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This post was last edited by yuchenchf on 2017-3-9 at 17:12. Practice has shown that the method of laying insulated pipes directly in the ground offers many advantages over traditional trench or overhead installation methods. However, attention still needs to be paid to many aspects in design and construction. I. Pipeline Layout The layout of directly buried heating pipelines shall comply with the relevant provisions of the current standard **«Code for Design of Urban Thermal Networks» (CJJ34)**. The horizontal or vertical clear distance between pipes and related facilities shall comply with the requirements specified in Table 3.1.1. Direct-buried heating pipes belong to pressurized systems; in their design, it is necessary to give priority to non-pressurized systems over pressurized ones, as well as to larger pipes over smaller ones. For sewage and rainwater pipes, attention must be paid to their slope and depth, and they should be placed away from these heating pipes. Pay attention to the slope: a vent valve well should be installed at the highest point, and a drain valve well at the lowest point. Valves are very important in pipelines, and it is essential to pay attention to their specifications and quality, as they play a crucial role in both transportation and maintenance. II. Compensation Methods 1. During design and construction, it is essential to truly understand that the direct burial method for heating pipelines consists of two types: compensated direct burial and uncompensated direct burial. It is important to grasp the working principles, characteristics, and applicable scenarios of each type, so as to make appropriate choices during design and ensure safe, reliable, and economical construction. 2. Understanding the concepts: The compensated direct burial installation method involves using natural compensation of the pipelines along with compensators such as square and bellows type compensators to address the thermal expansion of the pipes, thereby minimizing thermal stress. In contrast, the uncompensated direct burial installation means that no compensation measures are employed when the pipes are exposed to heat; instead, the strength of the pipe material itself is relied on to absorb the thermal stress. 3. Basic principle of the uncompensated laying method: During pipeline installation, the pipeline is first heated to a certain temperature and then welded in place; when the pipeline returns to its installation temperature (i.e., the temperature drops), it is subjected to a certain tensile stress in advance. When the pipe is under heating, as the temperature rises, the stress in the pipe becomes zero; as the temperature continues to increase, the compressive stress in the pipe rises. By the time the temperature reaches the operating temperature, the compressive stress (thermal stress) in the pipe is still less than the allowable stress. In this way, the pipeline can operate properly without a compensation device. When constructing in this uncompensated manner, it is necessary to preheat the pipes, which makes the process rather complicated. However, there are numerous engineering practices both domestically and internationally, and the theoretical calculations are reliable, ensuring safety. Another uncompensated method is to fully utilize the plastic potential of steel, which is convenient for construction and requires no preheating. III. Excessively high return water temperature: The circulation valve on the return pipe at the inlet device of hot water users is not closed or is not closed properly; in such cases, each inlet device should be checked to ensure that the circulation valves are properly closed. When the system’s thermal load is low, the circulation water volume is high, and the amount of heat supplied is large, it is necessary to adjust the main inlet and return water valves in order to increase the system resistance, thereby reducing the circulation flow rate. If the heating capacity of the boiler is too high and the consumption of the heating system is low, this results in excessively high temperatures of the supply and return water; in such cases, the upper limit for the supply water temperature should be controlled. When the water supply temperature reaches a certain value, corresponding measures are taken in the boiler room, such as shutting down the blowers and exhaust fans. IV. The return water temperature of the system is too low. The main reasons for such a low return water temperature are as follows: the boiler installed as the heat source is unable to supply sufficient heat, resulting in the supply water temperature not meeting the design requirements. At this point, the boiler should be modified or an additional one installed to increase the water supply temperature. If the flow rate of the circulation pump is low or its head is insufficient, the circulation of the heat medium in the system will be slow, and the temperature difference between the water supplied and the water returned will be large. In such cases, an appropriate circulation pump should be selected to replace the existing one. There is severe leakage in the outdoor piping network; the pressure in the boiler room drops too rapidly, and the amount of water required to feed the boiler far exceeds normal levels. In such cases, it is necessary to inspect the outdoor piping network to locate the leakage points and repair them promptly. The high heat loss in the external piping system can sometimes be the main reason for excessively low return water temperatures. Factors contributing to such high heat losses include poor quality insulation in the external piping system, partial or complete lack of insulation on certain pipes, as well as the use of insulation materials with poor performance. In addition, if the covers over the gutters are not installed properly, water from the ground can flow into the gutters, or leaks in the pipes within the gutters can result in an excessive amount of water there. This causes both the supply and return pipes to be submerged in water, turning the gutter into a large-scale heat exchange station. In such cases, it is necessary to improve the insulation and maintenance of the outdoor piping system, and to remove any accumulated water from the gutters promptly. The flow rate of water in the circulation system is too low; at this point, it is necessary to check whether the water pump is running in reverse, whether the pipelines, orifice plates, valves, etc. are blocked, or whether the valves are not fully open. The valves should be opened, and any dirt and sediment present in the system should be removed. V. Other faults and troubleshooting methods (I) Issues in construction and operation 1. The installation techniques for pipe joints and fittings are poor; they do not meet welding standards, and the welded joints have not been subjected to 100% flaw detection as required. 2. After the installation of the fittings and pipe joints, no anti-corrosion treatment was carried out; due to the poor construction conditions, dirt or soil accumulated at those areas and was not cleaned away; the insulation cover was not properly packaged before heat-shrink tape was used to proceed with the next steps. 3. The outer packaging of the interface was not installed in accordance with the standards. (1) The seal at the opening of the outer packaging was not tight; if there was dirt or mud on that opening, heat-shrink tape was used to seal it, which resulted in a poor seal and allowed air and water to leak in. (2) At the pipe fitting joints, insufficient insulation foam is injected, resulting in air remaining inside, or too much foam is used, causing it to overflow and leading to cracks in the seal and air leakage. (3) After insulating foam is injected into the joint, it is not sealed properly. 4. After the buried pipes and fittings are installed, qualified sand should be backfilled in accordance with the design requirements. Insufficient backfilling or sand of substandard quality, with impurities and foreign objects present in the sand; poor quality of the backfill soil, containing large foreign particles that puncture the insulation layer of the pipes, resulting in leaks and water intrusion. 5. The originally installed bellows (expansion joints) lack an outer insulation or anti-corrosion layer and are in direct contact with wet soil. The high groundwater level in the Tianjin area causes corrosion of the bellows. 6. Poor water quality in the pipes causes corrosion on their inner walls. There is a lack of methods for monitoring water quality and for treating it. 7. After the pipeline was taken out of service, it was not maintained in accordance with standards. (II) Quality issues with directly buried insulated piping fittings: 1. The quality of the plastic coating on the outside of directly buried insulated pipes is poor; in some cases, such coatings crack even before the pipes are buried, when exposed to direct sunlight. 2. The outer cover of directly-buried pipe fittings is not injection-molded as a single unit; instead, it is welded in segments at specific angles. If the quality of this welding is poor, if the welds are not secure, or if the welding materials are of inferior quality, this can lead to cracks, air leakage, and water intrusion. 3. The density of the polyurethane foam in the insulation pipes does not meet the standard requirements, resulting in the insulation pipes being unable to withstand external pressure. 4. The density of the polyurethane foam liquid inside the insulation pipe is too high; as a result, the insulation pipe fittings are prone to having their outer coating rupture when subjected to external high pressure. (III) Implementation of countermeasures to prevent failures 1. Strict quality management. In the construction of directly buried pipeline projects, it is essential to firmly adhere to the principle that \"quality comes first in long-term projects\"; strict quality control must be implemented at all stages, including design, construction, material procurement, and equipment manufacturing. 2. Implement bidding to ensure quality through the rule of law. For design, construction, material supply, and equipment manufacturing, the bidding system must be implemented to ensure quality through the rule of law. For losses caused by quality issues, compensation must be paid based on the cost of the materials, and in severe cases, legal action will be taken. 3. The project management system is implemented for project construction. Formulate a feasible on-site construction plan in accordance with the requirements of project construction standards and standardized management. A project schedule should be established at the construction site, and a system of technical responsibility for professional workers should be put in place. The construction log (mandatory for the project manager, on-site specialists, and project supervisors) should detail technical markings such as welds, insulation, joints, tees, elbows, bends, and obstacles, as well as the names of the construction workers, and these details should eventually be included in the completion documents. 4. Engineering construction supervision system. Engineering supervisors should be technical personnel with a high level of professional expertise, as well as possessing the necessary skills in the construction of directly buried pipeline projects. Supervisors shall exercise strict control and management over all aspects, including design, material selection, construction, completion, and commissioning. It is necessary to fulfill duties strictly; upon discovering quality issues or instances of non-compliance with design standards at the construction site, work must be halted immediately and a notice for rectification issued. The authority to make rulings on serious issues exists. 5. Protective measures must be taken for the construction of directly buried pipelines in special areas. Especially in areas where there is need to cross obstacles, ditches, or culverts, or in locations prone to heat and corrosion, effective protective measures should be taken, such as using sealed and waterproof pipe trenches. The external pressure exerted on the directly buried insulated pipes and fittings should not exceed the allowable load. 6. During operation, it is necessary to strengthen the monitoring of the water quality in buried heating pipes to prevent scaling and corrosion on the inner walls of the pipes due to poor water quality, thereby avoiding impacts on the pipeline’s lifespan as well as the safe and stable operation of the heating network. If the water quality is unsatisfactory, it can be treated by methods such as filtration or the addition of chemical reagents. 7. Clear markings should be provided at the sections where the heating pipes are buried directly in the road, to prevent damage to the pipes caused by other construction activities. Especially in special areas, where valve chambers, fittings, and pipes are at a low height above the ground, signs prohibiting the passage of heavy vehicles should be installed. (IV) Fluctuations in the water supply temperature, with large variations, can cause leaks in the radiators and pipe fittings due to thermal expansion and contraction. In such cases, appropriate measures should be taken to maintain a stable temperature of the water supplied to the boiler. The heights of the buildings vary greatly; in some of the buildings in the system, excessive pressure during operation causes damage to the cooling equipment and accessories, leading to leaks. In such cases, it is necessary to ask the technical department to install pressure regulators on the water supply pipes at the heating inlet devices of those buildings, based on the water pressure required for each building. For those already equipped with pressure regulators, the diameter of these regulators should be reselected. If possible, automatic pressure relief devices can be installed at the system inlet in the lower-rise buildings. With further advancements in science and technology, hot water heating technologies will continue to improve, and heating systems will become more sophisticated, thereby providing a comfortable environment for people’s work and living spaces, ensuring human health and promoting the modernization of our country. VI. Conclusion Practice has shown that the use of polyurethane and insulated directly-buried pipes offers many advantages over traditional trench and overhead installation methods. This is precisely the underlying driving force behind the rapid development of polyurethane-insulated direct-buried pipes in China’s heating projects. By paying attention to construction aspects, energy was saved, **increasing the economic efficiency of heating.