A manhole is an opening structure designed for personnel entry and exit for installation, maintenance, and safety inspections. It primarily consists of short tube sections (or short pipes), flanges, and a handle-equipped manhole cover. In terms of safety inspections, pressure vessels are required to have a certain number of manholes and their small sizes, which can be found in "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 based on their shape, with rectangular manholes only suitable for atmospheric equipment; and as horizontal or vertical based on their installation position. The manhole cover can be fixed with a swing cover or a lifting cover; the opening speed can be standard or quick. The manhole structure and dimensions are generally standardized and can be selected or designed according to relevant manuals.