Jackets on hot water pipe: the condensation problem
Outdoors, a hot water pipe can still condense on its surface. A jacket without water resistance loses performance from the first wet season.
GB/T 4272 sets requirements for insulation design, material performance and structure. For chemical industry water and drainage pipe, GB 50873 requires that the material, thickness and structure of the insulating layer be calculated rather than assumed.
Hot water pipe outdoors is a case where the environment, not the medium, decides the design. When ambient falls below the pipe surface temperature, condensation forms on the surface; a jacket that is not water resistant lets that moisture into the insulation, and conductivity climbs.
What the standard adds
GB 50873 requires anti-condensation construction to follow GB/T 11790, and requires the outer shell of the insulating layer to be sealed against water ingress. In practice that means a closed outer skin, sealed seams and a jacket that can be removed and refitted without breaking the seal.
Reference note. Figures and standard references are indicative and do not replace the project specification, the current edition of the standard, or product test reports for the specific supply.