Reducing wear particulates in a coal pulverizer gearbox

By Ken Nicholas, director of lubrication market services, Schroeder Industries; Richard Winslow, senior lead engineer, PacifiCorp – Naughton Plant; and Ted Naman, technical coordinator for industrial lubricants and greases, ConocoPhillips

A coal-fired power plant operating in the western U.S. was experiencing short gearbox life in its coal pulverizing operation. The AGMA 6EP (ISO 320) gear oil recommended by the OEM failed to provide adequate lubrication and protection based on oil analysis results and gearbox inspection after one year of operation. This was confirmed by excessive wear metals and lower viscosity in the used oil reports. Further analysis of the used EP gear oil indicated excessive buildup of particulate contaminants in the lubricant and depletion of the EP additive package. The contamination consisted primarily of dirt/coal dust and metallic particulates being generated by bearing and gear tooth wear, and a chain reaction of excessive wear was taking place.

Pulverizer gearbox description and operating costs
The pulverizer gearbox design dates back to the early 1960s. A steel worm gear driven by a large 800 rpm electric motor drives a bronze bull gear that is directly connected to a grinding table. The sump holds 255 gallons. The gear oil temperature is controlled by an integral water-cooled heat exchanger. The unfiltered ISO 320 EP gear oil is recommended by the gearbox OEM 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. Typical maintenance costs and intervals for each pulverizer gearbox were as follows:

• Oil changes required every 12 months at a cost of $5,000 in material and labor and $20,000 to $50,000 in lost electrical production, typical of most coal-fired power generation units of this timeframe. This particular plant had 13 of these coal pulverizers installed.

• After 10 years of operation, the bronze bull gear was rotated to expose the unworn gear teeth face side. This required four weeks of turnaround time, including maintenance work, at a total cost of $300,000 per unit.

• Every 20 years of operation, a complete rebuilding of the gearbox was required. Parts and labor for this effort exceeded $450,000 per gearbox, with lost production costing another $250,000 per pulverizer.

Breaking the wear chain reaction
Preliminary analysis of worn components indicated that the bronze gear face was experiencing significant sliding contact and spalling. As time after overhaul increased, 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 attributed to the following:

• High particulate loading of coal dust and dirt in the gear oil
• Chemical attack of the EP additive package during operation, most likely due to sulfur-phosphorus EP additive being active on the bronze bull gear, resulting in high levels of copper in the gear oil
• Catalytic reactions between the gear oil additives and some of the particulates generated

Plant personnel began to address these issues on multiple fronts:

• Search for methods to better seal the gearbox from particulate ingestion (primarily coal dust)
• Filtration methods/options for the gear oil to quickly capture the particulates and generated wear particulates
• Enhanced lubricant technology (both base oil and additive packages) to provide extended maintenance intervals without energy use penalties

Problem resolution
Success was achieved in the following areas in breaking the wear chain reaction:

Particulate ingress: This was successfully controlled through the use of breather desiccant filters on the gearbox vents and by very 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 as shown in Appendix 1 at the bottom of this article. Previous attempts at particulate counting were unable to establish the target ISO cleanliness level due to the very high levels of particulate.

Filtration method and customer requirements: Historically, the ability to filter ISO 320 and 460 gear oils in a coal pulverization environment was proven very difficult. Plant personnel determined that a kidney loop filtration system is one of the options to remove particulate contaminants from the pulverizer gearbox and to address the gear wear issue. The kidney loop filtration system must have the following characteristics:

• Adequate flow rate to handle the higher viscosity gear oil
• High dirt holding capacity
• Low maintenance; filter changes should not exceed once per month under normal operating conditions
• The ability to show gear oil cleanup within one week after maintenance is performed on the gearbox
• Continue to clean up the gear oil and maintain target cleanliness code of 18/15/11 per ISO 4406-1999
• Provide pre- and post-filtration sampling points for evaluation of filter effectiveness
• Skid mounting installation
• Suction and discharge locations designed to eliminate fire hazards, and the entire gearbox oil sump was turned over every 30 minutes
• Filtration skid size that did not interfere with normal maintenance activities

Figure 1. Kidney Loop Filtration System

Advances in filtration technology
An advanced filtration technology for this application was determined to be readily available for heavy gear oil that would meet the above requirements. An off-line kidney loop filtration package using a high-efficiency, high-dirt-holding capacity, synthetic filter media was procured and installed. The package uses two filter housings mounted in series, with a common-sized element in both housings as shown in Figure 1.

The filter elements initially recommended for the trial installation were rated at Beta 25=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 for a 460 centistoke (cSt) (2,500 SUS) gear oil. Temperature ranges of the system fluid varied from a low of 65 degrees Fahrenheit (18 degrees Celsius) when idle, up to 130°F (54°C) during normal operation. The filtration package is installed with the suction line coming into the filter bank directly from the bottom of the reservoir; the outlet, or filtered discharge line, is piped directly into the top of the reservoir.

The filter element condition is monitored 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 to begin opening. Other features of the filtration package include upstream and downstream sampling valves to allow gear oil samples to be taken without having to shut down the system.

Advances in lubrication technology
The OEM recommended 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 was causing premature wear in conjunction with the contaminants in the gearbox. Analysis of used gear oil samples confirmed that the EP additive package was being depleted. Depletion of the EP additive package was determined to be from continuous sliding of the bronze on steel gears and exposure to high temperature. This was confirmed with IR thermographic imagery. Very high dirt and particulate loading was confirmed by the ISO Cleanliness Code, as shown in Appendix 1. It was obvious then that the OEM-recommended EP gear oil did not provide adequate protection for the gears.

Based on these findings, and after consulting with the lubricant supplier, it was determined by all parties that AGMA 7 (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, and would extend the life of the gearbox, taking into account the temperature requirements and gearbox longevity. The physical properties of the synthetic gear oil are shown in Table 1:

ISO Grade 460

AGMA Grade 7

Density, lbs/gal 7.34

Flash Point (COC), °C (°F) 240 (465)

Pour Point, °C (°F) -29 (-20)

Viscosity,

cSt @ 40°C 460

cSt @ 100°C 37.2

SUS @ 100°F 2431

SUS @ 210°F 181

Viscosity Index 123

Acid Number, ASTM D974, mg KOH/g 0.20

Copper Corrosion, ASTM D130 1a

Foam Test, ASTM D892 Pass

Four-Ball EP, ASTM D2783, Weld Load, kgf 315

Four-Ball Wear, ASTM D4172, Scar Diameter, mm 0.40

FZG Gear Test, ASTM D5182, Pass Stage 12

Precipitation Number, ASTM D91, ml 0.001

Rust Test, ASTM D665 A&B Pass

Table 1. Physical Properties of ISO 460 Synthetic Gear Oil

 

In the past, plant personnel had evaluated the feasibility of using a synthetic gear oil in the pulverizer gearbox, but it was determined the high dirt loading in the gearbox made these uneconomical with frequent oil changes. However, with improved filtration now available, providing a potential oil life of at least three years, the economics of using a synthetic gear oil could be justified. The synthetic ISO 460 gear oil offered several benefits including:

• Enhanced pumpability at lower temperatures and, therefore, 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
The pulverizer gearbox was overhauled and all major rotating components were replaced, except the steel worm gears. The gearbox was wiped clean and dry with lint-free rags as part of the overhaul process. The steel worm and bronze bull gears were precision aligned and blue checked. The reservoir was flushed with an ISO 460 mineral oil and was then filled 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. The ISO Cleanliness Code result was 23/21/18. The pulverizer gearbox was put into service along with the filtration system. Following three hours of run time, the particle count was reduced to 21/19/11 as shown in Appendix 1.

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

Filter element service life was also monitored during the trial installation; results showed that the high-dirt-capacity media exceeded expectations, given the initial clean-up of the system, plus the service life during ongoing usage has been beyond the norm. Average service life to date, using the Beta 5=200 media is one year.

Figure 2.

During the trial installation, oil samples were taken and analyzed for physical and chemical properties, particle count and analytical ferrography. The results showed the wear metals were reduced significantly and the oil cleanliness was maintained.

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

• Significantly improved gear and bearing lubrication
• Minimal to non-existent wear metals in the gearbox to date based on the oil analysis reports
• No increase in drive motor energy consumption due to using a higher viscosity synthetic gear oil. Some plant instrumentation measurements indicated a 1 percent drop in motor amperage (4160 VAC motors)
• Particle count and analytical ferrography are now realistic options for accurate predictive/proactive maintenance.
• Gear oil life is extended and provides the additional benefits of reduced disposal costs and reduced environmental impact / waste oil generation.
• Gearbox life is significantly extended
• Contamination-related downtime is eliminated
• Maintenance intervals are extended
• Since applying the lubricant upgrade and the first filtration package and closely monitoring the results, the power plant has since purchased and installed their second unit

Acknowledgements

• John Kinion and maintenance personnel, Pacificorp Naughton Plant, South U.S. Highway 189, Kemmerer, WY 83101
• Chris Tully, project engineer, Schroeder Industries LLC, 580 West Park Road, Leetsdale, PA 15056
• Ken Knochel, technical services, Schroeder Industries LLC, 580 West Park Road, Leetsdale, PA 15056

References

1. ISO 4406:1999. Hydraulic fluid power. Fluids. Method for coding the level of contamination by solid particles
2. ISO 16889:1999 Hydraulic fluid power filters. Multipass method for evaluating filtration performance of a filter element
3. Ivan Sheffield, Schroeder Industries, “Changes in Filtration and Contamination – Switching Directions for the Filtration Industry”. Machinery Lubrication magazine, January 2005

Appendix 1