A fluid flow control device, comprising: a valve body defining an inlet, an outlet, and a throat disposed between the inlet and the outlet; a control element disposed within the throat of the valve body and adapted for displacement between a first position and a second position for controlling the flow of fluid through the valve body; a trim assembly comprising: a valve seat fixed against axial displacement relative to the valve body and comprising a seat ring portion the seat ring portion defining a first seating surface adapted to be engaged by the control element when the control element is in the first position, and a retention ring connected to the valve seat, characterised in that: the valve seat further comprises an upper ring portion disposed opposite the throat from the seat ring portion, and the retention ring threadably connected to the upper ring portion of the valve seat, the retention ring defining a second seating surface adapted to be engaged by the control element when the control element is in the second position; and a fluid tight seal defined at an interface between the trim assembly and the valve body at a location adjacent to the retention ring, the fluid tight seal provided by at least one of the following: an annular seal comprising a graphite material that is compressed between the retention ring and the upper ring portion of the valve seat, the annular seal sealingly engaging the valve body, and a metal-to-metal contact between an external surface of the retention ring and the valve body. a valve body defining an inlet, an outlet, and a throat disposed between the inlet and the outlet; a control element disposed within the throat of the valve body and adapted for displacement between a first position and a second position for controlling the flow of fluid through the valve body; a trim assembly comprising: a valve seat fixed against axial displacement relative to the valve body and comprising a seat ring portion the seat ring portion defining a first seating surface adapted to be engaged by the control element when the control element is in the first position, and a retention ring connected to the valve seat, a valve seat fixed against axial displacement relative to the valve body and comprising a seat ring portion the seat ring portion defining a first seating surface adapted to be engaged by the control element when the control element is in the first position, and a retention ring connected to the valve seat, characterised in that: the valve seat further comprises an upper ring portion disposed opposite the throat from the seat ring portion, and the retention ring threadably connected to the upper ring portion of the valve seat, the retention ring defining a second seating surface adapted to be engaged by the control element when the control element is in the second position; and a fluid tight seal defined at an interface between the trim assembly and the valve seat further comprises an upper ring portion disposed opposite the throat from the seat ring portion, and the retention ring threadably connected to the upper ring portion of the valve seat, the retention ring defining a second seating surface adapted to be engaged by the control element when the control element is in the second position; and a fluid tight seal defined at an interface between the trim assembly and the valve body at a location adjacent to the retention ring, the fluid tight seal provided by at least one of the following: an annular seal comprising a graphite material that is compressed between the retention ring and the upper ring portion of the valve seat, the annular seal sealingly engaging the valve body, and a metal-to-metal contact between an external surface of the retention ring and the valve body. an annular seal comprising a graphite material that is compressed between the retention ring and the upper ring portion of the valve seat, the annular seal sealingly engaging the valve body, and a metal-to-metal contact between an external surface of the retention ring and the valve body.