One of the claimed advantages of centrifugal pumps over positive displacement pumps is their ability to operate over a wide range of flow rates. Because a centrifugal pump operates at the intersection of the pump curve and the system curve, varying the system curve can easily change the operating point of the pump. The convenience and simplicity of flow control by discharge valve throttling comes at a price because the pump is forced to run either to the left or to the right of its best efficiency point (BEP). However, the real danger of operating the pump too far off-peak comes from suction side considerations: too far to the right, and you risk running out of net positive suction head available (NPSHA ), causing cavitation problems; too far to the left, and flow recirculation at the impeller eye will become evident through noise, vibration and damage. As a result, the flow must be limited on both sides of the BEP.
Pumping Prescriptions
07/20/2016
Figure 1. Flow control of the centrifugal pump by the discharge valve (Graphics courtesy of the author)
Figure 2. A pump operating range has limits.
Figure 3a. Open tank Figure 3b. Pressurized tank Figure 3c. Tank under vacuum
Figure 4. Development of cavitation
Knowing vapor pressure without relating it to a corresponding temperature is meaningless. Sometimes it is good to have a tabulation, or a graph, showing the relationship between the vapor pressure and temperature—the higher the temperature, the higher the vapor pressure.
A centrifugal pump is a “pressure generator,” produced by the centrifugal force of its rotating impeller. The pressure gets higher as flow progresses from the suction to discharge. This is why vaporization of liquid is most likely to happen in the inlet (suction) region, where the pressure is lowest.
In practice, it is difficult to know exactly when vaporization (cavitation) happens, so it is wise to keep some margin of available suction pressure over vapor pressure. The pressure (expressed in feet of water) is called discharge head at the pump exit side, or suction head on the inlet side. The difference is a pump-developed head, also called total dynamic head (TDH). These heads must include both static and dynamic components.
For water and other low-viscosity liquids, suction losses are usually low and often disregarded. For more viscous substances such as oils, these losses can be substantial and cause the pressure in front of the pump to drop below the vapor pressure, leading to cavitation. The inlet velocity must be minimized because the losses depend on velocity squared, which is essentially suction head dynamic energy.
Longer pipe runs, bends, turns and other restrictions add to inlet losses, leading to further pressure reduction in front of a pump. To avoid cavitation, what matters is not the suction pressure but how much higher the suction pressure is than the vapor pressure of the liquid being pumped. This is where the concept of NPSH comes in handy. NPSHA is simply the difference between this total suction head and vapor pressure, expressed as head, in feet.
Pump manufacturers conduct tests by gradually lowering suction pressure and observing when things get out of hand—the pump head would eventually begin to drop. For a while, as pressure decreases (i.e. NPSHA gets smaller), nothing obvious happens. A pump, operating at a set flow, keeps pumping and develops constant head. At some point, when the suction pressure (and corresponding NPSHA) reaches a certain value, pump head begins to drop. This typically happens rather suddenly.
Figure 5. Ample margin of NPSHA is important.
Figure 6. Problems come up when a pump operates at too low flow.
In addition to obvious mechanical problems with recirculation, the flow undergoes a complex vortexing motion at the impeller inlet, with localized high velocities of the vortex causing horseshoe-shaped cavitation damage. This damage usually occurs on the “blind” side of the blade, as compared with damage caused by high-flow cavitation. Another potential problem is radial thrust, which skyrockets at low flow and causes shaft deflections, which can lead to seal leaks, bearing life reduction and even shaft breakage.
Troubleshooting methods and failure analysis techniques help to pinpoint cavitation problems with a particular pump. The indications of high-flow cavitation are different from those of low-flow recirculation damage.
The extent and shape of the cavitation trough can be helpful in determining the causes of each individual problem.
Visit http://pumpingmachinery.com/pump_school/pump_school.htm for information regarding cavitation and air entrainment demonstrations.
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