This damage potential is why an NPSHA margin (M = NPSHA — NPSHR) is important. This margin is typically at least 3 to 5 feet, and if possible, it should be higher (see Figure 3).
Figure 3. Ample NPSHA
margin is important.
The NPSHR was limited to a particular flow on a pump performance curve. At higher flow, the internal fluid velocities are higher, and according to Bernoulli, the static pressure (or static head) decreases closer to vapor pressure. The static pressure, therefore, must be increased externally—a higher NPSHR value is needed for higher flows. Figure 4 shows an example of the NPSHR curve shape.
A pump was procured and designed to deliver between 350 to 500 gallons per minute (gpm), and the manufacturer quotation indicated 16 feet of NPSHR at 500 gpm. Because the process later changed, more flow was required, and the discharge valve was opened to allow this pump to deliver more flow (750 gpm). However, as can be seen in Figure 4, at about 700 gpm, the NPSHR exceeded the NPSHA. The pump began to experience typical NPSH problems—noise, loss of performance and impeller cavitation damage.
Instinctively, a solution for the cavitation was to replace the original pump with a larger one so that the flow would remain to the left of the BEP. This larger pump provided the same 16 feet of NPSHR. However, at a flow rate of 750 to 800 gpm, the larger pump would never run out of NPSHA.
When a centrifugal pump operates below a certain flow point, flow recirculation in the impeller eye begins. This depends on several design factors, such as suction specific speed, but generally recirculation begins at less than 80 to 60 percent flow. It becomes quite severe at less than 40 to 20 percent. At even lower flows, recirculation may become especially severe and is known as surge-violent, low-frequency sound, accompanied by strong low-frequency vibration of the pump and piping (see Figure 4).
Figure 4. Problems arise when a pump operates at flows that are too low.
In addition to obvious mechanical problems with recirculation, the flow experiences a complex vortexing motion at the impeller eye with localized high velocities of the vortex causing horseshoe-looking cavitation damage, usually on the blind side of the blade, as compared to high-flow cavitation.
Troubleshooting methods and failure analysis techniques can help pinpoint a cavitation problem with a particular pump. The indications of high-flow cavitation are different from low-flow recirculation damage. The side of the blades and the extent and shape of the cavitation trough can be helpful in determining the causes of each problem.
At the next Pump School, I will cover specific examples that compare static-head-dominated, friction and combined systems. To register, visit www.pumpingmachinery.com.