A manhole refers to an opening structure designed for personnel access and egress, as well as for installation, maintenance, and safety inspections. It is primarily composed of short筒 sections (or short pipes), flanges, and a handwheel-equipped manhole cover. In consideration of safety inspections, pressure vessels require a specified number of manholes and their small sizes, which are detailed in the "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure as either atmospheric or pressure manholes; by shape as circular, elliptical, or rectangular (with rectangular manholes suitable only for atmospheric equipment); by installation position as horizontal or vertical; and the manhole cover is fixed either by a swivel cover or a suspended cover. The opening speed of the manhole can be standard or quick-open. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.