A valve assembly includes a valve body with an upstream inlet and a downstream outlet and a valve element which is engageable with and disengageable from a valve seat to open and close the valve assembly. The valve body defines a bore with an axis for receiving the valve element. At least part of the bore and of the valve element have cooperating shapes which permit axial movement of the valve element in the bore and prevent rotation of the valve element in the bore. A rotatable actuator is provided, with a connection between the actuator and the valve element operable to convert rotation of the actuator into axial translation of the valve element.