When liquid is pulled through a pipe, the fluid may swirl. This swirl can extend to the air surface when a pump is pulling from an open reservoir. Free-surface air core vortices can form which will disrupt normal pump operation if they reach a pump’s inlet. Therefore, submergence –the distance between suction inlet and free liquid surface—is an important parameter to consider when designing pumping systems.
This is an additional consideration on top of NPSH requirements – it is possible that a pump meeting NPSHR will not meet the required submergence.
AFT Fathom calculates the required submergence at system boundaries with the following equation, pursuant to ANSI-HI 9.8.7 1998:
S = D+2.3V√(D/g)
Figure 1 – Submergence Calculation
