Manholes are openings designed for personnel entry and exit to facilitate installation, maintenance, and safety inspections. They are primarily composed of short pipe sections (or tubes), flanges, and a handwheel-equipped manhole cover. Regarding safety inspections, pressure vessels require a specific number of manholes and their small dimensions, which can be found under "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized into atmospheric and pressurized types based on pressure conditions; into round, oval, and rectangular types based on shape, with rectangular manholes suitable only for atmospheric equipment; into horizontal and vertical installation types based on installation position; and the manhole cover fastening methods include swing and lifting types. The opening speed of manholes can be standard or quick-release, and the manhole structures and dimensions are generally standardized, allowing selection from relevant manuals or custom design.