Optimised lubricants for longer maintenance cycles

Wed, 12/07/2017 - 09:24

Oils and greases protect the most important components of a wind turbine from wear and corrosion. However, demands on the lubricants have risen due to higher loads and longer oil-change intervals.

The increased cost pressures in the wind power sector can be seen in the ever-longer maintenance cycles. This is also creating new challenges for lubrication manufacturers. In demand are products which operators can benefit from in the long term. “When turbine operators are under pressure to lower the maintenance costs, lubricants are often one of the first items to face cutbacks,” observes Shell manager Dr. Richard Tucker. Although choosing a less suitable lubricant only rarely directly causes a turbine to break down, in the long term there may be higher maintenance costs as a result, says Tucker.

Indeed, the quality requirements for gearbox oils and greases have risen considerably in the last few years, especially in the wind power sector. While the mineral oils used in the past required replacement intervals of three to four years, today’s synthetic gearbox oils only have to be changed every five to seven years.

And the trend is continuing: the most modern synthetic oils are expected to be usable for ten years and above, thus also fulfilling the increased requirements in the offshore wind power sector. “Every oil change is a potential source of contamination and danger, which can affect the reliability of a wind turbine,” says Marcel Jansen, head of industrial sales at the lubricant manufacturer Total. “Offshore turbines are designed to have an operational lifetime of approx. 25 years. The task set by turbine manufacturers was to develop oils which can last for ten years and above,” he continues. The reasons for this are clear; while an oil change on an onshore turbine can cost approx. € 10,000 through yield losses alone, the costs in the offshore sector can quickly climb to six-figure values.

High-quality lubricants help to reduce maintenance costs

In mid May, lubrication experts and industrial users discussed the correct selection and application of lubricants at a symposium in Amsterdam. The basic function of oils and greases is to protect the gearbox and bearings from wear – independently of external conditions and loads. According to a study carried out by the oil company Shell, industrial users can considerably lower their overall operating costs through suitable lubrication management. “We have determined that many turbine operators recognise there are potential effects of choosing the optimal lubricant and of a good management programme on their operation costs, but that they underestimate these in the extreme,” reported Shell.

Ignoring these links means that many turbine operators are practically “throwing money away” and are not exhausting the savings potential. According to the Shell study, 60 % of the companies surveyed from eight different industrial sectors admitted that bad choices when it came to lubrication had led to unplanned downtimes. 51 % of the companies surveyed do not expect, however, that higher-quality lubricants help to reduce maintenance costs. In response to its customers, Shell states that optimising lubrication processes has led to savings of 139 million US dollars over the last five years.

A cost comparison by the lubrication manufacturer from Munich, Klüber Lubrications SE, also provides clear results. Although the modern synthetic high-performance gearbox oils are somewhat more expensive than petroleum-based products, the lower maintenance costs compared to mineral oil can result in cost savings of over 30 %.

Loads are increasing, additives can help

Due to the ever-increasing power density of wind turbines, the loads on the gearbox and especially the bearings are rising too. Problems with bearings are still one of the main causes of operation downtime in the wind power sector. Additionally, turbine power and rotor rotation speeds are meanwhile at levels at which gear tooth areas are seeing increased occurrences of grey speck damage, so-called micro-pitting. At the main shaft bearings, operating conditions are also far from optimal for creating a perfect lubrication layer. Because of the maximum operating rotation speed of approx. 20 revolutions per minute, speeds in the main shaft bearing are often not high enough to keep the roughness peaks of the components apart. The lubrication industry is thus doing everything it can to reduce wear and standstill marks.

At the energy company Total they are going with a phosphorous additive aimed at protecting the wind turbine components. “Phosphorous is polar and sticks to the surface, so that a protective layer forms on the metal,” explains Jansen on how it works. Additives containing sulphur or chlorine are also something to be considered. The additives in the lubricant have to be carefully harmonised with oneanother, however, so that a fusing of the turbine parts is prevented under extremely high pressures or loads. In this, the additives undergo a chemical reaction with the surfaces of the turbine components and replace metals from the materials.

The residuals of the oxidisation contaminate the fluids, however, and must be filtered out. Specialist suppliers such as the Danish filter manufacturer C.C.Jensen A/S (CJC) or the hydraulics specialist Hydac have fine filtration systems which protect the gearbox oil from contamination and aim to achieve the highest possible level of oil purity. The problem with the filtering is that the additives can also be filtered out, with the lubricants thus losing their effect. “Thanks to an ester-phosphorous combination we have achieved a better filtration ability to 3 micrometres,” explains Jansen.

That wind turbines are becoming ever larger is also confronting lubrication manufacturers with new challenges. Because of the increased height of the turbines and the extreme locations, the fluids are having to be able to withstand larger temperature ranges. The temperature can have a direct effect on the viscosity of the lubricants. It is important here that the oils remain in a pumpable state even in harsh conditions. “With a high viscosity index we achieve better low temperature properties,” explains Jansen and promises his products can be used over a temperature range of -40 to over 100 °C. Additionally, high-quality oils should be structured in such a way that any water present cannot destroy them. If oil in a wind turbine mixes with water, from condensation for example, then the oil may age faster. Lubrication manufacturers are trying to reduce this effect as much as possible and are specialising their products towards having long oil-change intervals.

Regular oil checks for protection

Turbine operators can prevent oil ageing too, however. If they have a Condition Monitoring System installed in their wind turbines, the condition of important turbine components is continuously monitored. Such a permanent turbine monitoring, especially of the bearings, is a basic requirement for increasing the profitability of a turbine. By using special sensors, the condition of greases and gearbox oils may be monitored, potentially providing an informational advantage.

Depending on the condition, additional greasing can be carried out. Problems which would otherwise only be recognised during a routine check can thus be avoided. Additionally, such a way of doing things also avoids the possibility of over-greasing the turbine with lubricants. At the same time, wear and tear of the bearings and gearbox can be recognised early on if such systems determine during the monitoring process that there are abrasion particles in the oil.

Furthermore, a continuous monitoring of the oil condition enables an eye to be kept on the viscosity. If it falls, then the addition of small amounts of fresh oil can quickly have a positive effect that is immediately recognised by the condition monitoring as an improvement. Experts also recommend six-monthly lab tests of the oil quality. For this a small oil sample is taken from the turbine and tested by a lab. This may be performed directly by the relevant lubrication manufacturer or by external service providers.