Radial thrust load skyrockets at low flow, causes deflection of the shaft and leads to seal leaks, bearing life reduction and even shaft breakage. In addition to obvious mechanical problems with recirculation, the flow undergoes a complex vortexing motion at the impeller inlet (eye), with localized high velocities of the vortex causing horseshoe-looking cavitation damage, usually on the "blind" side of the blade, in contrast to to high-flow cavitation. Problems arise when pump operates at too low flow This depends on several design factors, but recirculation generally begins below 80 to 60 percent flow, then becomes quite severe below 40 to 20 percent and appears as surge: violent, low frequency sound, accompanied by strong low frequency vibration of the pump and piping.įigure 6. When a centrifugal pump operates below a certain flow point, flow recirculation in the impeller eye starts. Ample margin of NPSHA is important for a complete range of operation-not just at a single rated flow point The NPSHA margin (M=NPSHA-NPSHR) is important and needs to be at least 5-ft if possible (see Figure 5).įigure 5. NPSHA must be greater than NPSHR for a pump to perform properly, i.e. Pump manufacturers establish NPSHR with tests, and may vary from one pump design to another. NPSHR = (Hsuction - Hvapor), required to maintain 3 percent TDH loss It is the NPSH required to maintain 3 percent TDH loss, per the Hydraulic Institute's specifications. The value of this NPSHA, at which a pump losses 3 percent TDH in excess of vapor pressure, is called net positive suction head required (NPSHR).
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