Hydraulic performance characterisation of variable rate irrigation technology

Hydraulic performance characterisation of variable rate irrigation technology

Variable-rate irrigation (VRI) technology for centre pivots and lateral moves has been commercially available for over three decades, but its uptake has been limited, with growers citing additional capital and operating costs, the detrimental impact on hydraulic and irrigation performance, as key issues. The hydraulic performance of VRI components are poorly understood in the irrigation engineering community, as evidenced by the common process of simply retro-fitting VRI technology onto existing centre pivots and lateral move machines, without any acknowledgement of the impact these have on the existing hydraulic system. The need to characterise the hydraulic performance of VRI components is essential to allow the accurate hydraulic design of this irrigation equipment for reduced energy consumption, and high irrigation uniformity. As well, the hydraulic performance of VRI components under unsteady hydraulic conditions are even more poorly understood, and this also warranted investigation.

An automated test-rig was developed to capture the hydraulic characteristics of modern VRI components for centre pivots and lateral moves. Highly accurate, calibrated flow and pressure sensors were incorporated into the test-rig. The ability to capture data at high frequency was a unique capability of the test-rig, and was developed specifically to study the unsteady nature of the hydraulic performance of VRI components, and 228 separate tests were completed.

The extensive measured data sets for VRI components, across three different manufacturers, two models, six discharges, and pressures ranging up to 30 m head, provided a unique opportunity to analyse the applicability and fit of standard hydraulic theory for these.

The results highlight that significantly greater head loss occurs through a particularly popular manufacturer’s VRI valve compared to others, and warrants special consideration in the selection of these, and if used, specific additional adjustments to machine hydraulics are necessary for optimal hydraulic operation.

The vital importance of the hydraulic characteristic of pressure regulator valves (PRVs) on the overall hydraulic operation of these machines warranted special investigation under steady and unsteady hydraulic conditions. Overall, six separate tests on PRVs were completed.

Due to the incompatibility of published peer-reviewed theories and the hydraulic characteristics measured for these pressure regulators, development of a novel hydraulic model was proposed to more accurately describe the performance, and to be validated against measured results.

Under steady conditions, average outlet pressures for different manufacturer and model combinations vary significantly from the nominal set pressures by up to 0.7 m head.

During unsteady conditions, the complex combination of the mechanical and hydraulic components within PRVs manifest themselves in a hysteresis of outlet pressure when inlet pressures change. A greatly improved explanation of the operation of modern PRVs is reported, for the full range of conditions these valves endure. A novel theoretical model of PRV hydraulic performance was developed based on traditional hydraulic theory and analysis of results from this study, which more accurately describes the performance of these devices.

Overall, an improved understanding of the performance of VRI components has been developed that will allow industry to more accurately design the hydraulic system of VRI equipped centre pivots and lateral moves. This provides the opportunity for improved irrigation performance and lower energy consumption of centre pivot and lateral move systems.

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