gate valve is opened by angularly rotating the valve stem 18 angular rotation of the valve stem 18 in a counterclockwise direction as viewed from the top of the control wheel 46 axially moves the valve stem relative to the flow restriction members x y and z the pitch of the valve stem threads 45 initially unseats the valve 40 and further angular rotation of the stem positions the upper end limit of the respective valve stem slots 68 above the top end surface 55 of the respective restriction member x y and z for initial opening or 34 cracking 34 of the gate valve fluid initially flows through the respective valve stem slot 68 and recess 56 while further rotation of the valve stem disposes the upper end limit of the respective valve stem slots 69 above the plane of the top surface 55 of the members x y and z the volume of fluid flow through the passageway is controlled by the width and depth of the slots and their position with respect to the end limit 55 of the members x y and z the maximum flow volume being obtained when the medial portion of the respective slots are in substantial alignment with the surfaces 55 as indicated by dotted lines fig1 this step by step successive positioning of all the slots 69 71 in a flow opening mode results in accurate control of fluid pressure differential across the valve which may.