A manhole is an opening structure designed for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It primarily consists of short筒节 (or short tube), flanges, and a hand-operated manhole cover. In consideration of safety inspections, pressure vessels require a specified number of manholes and their small dimensions, which are detailed in the "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized as atmospheric or pressure manholes based on the pressure conditions; as circular, elliptical, or rectangular manholes based on their shape, with rectangular manholes suitable only for atmospheric pressure equipment; and as horizontal or vertical installation manholes based on their installation position. The fixing methods for manhole covers include revolving covers and lifting covers; manhole opening speed options include standard and quick-open designs. The structures and dimensions of manholes are generally standardized, and can be selected or designed according to relevant manuals.