A manhole is an opening structure used for personnel entry and exit, as well as for installation, maintenance, and safety inspections of equipment. It primarily consists of short pipe sections (or short tubes), flanges, and a handle-equipped manhole cover. Considering safety inspections, pressure vessels are required to have a specified number of manholes and their small dimensions can be seen in the "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as either atmospheric pressure or pressurized based on the pressure conditions; as circular, elliptical, or rectangular based on their shape, with rectangular manholes suitable only for atmospheric pressure equipment; and as horizontal or vertical installation based on their installation position. The types of manhole cover fastening include swinging and lifting covers; and the opening speed of manholes can be either normal or quick. The manhole structure and dimensions are generally standardized, and can be selected or designed according to relevant manuals.