Rapid welding technology for polyethylene gas pipes at temperatures below standard

Abstract

Rapid polyethylene pipes welding technology at temperatures below standard is proposed. This technology consists of preheating, equalization to acceptable temperature, reflow in regulated mode, and cooling under heat insulation layer or in heat-insulating chamber. Additional welding parameters, including heating and temperature equalization duration, heat-insulating layer thickness or heat-insulating chamber dimensions, are calculated from condition of ensuring heat process flow according to welding pattern at acceptable temperature. For mathematical model adequacy to actual heat welding process, it is proposed to consider polyethylene phase transformation heat in temperature range. When solving problem using end-to-end counting method, effective heat capacity coefficient is introduced, which is determined using temperature dependence of polymer material crystallinity degree. Crystallinity degree in welding process is determined by differential scanning calorimeter. Results of calculating cooling rates for polyethylene pipes electrofusion welding at various temperatures are given. Significant increase of cooling rate in heat-affected zone during welding at low temperatures, at which fine-crystalline structure is formed, which causes weld material plasticity, is shown. It is shown that when welding polyethylene pipes in low ambient temperatures, preheating, temperature equalization and cooling with thermal insulation with calculated parameters lead to thermal process in heat-affected zone according to welding patterns at acceptable temperature.