Manhole refers to the opening structure used for personnel to enter and exit equipment for installation, maintenance and safety inspection. It is mainly composed of short tube section (or short tube), flange and manhole cover with handle. For safety inspection, the number of manholes required for pressure vessels and their minimum size are shown in "Inspection Holes". Manhole materials are carbon steel and stainless steel. Manholes are divided into normal pressure and pressure manholes according to the pressure condition; manholes are divided into circular, oval and rectangular manholes according to shape, and rectangular manholes are only suitable for atmospheric pressure equipment; manholes are divided into horizontal manholes according to the installation position and vertical installation of manholes; the fixing methods of manhole covers include swivel cover and hanging cover; the speed of manhole opening is ordinary and quick-opening, and the structure and size of manholes are generally standardized.