What to expect from oil sensors and sampling

The three drive-train components that fail most frequently are the main bearings, gearboxes, and generators. All are lubricated, yet oil analysis focuses only on the gearbox. Keep in mind that a wind-turbine gearbox is a box with gears, bearings, and oil. So what’s to fail? Gears, bearings, and of course, the oil.

Two techniques in the condition-monitoring toolbox stand out for providing insight to oil condition in a wind-turbine gearbox. One method draws a sample of oil for testing while the other uses an oil sensor. The major differences between these two are sample frequency and the type of data provided. There are pros and cons to each.

On-line oil sensors

These come in two types: Sensors that monitor the oil’s condition or chemistry are capable of detecting viscosity, oxidation, and water contamination. Sensors that detect the condition of the gearbox do so by detecting wear debris, contaminate, or particles carried in the oil.

On-line oil particulate systems (wear debris sensors) are installed on the gearbox on either a primary oil-feed line or secondary oil-feed line to monitor the presence of particles in the oil. As bearings and gears degrade, they produce wear debris. If there are metal pieces in the oil, there must be a problem with either a bearing or a gear.

Problems with the technology as it applies to wind turbines, revolve mainly around their operation. Oil debris sensors which perform well in a steam turbine or other steady-speed application, produce very linear trends because they are in continuous operation for long periods. Wind turbines work on different schedules. They start and stop, run fast and then slow. They operate at an unsteady state, so trends are more difficult to interpret.

Operational conditions that affect oil-sensor readings include:

• About 60% of the time, wind turbines are not running
• Wind speed varies from season to season
• Temperatures, which affect viscosity, vary greatly from season to season
• Viscosity also varies from oil manufacturer to oil manufacturer
• Temperatures change greatly from stopped to running in day-to-day operations
• Physical access to maintain or clean the sensors varies among turbine manufacturers

A criteria based upon speed (rpm) or temperature stabilization would greatly assist in qualifying the oil-condition data. A speed signal would allow taking data only during the 40 to 45% of the time the turbine is running, thereby eliminating the 55 to 60% of the time when it is not running. And temperature stabilization would reduce the amount of variability in sampling due to viscosity. Cumulative counts of wear debris can also be helpful as a quantifying tool.

This is not to say oil sensors are inappropriate in wind turbines. On the contrary, they provide useful data on the oil condition or gearbox condition, or both. Wind turbines are not easy on the condition-monitoring analyst.

Six to eight companies manufacture oil debris sensors with a range of different features, installation locations, and type of data generated. Some sensors can discern between ferrous, non-ferrous, particle size, and type of contaminates.

Oil testing

While oil debris sensors continuously monitor for evidence of debris, oil testing involves drawing a sample and having a laboratory run several tests on the oil’s condition, and possibly the condition of the gearbox as well. Oil is typically tested for the presence of wear metals, contaminants, and the manufacturers wear-additive package. Several tests provide a range of information. The most common includes a total-acid number (TAN), total-base number (TBN), viscosity, particle count, wear debris and contamination.

Oil monitoring gets more complicated when you consider there is no single standard test for all wind turbines. “The suite of tests we perform varies from client to client,” says David Frycki of Herguth Labs, an oil-testing company. “For example, a GE 1.5-MW turbine may use a different test for each different gearbox used in that model. Castrol has a specific test just for its A320 synthetic Optigear oil. And Vestas turbines will use four to five different tests specific to the four to five different oils it uses.”

The ASTM D2270 viscosity test, for instance, determines whether or not a correct oil has been used. This test involves measuring the viscosity index (VI) of oil between 40 and 100°C.

Other challenges to getting good oil-test results include:

• Having access to the tower during production periods
• Difficulty in regularly accessing gearboxes (They are 60 to 90 m up a tower)
• Time delays between acquiring a test sample and getting its test report
• Getting consistent gearbox conditions from turbine to turbine

These challenges limit the frequency of sampling. In addition, another difficut-to-manage variable comes by comparing data from a turbine that was running to one that was not. The detailed data, however, can be quite good.

Be mindful that these sensors and tests focus only on the gearbox and not the main bearing or generator. The data gathered will point to a problem or confirm a problem. In other words, using either a test or sensor may also indicate a failing bearing. However, you may not know which bearing is failing in the gearbox, or if it is an up-tower repair.WPE

How to Keep Them Working 20 Years and Longer

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.

Technicians from COT-Puritech will winch the lubricant hose up tower so they can draining and refill the gearbox with fresh oil.

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.

Viscosity

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.

Iron wear

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 contamination

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.

Oil  cleanliness

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.