Standard wind turbine gearbox warranties generally last two to three years, and when they expire, operations and maintenance professionals become responsible for keeping the turbines running for the remainder of their service lives. Considering that the average wind turbine is expected to operate for up to 20 years, maintenance professionals are challenged to maximize equipment performance and minimize maintenance.
Maintenance staffs are typically lean and must be as efficient as possible to complete the recommended maintenance in the proper intervals. Streamlined gearbox flushing and oil analysis strategies can help maintenance professionals prolong the life of a wind-turbine gearbox, minimize unscheduled repairs, and simplify maintenance procedures.
Do more than change the oil
The main gearbox drives the generator marking it a critical piece of equipment. Advanced designs and overall importance to system performance makes gearboxes costly to repair or replace after the warranty expires. For example, replacing a gearbox in a 1.5-MW turbine can cost a company more than $500,000 when you add in the price of a new gearbox, labor, crane rental, and lost revenue from turbine downtime.
To minimize the chance of a premature gearbox failure, OEMs recommend routine, scheduled maintenance, oil-circulation equipment, and an oil analysis. Although the maintenance staff traditionally changes the oil, there are many physical and logistical challenges, including the climb up-tower, remote turbine locations, weather extremes, limited available utilities, numerous trips lugging down used oil, hefting 85 gallons of new lubricant up a 60-m tower, and finally pouring it into a gearbox. Following established OEM procedures, the oil change for a wind turbine gearbox generally takes about eight hours and requires three maintenance technicians.
Industry averages suggest a 1.5-MW turbine generates roughly $1,200 worth of electricity every 24 hours. Thus, halting a turbine for an oil change costs a wind-plant operator $400 in revenue and may drive additional expenses for the use of outsourced maintenance crews to handle general maintenance activities and troubleshoot more pressing issues while the internal maintenance personnel change the gearbox oil.
To make matters worse, field experience indicates that routine oil-change procedures remove only about 70% of the used oil. When adding the new wind-turbine gear lubricant to the system, it mixes with the 30% residual used oil, which contains contaminants and wear metals. Mixing contaminated oil with new oil shortens the life of the new oil and could also lead to premature component wear and potential equipment breakdown.
To address these issues and help wind companies maximize turbine output, outsourced maintenance companies are starting to offer gearbox-oil-change services. The best of these service providers use a proprietary protocol and specialized equipment. This specialized equipment moves oil safely up and down the turbine using high pressure pumps and hoses. This equipment also assists in removing up to 97% of the used oil from the gearbox, while helping to remove contaminants and cleaning critical gearbox components.
The new wind-turbine gear lubricant then delivers better equipment protection than contaminated oil, thereby extending oil life and reducing lubricant expenditures. What’s more, the entire oil change, flushing, and cleaning also takes about half of the time it would take for a maintenance staff to perform a conventional oil change, and gets a turbine back to work sooner. This service makes minimal demands on the time of maintenance staffs and lets them focus on routine maintenance protocols. Considering the cost analysis, the expense of employing an outsourced partner to perform a gearbox oil change is quickly recouped in the cost-saving benefits of extended gear-oil drain intervals for wind turbines and it potentially extends wind-turbine gearbox life.
After filling the gearbox with fresh oil, it is imperative for maintenance personnel to monitor it. Routine oil analysis is one of the most widely used proactive maintenance strategies for wind turbines and employs a slate of tests to evaluate the condition of the in-service lubricant and help evaluate the condition of internal hardware. Routine oil analysis as part of a preventive maintenance program lets maintenance staffs extend the useful lives of the gear oil and gearbox by detecting and acting on early warning signs such as contamination or increasing wear metals.
For the greatest benefit from oil analysis, it is imperative to work with an expert lubricant manufacturer and conduct an oil analysis every three to six months. Identifying trends in the data will help maintenance personnel make more informed oil-suitability decisions. When analyzing oil samples from the wind-turbine gearbox, the maintenance staff must test for viscosity, iron wear, total acid number, water contamination, and oil cleanliness. Each deserves more discussion.
It measures a lubricant’s resistance to flow. This variable is the most critical parameter for most applications and can change over time, more quickly in equipment that sees extreme temperatures or high pressures or high speeds. Ensure that lubricant viscosity is within its targeted range. Doing so minimizes wear between critical equipment components.
Typically wind-turbine gearboxes require a viscosity around 320 cSt (centiStokes) but it can be higher based on service with variable temperature, speed, and loads. When viscosity changes by ±15% of its original value, monitor the oil more frequently. Equipment can perform normally if the lubricant is outside of this range, but it should be watched more closely because it usually indicates changes are needed. If oil temperature significantly varies, particularly trending higher, it is recommended to review the ISO grade of the oil to ensure it provides an appropriate film thickness for efficient wear-free performance.
When a lubricant’s viscosity increases, there is a chance it has been contaminated by oil with a higher viscosity or has started to oxidize. However, when viscosity decreases, inspect the oil filter for wear debris, monitor the box for higher operating temperatures, and check for other lubricant contamination.
Steel is used in nearly every piece of equipment, especially wind turbine gearboxes, so it is imperative to monitor its presence. Don’t be overly alarmed when elevated levels of iron appear in the oil analysis at the beginning of the gearboxes’ service life. This is typical during “break in” periods for new equipment and should reach a steady state over a few months.
Two tests can identify iron content in oil: ICP spectrography and a PQ Index. ICP spectrography identifies iron particles in oil ranging up from about 10μ (micron). For comparison, a human hair has about a 40μ diameter. Hence, the smaller particles could pass through an oil filter and potentially be a precursor to increased gear wear.
The PQ Index test quantifies the size of ferrous material in the oil sample. For the most accurate assessment of the oil’s condition, it is recommended that maintenance personnel monitor the Index over time and compare it to ICP analysis. When the PQ Index is lower than ICP analysis, it indicates the presence ferrous particles smaller than 5 micron. If the PQ Index exceeds the ICP analysis, that could indicate that the ferrous particles are larger and a chance that wear has accelerated.
When high iron-particle readings coincide with high silicon readings, the oil has become contaminated with dirt. The oil does not necessarily need a change at this point. It can be filtered. However if contaminant levels are not reduced or stabilized, change the oil and make sure to flush the bearings of wear debris that collects in oil pockets.
Total acid number
This number, called the TAN, indicates a thermal stability, oxidation rate, and amount of acidic material generated in the oil. Oxidation is the reaction of oxygen with the hydrocarbon molecules in the gear oil. The rate of oxidation increases exponentially as temperature rises and with the presence of metallic contaminants. An oil temperature increase of 10 Celsius degrees effectively doubles the oxidation rate. Copper, bronze, brass, and iron contaminants are typical materials that catalyze the oxidation reaction. Oxidation is typically the main contributor to sludge and varnish formation in gearboxes. Based on a turbine’s normal operating conditions, when the TAN is below 2.0, oxidation is occurring at a slower rate unless there is a mechanical problem such as plugged coolers, or bearing or seal failures.
PAO-based (polyalphaolefin) synthetic oils resist oxidation longer than their mineral-oil counterparts, but if the TAN is two points higher than its starting point, change the oil immediately.
Water and oil do not readily mix. Water tends to lead to reduced viscosity, an increased oxidation rate, and causes additives to drop out of the lubricant, potentially leading to component failure. Higher-performing oils are engineered to not hold water so contamination by it should not be an issue. However, if oil analysis indicates water levels above 200 ppm (parts per million), check all sources including oil for a contamination source. Oil containing as little as 200 ppm water can reduce bearing-fatigue life by up to 20%. An improperly stored drum of oil will breathe as temperatures change during the day and night, drawing standing water past the bung seals and into the drum. Store drums indoors, under cover, or tilt drums so that bungs are at 9:00 and 3:00 o’clock positions and water cannot collect on the top of the drum.
This measures insoluble dirt and hard particles in fresh or in-service oil. Several factors impact a lubricant’s cleanliness, most notably contamination and harsh operating conditions, such as, extremely high temperatures, press-ures, and operating speeds. Oil cleanliness is typically determined using a laser particle counter which can detect water and air bubbles in oil, even particles in sampling bottles. Hence, reported values can be higher than an oil’s actual cleanliness level. So, if oil samples come back as dirty with low wear rates, check the water content and then resample to confirm the results.
Under ISO 4406, an ISO code is determined by measuring and grouping particles into three ranges based on their size in microns (> 4μ, >6μ, and >14μ). From these results, oil is given an ISO classification between 00 and 24. The typical target oil cleanliness for gear oil is an ISO 16/13.
The long haul
Wind turbine gearboxes are one of the most challenging applications in the modern industrial world, and require proactive maintenance strategies to promote enhanced performance. The streamlined protocol presented here lets maintenance professionals prolong wind turbine gearbox performance, minimize unscheduled repairs, and simplify maintenance procedures long after a warranty expires.
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