A manhole refers to an opening structure designed for personnel access to equipment for installation, maintenance, and safety inspections. It is mainly composed of short筒节 (or short pipe), flanges, and a handle-equipped manhole cover. In terms of safety inspections, pressure vessels are required to have a certain number of manholes and their small sizes as specified in "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized into atmospheric and pressurized types based on the pressure they are subjected to; into round, oval, and rectangular types based on their shape, with rectangular manholes being suitable only for atmospheric equipment; and into horizontal and vertical installation types based on their installation position. The fixing methods for manhole covers include swinging covers and lifting covers; the opening speed of manholes can be standard or quick. The structures and dimensions of manholes are generally standardized and can be selected or designed according to relevant manuals.