Ultra-Supercritical Coal Fired Power Plant Coal Mill Pulverizer Gearbox Lube Filtration Improvement Success
A major independent power producer (IPP) in Malaysia has commissioned the one super-critical conventional coal-fired thermal power generating unit of 1,000MW. The plant is equipped with seven gravimetric raw coal feeders and seven pulverisers with dynamic classifiers.
The ultra-supercritical technology supplied by GE Power, which provides the most efficient coal combustion technology currently on the market.
The heart of the unit is a pulverized coal two-pass, utilizing GE’s latest technology with GE’s steam turbine and generator technology with the latest ultra-supercritical boiler and environmental control systems to generate electricity required for two million people.
Coal pulverizers typically operate under challenging conditions to perform the fundamental and important task of crushing the fuel to a uniform optimum size for proper combustion in coal-powered power plants. 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. A well-performing pulverizer also requires optimum airflows and good fuel fineness, among other performance characteristics, to achieve optimum combustion for the ultra-supercritical boiler.
The Productivity Opportunity
Coal pulverizers are continually exposed to heavy coal dust that operate in extremely dusty and dirty environments, making even routine maintenance a time-consuming and costly process. Critical components such as gears and roller element bearings often under performance challenges as these pulverizers often operate under high temperatures. Obviously maintaining high quality lubricant marked critical challenges for productivity reliability and availability, as well as personnel safety.
The plant is equipped with seven gravimetric raw coal feeders and seven HP 1103 pulverisers with dynamic classifiers. The gearboxes that run these coal mills are lubricated with extra high-performance gear oils having outstanding extreme pressure characteristics and load-carrying properties Mobilgear 600 XP320 high viscosity oil.
Traditionally, these large gearboxes have not enough filtration and the oil is changed on condition-based monitoring. 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 could potentially lead to premature gearbox failures and reduce the life cycle of the 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 this creates even more potential wear for the gearbox.
Meeting OEM recommended operating gear oil cleanliness target of ISO -/19/16 is obviously tough challenges ahead. Plant personnel began searching for a better lubrication system to break the wear chain reaction.
An offline filtration was installed and commissioned by a local filtration company representative unfortunately fluid cleanliness could not meet neither system OEM nor customer set target even though experienced high filter replacement. Plant personnel began to address these issues on multiple fronts.
Local Hy-Pro Filtration representative, Proactive Strategic Reliability Sdn Bhd (PSR) had a site assessment and thorough site system audition. Preliminary analysis indicated pulverizers are experiencing significant high coal dust ingression. It is necessary and practical to applying a total system cleanliness approach 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 – Make a commitment to fluid cleanliness. Immediately make a stoppage to contamination ingression into the gear box.
Step 2 – 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
. Table 1 details potential life extension of components as fluid cleanliness is improved. A review of how ISO codes are determined as provided.
Step 3 – Install Hy-Pro high efficiency glass media or desiccant breathers on all gearbox vent ports. Any 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.
Step 4 – Utilizing the condition-based monitoring from replacing the oil on 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.
Step 5 – While ingression can also come from unfiltered new oil, filtering all new oil will also help to minimize contaminant ingression.
Step 6 – Install a dedicated filtration system on each individual gearbox to filter the oil continuously during operation. Photos below show the customised PSR filtration system fitted with Hy-Pro DFE rated high-performance cost-effective filter element. There are many important features to consider when selecting a side loop unit: the unit should feature a DFE tested and rated high performance cost effective filter element because they provide good performance in cold start situations, they have a longer element life compared to similarly rated elements 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).
Immediate Measurable Results
A fluid cleanliness target was established. System OEM has set a cleanliness target of ISO -/19/16 and the customer has set a cleanliness target of ISO -/17/14. PSR, the distributor and representative of Hy-Pro Filtration has set a target of ISO 18/16/13.
Pulverizer Mill F has changed new oil. After one (1) month pulverizer in operation an oil sample was taken for cleanliness analysis and the result was ISO 21/20/14 or NAS10. The dedicated customised PSR on-line side loop filtration system was installed and commissioned. Less than two (2) weeks after the installation, gearbox oil contaminant levels were reduced significantly and the cleanliness target was achieved (see table 2). It is important to have a solid grasp on ISO codes to understand the scale of dropping the ISO code from 21/20/14 to 18/15/12 and lower. In this case the number of particles per milliliter > 4 micron dropped from 17,278 to 1902 and the number of particles per milliliter > 6 micron dropped from 5387 to 238, whereas the number of particles per mililiter > 14 micron dropped from 110 to 23. This represents a 808% drop in particles > 4 micron, a 2,163% drop in particles > 6 micron and a 378% drop in particles > 14 micron respectively.
The final result showed the fluid in the specified gearbox had achieved double component life quality cleanliness. A quick recall of the component life extension table previously referenced in this study should mean that those concerned with reliability have one less thing to worry about.
Graph 2 below provides a visual representation of Table 2 and shows the dramatic change in contaminant levels that resulted from installing the PSR total cleanliness system. Graph 1 shows the particle counts for the 4 micron, 6 micron and 14 micron channel. In this case the system OEM and the customer did not include the 4 micron channel in the fluid cleanliness target which is the first reported channel in the current ISO Code.
The Technology That Works Dynamic Filter Efficiency (DFE)
The PSR on-line filtration skid has come with proven high performance and cost-effective Dynamic Filter Efficiency (DFE) filter technology for the area of coal pulverizers.
The challenge is that the industry specified standard (ISO 16889) used to rate and compare relative filter performance does not subject test filters to the dynamic stresses they see in today’s systems. Instead, the ISO 16889 multipass performance test measures capture efficiency and dirt-holding capacity in a steady-state environment run at one flow rate, under ideal lab conditions without subjecting the filter to hydraulic actuation or system restarts.
In contrast, DFE introduces a range of duty cycles throughout the test, bridging the gap between the lab and real world. The DFE flow rate is not constant but, rather, hydrostatically controlled so full flow through the test filter can quickly change to simulate various hydraulic and lube duty cycles. Flow across particle-counter sensors remains constant during all readings and no intermediate reservoirs collect fluid prior to measurements. This ensures that the fluid counted accurately represents real-time system contamination levels. Counts are made before, during, and after each flow change, with results reported as filtration ratio (beta), efficiency, and actual number of particles per milliliter upstream and downstream of the filter.
DFE testing provides an inside look at the vital signs of a filter through a range of dynamic conditions to better understand how well a filter will capture and retain contaminant, and in real time.
Raw data is digitally tagged so filter efficiency is gauged for various combinations of flow conditions and differential pressures across the filter element. Typical particle counts are taken at maximum and minimum flows, and when flow changes (low to high or high to low). Rapid particle counting with proper timing provides a real-time understanding of the capture efficiency and retention characteristics of a filter.
It’s not surprising that many elements get higher ratings according to ISO 16889 than per DFE tests. This is troubling because OEMs often select filter media based on ISO beta ratios published by filter manufacturers. A common result is a hydraulic system that suffers from premature contamination-related failures, even though it is protected by filters that, in theory, should prevent such failures — causing downtime, unreliable equipment performance, and expensive component repair and replacement costs.