A manhole refers to an opening structure used for personnel entry and exit to facilitate installation, maintenance, and safety inspections. It is mainly composed of short筒节 (or short tube), flanges, and a manhole cover with a handle. For safety inspection considerations, pressure vessels require a specified number of manholes and their small dimensions, see "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressurized manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes suitable only for atmospheric equipment; by installation position into horizontal and vertical installation manholes; and by cover fixing method into swivel covers and hinged covers. The opening speed of manholes can be either standard or quick-open. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.