Teresa Hansen, Section Editor

A coal fired power plant operating in the Western United States was experiencing short gearbox life in its coal pulverizing operation. An annual gearbox inspection and oil analysis results indicated that the AGMA 6EP (ISO 320) gear oil recommended by the original equipment manufacturer (OEM) failed to provide adequate lubrication and protection after one year of operation. The used oil reports indicating excessive wear metals and lower viscosity levels confirmed this. Further analyses of the used EP (extreme pressure) gear oil showed that excessive build-up of particulate contaminants in the lubricant and depletion of the EP additive package were responsible for the faulty gear boxes. The particulate contamination consisted primarily of dirt, coal dust and metallic particulates being generated by bearing and gear tooth wear, resulting in a chain reaction of excessive wear.

Pulverizer Gearbox Description and Operating Costs

The plant’s pulverizer gearbox design dates back to the early 1960s. The gearbox consists of a steel worm gear driven by a large 800 rpm electric motor that drives a bronze bull gear directly connected to a grinding table. The sump holds 255 gallons of gear oil, with temperature controlled by an integral water cooled heat exchanger. The OEM recommended that the unfiltered ISO 320 EP gear oil be used to provide lubrication to the bronze-on-steel gears and bearings.

Although this gearbox design is rugged and simple, maintenance costs were becoming excessive and maintenance outage/overhaul intervals did not support power generation schedules at the plant. In fact, typical maintenance costs and intervals for each pulverizer gearbox included:

• Oil changes every 12 months at a cost of $5,000 in material and labor and $20,000 to $50,000 in lost electricity production.
• Bronze bull gear rotation after 10 years of operation to expose the unworn gear teeth. This required four weeks of turnaround time and included maintenance work at a total cost of $300,000 per unit.
• Complete gearbox rebuild after 20 years of operation. Parts and labor for this effort exceeded $450,000 per gearbox, with lost production costing another $250,000 per pulverizer.

With 13 coal pulverizers operating at the plant, maintenance and downtime costs quickly added up.

Breaking the Wear Chain-Reaction

Careful preliminary analysis of worn components indicated that the bronze gear face was experiencing significant sliding contact and spalling. As time progressed, the bronze gear face wear became more and more significant. Plant personnel began to search for a better lubrication system to break the wear chain reaction.

Plant personnel suspected that the wear patterns on the bronze bull gear faces were caused by several factors including high particulate loading of coal dust and dirt in the gear oil and catalytic reactions between the gear oil additives and some of the particulates generated.

Additionally, personnel suspected that due to high levels of copper in the gear oil, the EP additive package was chemically attacking the gear oil during operation. Personnel believed the high levels of copper in the gear oil were most likely due to sulfur-phosphorus EP additive being active on the bronze bull gear.

Plant personnel quickly decided that these issues needed to be addressed. They searched for methods to better seal the gearbox from particulate ingestion, primarily coal dust. Additionally, they implemented filtration methods and options to quickly capture the particulates and generated wear particulates in the gear oil and they applied enhanced lubricant technology (both base oil and additive packages) to extend maintenance intervals.

Problem Resolution

After a period of trial and error in which personnel tested a variety of solutions for effectiveness, they were successful at breaking the wear chain-reaction via particulate ingress, filtration and customer requirements.

Particulate ingress was successfully controlled through the use of breather desiccant filters on the gearbox vents and by paying close attention to the grinding table seals. Initial ISO cleanliness code of 23/21/18 (per ISO 4406-1999) was achieved with aggressive breather filtration. Prior to implementing particulate ingress, employees were unable to establish the target ISO cleanliness level due to the high particulate levels.

Historically, it has been difficult to filter ISO 320 and 460 gear oils in a coal pulverization environment. Plant personnel determined, however, that a kidney loop filtration system would be one of the better options for removing particulate contaminants from the pulverizer gearbox and addressing the gear wear issue.

An off-line kidney loop filtration package using a high efficiency, high dirt holding capacity, synthetic filter media was installed using two filter housings mounted in series.Click here to enlarge image

An effective kidney loop filtration system must have the following:

• High dirt-holding capacity
• Low maintenance requirements. (Filter changes should be needed no more than monthly under normal operating conditions)
• Obvious gear oil improvement within one week of preformed maintenance
• Continued gear oil clean-up and effective maintenance of target cleanliness code 18/15/11 per ISO 4406-1999
• Pre- and post-filtration sampling points for evaluation of filter effectiveness
• Skid mounting installation
• Suction and discharge locations designed to eliminate fire hazards and a turnover of the entire gearbox oil sump every 30 minutes
• Filtration skid size that does not interfere with normal maintenance activities.

Filtration Technology Advances

Plant personnel determined that advanced filtration technology that would meet the kidney loop filtration requirements for this application was available for heavy gear oil. An off-line kidney loop filtration package using a high efficiency, high dirt holding capacity, synthetic filter media was installed using two filter housings mounted in series with a common sized element in both housings.

The filter elements initially recommended for the trial installation were rated at Beta25=200 in the first stage and Beta10=200 in the second stage. Oil flow was delivered by a vane pump rated at 10 gallons per minute (GPM) for a 460 centistokes (cSt) (2,500 SUS) gear oil. Temperature ranges of the system fluid varied from a low of 65 F when idle up to 130 F during normal operation. The filtration package was installed with the suction line coming into the filter bank directly from the bottom of the reservoir and the outlet, or filtered discharge line, was piped directly into the top of the reservoir.

Presently, plant personnel monitor the filter element condition by differential pressure gauges installed on each filter housing, with a target of 25 to 28 psig as an indicator of element loading; the elements were changed out prior to allowing the internal bypass valve. Other features of the filtration package include upstream and downstream sampling valves to allow gear oil samples to be taken without having to shutdown the system.

Lubrication Technology Advancements

Over the course of the system update, personnel recommended that the system design include an AGMA 6EP (ISO 320) gear oil for the pulverizer gearbox. Evaluation of the wear patterns on the gear teeth indicated that the EP additive package in this gear oil was too active on the bronze bull gear and in conjunction with the contaminants in the gearbox, was causing premature wear and failing to provide adequate protection for the gears. Analysis of used gear oil samples confirmed that the EP additive package was being depleted from extended contact between the steel on bronze gear face and sliding wear. Infrared thermographic imagery indicated that this extended contact resulted in higher gear face temperatures. What’s more, very high dirt and particulate loading was confirmed by the ISO Cleanliness Code.

By addressing particulate ingress issues, adding filtration and switching to a synthetic gear oil in the pulverizer gearbox, the power plant saw significant benefits, including extended gear oil life. The benefits are apparent when an old sample and a new oil sample, pictured here, are compared.Click here to enlarge image

After consulting with the lubricant supplier, all parties determined that AGMA 7 R&O (rust-and-oxidation inhibited) (ISO 460) synthetic gear oil would best protect the gearbox in this application. The higher viscosity grade and improved lubricity of this synthetic gear oil, coupled with R&O additive chemistry, provides a higher oil film strength than that recommended by the OEM, which should extend the life of the gearbox. The physical properties of the synthetic gear oil are shown in Table 1.

Click here to enlarge image

In the past, plant personnel considered the feasibility of using a synthetic gear oil in the pulverizer gearbox, but determined the high dirt loading in the gearbox made these uneconomical with frequent oil changes. However, with recent filtration improvements enabling a potential oil life of at least three years, the economics of using a synthetic gear oil could be justified. The synthetic ISO 460 R&O gear oil offers several benefits, including:

• Enhanced pumpability at lower temperatures and with enhanced filterability
• Higher oxidation resistance and thermal stability
• Higher film strength at high and low temperatures
• Extended service life in a clean, filtered environment.

Operational Results

In the end, personnel overhauled the pulverizer gearbox and replaced all major rotating components except for the steel/worm gears. They wiped clean and dried the gearbox with lint-free rags as part of the overhaul process. They precision aligned and blue checked the steel worm and bronze bull gears and flushed the reservoir with an ISO 460 mineral oil and then filled it with the synthetic ISO 460 gear oil. A baseline gear oil sample was drawn from the reservoir and analyzed for particle count per ISO 4406-1999, with ISO Cleanliness Code results coming in at 23/21/18. Finally, personnel put the pulverizer gearbox into service along with the filtration system. Following three hours of run time, the particle count was reduced to 21/19/11.

After 48 hours of run time, the plant installed a set of Beta5=200 filter elements in each housing to further reduce the system contamination and achieve target ISO Cleanliness Code 18/15/11 more quickly. The pulverizer gearbox and filtration system continued to run for another two weeks with element condition being monitored using differential pressure gauges. As a result of using the 5-micron media during these two weeks, the target ISO Cleanliness Code 18/15/11 was reached.

Personnel also monitored filter element service life during the trial installation; results showed that the high dirt capacity media exceeded expectations given the system’s initial clean-up. Plus, the service life during ongoing usage has been beyond expectation. Oil samples analyzed during the trial installation showed that the wear metals were reduced significantly and the oil cleanliness was maintained.

Given the success of this initial installation, the power plant continues to achieve the following benefits by using the ISO 460 R&O synthetic gear oil and new filtration system:

• Significantly improved gear and bearing lubrication
• Minimal to non-existent wear metals in the gearbox
• No increase in drive motor energy consumption thanks to the higher viscosity synthetic gear oil (Some plant instrumentation measurements indicated a 1 percent drop in motor amperage with 4160 VAC motors)
• Accurate predictive/proactive maintenance using particle count and analytical ferrography
• Extended gear oil life, resulting in reduced disposal costs and environmental impact/waste oil generation
• Significantly extended gearbox life
• No contamination-related downtime
• Extended maintenance intervals.

Since applying the lubricant upgrade and filtration package and closely monitoring the results, the power plant purchased and installed a second unit.

Acknowledgements: Chris Tully, John Kinion and maintenance personnel at Pacificorp Naughton Plant, and Ken Knochel, Schroeder Industries.