A manhole is an opening structure used for personnel entry and exit of equipment for installation, maintenance, and safety inspections. It is primarily composed of short筒节 (or short pipe), 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 by pressure conditions into atmospheric and pressure manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes only suitable for atmospheric equipment; by installation position into horizontal and vertical installation manholes; and by cover fixing methods into rotating covers and hinged covers. The opening speed of manholes is available in standard and quick-open models. Manhole structures and dimensions are generally standardized, and can be selected or designed according to relevant manuals.