Marine outfall constructions are commonly used for domestic wastewater discharge into marine environment. Outfall systems are designed and operated according to their working lifetimes. Outfall designs could be affected by many factors. The hydraulic performance of a diffuser pipe, which is the most important part of an outfall, depends on its correct hydraulic design and suitable practical operation. One of the principal design factors is the discharge coefficient that is determined as port (diffuser hole) type. There are a few sources for obtaining the values of discharge coefficient. One of these is the usage of experiment-based graphics, while the other is via the equations modeled from those graphics. The obtaining of discharge coefficient depends on very strict rules and equation-based discharge coefficients only find limited use in design stage. However, obtaining those coefficients by using the equations only is the commonly preferred way in practice, due to its simplicity. This situation may cause some differences from those expected in both design and practice stages. They even may cause the operation lifetime of the plant to be less than the aimed lifetime in its design.