* max torque of the bolts at the flange / PTFE. 我需...

以下回复基于常见塑料（尤其是 PTFE）法兰连接的行业经验值和通用选型数据供参考，实际最大可用扭矩仍需结合具体工况（压力、温度、介质、垫片类型等）及法兰/螺栓生产商建议来最终确定。

1. PTFE 法兰特点与扭矩限制：

• PTFE（聚四氟乙烯）材料相对金属而言较为柔软，蠕变性大，且强度较低；如果拧紧力矩过大，容易造成法兰变形或损伤，紧固力矩一般要低得多，需要谨慎施加。

2. 3/4" UNF 螺栓的参考扭矩范围：

• 对于 DN200 的 PTFE 法兰，常见经验参考值会将 3/4" 螺栓的最大紧固力矩控制在大约 40～60 N·m 左右。
• 部分厂商或工程手册可能给出略高或略低的数值（例如 35～55 N·m、50～65 N·m 等），但大体范围相近。
• 具体数值应根据：
  • 法兰设计压力与温度等级
  • 管线介质性质（是否易渗透、易挥发等）
  • 垫片类型和压紧面要求（如 PTFE 垫片、柔性石墨、改性 PTFE 等）
  • 连接螺栓材质和级别
  • 现场安装工艺与安全系数

3. 安装及紧固建议：

1. 分级、对称交叉拧紧： 先按 30%→60%→100% 的顺序分阶段拧紧，并采用对称的“十字”或螺旋方式，保证法兰均匀受力。
2. 使用平垫或垫圈： 针对 PTFE 法兰，建议采用合适的垫圈（如金属垫片可能并不适用），并可考虑在螺母端配平垫或弹簧垫来分散载荷。
3. 关注实际工况变化： 如果操作温度、压力波动大，或介质会侵蚀垫片/法兰材料，需在运行一段时间后进行二次复紧（尤其是塑料法兰蠕变性更高）。
4. 遵循厂商/标准的具体要求： 优先参考法兰或螺栓制造商、ASME、DIN 等相关标准给出的扭矩指导值。如有法兰设计手册或第三方测试数据，亦应从其建议的数值中取值。

简要结论：
对于 DN200、材质为 PTFE 的带颈平焊法兰，选用 3/4" UNF × 90 mm 的六角螺栓时，一般建议最大紧固力矩约控制在 40～60 N·m 范围内（具体数值还应结合实际工作条件和厂商推荐值微调）。这样既能保证足够的密封力，又能尽量避免对 PTFE 法兰造成过量的蠕变或损伤。

Below is an English response based on typical industry practices and general data for plastic (especially PTFE) flange connections. Please note that the actual maximum allowable torque depends on specific operating conditions (pressure, temperature, medium, gasket type, etc.) and manufacturer recommendations.

1. Characteristics of PTFE Flanges and Torque Limitations

• PTFE (polytetrafluoroethylene) is softer compared to metal and has relatively high creep. Its strength is lower, so excessive torque can easily deform or damage the flange. Therefore, the allowable tightening torque for PTFE flanges is significantly lower and must be applied with caution.

2. Reference Torque Range for 3/4" UNF Bolts

• For a DN200 PTFE flange, common experience suggests limiting the maximum tightening torque of 3/4" bolts to roughly 40–60 N·m.
• Some manufacturers or engineering handbooks might recommend slightly different ranges (e.g., 35–55 N·m or 50–65 N·m), but overall they will be in a similar ballpark.
• The exact torque value should be determined based on:
  • Design pressure and temperature rating of the flange
  • Properties of the conveyed medium (e.g., volatility, permeation, corrosion)
  • Gasket type and required sealing stress (e.g., PTFE gasket, modified PTFE, flexible graphite, etc.)
  • Bolt material and specification
  • Installation procedures and safety factors

3. Installation and Tightening Recommendations

1. Tighten in multiple passes, using a symmetrical, crisscross pattern: For example, tighten to 30%, then 60%, then 100% of the final torque value, ensuring even load distribution around the flange.
2. Use suitable washers or gaskets: PTFE flanges typically call for non-metallic or specific gaskets to ensure effective sealing, and washers help distribute the load.
3. Monitor operating conditions: If the system undergoes significant temperature or pressure fluctuations, or if the medium could degrade the gasket/flange, a re-torque might be necessary after a period of operation—especially considering PTFE’s creep.
4. Follow manufacturer or standard guidelines: Always consult the flange/bolt manufacturer, as well as relevant ASME, DIN, or other standards for recommended torque values. If a design handbook or third-party test data is available, reference those recommended values accordingly.

Summary
For a DN200 raised-face weld-neck flange made of PTFE and using 3/4" UNF × 90 mm hex bolts, a recommended maximum tightening torque is typically in the 40–60 N·m range (further refined by specific operational and manufacturer guidelines). This range helps ensure adequate sealing without causing excessive deformation or damage to the PTFE flange.