Threaded Flanges are used in special requirements to work under high temperature and pressure, making sure no welding joint is made at the assembly.
With their main advantage being that they can be attached to the pipe without welding. Sometimes a seal weld is also used in conjunction with the threaded connection.
Although still available in most sizes and pressure ratings, screwed fittings today are used almost exclusively in smaller pipe sizes.
A threaded flange or fitting is not suitable for a pipe system with thin wall thickness, because cutting thread on a pipe is not possible. Thus, thicker wall thickness must be chosen.
ASME B31.3 Piping Guide says:
Where steel pipe is threaded and used for steam service above 250 psi or for water service above 100 psi with water temperatures above 220° F, the pipe shall be seamless and have a thickness at least equal to schedule 80 of ASME B36.10.
Standard Ratings: 150LB, 300LB, 600LB, 900LB, 1500LB, 2500LB, JIS 10K, 20K, PN 10, PN 16
• ANSI B16.5
• ANSI B16.47
Carbon Steel : ASTM A105, A350 LF2, LF3
Stainless Teel : ASTM A182 F 304/ L, 316/ L