I have installed 2 x large pumps 75KW (111L/s) under 2 x open circuit cooling towers
The pumps are on VSD's
If I switch on and ramp up the pumps slowly 1 pump or 2 pumps at the same time everything works very well...no noise, with no apparent cavitation
If however I ramp up a pump to full speed fast, the pump make noise and it appears there is air in the pipes and pump impeller (the gauges shake)
There is also noise and shaking if 2 pumps are running with no issues and suddenly I ramp down and switch off one of the pump, the one remaining ON makes noise, and appears to have cavitation
I have checked the NPSH and all looks good according to calculations.
Why is it an issue when the pumps are stopped/started suddenly and not when it ramps up or down slowly