Precast direct-buried insulated pipes consist of three layers: a working steel pipe, a polyurethane insulation layer, and an outer protective pipe. The strength of the bond between these three layers directly determines the pipe's lifespan. Shot blasting is the first step in ensuring a tight bond between the working steel pipe and the insulation layer. Shot blasting involves using high-speed shot to "grind" the surface of the steel pipe. Inside the shot blasting machine, steel shot is propelled at high speed onto the pipe surface, using impact and friction to remove contaminants such as rust, scale, rolled steel flakes, grease, dust, and moisture. Simultaneously, the high-speed shot creates uniform micro-pits on the pipe surface, known as "anchor patterns." A clean surface and rough texture allow the polyurethane foam to adhere more firmly during foaming.
There are clear industry standards for the surface treatment of steel pipes. The shot blasting rust removal grade should reach Sa2.5, and the anchor pattern depth should reach 50 to 90 micrometers. Insufficient anchor pattern depth will result in poor foam adhesion; excessive depth may weaken the steel pipe wall thickness, so it needs to be controlled within a reasonable range. After shot blasting, the subsequent foaming process should be carried out within 4 hours to prevent the treated steel pipe surface from becoming damp and rusting again.
Skipping the shot blasting process will directly impact the long-term operation of the pipeline. Failure to shot blast the working steel pipe is one of the main reasons for the insulation layer peeling off due to heat. Once the insulation layer and steel pipe separate, the pipeline loses its integrated structure, the soil's anchoring force cannot be effectively transferred to the steel pipe, and the pipeline is insufficiently constrained during thermal expansion and contraction, easily leading to localized deformation or even damaging the overall stability of the pipeline network. What appears to be an adhesion problem ultimately affects the safety of the entire pipeline.
Many insulated pipe manufacturers are equipped with fully automated shot blasting production lines to process steel pipes of different diameters and use professional instruments to test whether the anchor pattern depth meets the standards. However, some small manufacturers may not have shot blasting equipment at all, or even if they do, they may only perform the procedure without thoroughly removing rust and impurities from the steel pipe surface before proceeding to the next process. This process may seem basic, but whether it is done properly will become apparent after a few years of operation.