Five-layer structure steel-plastic composite pipe socket connection technology

1. Overview (1) Five-layer structure steel-plastic composite pipe. The five-layer structure steel-plastic composite pipe organically combines the advantages of high strength, strong pressure-bearing capacity, and low cost of steel pipes with the corrosion resistance and light weight of plastics. It is the latest generation of chemical building materials pipelines and is gradually becoming more and more popular with its superior performance. Promoted and applied in various industries such as urban construction, petroleum, chemical industry, mining, and pharmaceuticals. The five-layer structure steel-plastic composite pipe is made of butt-welded steel pipe as the middle layer, and the inner and outer layers are made of polyolefin plastic, using special hot-melt adhesive, and are compounded by extrusion molding, including the inner layer of plastic, special hot-melt adhesive, A new type of pipe with a five-layer structure including steel layer, special hot melt adhesive, and outer plastic layer. The five-layer structure steel-plastic composite pipe not only has the characteristics of high pressure bearing, good strength, high rigidity, impact resistance, and small thermal expansion coefficient of steel pipes, but also has the characteristics of corrosion resistance, light weight, clean and non-toxic fluid resistance of plastic pipes, and due to the use of plastic The pipe and the metal pipe are perfectly combined, and have some unique advantages such as high roundness, good air tightness, anti-penetration, low noise, good thermal insulation performance, both insulation and easy detection, and the best performance-price ratio. In the past few years, through the joint efforts and technical research of project development units and pipeline production enterprises, some key technical problems existing in the production and application of steel-plastic pipes have been continuously solved: such as the gaps between the layers of five-layer steel-plastic pipes. The bonding strength is much higher than the relevant standards, and the problem of metal layer exposure at the end of the steel-plastic pipe has been solved by adopting high-frequency heating and hot-melt sealing technology. A key difficulty currently existing is to develop a cheap, convenient and reliable pipeline connection method, which has been better solved through the flared connection developed by the pipeline manufacturer and the socket connection proposed in this article.

(2) Connection of steel-plastic composite pipes. Since the five-layer structural steel-plastic pipe uses thin-walled steel pipes as the pressure-bearing layer, the inner and outer plastic layers are anti-corrosion layers, and each layer is a thin-walled structure, ordinary connection methods are not applicable to steel-plastic pipes, so whether its connection technology is suitable Reliability, whether the pipe fittings are low-cost and easy to manufacture determine the success of the promotion of this new type of pipe. At present, there are mainly the following connection methods for medium-diameter steel-plastic composite pipes: The first is the flared connection. Use a special expander to expand the connecting end of the composite pipe, then insert the bushing equipped with a sealing ring into the expanded pipe end, and then use two expanded flanges or nuts to tighten the three. Together. According to different fastening forms, the flared connection can be divided into two types: flange type and nut type. The nut type is mainly used for the connection of small and medium diameter composite pipes, while the flange type is mainly used for the connection of medium and large diameter pipes. The flared connection is reliable, has good sealing performance, and has strong resistance to pull-out. The second type is a snap-on connection. First peel off the outer plastic at the end of a section of the composite pipe, then insert an open snap ring into the outside of the metal layer from which the outer plastic skin has been peeled off, then insert the inner liner with the sealing ring into the inner hole of the composite pipe, and finally Use two halves of Huff-type chucks to clamp the split snap ring inward, causing the split snap ring to shrink inward. The annular protrusion inside the split snap ring deforms the metal laminate at the end of the composite pipe to achieve connection. Through sealing ring to achieve sealing. This connection method is mainly used for connecting small and medium-diameter composite pipes. Large-diameter composite pipes are more difficult to deform due to their thickened steel layers.

(3) New connection method. More suitable for ground installation of steel plastic pipes. It is slightly insufficient in application in a large number of underground water supply pipe networks. In order to develop a simple and reliable connection method for medium-diameter five-layer structural steel-plastic composite pipes suitable for underground water supply pipe network applications, a large amount of project research work was carried out to understand the connection methods of various pipe materials, analyze, organize and summarize . And focused on the analysis of the socket connection method, which is the most commonly used connection method in urban water supply pipeline projects. It has many advantages such as convenient construction, reliable connection, durability, and resistance to leakage. Therefore, it is necessary to use this connection method for steel-plastic pipes. The feasibility and implementation methods were studied. Socket connections can be divided into socket hot-melt pipe fittings, socket electrofusion pipe fittings, socket adhesive pipe fittings and socket elastic sealing pipe fittings according to the method of achieving connection sealing. At present, various pipe materials such as cast iron pipes, cement pipes, rigid polyvinyl chloride (UPvc) pipes, polyethylene (PE) pipes, and random copolymer (PP-R) pipes all use socket-type connection methods. Among them, the socket connection of cast iron pipes and cement pipes is to add filler or rubber rings in the gap between the socket and the socket to make it dense and reach a certain strength to achieve the purpose of sealing the pressure medium. The socket connections of PE pipes and PP-R pipes mainly use socket hot melt and socket electrofusion welding. The pipe is inserted into the fitting and heated to weld the two together. The connection method used for buried UPVC pipes is a flexible connection with an elastic rubber seal, also called an R-R type interface, which is generally used for the connection of medium and large diameter buried pipes. It is to expand the annular sealing ring groove in the socket, put the special sealing ring into the groove, first chamfer the end of the pipe to be connected during installation, and then insert the pipe into the pipe fitting socket (small diameter pipes are inserted manually , large-diameter pipes can be connected using a special tightener to pull them in to a predetermined depth). Can steel-plastic composite pipes be connected by socket type? The socket connection method of cast iron pipes and cement pipes is not suitable for thin-walled composite pipes with PE outer layers; since the plastic layer of steel-plastic composite pipes mainly plays an anti-corrosion role, the wall The thickness is relatively thin, and the hot-melt socket connection cannot guarantee the strength after connection; using elastic rubber sealing rings is a theoretically feasible solution, and this connection method using flexible connections and rubber ring seals is technologically advanced and economically effective Obviously, it has decades of application experience abroad and has also been widely used in China. In order to explore the feasibility and reliability of this connection method for steel-plastic composite pipes, a hydrostatic sealing performance test of two pipe fittings was conducted.

Test 1: Use UPVC pipe fittings and steel-plastic composite pipes to conduct a hydrostatic sealing pressure test. It uses 0110 steel-plastic pipe, one end is sealed with UPVC flat seal, and the other end is sealed with UPVC straight-through and flat insert combination. When the pressure reaches 5.5MPa, the flat pipe fitting bursts. It uses 0110 steel-plastic pipe, one end is sealed with a flared pipe fitting, and the other end is sealed with a trPVC machining straight-through and flat insert combination. When the pressure was increased to 5.5MPa, the pressure dropped to 5.1MPa. When the pressure continued to be increased to 6MPa, the straight pipe burst.

Test 2: Due to the low strength and high brittleness of UPVC pipe fittings, we made homemade socket-type pipe fittings with metal structures and continued the test. After many tests, we achieved relatively satisfactory results. The test diagram is shown in Figure 1. Its connection method has many advantages:

① The structure is simple, each pipe fitting is composed of only one pipe fr-~K} sealing ring;

② The entire pipeline system becomes a flexible pipeline system, which effectively solves the problems of pipeline earthquake resistance, earthquake absorption, uneven settlement of structures and pipeline foundations, etc.;

③ Effectively solve the problem of thermal expansion and contraction of pipelines without adding additional expansion joints;

④ The price of pipe fittings is low, and the cost of socket-type pipe fittings is more than half the cost of other pipe fittings;

⑤ The end of the steel-plastic pipe is sealed with a special hot-melt sealing machine, which is simple to operate. The end of the steel-plastic pipe is heated and melted to form a reliable sealing surface to avoid corrosion of the steel layer opposite the medium and contamination of the medium;

⑥ Easy to install and low construction cost;

⑦ Less maintenance and simple maintenance methods;

⑧ There are corresponding standards and regulations that are easy to be accepted by users.

2. Socket fittings for five-layer structural steel-plastic composite pipes The structural steel cnc machining plastic  composite pipe is characterized by high pressure bearing and is not affected by temperature, so its connecting pipe fittings are also required to have the same performance.

(1) The form of direct connection of pipes in the socket-type connection of steel-plastic composite pipes can be divided into the composite pipe end forming method and the straight-through pipe connection method. The end forming method of the composite pipe is shown in Figure 2. The end of the composite pipe is expanded into a socket, the sealing ring is placed into the expanded socket sealing groove, and then another pipe is inserted into the socket. This structural type can save some straight-through pipe fittings, but the composite double O-shaped seal ring lip seal ring homemade socket pipe fittings tie rod connecting pipe central tool Figure 1 Test diagram During the test, insert the two connecting pipes (diameter 0110) respectively The other ends of the homemade socket pipe fittings with lip seals installed are inserted into the test fixtures equipped with double O-rings, and the positions of each component are fixed with tie rods. The test medium is water, the test environment temperature is 25 cI:, and the pressure is applied from one end of the clamp connected to the pressure test pump. The test results show that the socket connection can fully meet the fluid transportation requirements with nominal pressure below 2.5MPa and test pressure below 10MPa. The maximum working pressure of urban water supply and drainage pipelines generally does not exceed 1.6MPa. The pipeline hydraulic pressure test pressure of water supply pipelines is generally 1.5-2 times the nominal pressure of the pipe. According to the maximum nominal pressure of steel-plastic composite pipes of 2.5MPa, the test The pressure is generally 5MPa. The nominal pressure and test pressure of the steel-plastic composite pipe and the nominal pressure and test pressure of the socket connection can meet the above requirements.

(4) Advantages of socket-type connection for five-layer structural steel-plastic composite pipes. Under the condition that the engineering technical requirements are met, the socket-type connection is better than the steel-plastic pipe.