Prüfung von Fernwärmerohren und Muffenverbindungen : Neues infrarotoptisches Prüfverfahren mit lokaler Wärmeimpulsanregung

In preinsulated bonded pipes the space between medium pipe and plastic jacket pipe is filled with PUR foam. The foam serves as thermal insulation and as mechanical connection between medium pipe and jacket pipe. An imperfect structure of the foam leads to weak mechanical connection and poor thermal insulation properties which cause energy losses. Especially, the sleeve joints are frequent leak sources in district heating (DH) network systems. The 3rd Division of the Bavarian Centre for Applied Energy Research (ZAE Bayem) in Erlangen/Germany developed a novel method suitable for non destructive quality control of DH pipelines during or immediately after production and of sleeve joints during construction. The method is based on the measurement of the heat transfer through the thermal insulation of the pipe and the sleeve joint respectively. For a short time the interior of the medium pipe is heated locally. Heat conduction occurs and depending on the thermal properties of the foam the jacket pipe heats up after certain delay time. Where the foam structure is imperfect the heat transfer is faster because the thermal insulation is worse there. The temperature distribution on the surface of the jacket pipe is measured by a thermal imaging system based on a staring infrared focal plane array detector. This detector resolves temperature differences of the order of 0.01 °C. Damages of the insulation are resolved as »hot spots« on the surface of the heat pipe. Numerical simulations and experimental tests show that the interior pipe has to be heated to a temperature level which is only approx. 15 °C above the surrounding temperature in order to detect damages at a size down to a few cubic centimetres. Infrared images having a detectable contrast are usually observed with some delay (up to a few minutes) after the heat pulse. Testing district heat pipes This novel method permits to test preinsulated bonded DH pipes during or immediately after the fabrication. At constant speed a heater runs through the pipe and heats up its interior and afterwards the jacket pipe temperature is measured with an IR camera. After a few minutes time defects in the insulation become visible as »hot spots« in the thermal image. The power of the heat source and their speed through the pipe are optimised in order to achieve a short measuring time and a high sensitivity for the detection of defects in the insulation. This method requires no harmful radiation sources (e.g. radioactive elements or x-ray tubes) and is applicable for a fast and economic quality control of 100 % of the fabricated pipes. The method can be varied for the quality control of modern DH pipe designs with two medium pipes within one jacket pipe. Testing sleeve joints With the most recent pipe laying techniques a few pipes are laid into the excavated pipe trench and welded together. The slip on sleeves are mounted using PUR mounting foam. Immediately after sleeve mounting the pipe trench is refilled and the pipeline construction is continued at one open end until the entire pipeline network is complete. During the pipeline construction tests of the sleeve joint insulation are performed