When selecting a surface treatment for fasteners, consider the following:

1. Environment: Dry indoor conditions → electro-galvanizing or black oxide; humid outdoor conditions → hot-dip galvanizing or thermal diffusion galvanizing (zinc diffusion); high salt-spray environments, automotive, or chemical industries → zinc-nickel alloy or Dacromet/zinc-aluminum coatings.

2. Strength and hydrogen embrittlement: Electro-galvanizing is suitable for strength grades below 8.8; for grades 10.9 and above, avoid acid pickling and electroplating—prefer Dacromet, mechanical galvanizing, or zinc-aluminum coatings; if electroplating is used, a de-hydrogenation process is mandatory.

3. Threads and assembly: Hot-dip galvanized coatings are thick, requiring tolerance allowances or post-plating thread tapping; if torque specifications apply, clearly define the friction coefficient and lubrication requirements.

4. Application and cost: Preparation for painting → phosphating; stainless steel → passivation; decorative/wear-resistant → chrome or nickel plating.

Selecting a flange involves balancing operating conditions, connection requirements, and costs.

1. Determine operating parameters

Define the design pressure, design temperature, fluid characteristics (corrosivity, toxicity), and nominal diameter (DN). Note: A flange's allowable working pressure decreases as temperature rises; selection must be based on the pressure rating at the design temperature.

2. Select the flange type

Plate-type slip-on flange (PL): Lowest cost; used for low-pressure, ambient-temperature applications without strict requirements (e.g., water pipes with PN ≤ 16).

Slip-on flange with hub (SO): Strength is slightly superior to PL; used for low-to-medium pressure piping.

Weld-neck flange (WN): High strength; used for hazardous conditions involving high pressure, high temperature, or flammable/explosive media (e.g., PN ≥ 40 or Class 300 and above).

Socket-weld flange (SW): Used for small-diameter, high-pressure piping (DN ≤ 40).

Threaded flange (Th): Used for small-diameter piping where welding is prohibited or frequent disassembly is required.

3. Select the sealing face type

Raised Face (RF): Most common; suitable for low-to-medium pressure and general media.

Male-Female (MFM) / Tongue-and-Groove (TG); Ring Joint (RJ/RTJ).

4. Select the pressure rating

Domestic standards typically use PN (e.g., PN16, PN40), while US standards use Class (e.g., 150, 300). Ensure the flange's maximum allowable working pressure at the operating temperature is greater than or equal to the design pressure.

5. Select the material

Carbon steel (e.g., 20#, A105): Low cost; used for ambient temperature, low pressure, and non-corrosive media.

Stainless steel (e.g., 304, 316L): Corrosion-resistant; used in corrosive environments such as chemical processing and pharmaceuticals.

Alloy steel (e.g., 15CrMo): Used for high-temperature, high-pressure steam and thermal piping networks.

6. Select the standard system

National Standard (GB) / Ministry of Chemical Industry Standard (HG): Commonly used for domestic projects. ASME Standards (ASME B16.5/B16.47): Commonly used in international projects or for US-manufactured equipment; note that dimensions may not be interchangeable with those of Chinese national standard (GB) flanges.

7. Matching Gaskets and Fasteners

Gaskets must be selected based on temperature, pressure, and the medium: rubber gaskets for low-temperature water or steam service, spiral-wound gaskets for high-temperature and high-pressure applications, and metal ring-joint gaskets for high-pressure, severe service conditions. Bolt strength must match the flange rating.

Flange selection is a systematic engineering decision-making process that requires careful consideration of operating parameters, standards and specifications, and material compatibility. Standardized procurement and selection processes can avoid assembly errors and safety hazards.

1. Define the basic parameters and confirm the nominal dimensions. Accurate data needs to be obtained from the design documents to match the diameter of the connected pipes or equipment. At the same time, the additional forces or bending moments caused by the pipeline's own weight, thermal expansion, earthquakes, etc. must be considered. These factors will complicate the flange design, and if necessary, the pipe load should be converted into equivalent pressure for selection.

2. Select a flange standard system. The standard system determines the flange size, pressure rating, and marking method. Select the appropriate rating (e.g., Class 75, 150, 300, 400, 600, or 900) based on actual temperature and pressure conditions. If the project involves the American standard system, then ASME B16.5 should be referred to.

3. Confirm the flange type. Select weld neck flanges for pipe connections and blind flanges for end sealing and isolation. The flange type mainly depends on the pressure rating, pipe size, and operating conditions. For specific selection, please refer to the Complete List and Selection Reference of Industrial Pipe Fittings.

4. Confirm the flange material. The flange material must be compatible with the pipe material, medium, and design temperature.

5. Confirm sealing and connection requirements. Determine the flange end face (RF/FF/RTJ), gasket type, and bolt configuration; verify the availability of the selected NPS, grade, and series of RTJ.

6. Select matching bolts and gaskets. Match materials to the medium, temperature, and corrosion conditions.

When selecting a flange, ensure that its strength rating matches the pipeline and strictly verify the rated values according to the design temperature and pressure.

Chemical composition and mechanical properties of wear-resistant steel plates

C: Improves hardness and wear resistance, but too much carbon will reduce toughness and weldability.

Mn: Improves hardenability and strength, and aids in wear resistance.

Cr, Mo, Ni: improve hardenability, enabling thick plates to achieve high hardness.

P and S: harmful impurities; the lower the better, especially affecting toughness and weldability.