A turbine assembly having a hollow aerofoil and impingement device, the aerofoil having a first side wall from leading to trailing edge and a cavity arranged a distance to an inner surface of the cavity for impingement cooling and a flow channel for cooling medium from the leading to trailing edge, the impingement device has first and second pieces arranged side by side, the second piece downstream of the first forming a first flow passage providing passage from one side of the aerofoil towards an opposite side. A blocking element is arranged in the flow channel between the second piece and first side wall at a suction side for blocking flow of cooling medium from leading to trailing edge denying access to a section of the flow channel downstream of the blocking element while directing cooling medium in the first flow passage away from the suction side towards pressure side.