What are the requirements for heat‑resistance performance of insulated pipes? Key Points for Material Selection of Municipal Thermal Pipe Networks

In urban centralized heating, industrial‑steam transmission and new‑energy thermal‑transmission pipe‑network projects, the heat‑resistance performance of insulated pipes directly affects pipe‑network operational safety, heat‑loss level and actual service life of pipes. Mismatched heat‑resistance parameters may easily cause shrinkage, ageing, deformation, cracking and peeling of insulation layers, resulting in substantial heat loss. In severe cases, damage to outer pipe jackets will be accelerated, creating potential safety hazards for underground pipe networks. Therefore, during the design, procurement and construction phases, heat‑resistance indicators of insulated pipes shall be specified in accordance with national standards, and material selection shall be conducted rationally based on the temperature of conveyed media.

The heat‑resistance performance of insulated pipes covers three core dimensions: long‑term continuous operating temperature, short‑term peak withstand temperature and thermal‑cycle stability. Heat‑resistance indicators vary greatly among different insulation materials. For municipal centralized‑heating hot‑water pipe networks, prefabricated directly‑buried polyurethane‑insulated pipes are widely used. In compliance with GB/T 29047‑2021, the long‑term continuous working temperature of polyurethane insulation layers shall not exceed 120 ℃, while the short‑term peak temperature can reach 130‑140 ℃, which suits hot‑water transmission conditions for most primary and secondary municipal heating networks. When the medium operating temperature approaches the upper temperature limit for a long time, the thermal‑ageing performance of insulation materials shall be reviewed to ensure no obvious shrinkage or pulverization of insulation layers during long‑term service.

For transmission of high‑temperature media such as steam and high‑temperature heat‑conducting oil, polyurethane materials can no longer satisfy working conditions. Composite insulation structures made of inorganic insulation materials such as rock wool and aluminium silicate shall be adopted. Rock‑wool‑based insulation materials can sustain long‑term service at approximately 600 ℃, whereas aluminium silicate materials are applicable for even higher‑temperature conditions and are mostly used for high‑temperature thermal pipelines in power plants and industrial plants. Specifications stipulate that the maximum allowable service temperature of insulation materials in direct contact with service steel pipes shall be higher than the temperature of internal pipe media with sufficient safety margin reserved, so as to prevent rapid material ageing under long‑term critical‑temperature exposure.

Apart from temperature‑resistance values, thermal‑ageing resistance and thermal‑cycle stability are also non‑negligible indicators. In actual operation, pipes undergo repeated heating‑and‑cooling alternations, namely thermal‑cycle working conditions. After multiple temperature cycles, insulation materials shall keep their thermal conductivity and volume‑shrinkage rate within permitted standard ranges without delamination, hollowing or cracking. Some projects only focus on static temperature‑resistance values while ignoring thermal‑cycle endurance. A few years after commissioning, insulation layers become damaged, pipe‑network heat loss keeps rising, and subsequent maintenance costs increase significantly.

Heat‑resistance of outer‑protection structures also deserves attention. The polyethylene outer jackets of buried insulated pipes feature lower heat‑resistance than internal insulation layers. Local over‑temperature may easily cause softening and deformation of outer jackets. Direct‑buried steel‑jacketed‑steel insulated pipes for steam adopt steel outer jackets to overcome insufficient temperature resistance of plastic outer casings, making them suitable for high‑temperature‑media scenarios. During design, the interface temperature of composite insulation layers shall not exceed 0.8 times the allowable service temperature of outer‑layer insulation materials. Multi‑layer insulation design realizes gradual temperature transition and protects outer structures from high‑temperature impacts.

Insulated pipes for centralized‑cooling pipe networks have different heat‑resistance requirements from heating‑supply pipes. They lay more emphasis on low‑temperature cold‑insulation performance yet still need to withstand high ambient temperatures. For buried cold‑insulated pipes, soil temperature rises in summer. Insulation materials shall resist high ambient temperatures while maintaining stable closed‑cell structures to avoid water‑vapor penetration and condensation‑induced failure.

In engineering applications, material selection shall not merely pursue higher temperature‑resistance parameters. Comprehensive evaluation shall be made combining actual medium temperature, operation mode and laying environment. Blind adoption of ultra‑high‑temperature insulation materials for hot‑water pipe networks will raise project investment; application of ordinary polyurethane‑insulated pipes for high‑temperature‑steam pipelines will lead to rapid damage. In early‑project stages, product test reports shall be checked for data on thermal‑ageing resistance and thermal‑cycle tests to confirm all indicators comply with current national standards.

To sum up, the heat‑resistance performance of insulated pipes is not a single temperature figure but a complete indicator system. Multiple indicators including long‑term service temperature, peak withstand temperature, thermal‑cycle stability, interface‑temperature control and outer‑jacket temperature‑resistance shall coordinate with one another. Only in this way can long‑term stable operation of thermal pipe networks be guaranteed, energy consumption reduced and subsequent operation‑and‑maintenance investment cut, laying a reliable foundation for urban heating and new‑energy thermal‑transmission projects.