Polyurethane insulated direct-buried pipe has become a relatively mature advanced technology in some developed countries. In the past decade or so, China's heating engineering and technical personnel have promoted the development of domestic pipe network deployment technology to a higher level by digesting and absorbing this advanced technology. The results of more than ten years of practice have fully proved that the polyurethane embedding insulation directly buried pipe laying method has many advantages compared with the traditional trench and overhead laying. This is also the internal impetus for the rapid development of the Polyurethane Direct Insulation Pipe in China's heating engineering.

The advantages of polyurethane insulation directly buried pipe:

1, reduce the project cost. According to calculations by related departments, dual-control heat supply pipes can generally be reduced by 25% of the construction cost (using FRP as a protective layer) and 10% (using high-density polyethylene as a protective layer).

2, low heat loss, energy conservation. As the direct-buried pipe is made of polyurethane foam, the thermal conductivity is: λ=0.013-0.03kcal/m·h·oC, much lower than other commonly used pipe insulation materials, and the heat insulation effect is improved. 9 times. In addition, its water absorption is very low, about 0.2 kg/m2. The reason for the low water absorption is that the closed cell ratio of the polyurethane foam is as high as about 92%. Low thermal conductivity and low water absorption, coupled with high density polyethylene or FRP shells with good thermal insulation properties and good external water-repellent properties, have changed the status of the traditional trench laying heating pipeline “wet cotton padded jacket”, and greatly reduced the number of heating pipes. The overall heat loss, thermal network heat loss is 2%, less than the international standard of 10%.

3, anti-corrosion, good insulation properties, long service life. Directly-buried heat preservation pipes have a good anti-corrosion effect because the rigid polyurethane foam insulation layer is tightly bonded to the steel pipe outer skin, blocking the infiltration of air and water. At the same time, its foaming pores are closed and the water absorption is small. The high-density polyethylene shell and glass fiber reinforced plastic shell have good anti-corrosion, insulation and mechanical properties. Therefore, the outer skin of the working steel pipe is hardly affected by the outside air and water. As long as the internal water quality of the pipeline is well handled, according to the introduction of foreign materials, the service life of Polyurethane insulation pipe can reach more than 50 years, which is 3 to 4 times longer than the traditional trench laying and overhead laying.

4. It occupies less land, is quick to construct and is conducive to environmental protection. Directly buried heat pipes do not need to build huge trenches. They only need to insulate the underground heat insulation pipes, which greatly reduces the project's land occupation, reduces earthwork excavation volume by more than 50%, and reduces civil engineering masonry and concrete volume by 90%. At the same time, the insulation pipe processing and the trenching in situ are carried out in parallel. Only the on-site joints can shorten the construction period by more than 50%.

5. Polyurethane insulation prefabricated pipes produced abroad are equipped with leaking alarm lines. Once leakage occurs in certain places of the pipeline, through the conduction of the alarm lines, the exact position of the water leakage and water leakage of the thermal insulation pipe can be displayed on the dedicated detection instrument. And the degree of leakage, in order to inform the infiltration personnel to quickly deal with leaking pipe sections to ensure the safe operation of the heating pipe network. The domestic production of thermal insulation pipe is currently equipped with a leakage alarm line, which is yet to be filled. In short, the polyurethane insulation directly embedded pipe not only has the advanced technology and practical performance that are difficult to compare with traditional trenches and overhead laying pipelines, but also has significant social and economic benefits, and is also a powerful measure for heating and energy saving. The use of direct-buried heat supply pipeline technology indicates that the technical development of China's heat supply pipeline has entered a new starting point. With the further improvement and development of this advanced technology, it is imperative to replace the trenches and overhead with direct burying of heating pipes. Polyurethane Insulation Directly Buried Heat Pipes Design and Construction Issues for Attention Polyurethane Insulation Directly-buried Pipes are not only inconvenient in terms of energy conservation, cost reduction, shortening of construction period, and protection of the environment, but also the traditional trenches and overhead pipes. Technical and practical performance, but also has significant social and economic benefits, but a high-quality direct-buried heating pipeline project must also have a well-designed, reliable insulation pipe quality 1 well-built 3 conditions.

Since the direct-buried heating technology started relatively late in China, the above three conditions need to be continuously improved. From the perspective of quality problems in engineering practice, we should pay particular attention to the following issues in design and construction: First, in the design and construction, we must truly understand the direct burial method of heating pipelines is divided into compensated direct burial laying And with no compensation for direct burial and laying in two ways, it is true to master the working principles, characteristics and applications of the two methods, so that they can be reasonably selected in design and safe, reliable and economical in construction.

1. The concept must first be grasped: there is compensation for direct burial laying, and the pipeline is compensated by natural compensation and compensators (such as square and bellows compensators) to solve the thermal elongation of the pipeline so that the thermal stress is minimized; Buried installations, simply put, do not have any compensation measures when the pipeline is heated, but rely on the strength of the pipe itself to absorb thermal stress.

2. Basic principle of no compensation laying method: When installing pipelines, the pipeline is first heated to a certain temperature, then the pipeline is welded and fixed. When the pipeline is restored to the installation temperature (the temperature is lowered), the pipeline is previously subjected to a certain tensile stress. When the pipeline is hot working, with the increase of temperature, the pipeline stress is zero. When the temperature continues to rise, the pipeline compressive stress increases. When the temperature rises to the operating temperature, the pipeline's compressive stress (thermal stress) is still less than the allowable stress. In this way, the pipe can work normally without the compensation device. This kind of non-compensation method applies the fourth strength theory. The pipeline needs to be preheated during construction and the construction is troublesome. However, there are a lot of engineering practices at home and abroad, and the theoretical calculation is reliable and can ensure safety. Another non-compensation method is the calculation method and stress classification proposed by the Beijing Gas Thermal Design Institute in China in recent years. The stability analysis is used and the third strength theory is applied. This way to give full play to the plastic potential of steel, convenient construction, without preheating. 3. The depth of the two laying buries takes into account different factors.

First, when it is determined that there is a compensation and direct burial laying method, the depth of burial will only consider the stability of the pipeline due to the effect of the ground load, and it can be considered from the aspects of economy and construction convenience. When adopting the method of compensating for direct burial, it should be buried as shallow as possible. Generally, the thickness of the cover soil is more than 0.6 meters, and it is not related to the size of the pipe.

Second, when using uncompensated direct burial laying, the depth of burial shall take into account the stability requirements of the pipeline. The stability is mainly related to the thickness of the cover soil. Generally, it is buried deeper than the compensation. When the pipeline is laid without compensation for direct burial without compensation, The minimum soil cover depth shall be implemented in accordance with Article 7.2.15 of the “City Heating Network Design Specification” (CJJ34-90). The thickness of earth covering shall be proportional to the size of the pipe diameter. 4. Whether there is no compensation or laying with compensation in the design, the principle is that the straight pipeline is long and the middle branch is small. When the heating medium does not exceed 100°C, the non-compensation laying method should be preferred; otherwise, the compensation laying method should be considered. . The specific heat network main line should adopt the uncompensated laying method, while the branch courtyard pipe network should adopt the compensation laying method. However, some designers prefer to have the compensation laying and should promote the optimal design. 2. Before the construction, manufacturers must make investigations and researches on the production of prefabricated poly-amino-toluene insulation and direct-buried pipes. After entering the site, they must carry out serious inspections and refuse the use of unqualified insulation pipes. Third, in the construction of buried pipelines, welding is a key task to ensure project quality. 1. A welder who has obtained a certificate of conformity must be allowed to weld within the scope permitted by the certificate. A welder who does not have a certificate of qualification must not participate in the welding work. 2. When welding pipe joints, work pits should be well done, and the joints beveled and joint welding quality should be noted. Fourth, fixed brackets, construction quality of a variety of wells directly affect the project quality and the service life of the pipeline, such as poor well water, will make the parts damaged due to flooding. Therefore, serious construction should be carried out to ensure construction quality.

Fifth, we must pay attention to the pressure of direct-buried pipelines. Under the conditions of pressure suppression, the first step is to discharge the air and then divide it into two steps:

1. Strength test: After the pressure in the pipeline is increased to 1.5 times the working pressure, there is no leakage within 10 minutes of the pressure regulation.

2. Tightness test: When the pressure in the pipe is reduced to the working pressure, a 1 kg hammer is used to check the welds one by one around the weld. No leakage and no pressure drop of more than 0.2 atmospheres is acceptable after 30 minutes. . 3. The test pressure records should be made according to the specifications. Sixth, site joint insulation construction. This item is unique to the construction of direct-buried pipelines. The construction quality will directly affect the service life and must be given sufficient attention. Insulation pipe joint insulation on site shall be conducted only after qualified pressure test. There are two methods for insulation layer: on-site foaming construction and insulation tile construction. No matter which method is used for construction, ring space, cracking, delamination, and other defects cannot be protected. There are many layers of practice (such as high-density polyethylene and fiberglass protective layer), but all must ensure the integrity of the joint, tightness, waterproof. Seventh, the backfill soil should be tested in pipelines, joints, and completion measurements. After completion of sweeping, the backfill soils must be backfilled with prescribed thickness sand in accordance with the construction characteristics of the direct buried pipes. Do not cut corners.