The operation of marine outfalls is connected with their high hydraulic performance. However, it could be encountered at low hydraulic performance, especially in some old outfall systems, which were designed as sharp edged ports' diffuser pipes. When these sharp edged old systems are converted to the bell mouthed ports their hydraulic performance could be increased more than their usual performance. Due to the increase of flow capacity of systems their service life would be higher than their initial one. In this study some sharp edged ports' outfall. systems were designed according to various flow rates, then the same systems were converted to the bell mouthed ports' systems, and, the differences of flow rates of the systems were studied theoretically. In projecting of diffuser pipes Black Sea parameters were used. The results, which were taken after changing shape of the ports, were analyzed according to the increase of outfall systems' service lifes and conversion of the sharp edged ports to the bell mouthed ports in outfall systems were discussed.