A manhole is an opening structure used for personnel entry and exit of equipment for installation, maintenance, and safety inspections. It is mainly composed of short tube sections (or short pipes), flanges, and a handle-equipped manhole cover. For safety inspection considerations, pressure vessels require a specified number of manholes and their small dimensions are referred to as "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric and pressure manholes based on pressure conditions; as circular, elliptical, and rectangular manholes based on shape, with rectangular manholes only suitable for atmospheric pressure equipment; and as horizontal and vertical installation manholes based on installation position. The manhole cover fixing methods include revolving covers and hinged covers; manhole opening speeds include standard and quick-opening types. Manhole structures and dimensions are generally standardized, and can be selected or designed according to relevant manuals.