Vertical multistage (VMS) pumps have become popular because they provide increased pumping efficiency along with energy savings. With a variety of nominal flow ratings and the benefit of multistaging, the technology features a wide range of discharge pressures and pump horsepower ratings to cover a multitude of hydraulic applications such as water supply and pressure boosting; heating, ventilation and air conditioning; light industrial; irrigation and agriculture; and water treatment.
Combine one of these fixed-speed pumps with a variable frequency drive (VFD) controller, and the affinity laws will provide even more pump versatility. There is, however, a misconception that a VFD allows the VMS pump to function properly without having to appropriately size it according to the application. While pump sizing is key, users should consider a few other tips to ensure proper safeguards.
Examining a common example of how VMS pumps can be misused and overloaded helps demonstrate the specific effect pump sizing can have in the field. A series of identical VMS pumps, each controlled by a separate VFD and all installed at a single site, had experienced premature motor failures on a number of the pumps. None of the motors had been taken to a motor shop for evaluation. The units purchased were small, 1-horsepower (hp), 2-stage pumps with three-phase, standard-efficient, open drip-proof motors with Class B insulation, capable of 25 gallons per minute (gpm).
The first question asked about these 460-volt VFDs was how far they were located from the motors they controlled and whether there was an output filter included with each VFD. The response was that the pumps were about 50 to 60 feet away from their drives, and no filtering was in place.
An internet search of the VFD's model number provided the owner's manual, which stated that a VFD supplying 460 volts to a motor needed output filtering if more than 25 feet separated the device and the motor. Personnel insisted that the operation had many similar installations but had not experienced multiple motor failures after a few weeks of run time.
The operation had replaced one of the failed motors with an inverter duty motor, and it was still having issues. Personnel also reported that the pumps had not experienced any mechanical issues (tight turning pump/motor shaft, grinding of stages, etc.), so a field service engineer was dispatched to the site to gather more data.
Finding the Cause
The pumps were taking water on a flooded-suction basis from an open-to-atmosphere inlet vessel that collected washdown from a manufacturing process. The pumps transferred the water up into large, low-pressure media filters. The liquid-level transducer tied to the VFD monitored the inlet vessel fluid height and adjusted the pump speed to maintain a specific level in that vessel.
On the pump's discharge side, there was a small elevation change up to the filter inlet and no obvious friction loss. The filters offered little backpressure to the pump flow, even when they were fully loaded just before backwashing.
For the units that remained operable, amp draw was high and the motors ran hot. The cable between each VFD and its pump motor was 125 feet—double the initial estimate received—which further stressed the need for output filtering on each VFD.