Equipment for the Induction Heating of Pipes

Welding girth joints of main pipelines and flow lines (especially those of high strength and high alloy steels) often require preheating the metal to a specific temperature and maintaining that temperature throughout the welding process. PIH’s systems use a tried and tested method of induction heating of both ends of the pipe before line-up and welding of the root pass.

In certain circumstances, especially in off-shore projects where automatic welding is being used, unforeseen events can disrupt the welding process on the laybarge. Such delays can lead to degradation of the partially welded joint. To prevent this degradation and embitterment of the weld joint, which can lead to negative NDT results, PIH developed a special “re-heat” induction heating system, which consists of a hinged coil, designed for the local heating of only the narrow zone immediately around the weld joint.

If hydrogen remains in the joint material after welding, cold or hydrogen cracking is likely to occur. Induction heating removes traces of hydrogen from the metal.

In addition, welding under high temperatures affects the structure of the metal which becomes more brittle. Induction heating allows the metal to be tempered by controlled heating and cooling in order to restore its proper grain structure.

Induction heating is also used on the pipe before coating application.

The difference between induction heating and resistance heating, is that in the latter contact is necessary between the heating element and the work. Also resistance heating takes place from the surface and as a result, part of the heat is lost on heating the environment.

Induction heating takes place without contact. In order for this to work, the material which is to be heated must be electrically conductive. The material to be heated is the placed in a high-frequency magnetic field. The inductive eddy currents in this field agitate the molecules in the work piece which leads them to generate heat. As the heating takes place from within, under the surface of the metal, the heat loss is minimal.

The depth of heating by induction methods is dependant on the frequency. Heating at low frequencies (tens of Hz) provides penetration deep into the work. The heating zone is at a depth of approximately 3mm which means that greater pipe wall thicknesses can be heated. The induction coil during this process remains cold.

induction sleeve on a pipe

Advantages of PIH’s induction heating system:

  • Much faster temperature rise, then when using gas or resistance heating;
  • The system’s efficiency is not effected by low ambient temperatures;
  • Complete control over the rate of temperature rise and ability to ensure that the temperature is within the specified range;
  • Accurate, uniform heating of the metal to the specified temperature through the required width and around the full circumference of the joint;
  • Accurate and localised heating;
  • Virtually no damage to the factory coating;
  • No hydrogen residue (which would decrease the service life of the joint) in the joint metal;
  • No naked flame or exposed elements;
  • No requirement for gas and therefore no need to store large numbers of gas cylinders;
  • designed with the requirements of contractors in mind, the equipment is durable and reliable whilst remaining light and easy to use in the field, providing a fast, accurate, reliable and economically effective means of carrying out the work.