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Sealing wind turbine actuators

01 July 2008

Trelleborg has supplied a specially engineered sealing configuration to the Danish wind turbine hydraulic actuator manufacturer, AVN Energy A/S. Meeting the dynamic criteria due to constant movement of the actuator was a major design challenge for the seal specialist

The replacement of stall controlled turbines with continuous pitch systems has put extreme demands on hydraulic actuators. The position of stall controlled machines shift just once every ten minutes but in continuous pitch wind turbine systems, the nacelle and angles of the blades constantly change in small amounts once every rotation, on average 15 times per minute. This means that instead of producing six long strokes per hour, the turbine’s hydraulic actuator must now initiate nine hundred short strokes in the same period, 24 hours a day, seven days a week, for a targeted 20-year period.

“Customers have high expectations from our products, and the number one requirement of the wind turbine manufacturer is reliability,” says Poul Kristensen, export sales manager at Denmark based AVN Energy A/S, one of the world’s leading suppliers of actuators for wind turbines. “Maintenance of turbines is difficult and costly. On land, it is hard enough, but offshore it is really tough. And when a windmill is switched off for maintenance, it is not producing energy and losing income. On top of that, operators are often penalised if supply targets are not met. So a primary objective for them is to minimise routine downtime, while stoppages due to component failure have to be avoided at all costs.”

The seals within hydraulic actuators are critical to their performance, and optimising seal life is essential to the long-term effectiveness of the turbine system. Seals must operate at 250bar with constant pressure on the rod from behind and differential side loads that control positioning. Wear has to be minimised and dynamic movement - the continuous short strokes described above - have to be accommodated. Moreover, the seal arrangement must be able to tolerate a wide operational temperature range - between -30ºC (-40ºC in Arctic conditions), and +60ºC.

Having previously worked with Trelleborg Sealing Solutions on stall machines, AVN approached the company with a proposal that they jointly develop a sealing system to meet the challenging requirements of continuous pitch systems.

The final configuration is a complex arrangement of seals ranging from O-rings to specialist Turcon PTFE based energised seals in a variety of geometries plus piston rod wear/guide rings (Slydrings) manufactured from Orkot, a resin impregnated fine weave fabric material with added lubricant. Slydrings are able to withstand very high side loads and are capable of damping vibration and embedding particles that might otherwise accelerate wear. This novel configuration is engineered to enhance lubrication and optimise friction characteristics, while preventing external leakage.

Extensive testing concentrated on all aspects of assembly design in conjunction with the seal configuration. The aim of this was to maximise service life and achieve the operators’ 20-year target for turbine life with as little maintenance as possible. Some of the seals within the system are expected to achieve that time span without replacement, but it was impossible to guarantee this for all seal components. Thus the hydraulics had to be designed for easy replacement of the seal set.

The set is actually mounted in a module that can be just as quickly removed for seal replacement as it is installed. The minimum life expectancy of the sealing configuration, based on the seal with the shortest predicted life, is seven years, but replacement is recommended after five. Other than this and routine rod replacement, the actuators should be capable of running without maintenance apart from routine, systematic checking for leaks or losses of pressure.

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