Pulverizing Ball Mill Failures

Smart Lubrication Management and Oil Analysis Deliver Uptime and Reduced Costs

In recent years, the gap between electric generating capacity and demand for electricity has closed. In some areas, shortages exist. This has placed a new premium on unit availability. Likewise, with deregulation of the power generation industry looming, management is looking for ways to reduce operating costs and to ensure availability to remain competitive.

These forces have turned attention to maintenance and reliability teams. This is especially true with a 1300 mn coal-fired electric generating station based on the East Coast.

The maintenance team at this utility consists of the maintenance department and the PDM group. This group incorporates integrated technologies, which also include the dedication and tenacity of the maintenance department. With this group of knowledge the team focuses on delivering the reliability that management needs to remain competitive. The PDM group consists of the following members:

  • Mechanical Engineer
  • PDM Tech (Level III Vibration, Level I Thermography)
  • PDM Tech (Level II Vibration, Level I Thermography)
  • PDM Tech (Oil Analysis [OMA] Level I, Level I Vibration, Level I Thermography)

While each technician specializes in his respective technology, they are cross-trained on the companion technologies. If a problem is indicated with one technology, the team quickly incorporates information from partner technologies to discuss their findings as a group. Likewise, they work together to eliminate the root causes of failure, as is the case with the ball mill pulverizers, which had produced consistently high failure rates.

Focus on Ball Mills

Ball mills, or pulverizers, are critical to the operation of any coal-fired boiler. The pulverizers, as the name implies, crush coal into a fine powder so it can be blown into the boiler’s furnace to produce steam. No coal, no fire. No fire, no steam. No steam, no electricity. At this utility each of the two generating units employ six pulverizers, each on a continuous basis.

Each mill pulverizer is responsible for about 130 megawatts of power. When the coal quality is good, the unit can run at full capacity on five pulverizers. Therefore, in the event of a failure, the impact on production is minimal assuming a second pulverizer does not fail. The plant maintains two gearboxes as spares because this dual failure has occurred in the past.

Gearbox reliability on the pulverizers has historically been poor. Of the 12 boxes onsite, on average, ten were annually rebuilt, yielding a mean-time-between failure of about 1.2 years. About 35% of the rebuilds required replacement of the gearing, an expensive job that took 4,680 mw hours. In the other 65% of the failures, the gears were turned and the bearings replaced.

Major rebuilds cost around $20,000 to repair, while minor rebuilds cost $2,500 (Figure 1). On average, each rebuild costs $8,625, exclusive of any downtime figure. Actual records revealed repair expenditures of $137,000 and $81,000 for generating units 1 and 2 respectively.

An investigation of the pulverizer failures by the PDM team revealed the following factors contributed to the high failure rate in the gearboxes:

  • Excessive Load - The gearboxes, rated for 1060 HP, are coupled to 1200 HP motors.

  • Viscosity Problems - Because the gearboxes were splash lubricated, high viscosity (ISO 680) was great for the gearing, but starved the bearings. Lower viscosity gear oil was great for the bearings, but insufficient to lubricate the gearing.

  • High Temperatures - Due to excessive loads, the gearboxes run hot. The continuous duty lubricant temperature averaged around 200°F, causing it to rapidly degrade. In a gearbox, mating component surfaces produce hot spots, which increase the rate of degradation.

  • High Contamination - Due to the harsh and dusty conditions of a coal-fired power plant, the gearboxes were operating at an estimated ISO 26/24/21 to 27/25/22 cleanliness level. (Note: due to the absence of early oil analysis data, these levels were estimated based on related past experience).

Assessing the Options to Achieve Reliability

An investigation into possible corrective actions by the PDM team presented the two following options:

1. Change the specification of the gearboxes and redesign accordingly, or

2. Reduce fluid temperatures, control contamination and force-feed lubricant to the bearings.

Option one was quickly dismissed by the team, due to its high cost, in favor of option two. In addition to the initial cost of the gearbox, option one required expensive foundation changes and significant coupling redesign. Rather, the team elected to target the lubrication and contamination problem with following game plan:

  • Install a side-loop circulating system with air-cooling to reduce fluid temperatures.

  • Drill oil passages to force-feed lubricant to the bearings.

  • Install a high quality breather to restrict ingestion of contamination.

  • Install a filter on the circulating system to remove ingested and generated contaminants.

The team believed that delivering clean, cool oil in sufficient quantities to all wetted components would eliminate most of their problems and avoid an expensive redesign of the ball mill pulverizers.

Figure 2.Pulverizer Gearbox Modifications

The first unit installed was “home-made” from parts that were on hand (Figure 2). They reduced the fluid’s temperature by 50°F and decreased contamination levels to ISO 19/17/13. With the temperature drop, ISO VG 320 oil was adequate to lubricate the gears, and force-feeding the bearing ensured their protection.

Once the concept proved successful, commercially manufactured units were purchased and installed on all the pulverizer gearboxes. As a follow-up, the team also elected to upgrade the high maintenance, low performance lip seals with labyrinth-type bearing isolators.

In addition to providing superior contamination exclusion and lubricant retention performance, no seal failures have occurred. Bearing isolators cost about three-times as much up front, but the team believes the cost is quickly recuperated due to the longer life and improved performance.

Dollars and Cents

The cost to modify all the units with circulating oil, coolers and filters was $60,000, or about $5,000 per unit. In the year since implementing the change, no pulverizer gearbox failures have occurred. Only time will tell, but one can safely project from the data on-hand that the team’s efforts have reduced ten failures per year to less than one!

Assuming an average failure cost of $8,625 (actual data from the past two years is higher), the project achieved payback (discounted) in 10.26 months, and produced an internal rate of return (IRR) of 126% (Figure 3). The financial analysis of the project produced a five-year net present value (NPV) of $232,304.

It is important to note that the benefits of the project will continue until such time that the asset is no longer required, or the system modifications are decommissioned. The project valuation is based on avoided repair costs alone. It does not include the most important, but most difficult to assess, value of improved
pulverizer availability.

The team assures the reliability of the pulverizers and other plant assets with integrated condition monitoring that includes oil analysis. Oil analysis ensures that the pulverizer lubrication is kept clean and in tip-top condition. Thermographic and thermometric analysis ensures that the oil runs cool. All technologies stand ready to detect the now infrequent pulverizer failures so damage can be kept to a minimum, and to ensure that the generation of electricity is uninterrupted.

Applying common sense, technology and technical know-how, a skilled team turned bad-actor pulverizer gearboxes into reliable assets for this utility despite the fact that the units operate at 13% above their rated load. Oil analysis, integrated with other condition monitoring techniques, was vital to the discovery of the failure root causes.

It helped light the path to a design modification that offset the overloading condition, a modification that provided payback in less than one year exclusive of any downtime costs. Oil analysis continues to serve the PDM team to maintain healthy lubricant conditions, and to detect and deal with failures before production is threatened.

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