Abstract: Disclosed is a gate valve for controlling flow, providing a dual seal, including a valve body having a discharge orifice. A gate assembly, located upstream of the discharge orifice, is movable between open and closed positions remote from and blocking the discharge orifice, respectively. The gate assembly includes upper and lower spaced flexible sealing disks which are attached to a relatively rigid upstream backup plate. Radially inner and outer valve seats are secured to the valve body adjacent the discharge orifice. When blocking flow through the valve body, each gate is wedged in sealing engagement with a respective valve seat and a chamber is formed between the two seals to receive seepage past the upstream seal. The chamber may be either evacuated or pressurized to prevent flow of controlled material past the lower or downstream seal.