On small-diameter heat-transfer crude-oil or heating pipelines, a composite insulation layer is added to the outside of the pipe to reduce heat dissipation to the soil. The commonly used insulation material is rigid PU foam, with a temperature range of -185 to 120°C. The material is relatively soft. To increase its strength, a layer of HDPE is applied to the outside of the insulation layer to form a composite structure that prevents groundwater from penetrating the insulation.
High-pressure finished PU foaming equipment is used to shot-blast the outer surface of the steel pipe and corona-treat the inner surface of the outer jacket, improving the bonding performance of the insulated pipe. The insulation is rigid PU foam with a density of 60kg / m3 to 80kg / m3, filling the gap between the steel pipe and the jacket with a certain bond strength so that the steel pipe, outer jacket, and insulation form a solid whole. PU foam has good mechanical and thermal-insulation properties. It can normally withstand 120°C; through modification or combination with other insulation materials, it can withstand 180°C.
Prefabricated PU insulated steel pipe consists of three layers from the inside out:
Layer 1: Carrier Pipe Layer
According to design and customer requirements, seamless steel pipe, spiral steel pipe, and LSAW steel pipe are typically used. After advanced shot blasting for rust removal on the steel pipe surface, per GB / T8923-1988 the rust-removal grade of the steel pipe reaches Sa2, and per GB6060.5-88 the surface roughness can reach R = 12.5 μm.
Layer 2: PU Insulation Layer:
A high-pressure foaming machine is used to inject rigid PU foam raw materials into the cavity formed between the steel pipe and the outer jacket, commonly known as “pipe-in-pipe foaming.”
Layer 3: HDPE Protective Jacket:
It is preformed as a black or yellow polyethylene plastic pipe of a certain wall thickness. Polyethylene is odorless, non-toxic, and waxy, with excellent low-temperature resistance, and can be used at low temperatures of −70 to −100°C. It has good chemical stability and is resistant to most acids and alkalis (not resistant to oxidizing acids); it is insoluble in ordinary solvents at room temperature and has low water absorption, but because it is a linear molecule it can dissolve slowly in some organic solvents without swelling, and it has excellent electrical insulation properties. Its functions are, first, to protect the PU insulation layer from mechanical damage by hard objects, and second, to provide corrosion protection and waterproofing. HDPE foam insulated steel pipe is executed in accordance with SY/T114-2000 and SY/T115-2001 standards.
