[Abstract] The characteristics, compensation types, advantages and disadvantages of the metal bellows expansion joints in direct-buried heating pipelines, their advantages and disadvantages, the setting steps, and the matters needing attention in design and use are introduced.

 

1 Introduction

Metal bellows expansion joint, also known as wave expander, is a kind of pipe compensation component widely used in the world in the past ten years. In recent years, this new type of component has begun to be widely used in different fields in China. The direct burying technology of heating pipelines is widely used due to its advanced and low construction costs. The compensation method and compensator for direct burying pipelines have also been developed. Corrugated expansion joints for direct buried pipeline compensation show obvious advantages over traditional "π" type compensators due to their easy installation, small size, small footprint, and large compensation amount.

The design and production of corrugated expansion joint products involve various disciplines such as materials science, mechanics, thermodynamics, etc., and it is a complex elastic mechanics problem. At present, there are many theoretical and empirical methods proposed abroad, and the differences between various methods are large, and the gap with actual experience is sometimes larger. This creates problems for product design. In fact, the design methods adopted by domestic manufacturers are not the same, so the design results are very different. In particular, the difference between corrugated expansion joints in directly buried heating pipes is even more dramatic. In order to avoid unnecessary waste and even cause engineering accidents, it is the purpose of this article to understand and master the characteristics, compensation types, setting procedures and precautions of the corrugated expansion joints of heating buried pipes.

2 Features of direct-buried corrugated expansion joints

Although the direct-buried corrugated expansion joints have the same function as ordinary corrugated expansion joints used in overhead and trenches, they are used to compensate for expansion of the pipeline caused by heat, and their materials are made of stainless steel. However, direct buried corrugated expansion joints must not be confused with ordinary corrugated expansion joints. Ordinary corrugated expansion joints cannot be immersed in sewage, nor can they be directly buried in soil. Because the ordinary corrugated expansion joint is made of stainless steel corrugated and carbon steel by hydrogen arc welding, when the chloride ion content immersed in sewage or directly buried in the soil exceeds-a certain amount, the stainless steel and carbon steel will be welded. Electrochemical corrosion occurs at the interface, which accelerates the damage of the weld joint, reduces the service life, and makes the corrugated expansion joint scrap. The direct-buried corrugated expansion joint adopts measures to isolate the expansion joint from the accumulated water, thereby achieving full direct burial, and the service life of the corrugated expansion joint and the pipeline is the same.

3 Types of Buried Expansion Joints

Directly buried corrugated expansion joints are divided according to compensation methods, and there are three main types.

3.1 One-time direct buried expansion joint

见图 The structure of the one-time direct buried expansion joint is shown in Figure 1. The working principle is that after the product is installed on the pipeline, the pipeline is heated to the required temperature, the pipeline is thermally extended, and the corrugated expansion joint is correspondingly compressed. In this state, the outer sleeve 4 is sealed and welded at the A ring seam to compensate The device becomes a rigid whole and no longer has the ability to compensate. In work, the pipeline is compensated by the tensile-compressive elastic deformation, so it is essentially buried directly without compensation for the pipeline. In this way, the corrugated expansion joint only compensates a part of the total deformation of the section of the pipeline under the design temperature difference. Its function only makes the equilibrium point of the tension and compression of the pipeline work at the total deformation under the design temperature difference. midpoint. Make the tensile and compressive stresses substantially equal, and reduce the tensile or compressive deformation stress of the pipeline.

This type of corrugated expansion joint generally has a small compensation amount, no life requirements, and no fatigue failure problems. The pipeline does not need a sliding support, and the cost is low. However, it is necessary to heat the pipeline and weld the girth weld at Compensator A during construction, which increases the difficulty of construction.

 

3.2 Freely compensated direct buried corrugated expansion joint

Free-compensated directly buried corrugated expansion joints are shown in Figure 2. This structure enables the bellows to achieve free telescopic compensation under the protection of the casing 4, and other performances are the same as those of non-directly buried corrugated expansion joints. Compared with one-time direct buried compensator, this type of compensator has less pipeline deformation stress, simple installation, and expansion joints with bending resistance, so the influence of pipeline sinking can not be considered. Disadvantages: Because it is buried directly underground, the compensator and pipeline are required to have the same life. The life is generally not less than 20 years; the volume is large and the cost is high.

 

3.3 Well-type corrugated expansion joint

The manhole-type corrugated expansion joint is shown in Figure 3. This corrugated expansion joint is freely compensated by the pipeline directly buried compensator installed in the well. Because the pipeline is buried in the ground and sinks with the soil layer, the axis of the pipeline deviates greatly from the axis of the corrugated expansion joint, which will seriously damage the corrugated expansion joint. Therefore, a sliding support should be added to the pipeline, the wall of the well should be the first sliding support, and then the sliding support 6 should be added. The bearing is 14D from the well wall. In this way, even if the soil layer subsides, it will not cause the pipeline to deviate from the corrugated expansion joint.

 

3.4 Characteristics of three types of corrugated expansion joints

Among the above three compensator forms, the one-time direct-buried corrugated expansion joint requires pipeline heating during installation and construction, and it must be equipped with a mobile boiler, which increases the difficulty of construction and affects its application and promotion. If the construction equipment is matched, the pipeline heating technology is solved, and the installation is simple, this compensation method will have great promotion significance.

Free compensation direct buried corrugated expansion joints have the characteristics of free compensation and simple installation on directly buried pipelines, so they are very popular with engineering designers. However, because the corrugated expansion joint is directly buried underground, the product structure and life must be reliable, the installation and construction must be correct, and the chloride ion content of the soil layer and groundwater must not exceed the standard. These aspects must be done first, otherwise the repair works in case of failure Large amounts and very troublesome. So choose the free compensation bellows carefully.

The well-type corrugated expansion joint is located in the well, and its working state is equivalent to the working state of the corrugated expansion joint installed in an overhead or trench. This aspect is mature in domestic engineering design, compensator products, and installation. Therefore, according to the current domestic situation, it is more realistic and reliable to use well-type corrugated expansion joints in directly buried pipelines. I recommend this type of compensation for directly buried pipelines.

 

4 Setting steps:

4.1 Pipeline segmentation No matter how complicated a pipeline can be, it can be divided into a number of relatively simple shapes of independent pipe sections by setting fixed supports, such as straight pipe sections, L-shaped pipe sections, Z-shaped pipe sections, etc .;

4.2 Determine the compensation amount According to the thermal expansion coefficient of the pipe material in the working temperature range and the length of the pipe section, calculate the working displacement of the divided pipe section, that is, between the two fixed supports;

4.3 Select the compensator Select the compensator according to the designed pipeline compensation method and the working temperature and pressure of the pipeline. Enable it to accommodate the calculated displacement of the pipe section;

4.4 Design of fixed support When the heating pipe is heated and extended, a certain thrust force will be applied to the fixed support. The thrust should be calculated during design to determine the section of the fixed support.

 

5 matters needing attention in the setting:

5.1 Due to the different compensation methods of direct-buried corrugated expansion joints, the production and processing, selection, and installation of the product are very different. Before choosing direct-buried corrugated expansion joints, you must first determine what compensation method to use. According to the requirements of the compensation method, relevant calculation, selection, installation, etc. are performed. Do not confuse all compensation methods.

5.2 The calculation of the compensation amount of the directly buried corrugated expansion joint is calculated according to the layout of the pipeline in the overhead or trench. In fact, the situation of the pipeline buried directly in the soil is different from that of the pipeline overhead or in the trench. Due to the direct burial of the pipeline, the friction between the pipeline and the backfill soil (sand) is increased, so that the force that should be carried by the two fixed supports is converted into one part applied to the fixed support and the other applied to the pipeline. Check whether the stress on the pipeline exceeds the allowable stress of the pipeline, especially at the bends of the pipeline. In addition, the compensation amount can also be selected by the lower limit.

5.3 From the structure of the directly buried corrugated expansion joints, it can be seen that most expansion joints are provided with guide sleeves. Its main role is to reduce the resistance of the flow channel of the medium. However, the shroud is directional, and if it is installed in the wrong direction, it will not serve its purpose. Therefore, attention must be paid to the direction of the guide sleeve. Especially when the corrugated expansion joint device of the water supply and return pipe is in the same position in parallel, the water supply and return water flow direction is opposite and it is easy to get the wrong direction.

5.4 The setting position of the corrugated expansion joint in the straight buried pipe shall be changed according to different compensation methods. Only one corrugated expansion joint is installed between the two fixed supports. For one-time directly buried corrugated expansion joints, it should be set in the middle of two fixed bearings; for free compensation, the buried corrugated expansion joint should be set close to one of the two fixed bearings, and away from the other fixed bearing (see figure 4). This is mainly considered from the external structure of the corrugated expansion joint itself, because the end (the guide sealing sleeve) far from the fixed support can freely expand and contract when heated, and the end near the fixed support increases the telescopic Resistance, making it impossible to compensate.

 

6 Conclusion:

Directly buried corrugated expansion joints have broad development prospects as a new type of pipeline compensation equipment. The author only gives a brief description of the characteristics, types and precautions of the direct buried corrugated expansion joints of various manufacturers. With the further development of directly buried corrugated expansion joints, it is expected that technical measures or specifications for the design, production and use of corrugated expansion joint products will be established.