Wind turbine pitch, yaw, and brake systems use either electric or hydraulic actuation. The hydraulic systems use high-pressure, gas-charged hydraulic accumulators. Typically, either a bladder or a piston accumulator is used. Maintenance-free, metal bellows accumulators can offer an advantage in these applications.
A piston accumulator consists of a cylindrical metal body with end caps and an internal piston, a gas charging valve at one end, and a hydraulic port at the other end. The sliding piston separates the charge gas from the hydraulic fluid. A bladder accumulator uses a rolling rubber bladder to separate the gas from the hydraulic fluid. Both piston and bladder accumulators rely on wearing elastomeric seals, so they’re both susceptible to leaking charge gas into the hydraulic fluid over time. Hence, they require routine inspection and maintenance for re-charging and periodic seal replacement.
There is an alternative to these conventional accumulators that can eliminate all maintenance and service activity, resulting in substantially lower total life cycles costs. These devices offer superior performance with greater wind turbine uptime and availability. Metal bellows maintenance-free accumulators ensure zero leakage and no seal wear providing for completely maintenance and service-free operation for the life of the platform.
The bellows accumulator uses an edge-welded metal bellows capsule that forms an internal chamber, which contains the hydraulic fluid and separates it from the precharge gas. The bellows collapses and expands to meet the volume or flow demands of the hydraulic system, including both operating and hydraulic-shutdown modes. During operation, the hydraulic system is at full pressure, and the pressures on both sides of the bellows are balanced. While in hydraulic-shutdown mode, the hydraulic fluid pressure reduces to zero and the bellows capsule fully nests or “bottoms out” with a differential pressure across the bellows equal to the accumulator gas precharge pressure. The hydraulic shutdown mode places the highest stress on the bellows. Maintenance-free bellows accumulators incorporate a proprietary bellows that withstands the large differential pressure while nested.
The bellows accumulator uses a high-strength stainless steel flow-formed housing. The bellows subassembly consists of the stainless-steel bellows capsule welded at one end to the fixed terminal fitting, and at the other end the movable sweeper fitting. A Teflon guide attaches to the sweeper to ensure smooth motion of the bellows inside the housing with negligible friction. This entire bellows subassembly slides into the housing and the end terminal permanently attaches to the housing with a full penetration electron beam weld. At the domed (closed) end of the housing is a small stainless-steel charge tube. After charging, the tube is crimped, welded shut, and coiled down into the protective recess shown in the diagram. This recess is then filled with potting material to prevent access or damage to the charge tube. The accumulator charge gas is normally nitrogen with a trace of helium or 100% pure helium. Helium allows for subsequent leak checking of the completed accumulator using a helium mass-spectrometer leak detector. The accumulator is certified to be leak tight to 1X10-7 standard cubic centimeters per second of Helium (SCC/sec He) or better. For an accumulator with a 1,500 psi (103 Bar) precharge this is equivalent to 1 cubic inch (16.4 CC) of charge gas leakage in 500 years. For all practical purposes, the bellows accumulator is considered absolutely leak free.
The metal bellows accumulator completely eliminates use of elastomeric or wearing seals used in traditional bladder or piston accumulators. Both bladder and piston accumulators experience seal deterioration over time, which can allow charge gas migration into the hydraulic fluid. This adversely affects hydraulic system performance and leads to significant down-time and maintenance. With a metal bellows accumulator, the charge gas is hermetically sealed and so it cannot migrate or leak. Additionally, bladder and piston accumulators are adversely affected by low and high temperature conditions, whereas the welded metal bellows accumulator is unaffected by these temperature extremes. Bellows accumulators have been used in high performance, safety critical hydraulic systems with pressures as high as 8,000 psi (550 Bar) and temperatures from –100 to 450°F (-73 to 232°C). Because of their all welded stainless steel construction, bellows accumulators are compatible with all hydraulic fluids. Also, because the bellows accumulator uses no wearing seals and operates almost without friction, its response time is far superior to bladder and piston accumulators. Bellows accumulators can be mounted in any orientation and are suitable for use with all standard hydraulic fluids.
For some applications it is beneficial to use a 100% pure helium gas precharge to optimize performance and reduce size and weight of the accumulator. Bellows accumulators, with their all welded hermetic construction, can take exclusive advantage of helium whereas traditional accumulators, with elastomeric seals would exhibit unacceptable levels of leakage with a helium gas charge. Prior to shipment, bellows accumulators are tested for leakage and performance and delivered with a permanently sealed gas charge that will last for the life of the system.
Recurring inspection, recharging and seal replacement, on piston and bladder accumulators is costly and adversely affects wind-turbine reliability and availability. Metal bellows accumulators provide completely maintenance-free service, resulting in increased system availability, significantly improved reliability, and substantially lower total life cycle costs. WPE
Metal bellows maintenance-free accumulators offer advantages in wind turbine pitch, yaw, and brake systems. These can ensure zero leakage and no seal wear providing for completely maintenance and service-free operation.
By: John Meyer, Sr. Business Development Manager at Senior Aerospace, www.senioraerospace.com
Filed Under: Components