Threaded or screwed flanges are made of steel and are confined to special applications. Their primary merit is that the threaded end allows them to be assembled without welding: this makes them ideal for use in extremely high-pressure systems. Threaded steel flanges are unsuitable for conditions involving any extreme temperatures or extreme stress from bending, especially if the flange is being used under cyclic conditions where leakage through the threads could occur in few cycles of heating or stress.
Slip-on flanges are generally the preferred option over weld neck flanges, this is mostly due to their lower initial cost, wider tolerance for inaccuracies in pipe cutting and the easier assembly alignment. However, their final cost one installed is probably not much less than that of welding neck flanges. Their strength under internal pressure is two–thirds that of welding neck flanges, while their lifespan under fatigue is approximately one–third. For these reasons slip on flanges are only available in sizes between 1/2” and 2 1/2” in the 1500lb standard, are not found in the 2500lb standard at all. The ASME Boiler Construction Code restricts their use to the 4” model only.
Blind or blank flanges are used to cap off; or blank the ends of piping, valves or pressure vessels. Internal pressure and bolt loadings on blind flanges are the most highly stressed of all ASME Standard flange types, especially in larger sizes. The maximum stresses in a blind flange are caused by bending and are found at the centre, because of this they can safely be permitted to be higher than other types of flanges. If extreme temperature and extreme or repeated water hammer is a factor, consideration should be given to closures made of welding neck flanges and caps.
Weld neck flanges are easily distinguishable from other types by their long, tapered hub and smooth transition of thickness in the region of the butt weld joining them to the pipe. The long tapered hub reinforces the flange increasing its strength and resistance to dishing. The smooth transition from flange thickness to pipe wall thickness affected by the taper is extremely beneficial under conditions of repeated bending which is caused by line expansion or other unpredictable forces. This gives weld neck flanged assemblies strength equivalent to that of a butt welded pipe. This type of flange is commonly used where severe service conditions are found including extremely high pressures or and temperatures, and whether conditions are constant or vary. Welding neck flanges are ideal for handling explosive, flammable or valuable liquids, where the failure of a seal can have severe consequences.