A seal assembly is provided for a rotary ball valve having a top-mounted bonnet, thereby allowing the seal assembly to be installed and removed without removing the valve from the pipeline. In an embodiment, the seal assembly may include a seal ring disposed within the valve interior and biased toward the ball element of the valve. A seal carrier may be disposed between the seal ring and the valve body, and the seal carrier may be removably secured to the valve body by a plurality of bolts disposed in the valve interior. A primary leak path may exist between the seal ring and the seal carrier and a secondary leak path may exist between the seal carrier and the valve body. A bellows seal may prevent fluid flow through the primary leak path, and an auxiliary seal may prevent fluid flow through the secondary leak path.