A manhole is an opening structure designed for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It primarily consists 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 sizes, which can be found in "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressurized based on pressure conditions; as circular, elliptical, or rectangular based on shape, with rectangular manholes suitable only for atmospheric equipment; and as horizontal or vertical based on installation position. The manhole cover fixing methods include swing covers and hinged covers; the opening speed of manholes can be standard or quick-open. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.