Polyurethane insulation direct buried pipes have become a relatively mature advanced technology in some developed countries. In the past ten years, China's heating engineering technicians have promoted the development of domestic pipe network technology to a higher level by digesting and absorbing this advanced technology. The practice results in more than ten years have fully proved that the laying method of polyamine cheese insulation direct buried pipe has many advantages compared with the traditional trench and overhead laying. This is also the intrinsic motivation for the rapid development of polyamine cheese insulation directly buried pipes in China's heating projects.
Advantages of polyurethane insulated direct buried pipe: 1. Reduce engineering cost. According to the calculation of the relevant departments, the double-regulated heating pipeline can generally reduce the construction cost by 25% (using FRP as the protective layer) and 10% (using high-density polyethylene as the protective layer). 2, low heat loss, saving energy. Since the direct buried pipe is insulated by polyurethane foam, the thermal conductivity is: λ=0.013-0.03kcal/m·h·oC, which is much lower than other commonly used pipe insulation materials, and the heat preservation effect is improved 4~ 9 times. Further, its water absorption rate is very low, about 0.2 kg/m2. The reason for the low water absorption rate is that the closed cell ratio of the polyurethane foam is as high as about 92%. The low thermal conductivity and low water absorption, together with the high-density polyethylene or FRP protective shell with good insulation and outer waterproof performance, have changed the condition of “wet cotton quilting†in the traditional trench laying heating pipeline, greatly reducing the heating pipeline. The overall heat loss, heat network heat loss is 2%, less than the international standard of 10%. 3. Anti-corrosion, good insulation performance and long service life. The direct-buried insulation pipe has a good anti-corrosion effect because the polyurethane rigid foam insulation layer is tightly bonded to the outer skin of the steel pipe to insulate the penetration of air and water. At the same time, its foaming holes are closed and the water absorption is small. The high-density polyethylene casing and the FRP casing have good corrosion resistance, insulation and mechanical properties. Therefore, the outer skin of the working steel pipe is hardly eroded by the outside air and water. As long as the internal water quality of the pipeline is well treated, according to foreign data, the service life of the polyurethane butter insulation pipeline can reach more than 50 years, which is 3 to 4 times higher than the traditional trench installation and overhead laying. 4. Less land occupation, faster construction and environmental protection. The direct buried heating pipeline does not need to build a huge trench. It only needs to bury the thermal insulation pipe underground, thus greatly reducing the project land occupation, reducing the excavation volume of the earth by about 50%, and reducing the civil construction and concrete volume by 90%. At the same time, the insulation pipe processing and the on-site trenching are carried out in parallel, and only the on-site joints can shorten the construction period by more than 50%. 5. Foreign production of polyamine cheese insulation prefabricated pipes, all of which are provided with leakage alarm lines. Once leakage occurs somewhere in the pipeline, the transmission line can be used to display the accurate position of water leakage and water leakage of the insulation pipe through the special detection instrument. And the degree of leakage, in order to inform the inspecting personnel to quickly deal with the leaking pipe section to ensure the safe operation of the heating pipe network. The domestically produced insulation pipe is currently equipped with a leak alarm line, which needs to be filled. In short, the polyurethane insulated direct-buried pipe not only has the advanced technology and practical performance that is difficult to compare with the traditional trench and overhead laying pipeline, but also has significant social and economic benefits, and is also a powerful measure for heating energy saving. The adoption of direct buried heating pipeline technology marks that the development of China's heating pipeline technology has entered a new starting point. With the further improvement and development of this advanced technology, it is imperative to directly replace the trench and overhead of the heating pipeline. Polyurethane insulation direct buried heating pipe design and construction should pay attention to the problem Polyurethane butter insulation direct buried pipe from the perspective of saving energy, reducing cost, shortening the construction period, protecting the environment, not only has the traditional trench and overhead laying pipeline is incomparable Technical and practical performance, and has significant social and economic benefits, but a high-quality direct-buried heating pipeline project must also have three conditions: reasonable design, reliable quality of insulated pipelines and careful construction.
Since the direct buried heating technology started late in China, the above three conditions still need to be continuously improved. From the perspective of quality problems in engineering practice, the following issues should be paid special attention to in design and construction:
First, in the design and construction, we must truly understand the direct laying method of the heating pipeline is divided into two modes: compensation direct burial and uncompensated direct burial. It is true that the two working methods, characteristics and application of the two methods are mastered. Occasionally, in order to make reasonable selection in design, construction is safe, reliable and economical. 1. First of all, we must master the concept: there is a compensation direct burial method, which is to solve the thermal elongation of the pipeline through the natural compensation and compensator of the pipeline (such as square and bellows compensator), so that the thermal stress is minimized; Buried laying, in simple terms, means that the pipe does not have any compensation measures when heated, but relies on the strength of the pipe itself to absorb thermal stress. 2. The basic principle of the uncompensated laying method: When installing the pipeline, first heat the pipeline to a certain temperature, and then weld the pipeline to fix. When the pipeline returns to the installation temperature (the temperature is lowered), the pipeline is subjected to a certain tensile stress in advance. When the pipeline is working hot, the pipeline stress is zero with the increase of temperature. When the temperature rises, the compressive stress of the pipeline increases. When the temperature rises to the working temperature, the compressive stress (thermal stress) of the pipeline is still less than the allowable pressure. stress. In this way, the pipeline can work without the compensation device. This kind of uncompensated method applies the fourth strength theory. It is necessary to preheat the pipeline during construction. The construction is troublesome, but 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 Beijing Gas Thermal Design Institute in China in recent years. The stability analysis is applied and the third strength theory is applied. This way gives full play to the plasticity potential of steel, and it is convenient to construct and does not require preheating. 3. The two laying depths consider different factors.
First, when it is determined that the compensated direct burial method is adopted, the burial depth is only considered because the ground load does not damage the stability of the pipeline, and is considered from the aspects of economy and construction convenience. When using the compensated direct burial method, it should be buried as shallow as possible. Generally, the thickness of the covering soil is greater than 0.6 m, and it has nothing to do with the diameter of the pipe.
Second, when the uncompensated direct burial method is adopted, the burial depth should consider the stability requirements of the pipeline. The stability is mainly related to the thickness of the overburden. Generally, it is buried deeper than the compensation. When the unburied uncompensated direct buried pipeline is used. The minimum covering depth shall be in accordance with Article 7.2.15 of the Code for Design of Urban Heating Network (CJJ34-90). The thickness of the covering soil shall be proportional to the diameter of the pipe. 4. Whether the design uses uncompensated laying or compensation laying method, the principle is that the straight pipe is long, the middle branch is less, and the heating medium does not exceed 100 °C, the compensation-free laying method should be preferred. Otherwise, compensation should be considered. Laying method. The specific heat network main line should adopt the uncompensated laying method, while the branch garden pipe network should adopt the compensation laying method. However, some designers prefer the compensation laying, and should promote the optimized design. 2. Before the construction, it is necessary to conduct research on the manufacturer of the prefabricated polyamine cheese insulation and direct buried pipe. After entering the site, the inspection shall be carried out carefully, and the unqualified insulation pipe shall be refused to be used.
Third, in the construction of direct buried pipelines, welding is a key task to ensure the quality of the project. 1. The welder who has obtained the certificate of conformity must be welded within the scope permitted by the certificate of conformity. The welder without the certificate of qualification must not participate in the welding construction. 2. When welding pipe joints, work pits should be done well, and the weld quality of joint bevels and joints should be paid attention to.
Fourth, the fixed bracket, the construction quality of various wells directly affects the quality of the project and the service life of the pipeline. If the well is not well waterproofed, the components will be damaged by water immersion. Therefore, construction should be carried out carefully to ensure the quality of construction.
5. It is necessary to pay attention to the pressure of the straight-buried pipe. Under the conditions of suppression, firstly, the water is irrigated and discharged, and then it is divided into two steps:
1. Strength test: After the pressure in the pipeline is raised to 1.5 times the working pressure, there is no leakage within 10 minutes of the voltage regulation. 2. Strictness test: When the pressure in the pipe is reduced to the working pressure, the weld bead is tapped one by one with a 1kg small hammer, and no leakage is observed for 30 minutes and the pressure drop does not exceed 0.2 atmospheres. . 3. Pressure test records should be prepared according to the specifications. Sixth, on-site joint insulation construction. This item is unique to the construction of direct buried pipelines. The quality of construction directly affects the service life and must be paid enough attention. Insulation pipe site joint insulation must be carried out after the test pressure is qualified. There are two methods of on-site foaming construction and thermal insulation tile construction. No matter which method is used, no annular space, cracking, delamination and other defects can be seen. There are many layers (such as high-density polyethylene and FRP protective layer), but the joint integrity, tightness and water resistance must be guaranteed. Seventh, the backfill should be carried out in the pipeline pressure test, joints, completion measurement, cleaning, and must be backfilled according to the characteristics of the direct buried pipe construction, do not cut corners.
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