In a nuclear steam generating vessel, a baffle plate disposed a slight distance above the tube sheet has a plurality of openings therethrough for respectively receiving the heat exchanger tubes. Each opening has a circular central portion and eight equally spaced peripheral lobe portions alternating with eight lands. The inner end of the lands lie on a circle have a diameter slightly greater than that of the associated tube. The upper and/or lower surface of the baffle plate is mechanically deformed adjacent to each lobe portion of each opening to displace material into the lobe portions and constrict the cross sectional areas thereof to limit liquid flow therethrough. The lands are dimensioned and arranged so that one or two at a time can contact the associated tube, while the constriction of the lobe portion flow areas limits liquid flow therethrough sufficiently to insure proper baffling operation of the baffle plate.