A manhole refers to an opening structure used for personnel entry and exit of equipment for 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 dimensions can be found in "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized into atmospheric and pressure manholes based on pressure conditions; into circular, elliptical, and rectangular manholes based on shape, with rectangular manholes suitable only for atmospheric pressure equipment; and into horizontal and vertical installation manholes based on installation position. The manhole cover can be fixed with a swing cover or a lifting cover; the opening speed can be standard or quick. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.