Manholes are openings designed for personnel access to equipment for installation, maintenance, and safety inspections. They are primarily composed of short筒sections (or short pipes), flanges, and manually operated lids with handles. For safety inspections, pressure vessels require a specified number of manholes and their small sizes are referred to as "inspection openings." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressure manholes; by shape into round, oval, and rectangular manholes, with rectangular manholes being suitable only for atmospheric equipment; and by installation position into horizontal and vertical installations. Manhole cover fixing methods include hinged covers and lifting covers; opening speed includes standard and quick-open styles. The manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.