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).
Figure 1. Pump operating range limits
To avoid cavitation, suction pressure alone is not what is most important. How much higher the suction pressure is than the vapor pressure of the liquid being pumped is what must be considered. Net positive suction head (NPSH) is used. The NPSHA, therefore, is the difference between NPSH and vapor pressure, expressed as head in feet.
Pump manufacturers conduct tests by gradually lowering the suction pressure. As pressure decreases (the NPSHA lowers), nothing obvious happens. A pump, operating at a set flow, continues to pump and develops constant head. When the value of the suction pressure and corresponding NPSHA reaches a certain value, the pump head begins to drop, which typically happens suddenly (see Figure 2).
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.
The Hydraulic Institute (HI) has established a special significance to a particular value of NPSHA at which the total developed pump head drops by 3 percent. The value of this NPSHA, at which a pump loses 3 percent total dynamic head (TDH), in excess of its vapor pressure, is the net positive suction head required (NPSHR) to maintain a 3 percent TDH loss.
NPSHR = (Hsuction – Hvapor), required to maintain 3 percent TDH loss
NPSHR is, therefore, established by a test and may vary from one pump design to another. In contrast, the NPSHA is not related to a pump type but is strictly a calculated value of total suction head over vapor pressure. Clearly, the NPSHA must be greater than the NPSHR for a pump to deliver a TDH at a given flow.
Detecting NPSH problems is easy—a pump stops pumping. However, the vapor bubbles do not need to be dramatically developed to cause TDH drop—even smaller bubbles can cause pumping issues. If the pressure shock from the bubbles’ collapse occurs near the metal impeller blade, it causes a microscopic impact, eroding a small particle of metal. With enough bubbles and enough time, the impeller vanes can be eroded quickly, a phenomenon known as cavitation damage.