This is typical of the problems with soft start or soft stop. Head is lost or created by the square of the pump speed. A pump does not produce enough pressure to open the check valve until the RPM is at 80 to 90% of full pump speed. When a pump is ramped up slowly to full speed, it is only the last couple of seconds that actually produces enough head to move water. This makes ramping up the pressure a delicate procedure. First you must determine at what RPM the pump will actually build enough pressure to buck the static head. Then you must convert this calculated RPM into hertz. Now you must bring the pump up to this speed very quickly, then slowly ramp up from this speed to the required speed. The slightest miscalculation of RPM, either to high or too low, or any change in the static pressure, will cause the pressure to surge. Any delay in producing the flow required after start up, will cause a negative pressure wave, and a subsequent pressure surge. The same thing happens with a soft stop, only in reverse.
A Cycle Stop Valve has a hydraulic soft start and soft stop. It starts and stops the pump at 5 GPM, regardless of the static pressure. This eliminates pressure surges or water hammer, that can't be eliminated with electronic soft start or ramping up of a VFD. After start up, the CSV quickly opens up to maintain a constant pressure, before a negative pressure wave can accumulate. Hydraulic soft start and stop are just that. After all, isn't the fluid what needs to be started or stopped softly, not the motor?
