High-temperature prefabricated direct-buried heat preservation pipes offer significant advantages over traditional trench and overhead pipeline systems, both in terms of technical performance and practical application. These modern solutions bring substantial social and economic benefits, including energy conservation, cost reduction, shorter construction periods, and improved environmental protection. However, to ensure the success of a high-quality direct-buried heating pipeline project, three essential conditions must be met: proper design, reliable insulation materials, and skilled construction. In China, where direct-buried heating technology is relatively new, these factors require continuous improvement.
From the perspective of quality issues observed in real-world projects, special attention should be given to several key aspects during both the design and construction phases.
First, it's crucial to understand that direct burial can be implemented in two main ways: compensated direct burial and non-compensated direct burial. Each has its own principles, characteristics, and applications. Designers should choose the most suitable method based on specific project requirements, ensuring safety, reliability, and cost-effectiveness.
In compensated direct burial, the pipeline uses natural expansion or compensators (such as square or bellows types) to manage thermal elongation, reducing stress. In contrast, non-compensated direct burial involves preheating the pipe before installation, allowing it to expand and contract naturally without additional devices. This method relies on the fourth strength theory, requiring careful preheating and precise calculations. Another non-compensated approach, developed by the Beijing Gas Thermal Design Institute, uses stability analysis and the third strength theory, enabling easier construction without preheating.
The burial depth also varies depending on the method used. For compensated direct burial, the depth is mainly determined by economic and construction considerations, with a minimum cover soil thickness of 0.6 meters. For non-compensated methods, the depth depends more on the pipe diameter and the need for stability, often requiring deeper burial.
When designing, the general principle is to use non-compensated laying for long straight sections and compensated laying for areas with many branches. If the heating medium temperature is below 100°C, non-compensated laying is preferred. Otherwise, compensation should be considered. Main pipelines are typically designed with non-compensated methods, while branch networks use compensated ones. It’s important to promote optimal design rather than defaulting to compensative methods.
Before construction, it’s essential to thoroughly evaluate manufacturers of high-temperature prefabricated direct-buried thermal insulation pipes. Upon arrival at the site, all materials must be inspected carefully, and any substandard products should be rejected.
Welding is a critical part of the construction process. Only certified welders should perform the work within their authorized scope. Proper joint preparation, beveling, and welding quality are vital for ensuring long-term performance.
Fixed brackets and various types of wells significantly impact the project's quality and pipeline lifespan. Poorly constructed wells can lead to water damage and structural failure, so strict construction standards must be followed.
Pressure testing is another crucial step. The process involves first removing air from the system, then conducting two stages: a strength test at 1.5 times the working pressure for 10 minutes, and a tightness test at working pressure using a hammer to check for leaks and pressure drops.
Site joint insulation is unique to direct-buried pipelines and directly affects service life. After passing the pressure test, insulation should be applied using either on-site foaming or pre-fabricated insulation tiles. Regardless of the method, defects such as cracking or delamination must be avoided. Multiple layers, like high-density polyethylene and fiberglass, should be used to ensure waterproofing and integrity.
Backfilling must be done according to specifications, with sand used as required. Cutting corners in this process can compromise the entire system.
Finally, project acceptance is essential. Since buried heat pipes are largely hidden, thorough inspection and detailed documentation are necessary to ensure long-term performance and future maintenance.
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