TWI Industrial Member Report Summary 964/2010
By S Willis and M Troughton
Butt fusion welding of polyethylene (PE) gas and water distribution pipes, although producing joints of high integrity, can be time-consuming, with overall weld cycle times exceeding one hour for pipes of wall thickness greater than 50mm. This is mainly due to the specified long cooling times, which are typically 80% of the overall weld cycle time. Such long cooling times are specified to ensure that the material at the centre of the wall thickness has solidified before the pipes are removed from the clamps; however, this greatly limits the number of joints that can be made in a day.
Forced cooling of joints in PE pipes, which involves deliberately cooling the weld area during the fusion stage by, for example, blowing cold air on to the outer surface of the pipe at the joint, has generated significant industrial interest over the past few years due to the potential increase in productivity and associated reduction in operating costs (estimated at over £12 per metre of pipe laid for 1.2m diameter pipe), and commercial equipment incorporating forced cooling has recently come on to the market.
This report summarises available published data on the effect of forced cooling of welds in PE and similar fast-crystallising thermoplastic materials.
To review published literature on the effect of cooling time and forced cooling on the mechanical integrity of butt fusion welds in PE pipes.