Manholes are openings designed for personnel entry and exit to facilitate installation, maintenance, and safety inspections of equipment. They are primarily composed of short tube sections (or short pipes), flanges, and hinged manhole covers with handles. For safety inspection considerations, pressure vessels require a specified number of manholes and their small sizes are detailed under "Inspection Ports." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressure manholes; by shape into round, oval, and rectangular manholes, with rectangular manholes being suitable only for atmospheric pressure equipment; by installation position into horizontal and vertical installation manholes; and by cover fixing methods into swivel covers and lift-off covers. The opening speed of manholes can be either normal or quick. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.