and the thermal expansion deformation cannot be fully released, the pipe wall will undergo axial compressive plastic deformation due to axial compressive stress during the heating process. However, the axial tensile plastic deformation of the pipe wall is caused by the axial tensile stress during the cooling process, i.e. axial cyclic plastic damage. In the case of cyclic plastic failure, stability analysis should be conducted on the primary and secondary stresses to control the basic stress range of the composite stress range of the primary and secondary stresses not exceeding three times. This can ensure that thermal expansion deformation cannot be released, and when the circulating temperature difference is large, the working pressure is high, and the large diameter pipe cannot be released, the stable pipe is prone to cyclic plastic deformation. In the design of directly buried pipes, plastic deformation of polyurethane directly buried insulation pipes should be prevented.
3. Low cycle fatigue failure and stress concentration usually occur in bends, grooves, size heads, and folding angles of pipelines. During temperature changes, stress is concentrated on the peak stress generated by discontinuous pipeline structures. This will cause fatigue damage to the polyurethane directly buried insulation pipe. Due to the low frequency of temperature changes in fatigue analysis, fatigue analysis should be conducted within the peak stress range. According to the temperature variation law of urban heating networks, the main failure mode of directly buried heating networks is fatigue damage caused by basic allowable stress, elbows, tees, size heads, and bending angles. The peak stress control range should not exceed about 6 times.