Abstract: A safety valve and system is disclosed for hydrocarbon storage tanks having a floating roof construction and provision for drainage of water from the floating roof. The floating roof is constructed to collect water on top and discharge the same through a drain connected to a flexible drain line leading to an outlet opening. If the flexible line should break or otherwise become open to the contents of the tank, there is danger of leakage of the contents from the tank with attendant environmental danger and danger of fire. This invention comprises a valve which responds automatically to the presence or absence of water in the drainage line and is closed in the absence of water or the presence of hydrocarbons in the valve. The valve is motor operated electrically by storage batteries which are recharged by a solar panel. When the valve is empty of water or filled with hydrocarbons, sensors cause the valve to close.