Operating Characteristics of a Centrifugal Pump

CENTRIFUGAL PUMPS Fluid Flow Operating Characteristics of a Centrifugal Pump Normally, a centrifugal pump produces a relatively low pressure increase in the fluid. This pressure increase can be anywhere from several dozen to several hundred psid across a centrifugal pump with a single stage impeller. The term PSID (Pounds Force Per Square Inch Differential) is equivalent to DP. In this context, it is the pressure difference between the suction and discharge of a pump. PSID can also be used to describe a pressure drop across a system component (strainers, filters, heat exchangers, valves, demineralizers, etc.). When a centrifugal pump is operating at a constant speed, an increase in the system back pressure on the flowing stream causes a reduction in the magnitude of volumetric flow rate that the centrifugal pump can maintain. Figure 7 Typical Centrifugal Pump Characteristic Curve Analysis of the relationship between the volumetric flow rate ( ) that a centrifugal V pump can maintain and the pressure differential across the pump (DP_pump) is based on various physical characteristics of the pump and the system fluid. Variables evaluated by design engineers to determine this relationship include the pump efficiency, the power supplied to the pump, the rotational speed, the diameter of the impeller and blading, the fluid density, and the fluid viscosity. The result of this complicated analysis for a typical centrifugal pump operating at one particular speed is illustrated by the graph in Figure 7. Pump head, on the vertical axis, is the difference between system back pressure and the inlet pressure of the pump (DP_pump). Volumetric flow rate ( ), on the horizontal axis, is the rate at which fluid is flowing through the pump. The V graph assumes one particular speed (N) for the pump impeller. Cavitation When the liquid being pumped enters the eye of a centrifugal pump, the pressure is significantly reduced. The greater the flow velocity through the pump the greater this pressure drop. If the pressure drop is great enough, or if the temperature of the liquid is high enough, the pressure drop may be sufficient to cause the liquid to flash to steam when the local pressure falls below the saturation pressure for the fluid that is being pumped. These vapor bubbles are swept along the pump impeller with the fluid. As the flow velocity decreases the fluid pressure increases. This causes the vapor bubbles to suddenly collapse on the outer portions of the impeller. The formation of these vapor bubbles and their subsequent collapse is cavitation. HT-03 Page 48 Rev. 0