11.3 Suction specific speed
The term suction specific speed (nss) expresses the suction capability of the pump impeller and is defined by:
High numerical values of nss are associated with large impeller diameters and improved suction capabilities around pump best efficiency (ВЕР). The large eye diameter can, however, lead to problems of inlet flow recirculation under off-duty conditions, with consequential pulsations in the pump and adjacent pipework. At flowrates away from ВЕР, incidence causes flow separation within the impeller. This separated flow is then directed backwards out of the eye. The larger the impeller eye, the nearer to the ВЕР is the flow at which this recirculation occurs.
To prevent the damaging effects of inlet flow re-circulation, leak-off flowrates need to be sized so as to ensure that pump throughput is always in excess of the amount at which recirculation commences. This applies particularly to both feed pumps and extraction pumps.
In the absence of detailed information of pump capability at the tender stage, the CEGB normally requires that the suction specific speed based on the NPSH available to the pump should be limited to 2.4 (ISO).