Monitoring wind turbine gear oils with RULER as part of reliability and maintenance program –

Oil Condition Monitoring in the wind turbine industry can be of large impact, certainly if we take into account the growth of the wind turbine-generated power in the last 10 years. Also here is the reliability management the central driver, where many field-operating failures are a direct consequence of gearbox bearing failure, as load factors can be extremely high.
With the latest trend of wind turbine parks, proactive measurement of the RUL of the lubricants becomes the new maintenance strategy, rather than the reactive strategy based on measuring acid number or viscosity as main parameter for oxidation.
The main stress factors for wind turbine gearbox oils are

Ø      Temperature – local overheating on the bearing surfaces

Ø      water (mainly from condensates)

Ø      and particle contamination,

 What is the direct result of oxidation? Formation of deposits, due to oxidation on the bearing surfaces. The gearbox lubricants have to be seen as oil circulation systems, where large quantities of heat must be removed, and where strong contamination of the oil in service occurs. These oils must possess good water and air separation properties and retain excellent to high oxidation stability over a long period of time.

Type of gearbox lubricants

We can clearly see an evolution from the mineral type of gearbox lubricants, toward synthetic long-life fluids for the larger wind turbines (> 1MW). As these windmills are getting bigger and bigger, but not necessarily the oil reservoir, the lubricant is working at higher temperatures and needs higher oxidative protection.
Mostly these fluids apply mixtures of ashless antioxidants in order to enhance high temperature oxidative protection. The figure below shows a typical graph from a 1,5MW wind gear lubricant, after 36 months of operating time and show high depletion of phenols (AO at 15 seconds), and still having high RUL through antioxidant protection by aromatic amines.
 

Figure 1: synthetic (PAO) wind turbine lubricant after 36 months lifetime on 1.5 MW windturbine

Figure 2: from 500 KW Danish Wind turbine after 24 months of operation.

Figure 3: 320 cST lubricant from 1 MW Wind turbine after 4 years been in service - wear metal increase with 7% RUL.

Case wind turbine power generation plant: a large wind turbine operator in Europe decided in 1998 to include RUL% data as part of their condition based drain program. Before starting this program the service company received 2 times a year an oil analysis report showing TAN, viscosity, particle counting, water and spectro-analytical analysis for wear metals, for the majority of their gearbox lubricating oils.
As the plant had experienced positive experiences from portable particle counters, and water analyzers resulting in optimized lubricant use, the missing link for the wind turbine operator was the information on (remaining) oxidation life. The plant had invested significantly in fine filtration (3 and 6 micron absolute), as well in vacuum dehydration to remove water from the lubricants (via kidney loop).
Once the RULER instrument was purchased the following improvements to the on-site condition-monitoring program were established:

• Quality control of all incoming oil batches for RUL estimation
• Used oil analysis for all gearbox lubricants (and some cases greases and hydraulic fluids)
• Oil analysis frequency was adapted in function of the RUL% (higher than 50% RUL / every 4 month sampling – for RUL% between 25 – 50 % every 1-2 months – critical limit at 25% RUL).
• Data were correlated with particle counting and water monitoring, and the plant operators were able to trend life of the oil.

The savings from this can be significant:

Lubricant savings => on-site oil analysis achieved significant savings in oil life extension – with the price of 2 to 3 Euro per liter of synthetic lubricant

Maintenance savings => by detecting proactively potential equipment problems (trend analysis ) figures not included, but on-site condition monitoring program allowed to plan maintenance = significant cost reduction – downtime costs. Especially with wind turbines were accessibility can be a problem (off-shore).

For wind turbine gear lubricant the economical contribution to the on-site condition-monitoring program has to be situated on 4 levels:

     Making the right decisions, on the right time, and for the appropriate equipment maximizing oil life without compromising equipment & components.

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