After thermal insulation pipes are buried underground, checking the sealing of joints, water ingress into the insulation layer, and leaks in the steel pipe are among the most difficult issues to assess during maintenance. By the time obvious signs are detected, steam often appears on the road surface, and customers complain, requiring emergency repairs overnight. The leak alarm line is designed to expose these potential problems early, while they are still manageable.
The working principle of the alarm line is not complicated. During the foaming process of the thermal insulation pipe, two conductors-usually a bare copper wire and a constantan wire-are embedded in the polyurethane foam layer, maintaining a certain distance from the working steel pipe, relying on the foam for insulation. Under normal dry conditions, the resistance between the two conductors and the steel pipe is very high. Once the outer protective pipe is damaged, allowing groundwater to seep in, or the working steel pipe cracks, causing leakage, water or moisture will damage the insulation properties of the foam, creating a conductive path between the conductors and the steel pipe, causing a sharp drop in resistance. The monitoring system continuously measures the resistance change, and when the value falls below a set threshold, it issues an alarm signal. According to the requirements of GB/T 29047 standard, the alarm wire must be continuous, not short-circuited to the steel pipe, and the distance between them should not be less than 10 mm.
So, is the alarm wire effective in actual engineering? Judging from some pipeline projects already in operation, the effect is clear. The installation of the alarm wire is carried out simultaneously with the insulation repair work. After the repair is completed, the tightness of the repair can be checked at any time using on-site testing instruments. Once an unqualified resistance value is detected, it indicates a sealing defect, and the construction party needs to rework immediately. This mechanism effectively constrains construction quality-to avoid rework, on-site personnel will pay more attention to every repair step. On a heating pipeline of approximately 25 kilometers in length, the leakage monitoring system has been operating continuously for multiple heating seasons, and the airtightness pass rate of the repair has remained high, with no insulation failure caused by water ingress into the repair.
The system can also be connected to a cloud monitoring platform. When the humidity inside the insulation layer exceeds the standard, or an internal leak occurs in the pipe, the platform will issue an early warning. Maintenance personnel can then use positioning equipment at the checkpoint in the alarm zone to determine the approximate area of the fault, and then precisely locate it within the section, scheduling a suitable time for targeted excavation and repair. This transforms the situation from "a problem has occurred, but we don't know where it is" to "we know where it is before we dig," avoiding blind road clearing and prolonged heating outages for emergency repairs.
Of course, the alarm line is not a panacea. The wire needs to be properly protected during laying and welding to prevent it from being burned by high temperatures or broken by being stepped on. Also, the alarm line can only detect existing seepage or leaks; it cannot provide direct information about the performance degradation caused by the slow aging of the insulation layer. In summary, the built-in leak alarm line is indeed effective-it cannot prevent leaks, but it allows leaks and water ingress to be detected and located immediately, significantly shortening fault response time. For newly built or renovated heating networks, this is a worthwhile basic investment to consider.