Centrifugal Pumps Selection
Centrifugal pumps are the most common type of kinetic pump, and are used most often in applications with moderate-to-high flow and low head. These type of pumps are almost always more economical to own, operate and maintain than other types of pumps.
The process engineer is normally responsible for specifying the process requirements of the pump, including the conditions and physical properties of the liquid, and, most importantly, the ﬂowrate, pressure, density and viscosity. The ﬂowrate determines the capacity of the pump, and the head depends on the density and viscosity of the liquid.
In general, the required ﬂowrate is determined by the material and energy balances. Design margins, typically between 0–25%, are added to the material-balance ﬂowrate to account for unexpected variations in properties or conditions, or to ensure that the overall plant meets its performance criteria. Also, minimum ﬂow protection is often added as continuous circulation.
During speciﬁcation, the maximum pressure a pump will develop during any aspect of operation, including startup, shutdown and upset conditions is determined. The shutoff pressure is the maximum pressure a pump will develop under zero-ﬂow conditions, which reﬂects a fully blocked outlet.
The energy that the pump imparts to the liquid, the total dynamic head, TDH, takes into account differences in pressure, liquid elevation and velocity between the source and destination. In addition, TDH accounts for line (friction) losses and the pressure drop through the different items in the ﬂow path of the liquid.
Further, the pump must be specified to avoid cavitation. Cavitation occurs when the suction pressure of the pumped fluid drops below its vapor pressure, leading to the formation of vapor bubbles. As the fluid becomes pressurized again in the pump, these bubbles implode, leading to pitting of the impeller and other pump components. In addition, since vapor has a lower density than liquid, cavitation leads to a reduction in the pump capacity and efficiency.
Net Suction Height
The net positive suction head (NPSH) is a measure of the proximity of a liquid to its bubble point (or vapor pressure). Pump suppliers set the NPSH required (NPSHR) for a given pump. The NPSHR takes into account any potential head losses that might occur between the pump’s suction nozzle and impeller, thus ensuring that the ﬂuid does not drop below its vapor pressure. NPSHA must exceed the NPSHR set by the supplier.
Source: “Understand the Basics of Centrifugal Pump Operation”
Kimberly Fernandez, Bernadette Pyzdrowski, Drew W. Schiller and Michael B. Smith