A manhole is 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), flanges, and a manhole cover with a handle. In consideration of safety inspections, pressure vessels require a specified number of manholes and their small sizes are listed under "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressure manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes only suitable for atmospheric pressure equipment; by installation position into horizontal and vertical installation manholes; and by cover fixing method into swing covers and lifting covers. The opening speed of manholes can be either standard or quick-open. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.