A manhole is an opening structure used for personnel access to equipment for installation, maintenance, and safety inspections. It primarily consists of short pipe sections (or short tubes), flanges, and a handle-equipped manhole cover. For safety inspection considerations, pressure vessels are required to have a certain number of manholes and their small sizes are specified under "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressurized 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 mounted manholes; and by cover fixing method into swivel covers and lifting covers. The opening speed of manholes can be standard or quick open. Manhole structures and dimensions are generally standardized, and can be selected or designed according to relevant manuals.