A manhole is an opening structure used for personnel entry and exit to facilitate installation, maintenance, and safety inspections. It is mainly composed of short tube sections (or short pipes), flanges, and a handle-equipped manhole cover. For safety inspection considerations, pressure vessels require a specified number of manholes and their small dimensions, which can be found in "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressurized based on pressure conditions; as circular, elliptical, or rectangular based on shape, with rectangular manholes only suitable for atmospheric equipment; and as horizontal or vertical based on installation position. The types of manhole cover fastening include rotating covers and lifting covers. The opening speed of manholes can be either standard or quick-opening. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.