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), flange, and a handle-equipped manhole cover. In terms of safety inspections, pressure vessels require a specific number and size of manholes, which can be found in "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure as atmospheric and pressurized; by shape as circular, elliptical, and rectangular, with rectangular manholes suitable only for atmospheric equipment; by installation position as horizontal and vertical; and the manhole cover fastening methods include hinged and suspended types. The opening speed of manholes can be standard or quick-release. Manhole structures and dimensions are generally standardized, and can be selected or designed according to relevant manuals.