Therefore, a lower NPSHR design may appear to be preferred, and a competing pump manufacturer might present a lower NPSHR design as one that automatically translates into construction cost savings because the NPSHA does not need to be increased. Because a lower NPSHR design means a higher suction specific speed (according to the definition above), the design with the highest suction specific speed may seem like the best option. However, this is not always the case.
Figure 3. The impeller eye size affects NPSHR
(the red curve is for the larger eye—better near BEP, worse at low flow).
The upshot of this is that a larger impeller eye decreases the NPSHR at the BEP, but it causes flow separation problems at off-peak, low-flow conditions. In other words, a high suction specific speed design is only better if a pump does not operate significantly below its BEP.
Figure 4. Plant experience indicates that impellers designed with suction specific speeds of more than 9,000 have a poor reliability record.
Suction Specific Speed & Reliability
With few exceptions, a centrifugal pump rarely operates strictly at its BEP, and pumps often do not operate close to their BEPs. Plant flow demands change constantly, and operators often use the discharge valve to control the pump flow. A high suction specific speed design often results in reliability problems because of frequent operation in the undesirable low-flow range. Based on values in the Hydraulic Institute guidelines, if suction specific speeds are above 8,500 to 9,000, pump reliability begins to suffer exponentially (see Figure 4).
Realizing this, around the mid-1980s, users started to limit the value of the suction specific speed, and the Hydraulic Institute uses a suction specific speed of 8,500 as a typical guiding value. More information on this topic will be presented at the next Pump School session.