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Discussion on Several Problems in the Application of FRP Pipes
Announcer:admin Time:2022-03-22 11:18:22 Read:103page

In my country, steel pipes, cast iron pipes, cement pipes, plastic pipes, etc. are mainly used in water supply and drainage pipeline projects. As a new type of pipe material, FRP pipes have gradually become known to people in recent years. In recent years, with the introduction and development of FRP pipeline technology and equipment, a number of FRP pipeline production plants have been built in my country, and FRP pipelines have been used in a considerable number of water supply and drainage pipeline projects. FRP pipes have broad application prospects due to their outstanding characteristics, but in the process of use, due to insufficient understanding of the performance of FRP pipes and targeted solutions, the quality of the project is affected, which in turn affects the promotion and application of FRP pipes. . According to the problems encountered in the application of FRP pipes in recent years, the author discusses and proposes ways to solve the problems for reference in the design and construction of similar projects.
1. The main characteristics of glass steel pipe FRP is also called glass fiber reinforced thermosetting resin pipe. In our country, there are two types of FRP pipes available: fiber winding sand pipes and centrifugal casting sand pipes. The fiber winding sand pipe is made of glass fiber yarn, quartz sand and unsaturated polyester resin as the main raw materials, and is made by the fiber winding method. Centrifugal casting sand pipe is made of unsaturated polyester resin, chopped fiber and quartz sand as main raw materials by centrifugal casting method. FRP pipes have the following main characteristics:
1.1 Strong product applicability
Due to the characteristics of the FRP pipe forming process, it is easier to change the specifications and dimensions of the products. Under factory conditions, the diameter of the pipe products can reach 4m and the length up to 12m.
1.2 Easy installation and low cost There are various installation methods, which can be flange connection, socket connection, adhesive connection, and can be easily realized between FRP pipes, FRP pipes and valves or Connections between other accessories and between FRP pipes and pipes of other materials.
Because of its light weight, it is not necessary to use large lifting equipment during installation, only manual or light lifting equipment can be used. Because the pipeline is smaller than the factory, the number of joints is reduced, so the installation efficiency is improved and the cost is reduced.
1.3 Light weight
The specific gravity of FRP is generally 1.8g/cm3, the tensile strength is 245~294Mpa, the specific gravity is small, and the strength is high. Under the same internal pressure as the pipe diameter, the FRP per unit length The weight of the pipe is only 1/5 of the carbon steel pipe, 1/8 of the cast iron pipe, and about 1/2 of the prestressed reinforced concrete. The long-distance transportation and construction handling are relatively simple and the cost is relatively low.
FRP pipe products have good chemical stability to the atmosphere, water and medium concentration of acid, alkali, salt and other media, especially the corrosion of water to FRP is almost zero, and the corrosion resistance is good. There is no need to adopt strict internal and external coatings or cathodic protection like metal pipes, and basically no protection is required during the service life.
1.4 Good hydraulic properties
Glass steel pipe has a smooth inner surface, and the coefficient of friction of water flow is small. The roughness coefficient of FRP is only 0.0053~0.0084, while that of concrete pipe is 0.013~0.014, a difference of 55%~164%. Under the same flow and the same available water conservancy conditions, the pipe diameter can be reduced, thereby saving investment. Under the condition of equivalent flow and the same pipe diameter, the head can be saved and the energy consumption of operation can be reduced.
2. Common problems and analysis in the use process
Because FRP pipes have the above characteristics and advantages, they have been vigorously promoted and applied in my country, but in the process of promoting and applying FRP pipes, some Some installation and construction units and owners have reported some specific problems in the use of FRP pipes. If they are not handled properly, it will cause engineering quality accidents, or it will be time-consuming and laborious to handle, which will affect the overall social and economic benefits. The following analyzes several common problems in the use of FRP pipes and proposes ways to solve them.
2.1 Leakage caused by improper handling of rock pipe grooves
It is a common geological condition for pipeline burial to excavate trenches in rock. It is easy, a little carelessness will cause local uplift and local overload after backfilling, and individual rock tips will crack or even top through the glass fiber reinforced plastic pipeline, resulting in pipeline leakage.
This is because the hardness of FRP material is relatively small. When there are local uplifts and individual rock tips, the backfill of the pipe groove and other external loads will cause the force transfer of the entire pipe section to concentrate in one place and form local stress. Concentration leads to damage to FRP pipes. Solution: First, excavate trenches in rock formations to be repaired and leveled in strict accordance with the design elevation, without local uplifts or rock tips at the bottom of the trenches; second, first over-dig the trenches by 0.3~0.5m, and then use coarse sand strictly. Leveling according to the design elevation. 2.2 Leakage at the joint with the reinforced concrete pool wall Reinforced concrete pools are the most common structures in water supply and drainage projects, and it is natural that glass fiber reinforced plastic pipes are used as the connecting pipes between pools. In the design of reinforced concrete pools, the method of pre-embedded casing is often used to solve the contradiction between civil engineering construction and pipeline engineering installation. The design unit generally adopts the current national standard drawing S312 when designing the embedded casing of the pool. The gap between the inner wall of the casing and the outer wall of the FRP pipe is compacted and solidified by the filler, and the connection is completed. However, after the project is completed and put into production, leakage often occurs at the connection between the reinforced concrete pool wall and the FRP pipeline, which is not easy to solve. This is because the rigidity of the wall of the reinforced concrete pool is large, while the rigidity of the inserted FRP pipe is relatively small. When the FRP pipe on the outer wall of the pool is deformed in the circumferential direction under the action of external force, it is easy to separate from the filler and cause leakage.
The solution: one is to use a short steel flange pipe to insert into the embedded casing of the pool wall, and the end of the glass steel pipe is flanged and then connected to the short pipe; the other is to directly use a short steel flange pipe to add After welding the water-stop ring, it is pre-buried in the reinforced concrete pool wall, and the end of the glass steel pipe is flanged and then connected with the short pipe.
2.3 Leakage caused by improper backfill material of pipe trench After the pipeline is installed and passed the pressure test and acceptance, the pipe trench needs to be backfilled to restore the rest of the ground. When it began to use mechanical equipment to roll, individual stones were inserted into the glass fiber reinforced plastic pipe at acute angles, causing leakage. The reason is also due to the small hardness of the FRP material. When the sharp angle of the stone is facing the FRP pipe and the back of the stone is against the rock wall of the pipe trench, the FRP pipe is easily crushed by the stone when the stone is rolled and squeezed down. The block is pierced at an acute angle. FRP pipes are different from steel pipes, cast iron pipes or reinforced concrete pipes in that they are hard enough not to be pierced by sharp angles of stones.
The solution: First, when encountering rock trenches, soil should be replaced and backfilled in the trenches below 0.5m above the top of the pipe. The second is to prevent foreign boulders from rolling into the pipe trench or hard stones mixed in the backfill when encountering a pipe trench with a general hard soil foundation.
2.4 Leakage at the joint with the steel pipe
FRP pipes are often connected with steel pipes or cast iron pipes in engineering practice. Leakage at the joints is a common phenomenon, and maintenance is also difficult to be effective. There are three ways to connect FRP pipes and steel pipes: FRP pipe sockets and steel pipe sockets, FRP pipe sockets and steel pipe sockets, and flange connections. According to the investigation, there are very few excuses for the connection method of flange connection and the connection method of FRP pipe socket and steel pipe socket. The stiffness of the pipes is different. Generally, the stiffness of glass fiber reinforced plastic pipes under normal pressure is SN=5000 or 10000, while the stiffness of steel pipe or cast iron pipe socket is much larger than this value. The tube is deformed in the circumferential direction, and there is a gap at the interface and water seeps.
The solution: First, use the flange connection method and the connection between the FRP pipe socket and the steel pipe socket; The 180° plain concrete pipe foundation is used within the range of 2m, and the lower part of the pipe of the remaining pipe sections should be backfilled densely, and compacted to the waist of the pipe in layers to prevent the FRP pipe from being deformed in the circumferential direction under the action of external force. The third is to use glass fiber reinforced plastic pipe products with a stiffness of SN ≥ 10000 as much as possible to reduce the possibility of leakage caused by the deformation of the glass fiber reinforced plastic pipe caused by external force.
2.5 Leakage caused by the defects of the FRP pipeline itself FRP pipe is a new type of pipe with many advantages, but in actual engineering, it is often encountered by the owner and construction unit that the quality of the FRP pipe is defective. The wall appears sweating; when cutting, it is easy to see that the structural layer is not dense and the pipe wall is loose; when laying in the open air, the surface of the pipe is prone to roughness, fading and peeling.
At present, there are two kinds of FRP pipes produced by centrifugal casting sand pipe and filament winding sand pipe on the market in my country. One of the main reasons for these phenomena is that the thickness of the isolation layer of the fiber wound sanding pipe is relatively thin, usually less than 0.5mm, and the gas cannot be discharged under mass production conditions. Sweat appeared on the pipe wall during the pressure test; second, the pipe wall of the filament wound sanding pipe is a multi-layer sand core structure, when the pipe is deformed in the circumferential direction and the pipe wall is subjected to bending force, delamination is easy to occur. The higher the sand content, the greater the delamination trend. Once the pipeline is delaminated, it means that the structural strength of the tube has been damaged, and it is not suitable for continued use as a pressure-bearing pipeline; third, in the production process of quartz sand It falls in the form of free fall and is wound between the fiber layers. The amount of sand added in this process is uneven, the pipe wall is loose, and it is easy to cause delamination. Fourth, the FRP pipe is installed in the open air, and it is exposed to ultraviolet rays for a long time. Rough, faded, peeling.
The solution: First, use underground laying as much as possible to avoid laying FRP pipes in the open air, or take appropriate measures to prevent FRP pipes from being directly irradiated by ultraviolet rays. The second is to use centrifugal casting sand pipe. Because the wall of the centrifugal casting sanding pipe is usually a multi-layer structure, layering designs with different functions such as enhanced anti-seepage, transition, isolation, central layer with increased rigidity, and inner lining can be carried out according to the needs, and the thickness of the inner lining can be According to the needs, it is determined that the tube wall is pressed under the action of large centrifugal force, and is very dense, solid, non-layered, and non-porous. The tube wall is an I-beam structure, with strong bending resistance and high rigidity. At present, most of the products in the market are filament wound sanding pipes, which have many defects in product quality. The market share of centrifugal casting sanding pipes should be gradually increased, and the project quality will also be improved.

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