Marshall, Brogden, Shepher
System Experience & Training # 1998 Gothenburg
Many early brittle failures in PVC water and sewerage pipelines, where repeated surge events are thought likely to induce embrittlement, were attributed to fatigue. The concerns of pipeline engineers led to the adoption of the British Standard Code of Practice CP 312 (ref. 1) by the UK water industry, which gives maximum and minimum pressure ratings for various pipe classes.
‘Surge and Fatigue’ are often combined as a collective term. However although both phenomena arise from the same events (valves closing quickly, pump shut down etc.) they should : be considered separately siice they describe different effects on the pipe material.
Rapid closure of a valve or a pump Starting up may give rise to a rapid increase in pressure to a level well in excess of the steady state condition. This is commonly known as surge. Surge ‘events’, generally occur over a very short time-scale and it is known that many PVCu pipe failures followed surges in pressure in pumped systems.
Fatigue is associated with the repetition of such events, where the fluctuatrons in pressure cause a loss in the long-term strength of the pipe material.
Because it was believed that the combined effect of over-pressurization from surge and loss in properties caused by fatigue could have serious implications on the lifetimes of PVCu pipelines, a design code was introduced. in 1977 (BS CP312). The-advice for design from both CP312 and the PMSM is that in a dynamic loading situation, the difference between maximum and minimum pressures must be less than 0.5*Static Rating. An additional criterion is that the maximum pressure must always be less than the pipe rating. In many pumping situations, the maximum pressure may rise to twice the steady state condition and drop to zero. This means that the stress range is twice the siatic case and hence via CP312 criteria the wall section must be increased by a factor of four. Increasing wall sections of pipes has clear cost implications, since the cost of plastics is directly proportional to the weight of material used. It is also well known that increasing wall thickness promotes the generation of a state of plane strain in the wall section and this will reduce resistance to crack growth under static loading.
The Water Industry adopted the CP312 code for PVCu in the PVC Guidance Manual and the Water Research Centre (WRc) extended the range of materials affected to include PE pipe materials in the Pipe Materials Selection Manual (PMSM). There was no service evidence to suggest that fatigue was a problem for PE; the criteria were adopted as a conservative measure awaiting experimental evidence to demonstrate whether the properties of PE are affected : by cyclic loading. Since there is also considerable evidence to suggest that the major problems with PVCu were associated with static overloading rather than fatigue, clarification of the U.K. design position was sought by the water utilities. Laboratory research on the fatigue of PVC and PE materials has thus been carried out by Pipeline Developments via a commission from UK. Water Industry Research Ltd. (UKWIR) to define the position for the range of tough polymers currently in use in the UK Water Industry.