A manhole refers to an opening structure for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It is mainly composed of short tube sections (or short pipes), flanges, and manhole covers with handles. In terms of safety inspections, pressure vessels require the number of manholes and their small sizes as specified in "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressurized based on the pressure conditions; as round, elliptical, or rectangular based on shape, with rectangular manholes only suitable for atmospheric equipment; and as horizontal or vertical based on installation position. The methods of fixing the manhole cover include swing and hinged covers; the opening speed of the manhole is either standard or quick-open. Manhole structures and dimensions are generally standardized, allowing for selection from relevant manuals or design.