First, what are the production processes for plastic-coated steel pipes?
(I) Strict Selection of Steel Pipe Substrate: 
High-quality steel pipe substrate is the foundation for creating high-performance plastic-coated steel pipes. When selecting materials, high-quality carbon structural steel or low-alloy steel is usually chosen. It possesses high strength and good pressure resistance. Simultaneously, its good weldability allows the steel pipe to be easily and firmly connected to other components during processing, much like Lego bricks, allowing for flexible assembly and laying a solid foundation for subsequent flange connections and other processes.
(II) Careful Construction of the Anti-corrosion Coating: 
After selecting the substrate, the steel pipe needs to be coated with an "anti-corrosion armor." This armor is carefully applied using anti-corrosion materials such as epoxy resin and polyurethane, mainly coating the inner wall of the steel pipe. In actual operation, technicians will precisely control the coating thickness and formula according to the pipeline's operating environment and the characteristics of the transported medium. This anti-corrosion coating acts like a sturdy shield, effectively resisting the invasion of various corrosive media, preventing rust and corrosion inside the steel pipe, and greatly extending the service life of the steel pipe. 
(III) Perfect Coverage of the Plastic Outer Layer: 
After applying the anti-corrosion coating, a layer of polyethylene, polyvinyl chloride, or other plastic material is applied to the outer surface. This process requires advanced technology and equipment to ensure that the plastic layer adheres evenly and firmly to the steel pipe surface. The plastic layer acts like a "protective suit" for the steel pipe, not only further enhancing its mechanical properties but also protecting it from external physical damage such as impacts and friction.

Second, Unique Advantages of Flange-Connected Plastic-Coated Steel Pipes
(I) Superior Sealing Performance: 
In piping systems, sealing is crucial; even slight negligence can lead to serious safety accidents. Flange connections act as reliable "sealing guardians," achieving excellent sealing through the tight fit of the gasket and bolts. When two flanges are joined, the gasket placed between them acts like a soft yet resilient "protective layer," adhering tightly to the flange's sealing surface and filling any tiny gaps. Subsequently, tightening the bolts applies preload, causing the gasket to elastically deform, further enhancing the sealing effect. 
(II) Convenient Installation and Maintenance: 
Compared to other connection methods, flange connections offer significant advantages in installation and maintenance, making them a "convenient tool" for pipeline installation and maintenance. During installation, simply align the two flanges, ensuring the bolt holes correspond one-to-one, then insert and tighten the bolts to quickly complete the connection. This rapid positioning and bolt tightening method greatly shortens installation time and improves construction efficiency.
(III) Excellent Durability: 
The synergistic effect of the plastic coating and the steel pipe substrate endows flange-connected plastic-coated steel pipes with excellent durability, making them the "top choice" for pipeline applications in harsh environments. The steel pipe substrate acts as the "skeleton" of the pipeline, providing strong pressure resistance, capable of withstanding the high pressure of internal fluids and the pressure of the external environment. The plastic coating acts as a robust "protective film," possessing excellent corrosion resistance, able to resist the erosion of corrosive media such as acids and alkalis, as well as the effects of harsh environments such as damp soil and chemical gases.

Third. Wide Range of Applications of Plastic-Coated Steel Pipes
(I) Reliable Choice for Municipal Engineering: 
Urban water supply is related to the daily lives of every resident, requiring extremely high standards for water quality and pipeline safety. This pipeline, with its corrosion-resistant properties, effectively resists the erosion of various chemicals in the water, ensuring long-term stable operation and preventing water quality from being affected by rust or corrosion. Its excellent hygiene performance, with a smooth inner wall, makes it difficult for bacteria and dirt to grow, acting like a clean "water channel" to deliver safe and hygienic drinking water to residents. It also performs exceptionally well in wastewater treatment. Urban sewage contains a large number of impurities, acidic and alkaline substances, and various microorganisms, which are highly corrosive to pipes. However, the flange-connected plastic-coated steel pipe, with its strong corrosion resistance, can easily adapt to this complex water environment, ensuring normal sewage discharge and avoiding environmental pollution caused by pipeline leaks.
(II) A Robust Defense Line for Fire Protection Systems: 
Fire protection systems are an important line of defense for protecting life and property, and flange-connected plastic-coated steel pipes are the "steel guardians" of this line of defense. Their excellent fire resistance allows them to withstand high temperatures during a fire, without deforming or cracking due to temperature increases, ensuring a stable supply of fire-fighting water. At a fire scene, time is of the essence, and the timely supply of fire-fighting water is crucial. The reliable sealing performance of this pipeline prevents leakage of fire-fighting water, ensures stable pressure in the fire-fighting system, and buys valuable time for fire fighting and rescue.
(III) A Powerful Assistant in the Industrial Sector: 
In the petroleum and chemical industries, pipelines need to transport various corrosive media, such as acids, alkalis, and salts, in extremely harsh working environments. This pipeline, with its excellent corrosion resistance, can operate stably for extended periods in such environments, ensuring continuous production. In the mining industry, pipelines transporting slurry and tailings need to withstand significant pressure and wear; their high strength and wear resistance enable them to perform this task, ensuring normal mining operations. In the power industry, the transportation of circulating water and cooling water places high demands on pipelines; its excellent corrosion resistance and sealing performance effectively prevent pipeline corrosion and leakage, improving power production efficiency.

Fourth. Installation Points and Precautions for Flange-Connected Plastic-Coated Steel Pipes
(I) Connection Steps and Standards: 
When installing flange-connected plastic-coated steel pipes, standardized connection steps are crucial to ensuring the stable operation of the pipeline system. First, flanges are welded to both ends of the steel pipe. This step is like installing a sturdy "connector" on the pipeline. The welding process must be strictly controlled to ensure a tight bond between the flange and the steel pipe, free from defects such as incomplete welds or leaks. Taking the installation of fire protection pipelines in a large building project as an example, technicians used advanced welding technology and high-quality welding rods to perform multi-layer welding on the welded areas and conducted rigorous flaw detection to ensure that the welding quality met national standards. After welding, a polyethylene coating is applied, giving the pipeline a "protective layer" and enhancing its corrosion resistance. During actual installation, a gasket is installed between the two flanges of the coated composite pipe. The selection of the gasket is crucial and needs to be determined based on the characteristics of the medium being transported and the operating pressure. For example, in pipelines transporting hot water, a high-temperature resistant gasket should be selected; in pipelines transporting corrosive media, a chemically resistant gasket should be selected. After installing the gasket, the two flanges are tightened with bolts. When tightening the bolts, a diagonal sequence should be followed to ensure a firm flange connection and reliable sealing. In this way, through rigorous connection procedures and standardized operations, the sealing and stability of the pipeline system can be guaranteed.
(II) Key Points for Pipeline Fixing
To ensure that pipelines do not experience axial movement due to pressure or water hammer during operation, effective fixing is crucial. Pipelines can be fixed using supports, brackets, hangers, etc. During fixing, the swaying and deflection of the pipeline under pressure must be fully considered to ensure that the pipeline does not shift. For vertically installed pipelines, the distance between two fixing points should not exceed 2m to ensure the pipeline's vertical stability. For horizontal installations, the spacing between fixing points should be determined based on the nominal outer diameter of the pipeline: when the nominal outer diameter of the coated composite pipe is less than 76mm, the spacing should not exceed 750mm; when the nominal outer diameter is not less than 76mm, the spacing should not exceed 1100mm.
(III) Safety Warnings and Protective Measures
Safety is paramount during pipeline installation. During installation, the warning line on the pipe must face outwards, forming a continuous line. This warning line acts as a "safety signal light" for the pipeline, playing a vital safety warning role during construction and subsequent maintenance. It can alert construction workers and others to the location of pipelines, preventing damage due to misoperation, and also facilitating later maintenance and modifications. In a municipal engineering pipeline construction project, because construction workers strictly adhered to the requirement of keeping the warning line facing outwards, other construction teams were able to clearly identify the pipeline's location during subsequent road excavation, avoiding damage and ensuring the safe operation of the entire pipeline system. During handling and installation, care must be taken to protect the pipes from collisions, scratches, and other damage. When handling, appropriate lifting equipment and tools should be used, and the pipes should be lifted and placed gently. During installation, contact with sharp objects should be avoided to prevent damage to the coating. If the coating on the pipe surface is damaged, it should be repaired promptly to ensure the pipeline's corrosion resistance.