Quality control of heat pipelines and sleeve joints by infrared measurements

Weak spots or damage areas in the thermal insulation of heat pipelines cause heat losses in district heating distribution systems. Especially, the sleeve joints are frequent leak sources. Traditional methods for testing the pipeline insulation require heating the whole heat pipeline after laying and are thus time consuming. In this paper we present a novel and simple method suitable for non-destructive quality control. The method may be applied during pipeline production. It also serves as a fast method for testing the sleeve joint directly after assembly, even in the case of a cold laying of the pipelines. The procedure only requires low-energy local heating in the test area which needs to be applied to the interior of the medium pipe. The method is based on a focal plane array thermal imaging system. The temperature distribution on the surface of the pipeline is imaged by the infrared camera after applying a heat pulse in the area under test. The camera used resolves temperature differences of the order of 10 mK, exhibits long-term calibration stability, and is robust for outdoor use. Damages of the insulation at a size down to a few cubic centimeters are resolved as 'hot spots' on the surface of the heat pipeline. The high sensitivity is achieved by dynamic measurements of the heat redistribution after applying the heat pulse. Images having an optimum contrast are usually observed with some delay (up to a few minutes) after the heat pulse. In the paper we will present numerical simulations of the leak detection and thus demonstrate the resolution. Examples of tests will be given.