An assembly is provided for performing start-up testing on a fluid flow control device. The assembly includes a retainer adapted to be disposed in a valve body of the fluid flow control device, and a sealing assembly coupled to a portion of the retainer and adapted to be disposed adjacent a valve seat of the fluid flow control device. The sealing assembly includes one or more sealing elements configured to provide a seal during start-up testing at a service temperature of between approximately −325 degrees Fahrenheit and approximately 1250 degrees Fahrenheit.