An air cannon has a pressure vessel, a discharge valve, and a discharge tube assembly. The discharge tube assembly comprises a fluid passageway and a blast guard valve. The discharge valve is capable of opening and closing and allows pressurized gas to be discharged from the pressure vessel into the fluid passageway of the discharge tube assembly only when open. The blast guard valve is capable of opening and closing and obstructs the fluid passageway of the discharge tube assembly when closed. The blast guard valve allows gas to pass through the fluid passageway when open. The discharge valve and the blast guard valve are operatively connected to each other in a manner such that the discharge valve can open only when the blast guard valve is open.