A manhole refers to an opening structure designed for personnel entry and exit for installation, maintenance, and safety inspections. It is mainly composed of short筒节 (or short pipe), flanges, and a manhole cover with a handle. In terms of safety inspections, pressure vessels require a specific number of manholes and their small dimensions can be found under "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressurized manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes being suitable only for atmospheric equipment; by installation position into horizontal and vertical installations; and the manhole cover fixing methods include swivel covers and lifting covers. The opening speed of the manhole can be standard or quick-release. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.