To attain their highest possible capacity factor, wind turbines must be sited where they will be exposed to as much consistent wind as possible. An assessment must often show a site to annually have about 2,500 hours of wind at about 25 mph to make it viable for a utility company. Consequently, all parts of a wind turbine are subjected to extremes of weather, ozone, unusual stresses, and vibration. Things get worse offshore where they must also withstand the rigors of the sea including exposure to saltwater from continuous sea spray.
Despite the abuse, utility companies have high expectations from their investments by setting their usage rates at 98%. Compounded to the rigorous low-maintenance expectation of many components is a 20-year life expectancy. It’s not surprising, diligent engineering and consistent quality manufacturing processes must accompany each component.
In addition to metal components in the structure of a wind turbine, there are many polymer-based components. Designing these parts to meet an operator’s expectations in terms of life expectancy and minimized maintenance is no easy matter. Standard elastomers and plastics can deteriorate when exposed to sea spray, ozone, aggressive chemicals, high pressures, and extreme temperatures, all of which are likely in an average wind turbine. Polymer-based compounds there must be engineered to withstand these challenging operating parameters.
In the design of wind turbine components, the importance of well engineered seals is often overlooked. A diligently engineered sealing system will increase the mean time between component failures, reduce manufacturing costs associated with the use of exotic coatings, and reduce the on-board power consumption due to unnecessarily high friction.
The right stuff
Intense research and development goes into finding the right materials for seals, bearings, mounts, grouts and hoses. One advantage comes with knowledgeable material development. For example, not all PTFE seals perform equally, and not all fluoroelastomers (FKMs) are the same. Each specially formulated recipe ensures control over a material’s wear rate, coefficient of friction, extrusion resistance, damping factor, and fatigue resistance. Test facilities can then ensure the right combination of, among other things, wear factors, friction and tan delta (for damping behavior) to provide the right material for each application.
For instance, polymer seals are used in the main and yaw brakes, main gear, main bearing, lock cylinder, pitch cylinder, and a turbine’s hydraulic accumulator. In addition, there are flexible oil hoses and vibration mounts. A colossal grout seal, about 16 feet in diameter, is located at the base of off-shore wind turbines.
Each of component is integral to the performance of the wind turbine. Take for example the seals in a hydraulic actuator. In continuous-pitch systems where the angle of the blades changes on average 15 times/min, the actuator will have to initiate close to 8,000,000 strokes annually, and failure is not an option.
In addition to high cycle rates, seals must operate at over 3,000 psi with constant pressure on the rod from behind and differential side loads that control positioning. Standard applications require a temperature resistance to -22F with -40F required in the Arctic, and a maximum standard capability of 140F. The lost revenue from reduced efficiency and the high cost to replace prematurely failed seals punctuates the need for proper seal choices. Hence, a lot of R&D goes into developing the right seal materials, as well as in testing their performance in these high pressure, wide temperature range, and heavy side loaded environments. The few application examples that follow show where materials and components work best.
Trelleborg Bakker is a manufacturer of grout seals. Offshore wind power stations are built using structures such as monopiles and tripods. The design philosophy for wind-power stations is based on the installation method which depends on soil properties, water depth, and contractor experience.
A monopile provides one application. After the pile has been driven into the sea bed, the top will be 16 feet under the water table. A transition piece, about 82-feet high, is lowered over the top of the pile and will clear the water by about 43 feet. The space between the pile and the transition piece is sealed by an inflatable grout seal. This seal closes a substantial gap and simultaneously resists an internal pressure of a few psi. Grouting in the space above the grout seal results in a very strong joint. Finally a floating crane is used to install the tower and the turbine.
Many seals a tight system make
Trelleborg Sealing Solutions worked with hydraulic actuator manufacturer AVN Energy A/S to specify an unusual configuration that would enhance lubrication and optimize friction characteristics, while preventing leaks. The solution is a complex arrangement of seals ranging from O-Rings to specialized Turcon PTFE based geometries and Slydrings in Orkot composite material.
Turcon is an advanced PTFE while Orkot is an advanced non-metallic reinforced polymer composite. And a Slydring is a bearing that prevents metallic contact between the sliding parts of a cylinder.
The goal of this was to maximize service life and hit the operators’ 20-year target for turbine life with as little maintenance as possible. Hence, testing concentrated on all aspects of assembly design in conjunction with the seal configuration. Some seals in the system are expected to work 20 years without replacement while others will need replacing. Consequently, the hydraulics are designed for the easy exchange of seal sets. The seal is mounted in a module that can be quickly bolted on and off.
The minimum life expectancy of the seal configuration, allowing for 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 are predicted to run without maintenance except for the systematic checking for leakage or pressure loss.
Split radial oil seals
Our company also provides rotary designs to the wind turbine industry. The seals previously used resulted in misalignments that made installation difficult, and then they leaked over time. The split TRJ Radial Oil Seal solved this problem for main-shaft bearing applications. Furthermore, the seals overcome the tedious and time-consuming bonding necessary by incorporating positioning pins and custom dovetail designs at the joining ends.
Gearboxes, generators, and nacelles occasionally need anti-vibration isolation elements under them. The RA/RAEM mounts from Trelleborg Industrial Anti-Vibration Systems are often preferred by windpower OEMs to reduce drivetrain generated torque and vibration. Reducing operation noise increases performance and lengthens service life.
Novibra RA mounts use a rubber profile in shear and compression modes to combine good vertical flexibility with horizontal stability. Meanwhile, heavy duty compact Spherilastik Bearings act as a resilient spherical joint, providing articulation and letting turbines pivot while suppressing noise and vibration. The Spherilastik bearing works without lubrication in maintenance-free operation and has been used in combination with Cushyfloats for significant reductions in structure-borne noise.
Larger and more flexible hoses
Don’t overlook oil coolers and filters for turbine gearboxes. When the manufacturer of one such system was sourcing hoses, it found conventional hydraulic hoses, which are from 0.24 to 0.79-in. diameter, were too small. Worse, metal reinforcements made the hoses too stiff.
The solution was an all polymer hose, PUMPADUCKT S from Trelleborg Industrial Hose, which has the same characteristics as a metal reinforced hose. Materials proven in chemical and hydrocarbon processing industries fulfilled the application’s requirements. These hoses tolerate cleaning solutions, are compatible with gear oil, and resist abrasive particles and debris that causes premature failure in conventional hydraulic hoses.