A polyurethane insulation layer should consist of a uniform, dense foam body; however, cutting open a pipe sometimes reveals localized gaps-voids within the foam. The presence of voids severely compromises insulation performance and poses a risk of premature pipeline failure.
The primary cause of voids is an insufficient quantity of injected foaming material. The pipe-in-pipe manufacturing process involves injecting liquid polyurethane precursors into the annular space between the steel pipe and the outer casing; if the injection volume is inadequate, the foam cannot fill the entire space. Some manufacturers cut corners on material costs by underestimating the required injection volume, resulting in localized gaps. Poor flowability of the polyurethane mixture can also prevent even distribution during injection, leading to void formation.
Insufficient injection pressure is another cause of voids. Polyurethane foaming requires specific injection pressure to ensure the raw materials disperse evenly into every corner of the annular space. If the pressure is too low, the material fails to reach distant sections, potentially creating voids at those far ends. High-pressure foaming equipment in factories typically operates at injection pressures of 10–20 MPa, ensuring uniform material distribution. Conversely, manufacturers using rudimentary equipment often lack sufficient injection pressure, leading to inconsistent product quality.
Inadequate preheating of the steel pipe is also a contributing factor. Polyurethane foaming is an exothermic reaction that requires a certain amount of heat to sustain the process. If the steel pipe temperature is too low, the foam contracts rapidly upon contact with the cold surface, and the curing process slows down, making the formation of shrinkage voids likely. The risk of voids is significantly higher when foaming is performed in winter without preheating.
Voids can also easily form at the interface where two separate foaming stages meet. Ideally, the foaming process involves a single, continuous injection without interruption. If the injection is interrupted due to equipment failure or operational error, the seam between the two foaming stages may fail to bond perfectly, leaving a void. Once formed, these voids might be detected during factory inspections or slip through unnoticed. Purchasing from manufacturers with comprehensive inspection capabilities can help reduce the risk of void-affected products reaching the job site.