Manholes are openings designed for personnel entry and exit, facilitating installation, maintenance, and safety inspections. They are primarily composed of short pipe sections (or short pipes), flanges, and lids with handles. For safety inspection considerations, pressure vessels are required to have a specific number and size of manholes, which can be found under "Inspection Ports." Manhole materials include carbon steel and stainless steel. Manholes are categorized as either atmospheric or pressurized based on pressure conditions; as circular, elliptical, or rectangular in shape; rectangular manholes are only suitable for atmospheric equipment; as horizontal or vertical in installation position; and the lid fixing methods include revolving covers and lifting covers. The opening speed of manholes can be either standard or quick-opening. Manhole structures and dimensions are generally standardized, allowing selection from relevant manuals or design.