A portable apparatus for operatively coupling a source of rotary torque to the rim of a valve handwheel comprises a coupling hub, to which three attachment arms are connected by hinge-type joints. The three attachments arms are equidistantly distributed around the perimeter of the coupling hub and extend radially downward and outward from the hub at an acute extension angle to the hub's central axis. The distal end of each attachment arm comprises a clamp, which is releasably attachable to the rim of the valve handwheel. A coupling stem extends upward from the coupling hub in alignment with the hub's central axis. The coupling stem is configured to connect to the source of rotary torque, so as to transmit the torque through the attachment arms to the rim of the valve handwheel. The length of the extension arms and/or their extension angle can be adjusted for various sizes of valve handwheels to be engaged.