A manhole is used for cleaning or maintenance personnel to enter and exit for inspection of storage tanks, oil tanks, container tanks, etc. Manholes are usually installed on the top or tank wall of container tanks. It is a safety device for protecting storage tanks. A manhole is also an emergency ventilation device installed on storage tanks, usually used in conjunction with flame arresters and mechanical breather valves. Manholes are divided into emergency pressure relief manholes, explosion-proof flame-retardant breathing manholes, tank-top manholes, tank-wall manholes and core manholes.