A manhole refers to an opening structure designed for personnel entry and exit, facilitating installation, maintenance, and safety inspections. It primarily consists of short pipe sections (or short pipes), flanges, and a manhole cover with a handle. For safety inspection considerations, pressure vessels require a specific number of manholes and their small sizes are detailed under "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressurized based on the pressure conditions; as circular, elliptical, or rectangular in shape; rectangular manholes are only suitable for atmospheric equipment; as horizontal or vertical installations; and the manhole cover can be fixed with a rotating cover or a lifting cover. 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.