Threaded (TH, THFF, THRF) Flanges are similar to the slip-on, this flange slides over the pipe but is popular because it can be attached without welding since it is designed with tapered threads to work with pipes that have external threads. Also low cost, it’s great for small pipes and highly combustible areas where welding is dangerous.
Threaded (TH, THFF, THRF) Flanges are used for special circumstances 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.