Design and application of the joint reinforcement

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The connection, reinforcement, design and application of UPVC Plastic Pipes

1 preface

UPVC Plastic (also known as PVC plastic without plasticizer, hereinafter referred to as UPVC) pipes are made by taking PVC resin as the main component, adding stabilizers, fillers, lubricants, pigments, etc., and then kneading, mixing, processing and molding. Due to the rich source of raw materials of PVC resin, So the price is lower than other plastics. UPVC is not only cheap, but also has high mechanical strength and good processability, especially it has excellent chemical corrosion resistance. Therefore, UPVC pipes are widely used as corrosion-resistant engineering materials

upvc, like other materials, also has its own inherent shortcomings: the coefficient of linear expansion is large, which is 5 ~ 6 times that of steel; The thermal deformation temperature and welding seam coefficient are low. These shortcomings also have adverse effects on the use of its materials in corrosive environments at higher temperatures. For example, the welded parts of UPVC pipes are low in strength and easy to leak. Professor huguorong, director of the Institute of light metals and industrial electrochemistry, School of metallurgy and environment, Nanjing University, pointed out that it is easy to deform in use for a long time. Such accidents often occurred in the past use of anti-corrosion projects. With the progress of science and technology, the improvement of the connection mode and pipeline design of UPVC pipes, as well as the outsourcing of FRP to increase the strength, have gradually overcome the shortcomings of UPVC itself, making the use of UPVC pipes in corrosive environments more and more reliable, and the scope of application has gradually expanded. The connection, reinforcement, design and application of UPVC pipes are discussed comprehensively for the reference of relevant personnel and units

2 pipe connection method

the problems of UPVC pipes in use mainly focus on the pipe connection part, rarely on the pipe itself. The quality of pipe connection is the key to the success of use, so the selection and construction of pipe connection method is very important

upvc pipe connection methods include: butt welding connection, socket connection, cold glue fusion connection, flat welding plastic flange connection, pier pipe looper flange connection, mouth protection looper flange connection, etc. The commonly used connection methods include butt welding connection, socket connection and cold glue fusion connection

2.1 butt welding connection

groove the pipe end of the pipe or pipe fitting joint, and then conduct assembly and butt welding. Due to the low cost of this connection method, the pipe manufacturing and processing is convenient and easy to implement, this method was often used in the past. However, there are many problems, mainly the leakage of butt weld, especially in the corrosive environment with high temperature, which is the weak link in the whole UPVC pipeline. This is because the UPVC weld coefficient varies from person to person, and the weld coefficient welded by the same person will fluctuate, with the highest of 0.9 and the lowest of 0.3. At present, there is no good non-destructive weld inspection method, and the quality of on-site welding construction is difficult to control. Generally, the weld coefficient can only be taken as about 0.5, so the UPVC weld coefficient is low, The strength of the joint is lower than that of the pipe itself, and the reliability is poor. The better solution is to improve the proficiency and working spirit of construction personnel

2.2 socket connection

one end of the pipe fittings and pipes is heated and flared into a socket, or the pipe fittings and pipes processed into the socket are directly purchased. During installation, the inner wall of the socket and the outer wall of the pipe joint of the inserted pipe are coated with a layer of PVC adhesive, and then the pipe is inserted into the socket and fixed and solidified. Finally, the pipe end of the socket is welded on the pipe of the inserted pipe. It has higher strength than butt welding connection, about 1.5 ~ 2.0 times that of the pipe itself; There are also two seals of bonding and welding, which reduce the possibility of leakage and improve the reliability of use. But the cost is higher than that of butt welding. In practical application, its application effect is better than butt welding connection

2.3 cold glue fusion connection

it is a new connection method, which is widely used in industrial, civil and other fields. The specific construction is: use a special cleaning fluid (the role of the cleaning fluid: on the one hand, clean the surface, on the other hand, soften the surface layer of the joint surface UPVC) to clean the outer surface of the pipe end of the inserted pipe about 50mm long and the inner wall of the socket of the pipe fitting, and then apply a layer of adhesive evenly on the bonding surface, Insert the pipe into the socket of the pipe fitting, and then knock it with a mallet, so that all the pipes are inserted into the socket, fixed and cured for a few minutes. The pipe fittings used in this connection method are quite different from those used in socket connection in the past: the socket of the pipe fitting (or pipe) used for socket connection is a taper with about 10 inside, that is, the gap between the two joint surfaces of the connection is uneven, and the thickness of the adhesive is uneven during bonding; The pipe fittings used for cold glue fusion connection are made of special molds, the bell and spigot have no taper, the gap between the two joint surfaces of the connection is uniform, the coincidence is high, the fit is tight, and the joint bonding surface is large. This connection method has the advantages of simple construction, fast speed, no special tools and professional construction team, and easy quality assurance; However, high-precision pipe fittings are needed. At present, such large-diameter pipe fittings are not developed for a long time, and the price is high. With the increase of usage, professional production is getting higher and higher, and the price is bound to decrease in the future. In practical application, its application effect is very good, which creates conditions for the further promotion and use of UPVC pipes

3 increase strength

because UPVC has much lower strength and thermal deformation temperature than steel, while FRP has higher strength and thermal deformation temperature, wood plastic new materials will replace wood nbsp; In the domestic wood plastic market, the strength of UPVC pipes is enhanced by outsourcing FRP in blue ocean. In this way, UPVC pipes mainly bear the impermeability and corrosion resistance, and the strength is mainly borne by the outsourcing FRP layer, and the service temperature is about 10 ℃ higher than that of UPVC pipes. This composite method not only gives full play to the advantages of good impermeability and corrosion resistance of UPVC pipes, but also gives full play to the advantages of high strength of FRP, It overcomes the shortcomings of UPVC and glass fiber reinforced plastics, and the price is also cheaper than the same grade of anti-corrosion pipeline. It is an economic and affordable composite method. The surface energy of UPVC pipe is low and the surface is very smooth. Improving the adhesion between the surface of UPVC pipe and the interface between glass fiber reinforced plastics is the key to the success of composite. With the advent of high-performance interface treatment agent and the increasingly mature technology of mechanical winding glass fiber reinforced plastics, now the quality of UPVC pipe wrapped with glass fiber reinforced plastics can be reliably guaranteed

in actual use, the wrapped FRP reinforced UPVC pipe rarely has problems. Most of the problems appear in the pipe connection joint, just like the UPVC pipe. This is because the wrapped FRP can only improve the long-term use strength of the UPVC pipe, but can not improve its instantaneous bursting strength, that is, it can not improve the damage strength and impermeability of the pipe connection joint. Whether the connection joint of UPVC pipe reinforced with FRP is easy to leak mainly depends on the form and construction quality of the connection joint of UPVC pipe. The situation in practical application is exactly the same. The impermeability of UPVC pipe socket connection joint wrapped with FRP is much better than that of butt welded connection joint wrapped with FRP

4 pipeline design

upvc linear expansion coefficient is 5 ~ 6 times larger than steel, the strength and stiffness are much smaller than steel, and the thermal deformation temperature is also low, which requires double careful design to overcome the shortcomings of physical and mechanical properties. The spacing and form of supports (or hangers) on UPVC pipeline should be reasonable: the spacing of supports (or hangers) should be 2 ~ 5 times smaller than that of steel pipes of the same specification, and soft pads should be placed at the support to prevent excessive deformation and impact damage. The number of expansion joints set on the UPVC pipeline should be determined according to the compensation calculated by the use temperature. In the past, the expansion joints were often installed in the form of type II UPVC pipe or welded soft PVC pipe. They were often damaged due to impact, aging, collision and other reasons, and now they have been almost eliminated. At present, polytetrafluoroethylene expansion joint is basically selected, which has the advantages of low resistance, large compensation, aging resistance, corrosion resistance, convenient installation and so on. UPVC pipes are selected according to the service pressure. 3. The wall thickness of safety belt breaking load test, for example, should be one grade higher than the pressure level specified in the national standard, because there are many factors affecting industrial production, such as the fluctuation of process parameter control, which will make the service temperature and medium concentration higher than the design value; Poor site construction conditions and varying levels of proficiency of construction personnel may reduce the quality of installation and construction; And the vibration and impact caused by other equipment may reduce the safety factor of use. Although this will increase some investment, it will improve the reliability of production and reduce the loss caused by shutdown and overhaul time. Therefore, it is worth increasing some investment

5 examples of use

(1) the pipes for transporting 20% sulfuric acid (65 ℃, 0.2MPa) in a smelter in Hubei Province and corrosive media adopt UPVC pipes, with specifications below DN200 and a total amount of about 4000m; The pipes are connected by butt welding. After being put into use, the butt weld often leaks, causing environmental pollution and material loss, which has become one of the main factors affecting production

(2) during the capital construction period of our company, the transmission pipeline of electrolyte in the electrolysis workshop (corrosive environment: 20% sulfuric acid, 65 ℃, below 0.2MPa) adopts UPVC wrapped FRP composite reinforced pipe (FRP adopts mechanical winding), the total amount is about 10000m, and the specification is less than dn400mm. The UPVC pipe connection adopts butt welding connection, wrapped with FRP (FRP is hand paste). Shortly after it was put into use, it was found that many butt welds leaked, and the electrolyte seeped from the surface FRP reinforcement layer. After several months of rectification, it was initially eliminated, but in the past five years, some butt welds still leaked; No obvious deformation is found in the pipeline; This shows that the reliability of butt welding connection of pipes is poor, and the outsourcing of FRP has a good effect on the improvement of strength and stiffness. In order to save investment, most of the electrolyte pipelines in the electrolytic workshop during the technical transformation period still use UPVC wrapped FRP composite reinforced pipes, with a total volume of about 4000m and a specification of less than dn350mm, but the pipe connection adopts UPVC pipe socket and wrapped FRP, and the pipelines (including pipe racks and expansion joints) are carefully designed and constructed to strengthen construction management, The use effect is much better than that in the capital construction period. After more than one year of use, no leakage is found at the socket connection of the pipe. During the technical transformation period, a small part of the electrolyte transmission pipeline in the electrolysis workshop adopts UPVC pipeline, with a total amount of about 400m and a specification of dn250mm. The pipe connection adopts the cold glue fusion method for connection. The on-site construction is very convenient and rapid, and there is no need for professional teams and professional equipment. It has been used for more than a year, and the effect is very good. No leakage is found in the cold glue fusion method connection joints of all pipes

(3) dilute sulfuric acid pipeline for sulfuric acid purification and transmission in a smelter in Jiangxi Province (operating environment: 15% sulfuric acid, 50 ℃, below 0.3MPa), specification below DN300, total price

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