The results of a study on the equalization of a non-uniform steam load using different submerged perforated sheets (SPSs) are presented for a full-scale horizontal WWER-1500 steam generator (SG). The calculations performed with the STEG code are used. A short description of the 3D thermal-hydraulic code STEG is presented. A new model of gravitational separation based on empirical correlations for the size distribution of water droplets and for the modal droplet diameter is implemented in the STEG code. Three variants of the distribution of an SPS perforation degree are analyzed. Various characteristics are compared: the distributions of the superficial velocity on the SPS and evaporation surface, the fractions of the steam flow rate with high velocity in the steam region and the steam moisture at the outlet from the SG. The SPS perforation degrees, which significantly improve the equalization of the non-uniform steam load, are found using the STEG code.