What are the failure mechanisms of PE pipe and how are they avoided?
Failure of any pipe system can occur when the strength, toughness or chemical resistance capabilities of the pipe are exceeded. The general performance of PE is comparable with, and frequently superior to, that of other pipe materials. Design procedures for PE pipe enable designers and specifiers to avoid failure mechanisms and to ensure an adequate factor of safety against each.
Strength; Strength is the ability of the pipe to withstand deformation. Such deformation can arise from, internal fluid pressure, ground loading, external water table etc. However for a buried PE pipe the initially imposed stresses are continually relaxing. PE pipe systems can normally resist ground movement and subsidence without a problem. An extreme example of this was the earthquake in Kobe, Japan. The PE gas and water systems survived and remained intact whilst multiple failures occurred in pipelines in other materials . Failure can occur when the design parameters are exceeded for example due to excessive loading or extreme temperatures.
Toughness; Toughness is the ability of the pipe to withstand fracture. Fractures can subsequently be divided into ‘slow crack growth' (SCG) and ‘rapid crack propagation' (RCP). Slow crack growth can occur if the pipe is subject to continuous bending forces as a result of, for example, ground movement. Rapid crack propagation theoretically results from a combination of over-pressurisation and, for example, third party impact damage. National and International standards include tests to determine the pipe's resistance to both of these fracture mechanisms. Design procedures are to be followed to ensure that fracture failure does not occur in normal operation. The exceptional track record of a much lower incidence of failures in PE systems underlines the durability and toughness of the material, and that the correct design procedures have been applied.
Chemical Resistance; Chemical resistance is the ability of the pipe to withstand the effects of chemicals, either being carried within the pipe or occurring in the adjacent ground, without reduction in performance characteristics. PE has excellent resistance to most chemicals. However some chemicals, notably organic solvents and oils, may have the effect of reducing the pipe strength through absorption into the pipe wall thus changing its characteristics. However such changes are normally reversible if the solvent or oil is allowed to evaporate away. If there is any doubt, or to ensure such a failure does not occur, a chemical analysis should be carried out and advice sought from the pipe manufacturer. The design can then be modified to take into account the possible effect of the chemical action.
ISO Technical Report 10358 includes an authoritative table identifying the resistance of PE, and other plastic materials, to a large range of chemicals.
ISO 13477:2008, Thermoplastics pipes for the conveyance of fluids - Determination of resistance to rapid crack propagation (RCP) - Small-scale steady-state test (S4 test)
ISO 13478:2007, Thermoplastics pipes for the conveyance of fluids - Determination of resistance to rapid crack propagation (RCP) - Full-scale test (FST)
ISO 13479,:2009Polyolefin pipes for the conveyance of fluids - Determination of the resistance to crack propagation - Test method for slow crack growth on notched pipes (notch test).
ISO 13480:1997, Polyethylene pipes - Resistance to slow crack growth - Cone test method
O'Rourke, T (1996) Lessons learned for Lifetime Engineering from Major Urban Earthquakes. Proc. 11th World Conference on Earthquake Engineering. Pergamon. Elsevier Ltd. Oxford, England. Paper no. 2172
ISO/TR10358:1993 Plastics pipes and fittings - Combined chemical resistance classification table
ISO 4433:1997 Thermoplastics pipes - Resistance to chemical fluids - Classification: Part 1: Immersion test method; Part 2: Polyolefin pipes