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During a recent refurbishment of the Allt na Lairige hydro-electric power station near Inverary in Scotland, the hydraulic control system was upgraded from the original low pressure system to a modern high pressure one. In-house designed multi-orifice restrictors were being used in the hydraulic actuators which operate the main water inlet valves, called spears, which are controlled by the governor to ensure that suitable control of the water jets to the turbines is ensured for all load conditions of the generator.
Due to the increase in system pressure and the variation in temperature throughout the year which was affecting the viscosity of the oil, it was determined that the restrictors were not providing sufficient restriction, which resulted in sporadic operating times of the spears. To compensate for this, adjustments had to be made to throttle the flow at the hydraulic power unit, since there were concerns about making the orifices smaller than 0.9mm diameter due to contamination.
Lee Products was able to solve the problem by providing high Lohm rate restrictors from their 250 series axial visco jet range. Visco jets contain many internal stages to provide a very small restriction to flow but still keep the internal passageway sizes large enough to resist blockage by contamination. Filter screens are fitted at each end of the visco jets to provide protection for the orifices. Another performance advantage of the visco jet is that by continually swirling the fluid to maintain turbulent flow, it compensates for the change in viscosity of the fluid due to any change in temperature. This was an important factor at the Allt na Lairige power station because the temperature of the hydraulic oil varies considerably during the year, such that the water valves were opening too slowly in winter with cold oil and too quickly in summer when the oil was hot.
