During on-site welding of insulated pipes, the exposed polyurethane foam at the steel pipe ends is situated very close to the weld joint. Given the high temperature of the welding arc, the foam is susceptible to burning or carbonization if left unprotected, which directly compromises the quality of the joint insulation.
The most straightforward solution is to install shields on both sides of the welding area. One common method involves installing movable shields on either side of the weld point; these are manually rotated as welding progresses to ensure they continuously shield and protect the insulation layer. These shields can be made of metal and installed flush against the inner wall of the outer casing, effectively isolating the foam from the weld joint.
The welding sequence is also critical. It is generally recommended to complete the steel pipe butt weld before performing the insulation work at the joint. Welding generates concentrated heat near the joint; if insulation is applied prior to welding, the foam will inevitably shrink or carbonize due to the heat. By welding first and insulating later, the foam is not directly exposed to high temperatures.
Attention must also be paid to protecting the waterproof end caps during welding. These caps protect the foam at the pipe ends during transport and storage; if they are burned or dislodged during welding, rainwater and debris could enter the pipe ends. Protective measures include temporarily removing the caps before welding and reinstalling them afterward, or covering the pipe ends with asbestos cloth during the welding process.
Proper preparation of the weld bevel and control of welding parameters are equally important. Excessive welding current or duration generates more heat, increasing the impact on the surrounding foam. Adhering to standard welding parameters ensures weld quality while minimizing thermal damage to the foam.
After welding but before applying the joint insulation foam, the foam at the pipe ends must be inspected for damage. If scorching or carbonization is detected on the foam surface, the damaged areas must be cleaned away to ensure proper bonding between the new insulation foam and the existing foam. The condition of the foam at the pipe ends directly determines the quality of the insulation at the joint.