Manhole is a safety emergency ventilation device installed on the top of a storage tank, usually used in conjunction with fire extinguishers and mechanical breathing valves. It can not only prevent accidents caused by sudden overpressure or vacuum in the tank due to unexpected reasons, but also play a role in safety fire prevention. It is a safety device for protecting the storage tank, especially suitable for storing materials in nitrogen capped dome atmospheric pressure tanks. It has the advantages of constant pressure discharge, constant pressure suction, flexible opening and closing, safe fire resistance, compact structure, good sealing performance, and safety and reliability.