What influence does environmental factors have on the performance of HDPE thread fitting

Temperature changes have a significant impact on the performance of HDPE thread fittings, especially under extreme temperature conditions. In high temperature environments (over 60°C), the thermal motion of the material's molecular chains is enhanced, resulting in a significant decrease in crystallinity. Experimental results show that the creep resistance of joints exposed continuously at 80°C decreases by more than 55% compared to that at room temperature. This thermal softening effect not only weakens the mechanical interlocking ability of the thread, but may also cause melt deformation. In a high-temperature medium transportation pipeline system of a petrochemical enterprise, thermal aging was confirmed to be the main cause of leakage accidents caused by joint failure. In contrast, low temperature environments bring the risk of brittle fracture. When the temperature drops to -20°C, the impact strength of HDPE material drops to 30% of that at room temperature, and a small stress concentration may induce crack propagation.

Erosion by chemical media is another important factor that leads to material performance degradation. In an industrial environment containing chloride ions, the chlorination reaction of HDPE molecular chains makes the material more fragile. When the chloride ion concentration exceeds 50ppm, the stress cracking resistance (ESCR) of the joint decreases at a rate that is three times that at room temperature and pressure. A coastal sewage treatment plant used ordinary HDPE threaded joints in the process of treating saline wastewater. After 18 months of operation, batch leakage occurred. The test results showed that pitting pits with a depth of 0.2mm formed on the inner wall of the joint. In addition, the pH changes in the soil environment should not be ignored. Acidic soil with a pH value below 5 can increase the material mass loss rate to 0.15%/year, far exceeding the 0.02%/year in a neutral environment.

Ultraviolet radiation is a key environmental factor that causes the performance degradation of outdoor exposed joints. When ultraviolet light with a wavelength of 290-400nm continues to act, oxidation products such as carbonyl and hydroxyl groups will form on the surface of the material. After 6 months of exposure, the impact strength may drop by up to 40%. In the scenario of overhead laying, this photooxidation effect is particularly obvious. In a leakage accident caused by aging of the joints in a water pipeline of a photovoltaic power station, ultraviolet aging was confirmed to be the main cause. The product of radiation intensity and action time (radiation dose) is the core parameter for evaluating the degree of material aging. When the cumulative dose exceeds 1500kJ/m², the surface of the material will show obvious powdering.

In addition, microbial corrosion also poses a potential threat under certain circumstances. Hydrogen sulfide produced by sulfate-reducing bacteria (SRB) under anaerobic conditions can react with HDPE molecular chains, resulting in significant degradation of material properties. Experimental results show that when the SRB concentration exceeds 10⁵CFU/mL, the impact strength of the joint decreases by 40% within three months. Organic acids produced by fungal metabolism can also accelerate the aging process of materials, especially in buried pipeline systems in humid environments, where biocorrosion is more significant. In a municipal drainage pipeline joint failure accident caused by microbial erosion, the biofilm thickness detection value reached 0.3mm.

The effect of the mechanical environment affects the performance of the joint through the stress transfer mechanism. During the operation of the pipeline system, pressure fluctuations (ΔP＞0.2MPa) will cause fatigue damage to the joint material. When the number of cycles exceeds 10⁵ times, the thread profile will show obvious wear. In addition, the lateral displacement caused by soil frost heave may cause buried joints to be subjected to shear stress exceeding the design value, which is particularly prominent in pipeline systems in some northern regions.