UNDERGROUND POLYETHYLENE PIPE MATERIAL FOR FUEL GAS USE FEEDWATER HIGH DENSITY POLYETHYLENE PIPE MATERIAL FEEDWATER LOW DENSITY POLYETHYLENE PIPE MATERIAL FEEDWATER RIGID POLYVINYL CHLORIDE FUEL GAS PIPE MATERIAL UNDERGROUND FUEL GAS PVC PIPE MATERIAL DIRECT BURIAL HEAT PRESERVATION ANTISEPSIVE STEEL-PLASTIC COMPOSITE PIPE MATERIA FEEDWATER HARD POLYVINYL CHORIDE PIPE PIECE IRRIGATION HIGH DENSITY POLYETHYLENE PIPE MATERIAL