Coal Fired Power Plant Case Study: Coal Crusher Gearbox Lube Filtration Total System Cleanliness Approach

The Problem - Coal fired power plants typically operate several ball mills or coal pulverizing mills to crush the incoming coal to the optimum size for combustion. This means that each power generation unit has five or six mills. The gearboxes that run these coal mills are usually lubricated with high viscosity oil, such as ISO VG320 or ISO VG460. Traditionally, these large gearboxes have no filtration and the oil is changed on a service interval. Oil analysis, and even visual inspection, revealed that the oil in the gearboxes was contaminated to levels not suitable for any lube or hydraulic system. Oil that is so highly contaminated leads to premature gearbox failures that, unfortunately, have become accepted by operators as the normal life cycle of their equipment. Even though the oil is drained and replaced with new oil, there is no protection for the gears and bearings in the time before the oil change occurs, causing them to suffer tremendous wear at the end of each service interval. There also exists an inherent problem with the drain and refill routine as a whole: while the gearbox is turned off in preparation for an oil exchange, the lubricant is no longer agitated, and suspended contaminants will fall out of suspension and collect in the sump area of the gearbox, which is usually slightly lower than the drain valve. A high quantity of that solid contaminant then remains in the gearbox to be stirred up and held in suspension by the new oil and, because “unfiltered new oil is dirty oil,” this creates even more wear for the gearbox.

The Costs & Opportunity - It only takes a moment of considering these problems to realize that poor maintenance of the lubricated gearbox can bring about high costs and long equipment downtime. A service interval flush program has its inherent cost in the rising price of oil, which for the ISO VG320 and VG460 oil used to lubricate the gearboxes is $538.41 per barrel. Another cost can be found in the 4.5 hours required by the drain and refill routine for the oil. Once drained, the oil must be disposed of properly, which again produces cost. The big, ugly expenses lurking in the background occur when the gearbox fails outright and must be rebuilt, often costing up to $610,000. By achieving and maintaining clean fluid there is a great opportunity to have a positive impact on productivity and the bottom line. This application also presents the opportunity to break the decades of stigma that “drain and replace” is the only way to maintain lubricated machinery.

The Solution - Applying a total system cleanliness approach is necessary to maximize uptime, extend gearbox life, extend fluid life, reduce limited maintenance resource demands, reduce fluid disposal costs and even save money on power consumption. Installing a dedicated filtration system and applying the following strategies can prove to be very effective.

Step 1 - Set a target for fluid cleanliness to be verified by periodic oil analysis with the results reported per ISO4406:1999. The ISO code results should be expressed in a three number code that uses the latest particle counter calibration 4µ[c] /6µ[c] /14µ[c] . Table 1 details potential life extension of bearings as fluid cleanliness is improved.

Step 2 - Install high efficiency glass media or desiccant breathers on all gearbox vent ports. Many coal mill gearboxes have no breather protection which allows coal dust to enter the unit after the oil has begun being filtered. Unprotected vent ports can be one of the worst sources of ingression. While ingression can also come from unfiltered new oil, filtering all new oil will also help to minimize contaminant ingression.

Step 3 - Change the criteria for replacing the oil from a time interval to a decision based on the oil condition determined by oil analysis. If the oil is kept clean and is not subjected to excessive temperature, the oil life should be defined based on the health of the oil and the additive package. Replacing the current oil with new oil at this point would also increase the speed at which results are noticed. It is also suitable to consider using a synthetic oil that will yield better performance when kept clean and lower amp draw for the coal mill.

Step 4 - Install a dedicated filtration system on each individual gearbox to filter the oil continuously during operation. Photo 1 shows the Hy-Pro FSL series self contained filtration unit that was used in the Edison St. Claire application. There are many important features to consider when selecting a side loop unit: the unit should feature a large filter element because they provide good performance in cold start situations, they have a longer element life compared to similarly rated elements of a smaller size, and they permit the use of finer filtration that can even be combined with water removal media. Maintenance friendly top loading filter housings are a plus for minimizing oil spills. Oil sampling ports should be included before and after the fluid passes through the filter element so that the true condition of the gearbox oil can be measured (before the filter) and the filter performance may be quantified (after the filter). It is also important to have a true differential pressure gauge with color coded indication so that true element condition may be monitored.