UPGRADING SYSTEMS


of excess discharge pressure. Various factors are used by designers for a variety of reasons:


• future pump wear – Pumping sand wears pump impeller and bowl material, eventually decreasing the pump capacity. An additional 5 feet of total dynamic head is typically used to account for this.


• safety factor – There is always some uncertainty in the design process, including hydraulic computations, manufacturer- provided pump curves and other factors. An additional 12 feet of TDH is typically used to account for these uncertainties.


Total dynamic head — TDH — is the pressure necessary to overcome the friction losses in pipes and fittings, changes in elevation between the water source and the point at which water is discharged, and the pressure required by the sprinklers or emission devices to operate correctly.


In total, that is an extra 17 feet of TDH added to the design operating point. Operating a slightly oversized single-speed pump consumes extra energy. With a VFD, the pump can be oversized but operated at a lower than normal speed to maintain the target flow rate or pressure. As things change, the VFD-controlled pump speed can be increased or decreased to meet demand.


For a typical 50- to 160-acre drip-irrigated field on flat ground, a typical booster pump discharge pressure is 45 psi, or 104 feet of TDH (ITRC Report No. R11-005, available at www.itrc.org/ reports/vfdanalysis.htm). By including additional TDH for lift, minor losses and pipe friction, a typical booster pump will have a design TDH of about 120 feet. Therefore, it is reasonable to select


Table 1. Scenarios where a VFD may be appropriate and should be considered


SCENARIO


There are several target irrigation/fertigation flow rates or pressures where


 a valve adjustment (throttling or bypassing the pump) is required to achieve a target flow or pressure. or


 extra pressure is developed by the pump and provided to all emitters, above and beyond the pressure needed to achieve good distribution uniformity.


Operating a well pump where


 the pumping level (static depth and/or drawdown) is expected to change over time. or


 stressing the well casing with rapid flow increases at pump startup is a concern.


Water hammer is a concern (old, fragile or low-pressure pipes). VFDs can provide slow (ramped) start and stop capabilities. VFDs can rapidly compensate for varying inlet pressures.


The water source pressure (district or municipal) fluctuates over time with a constant irrigation flow rate.


The irrigation pump is slightly undersized or worn but otherwise functional.


Across-the-line starting of the pump motor is prohibited by the utility.


irrigationtoday.org VFDs can overspeed motors.


Starting large horsepower motors requires large inrush currents and can affect local electric grid performance.


Fall 2019 | Irrigation TODAY 25 COMMENTS


Pumps are selected to meet the “worst case” demand, or the largest target flow or pressure. Excess energy is developed by the pump and consumed by throttled/bypass valves or by pressure compensating emitters. Instead, a VFD can be used to reduce pump speeds to achieve several target flows or pressures.


VFDs can compensate for varying pump levels and can provide slow start capabilities.


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