Manhole is a safety emergency ventilation device installed on the top of the storage tank. It is usually used together with a fire extinguisher and a mechanical breathing valve. It can not only avoid accidents caused by sudden overpressure or vacuum in the tank, but also play a role of safe fire resistance. It is a safety device to protect the storage tank, especially suitable for the vault atmospheric pressure tank with nitrogen sealed top for storing materials. It has the advantages of constant pressure discharge, constant pressure suction, flexible opening and closing, safe fire resistance, compact structure, good sealing performance, safety and reliability. The manhole of the non-metallic oil tank is set on the tank top, and the manhole of the metal oil tank is set on the lower ring plate of the tank wall. Most of them are round holes with a diameter of 600mm, and the center is 750mm away from the bottom plate. They are used for oil tank cleaning or maintenance personnel to enter and exit the oil tank, also known as doorway. The manhole shall be used for lighting and ventilation at all times during maintenance and cleaning of oil tank. If the capacity of vertical oil tank is less than 5000m3, 1-2 manholes shall be set, and if the capacity is more than 5000m3, 2 manholes shall be set. The manhole shall be installed at a distance of no more than 90 ° from the inlet and outlet oil pipelines. When only one manhole is set, it shall be placed opposite the light transmission hole on the tank top; When two manholes are set, one of them is set on the opposite side of the light transmission hole, and the other shall be at least 90 ° away from the manhole.