What Is a Threaded Flange?
A threaded flange has an internally threaded bore that screws onto externally threaded pipe. The threads create a mechanical connection that can be assembled without any welding or specialized equipment. Threaded flanges use standard NPT (National Pipe Taper) threads per ASME B1.20.1 for North American applications, or BSP (British Standard Pipe) threads per ISO 7 for European markets. The thread engagement length is designed to provide full thread strength without exceeding the material's capacity.
Design and Thread Standards
The flange body of a threaded flange is similar in external dimensions to a slip-on flange, but with a threaded bore instead of a smooth one. The threads are tapered to provide a mechanical interference fit that creates a seal when the joint is assembled. NPT threads have a 1:16 taper ratio, meaning the diameter changes by 1/16 inch per inch of thread length. The thread engagement length must provide sufficient overlap to achieve the rated pressure capacity. Some designs include a hub that extends beyond the thread length for additional reinforcement at the thread root.
| NPS | Threads per Inch (NPT) | Thread Length (in) | Hand-Tight Engagement (in) | Wrench-Tight Engagement (in) |
|---|---|---|---|---|
| 1/2 | 14 | 0.53 | 0.32 | 0.53 |
| 3/4 | 14 | 0.55 | 0.34 | 0.55 |
| 1 | 11.5 | 0.68 | 0.40 | 0.68 |
| 1-1/4 | 11.5 | 0.71 | 0.42 | 0.71 |
| 1-1/2 | 11.5 | 0.72 | 0.42 | 0.72 |
| 2 | 11.5 | 0.76 | 0.44 | 0.76 |
Pressure Ratings
Threaded flanges have lower pressure ratings than equivalent welded flanges because the threaded connection is inherently weaker than a welded joint. They are typically limited to Class 150 and Class 300, though some designs are available in Class 600. The actual pressure rating depends on the thread engagement length, pipe wall thickness, and the strength of the threaded section under internal pressure. Threaded flanges are not recommended for high-temperature service because thermal cycling can cause the threaded connection to loosen over time, leading to leakage.
Materials
Threaded flanges require materials with good machinability to produce clean, accurate threads. ASTM A105 carbon steel is the standard choice for general service. ASTM A182 F304 and F316 stainless steels are used for corrosive environments where the thread profile must resist galling during assembly. Ductile iron grades are available for lower pressure applications. The material must be compatible with the thread sealant used, as some sealants can cause stress corrosion cracking in certain materials.
Applications
Threaded flanges are essential in hazardous areas where welding is not permitted, such as gas plants, refineries with active process areas, and facilities handling flammable materials. They are used for high-pressure water and chemical injection lines where welding access is limited. Temporary or emergency piping repairs benefit from the quick installation of threaded flanges. Firewater systems and utility piping often use threaded flanges for their ease of installation. Small-bore piping requiring frequent removal for maintenance is another common application.
Threaded vs Welded Connections
The primary advantage of threaded flanges is that they require no hot work, eliminating the need for welding permits, fire watch, and weld inspection. They can be installed in any environment where a pipe threader can reach. However, welded connections provide higher pressure ratings and superior leak integrity. Threaded connections are faster to install initially but may require retightening over time. Welded joints can be inspected by nondestructive methods to verify integrity, while threaded joints rely on the quality of the thread engagement and sealant application.
Installation Best Practices
Apply thread sealant or PTFE tape to the pipe threads before assembly to lubricate the threads and fill the spiral leak path. Hand-tighten the flange onto the pipe, then use a strap wrench for final positioning. Avoid over-tightening, which can damage threads and cause the flange to crack. Torque must achieve proper thread engagement without galling the threads. After installation, perform a pressure test to verify the seal at the threaded joint. If the joint leaks, disassemble, clean, and reapply sealant rather than trying to stop the leak with additional torque.
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