The single most effective method of preventing cavitation in the operation of a centrifugal pump when handling refrigerants close to their saturation condition is to provide an adequate amount of net-positive-suction head (NPSH). The NPSH is defined as the difference in elevation between the liquid level of the centerline of the pump and the liquid level in the vessel supplying the pump. The function of the column of liquid is to provide the static head that increases the pressure above saturation enough to inhibit vaporization of liquid in the passages of the pump.
The pump manufacturer will determine by calculation or by laboratory test the minimum NPSH. The individual factors that would be included in a calculation are the following:
• pressure drop in the liquid leg, turn into the pump, and the open shutoff valve
• friction loss in the inlet connection of the pump
• acceleration of the liquid from entrance to just before the blade row
• influence of centrifugal force due to rotation of the impeller
• Coriolis force because of turn from axial to radial flow direction
The user of a pump can obtain from the manufacturer data such as shown in Figure 8.18, which provides insights in addition to the minimum values of NPSH at various operating points. The NPSH is essentially a function of flow rate and increases as the flow rate increases because all the individual terms mentioned above increase with flow rate. The first conclusion is that a pump does not have a unique NPSH, but the value depends where on its operating curve it is performing at the moment.
The pump whose performance is shown in Figure 8.18 can develop pressure differences that would typically be required in a liquid recirculation system, but the NPSH requirements are quite stringent, because some safety factor should be applied to the manufacturer’s recommendations, perhaps doubling them since the pump manufacturer has no knowledge of the pressure drops in the piping leading up to the pump. Furthermore, achieving more than about 1.5 m (5 ft) of positive suction head because of the physical restrictions in the plant is usually difficult. The pump is small with an impeller diameter of 152 mm (6 in), which is appealing, but in order to develop the necessary pressure difference it must operate at 3520 rpm. This leads to the practice of normally choosing an operating speed of 1750 rpm, which necessitates a pump of larger impeller diameter. The NPSH values for the slower-speed pump would be 1/3 to 1/2 the values shown in Fig. 8.18.
Also shown are the head-flow curves for two systems. Actually these curves may represent the same physical situation, but in one case many of the solenoid control valves are open and in the other most are closed. The balance points or
operating points are the intersections of the piping curve with the pump curve and are represented by points A and B. The insight provided by the positions of A and B on the map is that when the piping network is drawing a high flow
rate, operation moves to a condition requiring a high value of positive suction head. The pump may operate properly at the low-flow condition but cavitate at high-flow.
Usually designers try to select the maximum-flow operating point rather high on the performance curve, preferring Point A over Point B. Not only is the risk of cavitation reduced, but the pump will operate with greater efficiency higher on its curve. For the pump shown in Figure 8.18, Point A is close to its point of maximum efficiency (48%), while the efficiency at Point B is 35%. It is true that if maximum flow exists at Point A, which is at maximum efficiency, the efficiency drops off as the flow rate required by the piping network diminishes.