One of the supposed advantages of centrifugal pumps when compared to positive displacement pumps is their ability to operate across a wide flow range. Because a centrifugal pump operates at the intersection of a pump curve and a system curve, varying the system curve allows the operating point of the pump to change easily using the discharge valve. The convenience and simplicity of flow control by throttling the discharge valve comes at a price because a pump is forced to run either to the left or right of its best efficiency point (BEP). However, the real danger of operating the pump too far off the BEP is suction side issues. If it operates too far to the right, the pump may exhaust its net positive suction head available (NPSHA), which may result in cavitation. If it operates too far to the left, flow recirculation at the impeller eye will occur and cause noise, vibration and damage. Therefore, the flow must be limited on both sides of the BEP (see Figure 1).
07/02/2014
Figure 1. Pump operating range limits
Figure 2. The development of cavitation
The formation of cavitation begins inside the pump well before the sudden drop of head, but it is not initially obvious. First, at substantial suction pressure, small bubbles form. This is called incipient cavitation—similar to the tiny bubbles in the water in a kettle that begins to percolate before the water is fully boiling. These small bubbles form and collapse at very high frequency and can only be detected with special instrumentation.
As pressure decreases further, more bubbles form. Eventually, so many bubbles have formed that the pump inlet becomes vapor locked. No fluid can enter the pump, and the pump stops pumping. The head drops and quickly disappears. Ideally, enough pressure would always be available at the suction so that no bubbles ever form. However, this is not practical, and some compromise must be reached.
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.
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.