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Slip-on Flanges continue to be preferred to welding neck flanges by many users on account of their initially lower cost, the reduced accuracy required in cutting the pipe to length, and the somewhat greater ease of alignment of the assembly.
However, their final installed cost is probably not much, if any, less than that of welding neck flanges. This type of flange introduces irregularity to the internal bore which increases the resistance of flow in the line.
When installed, the pipe is set back from the face of the flange a distance equal to the wall thickness + 1/16".
Their calculated strength under internal pressure is of the order of two-thirds that of welding neck flanges, and their life under fatigue is about one-third that of the latter.
For these reasons, slip-on flanges are limited in sizes ½" to 2 ½" in the 1500 lb. Standard and are not available in the 2500 lb. standard.
The ASME Boiler Construction Code limits their use to the 4" size.
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