Abstract: A rotary valve comprising a spherical segment supported on arms extending radially from aligned stub shafts. Carried by the arms is a vane which is circular in outline and of maximum thickness along the diameter parallel to the axis of the shaft, tapering to lesser thicknesses in both directions transverse of that diameter. In addition, the vane is tilted about the maximum thickness diameter in the rotary direction opposite to valve opening movement, so that it remains exposed to the flow stream through full movement of the plug. The flow stream acting against the vane biases the plug toward closed position, cancelling the effects of the flow stream acting against the plug itself.