Ingeniero

Figure 1 shows the S calculation for an 8-inch, 1,780-rpm wastewater pump with a BEP flow and head of 3,000 gallons per minute at 135 feet and a specific speed (Ns) of2,450. BEP efficiency and NPSHr are 82 percent and 10 feet. The calculated value for S is 17,337.

Figure 1. Suction specific speed calculation for a wastewaterpump
Figure 2 shows the minimum continuous stable flow (MCSF) for pumps with a given Ns and S. MCSF is the flow at which the onset of suction recirculation can begin. TheY-axis is S and the X axis is percent of BEP flow. The three curves represent various pump Ns. Note that MCSF is dependent upon both Ns and S.

Figure 2. Minimumcontinuous stable flow
The example in this column has an Ns of 2,450, so the upper curve will be used. The red horizontal line at Y = 17,337 intersects the curve at X =80 percent. Therefore, this pump could potentially begin suction recirculation when the flow drops to just 80 percent of BEP flow. If the suction energy ratio (Pumps &Systems, February 2010) of this pump is examined, an NPSHa to NPSHr margin of 4.0 could be required to provide stable operation at or below 80 percent of BEP flow.Many higher flow pumps with relatively low values of S can still exhibit an increase in NPSHr as flow is reduced. For example, the performance curve for an 18-inch by16-inch wastewater pump (Ns= 2,735) with a BEP flow of 16,000 gallons per minute at 200 feet shows an NPSHr of 31 feet. The calculated value of S is 11,072, which predictsan MCSF of approximately 50 percent. However, at 13,000 gallons per minute (a flow reduction of just 19 percent), the NPSHr, as shown on the curve, increases to 44 feet.