Cavitation happens when the NPSHA is less than the NPSHR. The NPSHR is actually reduced as the flow is reduced. Many times just reducing the discharge flow rate of a centrifugal will eliminate cavitation. Ie; gravel noise from the pump, close discharge valve, gravel noise goes away. Reducing the discharge flow rate, no matter which kind of valve you use, will reduce NPSHR and not cause cavitation.
At 1 GPM, what you get is more recirculation than cavitation. Recirculation can cause cavitation like wear but, usually on different areas of the impeller. The fact that the CSV always allows 1 GPM, never 0 GPM, keeps enough cool water passing through the pump that the temperature rises very little. As long as the water stays cool, it doesn’t flash when re-circulating, which could cause wear.
Impellers made with materials of high tensile strength are more resistant to wear from cavitation. Soft brass might wear away when Stainless Steel and plastics, which are normal materials for impellers, work very well. Some impellers are made with an acetal plastic that is even self lubricating.
Actually it is the increase in the number of impellers that requires more flow for cooling. A single impeller in an end suction type pump doesn’t touch anything. The only heat produced is from the friction between the water and the tip of the impeller blades. Cool water has great lubricating factors. So again, as long as you have 1 GPM of cool water passing through, there is great lubrication and very little friction or heat.
With pumps that have multiple impellers, there is normally a rubber bushing touching the shaft between each impeller. The more impellers, the more bushings, and the more heat produced. So a pump with 20 impellers, needs more cooling flow than a pump with a single larger impeller.
A pump with multiple stages also builds more pressure than a pump with fewer impellers. The 1 GPM through a CSV is just an average. It will change from .4 to 1.8 GPM, depending on the differential pressure. So a pump with many impellers will get 1.8 GPM for cooling, while a low head pump will only have .4 GPM for cooling. This way the CSV keeps the minimum flow as low as possible, while automatically adjusting to match the pumps cooling requirements.