A hermetically sealed solids discharge centrifugal separator having a separator bowl with a cylindrical upper portion and a conical lower portion, an inlet port at the bottom of the bowl and an outlet port at the top of the bowl. A disposable liner assembly includes a bowl liner having a cylindrical upper portion and conical lower portion, such that the bowl liner can conform to the interior surface of the separator bowl. The liner assembly further includes a piston within the bowl liner, the piston having an upper portion that conforms to the bowl liner cylindrical upper portion and a lower portion that conforms to the bowl liner conical lower portion. A fluid pathway extends through the piston, with a shuttle valve disposed within the fluid pathway.