1. Welded steel pipe for conveying low-pressure fluid: The welded pipe specified in GB/T309 1 is the main pipe for manufacturing steel-plastic pipe for water supply, which is generally suitable for pipes with nominal pressure not exceeding 1.6MPa. When implementing this standard, two points should be paid special attention to:
(1), steel pipe wall thickness: In order to ensure that steel pipes can withstand greater pressure, the standard stipulates that steel pipes have two wall thicknesses, one is ordinary wall thickness, and the other is thick wall thickness. Common wall thicknesses are:
nominal dimension
Caliber15 20 25 32 40 50 65 80100125150.
The wall thickness is 2.8 2.8 3.23.53.84.04.04.04.04.5.
Generally speaking, enterprises engaged in the manufacture of steel-plastic pipes for water supply must abide by this wall thickness. Of course, due to various considerations of cost, the enterprise can't guarantee this wall thickness, but it should be within the deviation range of plus or minus 12.5% specified in the standard.
(2) Specification of steel pipe: For the old standard of GB309 1 (version 93), the specification of welded pipe only reaches DN 150, while for the new standard, the specification of steel pipe has been expanded to DN 1600, so the specification of steel-plastic pipe for water supply, especially the specification of plastic-coated pipe, can be expanded.
2. Spiral welded pipe: Spiral welded pipe is divided into spiral welded high-frequency steel pipe and spiral submerged arc welded steel pipe. Generally, the pressure of the former is below 1.6 MPa, and that of the latter is 2.5Mpa. The maximum diameter of high-frequency welded spiral pipe is 406mm, and the maximum diameter of submerged arc welded spiral pipe is 2220 mm ... SY/T5037 "Spiral seam submerged arc welded steel pipe for low-pressure fluid transportation pipeline" is the standard of spiral seam submerged arc welded steel pipe.
As mentioned above, the specification of longitudinal welded pipe can now be extended to DN 1600. Why do some users choose spiral welded steel pipes after the pipe diameter exceeds DN300? Mainly because the price of spiral welded pipe is lower than that of straight welded pipe.
Seamless steel pipe: The standard is GB/T8 163 Seamless Steel Pipe for Fluid Transportation. Seamless steel pipes are made of Q295, 10, 20 and 20 steels, which have no weld seam and are resistant to high pressure. They are not used as water supply pipes, but mainly used for industrial pipelines.
It should be noted that in the specification representation of steel pipes, the marking methods of welded pipes and seamless pipes are different, and welded pipes are represented by nominal caliber, such as DN25 and DN50. Seamless steel pipe is expressed by the outer diameter of steel pipe, such as < ￠57x3, 57x3.5, etc.
Second, the classification of plastics for steel-plastic pipes and the names of commonly used plastics
1, Thermoplastic: This kind of plastic can be softened or melted when heated, but hardened when cooled, and this process can be repeated. Thermoplastics undergo physical changes when heated. Although the shape of plastic has changed, the molecular structure has not changed. Commonly used thermoplastics include polyethylene, polypropylene and polyvinyl chloride.
2. Thermosetting plastic: This kind of plastic with curing agent will harden after heating, will not soften after heating, and will decompose at high temperature. Thermosetting plastics will change chemically when heated, not only the shape of the plastic changes, but also the molecular structure changes. Commonly used thermosetting plastics include epoxy plastics and phenolic plastics.
Commonly used plastic names in steel-plastic composite pipe:
PE polyethylene
HDPE high density polyethylene
MDPE medium density polyethylene
LDPE
LLDPE linear polyethylene
PE-RT heat-resistant polyethylene
PE-X variable polyethylene
polypropylene
PP-R random polypropylene
polyvinyl chloride
rigid pvc
epoxy
Thirdly, the bonding mechanism between steel pipe and plastic.
1. Viscosity classification of metal materials: According to the difficulty of adhesion between plastic and metal surface, metal materials can be divided into:
(1), active surface: The surface of this material is easy to adhere to plastic and has high adhesion. Representative materials are steel, iron and its alloys, copper and its alloys, manganese, aluminum and its alloys.
(2) Inert surface: The surface of this material is not easy to bond with plastic. The adhesive strength is low, and the representative materials are: stainless steel, zinc, pure aluminum, cadmium, gold, silver and so on.
(3), stagnant surface: the surface of this material can not be bonded with plastic. Representative materials are: surface anodizing, oxidation and electroplating metal.
2. Infiltration theory: "Infiltration" means soaking and wetting. The introduction of "wetting" theory between steel pipe and plastic means that in the bonding process of steel pipe, the bonding layer in contact with steel pipe must be melted, thus wetting the surface of steel pipe and making the molecules of bonding layer fully contact with metal molecules to obtain higher bonding force.
3. Three stages of applying plastic powder to steel pipe to form coating:
(1) Caking: refers to the dispersed powder particles, which contact the heated steel pipe and then adhere to the workpiece to form a continuous uneven coating.
(2) Leveling: After the powder is bonded to the steel pipe, it will melt and flow on the surface of the steel pipe after continuous heating, so that the uneven surface will form a smooth surface.
(3) Curing: the molten coating undergoes cross-linking reaction, and the viscosity is continuously enhanced, and finally a hard coating is formed (for the coating of thermoplastic materials, it is cooling hardening).