The calculation of the theoretical weight of plastic-coated spiral pipes needs to take into account the contributions of both the metal substrate and the plastic coating. The weight of the metal part can be calculated by the formula W = π×(D-t)×t×L×ρ, where D is the outer diameter (mm), t is the wall thickness (mm), L is the length (m), and ρ is the density of the steel (7.85 g/cm³). For example, for a spiral pipe of DN500×8 mm, the theoretical weight per meter is 3.14×(508 - 8)×8×7.85÷1000 = 98.7 kg 

The weight of the plastic coating depends on the coating process. Usually, electrostatic spraying or dip-coating processes are adopted, and the coating thickness is between 0.25 - 0.5 mm. The density of polyethylene material is about 0.94 g/cm³. Under the same specification, the weight of the coating is approximately 1.2 - 1.8% of that of the metal part. Although the proportion is not large, in large-diameter pipeline projects, when laying at the kilometer level, the cumulative weight difference of the coating can reach several tons.