A valve device includes a valve body having a cavity, a stopper member in the cavity, and a fluid passage in fluidic communication with the cavity when the valve device is in an open state. A stem is coupled to the stopper member for driving the stopper member to open or close the valve. The stem extends through a stem aperture in the valve body. A first self-expanding seal around the stem is used to seal off an annular gap around the stem. A second self-expanding seal and a third self-expanding seal are disposed around the fluid passage substantially concentrically to prevent leakage. A method for assembling and sealing the valve device, and a valve system are also provided.