Manholes are openings designed for personnel access to equipment for installation, maintenance, and safety inspections. They primarily consist of short tube sections (or pipes), flanges, and a handle-equipped manhole cover. For safety inspection considerations, pressure vessels are required to have a specified number of manholes and their small sizes are detailed under "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressurized manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes being suitable only for atmospheric pressure equipment; by installation position into horizontal and vertical installations; and the manhole cover fixing methods include rotating covers and lifting covers. The opening speed of manholes can be standard or quick-release, and the manhole structures and dimensions are generally standardized, allowing for selection or design based on relevant manuals.