Manholes are openings designed for personnel entry and exit to facilitate installation, maintenance, and safety inspections. They are primarily composed of short tube sections (or short pipes), flanges, and manually operated handholes with handles. In terms of safety inspections, pressure vessels are required to have a specified number of manholes and their small dimensions are detailed in the "inspection openings." Manhole materials include carbon steel and stainless steel. Manholes are categorized based on pressure conditions into atmospheric and pressurized manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes suitable only for atmospheric pressure equipment; and by installation position into horizontal and vertical installations. The types of manhole cover fastenings include swivel covers and lifting covers. The opening speed of manholes can be either standard or quick-opening. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.