the surfaces efficiently prevents fibers from accumulating in the flow space when the valve is open and liquid or pulp suspension is fed into the valve the flow passes the inlet passage 2 when the flow joins tangentially the space inside the valve body it forms a vortex and strong turbulence the clogging of the valve by fibers is prevented in this way in fig4 a it can be seen that when the closure member is turned to an angle of 40 u00b0 as shown in fig4 b the valve is fully closed at the position of 0 u00b0 the valve is half open and at the position of 35 u00b0 the valve is fully open as shown in fig4 c the opening of the valve takes place by rotating the closure member in a plane which is perpendicular to the center line of the valve body and the outlet passage the flow opening is first formed on that edge of the opening of the inlet passage which tangentially joins the outlet passage while the closure member is rotated further and the flow opening increases the tangential flow is maintained all the time as shown in fig4 d the flow volume is shown relative to the angular position of the closure member in the embodiment of fig5 and 6 the closure member 6 39 at the end of the plug 4 is sleevelike and has an opening 11 on its side the size and shape of the opening correspond to the size and shape of the end of the inlet passage the inner surface of the closure member 4 39 is.