A manhole refers to an opening structure designed for personnel entry and exit for installation, maintenance, and safety inspections. It is mainly composed of short tube sections (or short pipes), flanges, and a handle-equipped manhole cover. In terms of safety inspections, pressure vessels require a specified number of manholes and their small sizes, which can be found in "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 installations; and by cover fixing methods into rotating covers and lifting covers. The opening speed of manholes can be either standard or quick-open. Manhole structures and dimensions are generally standardized, allowing selection or design based on relevant manuals.