The sliding flange has a lower hub because the pipe enters the single flange before welding. It is welded inside and outside to provide enough strength to prevent leakage. Sliding flanges are matched with boring pipes with slightly larger outer diameters. They are better than the welding of neck flanges, because the initial cost is lower for many users, but the final installation cost is probably not low, because the number of welded neck flanges is less.
