A manhole refers to an opening structure designed for personnel entry and exit for installation, maintenance, and safety inspections. It is primarily composed of short pipe sections (or short tubes), flanges, and a manhole cover with handles. In terms of safety inspections, pressure vessels require a specific number of manholes and their small sizes, which are detailed in the "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 swivel covers and hinged covers. The opening speed of manholes can be standard or quick. Manhole structures and sizes are generally standardized, allowing for selection or design based on relevant manuals.