Plastic Pipes Conference Association # 1995 Edinburgh
Marshall, Birch, Morley
Many UK Water Companies are now using PE pipe systems because it 1s believed that use of an all-welded pipe will minimise the number of mechanical fittings employed and therefore will reduce potential sources of leakage. With pipes such as ductile iron and PVCu, which are joined in short lengths, it is often postulated that the rubber seal within the joint may eventually become ineffective and will allow leaks to develop. There is little hard evidence that this is true and indeed, the welded PE system has experienced considerable leakage problems due to the failure of electrofusion joints. Such failures do not need a sensitive test procedure to discern leakage, the fitting failure is normally of such a magnitude that the pressure drops rapidly to zero instantaneously. There have been almost no failures of butt fusion welds. The leakage problems with PE have centred on poor tightening of flange connections where the gasket provides a poor seal.
With ductile iron the test procedure is regarded as being straightforward and simple. The main is charged with water and raised to the test pressure and a valve closed to seal the system.
After allowing the main to stand for at least lhr any pressure decrease is noted and the volume of water necessary to raise the main back to the test pressure is measured. The current UK specifications allow for an acceptable leak rate of 0.02litres/day/bar pressure/mm dia./km length.
This test procedure 1s simple, but the interpretation of results may be in error if air 1s contained in the system. Because water has a very high bulk modulus a small leak will reduce the pressure very quickly. Where air is present however, the pressure will drop very gradually since gases decompress very slowly and a significant leak may reduce the pressure by only a small amount. In the UK, there is no requirement to ensure that iron mains are free of air prior to test and it is believed that many mains are declared to be leak-free when test results are rendered insensitive by the presence or air.
The iron pressure test 1s completely inappropriate for either PE or PVC materials which will creep and extend in dimensions under constant stress conditions and will suffer stress relaxation under constant strain. Hence the pressure would naturally decay without leakage and it is thus not possible to use the simple iron test for plastics pipes, since molecular rearrangement within the pipe wall will confuse interpretation of results.
Because of the non-linear behaviour of plastics under stress, it has been necessary to develop new test procedures for the testing of MDPE and HPPE pipe systems and this was recognised in the production of the WRc MDPE Manual. New procedures attempting to account for the viscoelastic deformation behaviour were proposed. Although PVCu was excluded from the new approach and WRc have decided to adopt the iron procedure, there is no reason in principle why the Type 2 test should not apply. PVCu does deform in a viscoelastic manner and has time dependent properties, albeit at a less marked level than for PE.
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