A manhole is an opening structure used for personnel entry and exit of equipment for installation, maintenance, and safety inspections. It primarily consists of short pipe sections (or short tubes), 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 the "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressure manholes based on pressure conditions; as circular, elliptical, or rectangular manholes based on shape, with rectangular manholes suitable only for atmospheric pressure equipment; as horizontal or vertical installation manholes based on installation position; and the manhole cover fixing methods include swinging covers and lifting covers. The opening speed of manholes can be standard or quick-open. The manhole structure and dimensions are generally standardized and can be selected or designed according to relevant manuals.