Study On Destructional Factors Of Plastic Water-delivery Pipe Under Internal Negative Pressure

Pipelining irrigation network construction is the important development direction in the field of agricultural water-saving irrigation. Plastic pipe is widely used in pipeline construction. But a lot of research and practice show that plastic pipe resistance ability to resist water hammer positive pressure is much greater than the ability to resist water hammer negative pressure. Among actual water-delivery pipe damage cases, quite large amount are caused by negative pressure. PVC-U pipe are widely used in water-delivery for agriculture irrigation. In this paper, negative pressure destruction model test and numerical simulation of pipe under internal negative pressure are conducted to research the destruction factors and analyse the mechanism of destruction under negative pressure. On the base of research, theoretical foundation is provided to reasonably optimize pipe wall shape, thickness and stiffness, then do some theoretical help for pipelining irrigation network construction. Specific content is as follows:Using Electronic universal testing machine and Strain resistance type extensometer, mechanical property of PVC-U pipe are tested. Test results show that Elastic modulus of pipe is 846 MPa, Poisson’s ratio is 0.35. And based on the stress-strain curve of pipe tensile test, the pipe stress-strain relationship of pipe material is fitted out by Ramberg- Osgood model, strengthening coefficient of pipe material is concluded at 14.5. These results provide material parameters for numerical simulation and theoretical analysis of pipe destruction under internal negative pressure.PVC-U pipe negative pressure destruction test was conducted with the Negative pressure test system. For pipes with different initial elliptical defects, experimental study results show that the critical destructional value of negative pressure notably reduces when pipe ellipticity increases; Orthogonal test are conducted to different diameter-thick ratio and length of pipe, test results show that the critical destructional value of negative pressure notably reduces as the increasing of diameter-thickness ratio of the pipe; Constraint at the both ends of the pipeline has obvious influence on pipeline buckling pressure; For wall thickness of 3.2 mm, pipe ribbed at intervals of 1.4m can avoid negative pressure buckling when pipe is vacuum; For the thickness of 3.5 mm, pipe ribbed at intervals of 1.2 m. These results provide basis for pipeline design.Based on the Small deflection theory, linear buckling equation of pipe is deduced, then the critical destructional value of negative pressure is figured out. Compared with test results, theoretical value of linear buckling is notably greater. Inaccuracy is mainly caused by ignoring yield strength of pipe material For a long pipeline, based on the structure statics analysis method, take out a circular cross section of pipe for analysis, plane strain classical solution of pipe buckling under negative pressure is figured out. Compared with test results, theoretical value of linear buckling is slightly greater. However, the linear buckling critical value can be used as a failure load limit of pipe buckling under negative pressure, and provides reference for pipeline design.Based on the ANSYS software, liner buckling load and destruction mode are achieved from the numerical simulation analysis of pipe buckling deformation under the action of internal negative pressure. The developing process of stress and strain in pipe wall is analysed, results of numerical simulation show that the pipe wall is circumferential pressed to yield and vertical section rotates, then pipe buckling occurred. The increase of the bending stress of the pipe wall is an important reason for the increase of the circumferential compressive stress. The influence of different diameter thickness ratio, length and ellipticity is analysed to study the PVC-U pipeline buckling under negative pressure. Results show that the collapse pressure notably decreases when initial ellipticity increases under internal negative pressure; For short pipe, the constraints at both ends of the pipe have a significant impact on the collapse pressure of pipeline. Compared with results of negative pressure model test,the collapse pressure achieved from numerical simulation of nonliner buckling goes well with test value. The collapse pressure achieved from numerical simulation of nonliner buckling can well predict the critical collapse pressure of pipe buckling under negative pressure.