Condition-based lubrication reduces asset management costs
2017 January 31, 09:00 GMT
Given the operating conditions a wind turbine faces over a typical 20-year service life, maintenance problems aren’t a question of “if,” but “when”. When those inevitable maintenance issues arise, wind farms are faced with the prospect of expensive crane mobilisation, lost energy production and the soaring costs of getting teams of maintenance technicians to site. The need to reduce overall operating costs is an important factor in any purchasing decision, especially now in the light of the planned withdrawal of the renewables obligation for onshore wind energy.
As a result, operators are showing increasing interest in more comprehensive asset management packages - implementing combined remote condition monitoring and automatic lubrication systems, for example, which can have a positive impact on operating life and maintenance cost reduction. Specifying such a condition-based lubrication system, either on a new turbine or as a retrofit, is becoming an attractive option for operators seeking to reduce running costs.
While sophisticated lubrication systems for wind turbines have typically been installed on larger turbines of 1.5MW and above, with manual lubrication costs accounting for as much as 10 per cent of the total servicing bill, they are now increasingly likely to be found on smaller turbines. Condition-based lubrication enables remotely monitored, automatic lubrication of a wind turbine’s hard-to-access bearing systems, based on real-time bearing condition monitoring, reducing the frequency of on-site service engineer visits and potentially wasteful periodic, manual lubrication.
The basic type of automated lubrication device is a single point gas-driven or electro-mechanical lubricator. However, while this offers a simple, easy-to-change solution, its life is normally limited to around twelve months. A more effective option is an automated lubrication system, combining a refillable reservoir, pump and metering devices, feeding multiple lubrication points through a network of pipes. Lubricants are applied in pre-determined volumes and time intervals by each valve, or may be progressively applied to all lubrication points while the main pump is running. Only occasional manual intervention is needed to recharge lubricant reservoirs and simple system maintenance as part of a normal routine.
When interfaced with a remotely accessible condition monitoring system, which uses vibration and temperature sensors mounted on a turbine’s main shaft bearings, drive train gearbox and generator to collect, analyse and compile a range of operating data in real-time, condition-based lubrication enables automatic applications of lubricant as and when they are necessary - and independently of any existing time-based cycle. A remotely located condition monitoring specialist can use the vibration/temperature data analyses to set appropriate alarm settings that trigger additional or less frequent lubrication cycles and volumes, as necessary.
A large wind turbine will typically have up to five lubrication systems, covering pitch bearings and gears, yaw bearings and gears, main bearings, gearboxes and generators. Each of these systems will be subject to different stresses and will therefore have different lubrication requirements; for example, main bearings are subject to fluctuating thrust loads, and generator bearings to high and low speed axial loads. Additionally, the turbine has to withstand demanding environmental conditions, with extremes of temperature, high and gusting winds and, in coastal locations, the effects of salt corrosion.
Pitch bearings require a constant re-supply of lubricant. The lubrication unit is mounted in the rotor, with which it continually turns, adjusting to the resulting vibration and centrifugal force. Grease pumps, such as those supplied by SKF are fitted with a grease follower plate to ensure that grease is maintained in the area of the pump elements, enabling suction even with rotation. From there, the grease arrives at the metering devices through a progressive or single-line lubrication system.
Lubrication for the pitch open gear can also be supplied via SKF pumps for the rotary application. Lubrication pinions are used to apply grease precisely to the area of contact on the drive pinion or the pitch open gear, and evenly lubricate the entire cog width.
An electrical grease pump introduces lubricant into the yaw bearing via progressive or single-line metering devices. Automatic lubrication achieves a sufficiently thick lubricant film to prevent excessive wear and the ‘stick-slip’ effect upon turning the nacelle. The automatic lubrication pinion meshes precisely with the gearing, distributing the lubricant uniformly in the area of contact across the entire cog width. Used grease is removed by a lubricant collector and delivered to a reservoir to prevent spillage on the nacelle floor, improving worker safety.
The main bearings of a wind turbine are subject to severe forces and usually require large quantities of lubricant. For stationary operation, a stirring and fixed paddle in the pump’s reservoir is sufficient. The automatic lubrication system continually supplies grease to the main bearings even while in motion.
The gearbox is lubricated and cooled by oil, which is recycled via filters. The oil can be electrically heated to achieve an optimal start-up temperature for the lubrication cycle in conditions of low ambient temperatures. Flow limiters maintain the prescribed oil volume flow independent of pressure and temperature changes and these can provide feedback on the current flow rates to the condition monitoring system.
Control and monitoring
Taking SKF’s condition-based lubrication concept as an example, here we have the interface between a condition monitoring system (SKF WindCon, in this case) and the automated lubrication systems (SKF Windlub or Lincoln Quicklub, for example). Combined, these elements are able to track and control lubrication system health, pump status and grease levels, as well as alerting operators to failures such as empty or blocked lubricant pumps or fractured feed lines. This monitoring data can also be uploaded via Internet connections to remotely located diagnostics centres, operating globally 24/7, for expert analysis and reporting.
By logging and tracking deteriorating component conditions in real-time, condition-based monitoring allows maintenance decisions to be based on actual machine conditions, rather than arbitrary maintenance schedules. Along with the possibility that maintenance intervals can be extended, the system provides a powerful tool for managing day-to-day maintenance routines and consolidating high risk, costly maintenance activities.
The above technical article was originally published in the January 2017 edition of Wind Energy Network magazine.