Manholes are openings designed for personnel access to equipment for installation, maintenance, and safety inspections. They are primarily composed of short tube sections (or short pipes), flanges, and hand-operated manhole covers. For safety inspection considerations, pressure vessels are required to have a specified number of manholes and their small sizes are detailed in the "inspection openings." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressure manholes based on pressure conditions; as circular, elliptical, or rectangular manholes based on shape, with rectangular manholes suitable only for atmospheric pressure equipment; and as horizontal or vertical installation manholes based on installation position. Manhole cover fastening methods include swivel covers and hinged covers; manhole opening speed options include standard and quick-open; the manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.