Alferink, Blom, Venema, Wolters
# 1985 York
Joints in PE casings of district heating pipelines are made by extrusion welding, butt welding or sleeves in which sealing is obtained by an adhesive. To set up guidelines for making these joints a lot of research has been performed, part of which is described in this paper.
Joints of a good quality are obtained if extrusion and butt welding are performed under the prescribed conditions using good equipment. In some tests in which the effects of soil conditions on the joint are simulated, welded joints proved to be better than joints containing an adhesive.
District heating is a system by which the required heat for room heating and hot the generation of electrical energy, industrial processes, incineration of domestic waste, and central combustion of fuels. The heat is usually transported and distributed in steel carrier pipes at temperatures between 100 and 140 “Cc and pressures of 10-16 bar. To avoid excessive heat losses this steel pipe must be insulated. There are several ways to install the system either above ground or underground. In the Netherlands normally underground systems are used.
To ensure reliable distribution with a buried pipe system attention should be paid to protection of the insulation against mechanical damage and water ingress. This is often done by means of a PE casing pipe. In most of the systems polyurethane rigid foam (insulation) bonded to a PE casing pipe is used. The system is built up of prefabricated pipe lengths of 6, 12 or 16 metres each, and fittings like bends and branches.
For the straight pipe lengths an uninterrupted casing is used. But for the fittings the casing is mostly composed of several parts (segments) jointed by butt welding prefabricated in the factory.
To connect the prefabricated pipe sections and fittings steel welds are used for the carrier pipes, while the outer casing pipes are often jointed by means of sleeves and tightened by welding or using an adhesive.
By direct burial of the bonded steel - PUR - PE system, the soil friction is used to reduce the thermal expansion of the pipe system. Small parts of the system still move in the ground. To withstand these movements jointed casing pipes shall have sufficient strength to resist axial movements and water pressure.
To obtain a watertight and strong PE joint, butt welding is normally used for fittings while extrusion welding is used for the connection of the prefabricated parts.
This paper reports on research carried out to define the conditions by which acceptable butt welds in fittings and extrusion welds in joints are obtained.
In extrusion welding a distinction is made between fillet welds using a sleeve and V-shaped groove welds (Fig. 1).
The aim of the investigations performed was to set up a standardized procedure for making butt welds in fittings and extrusion welds in joints.