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coal mill in operation and maintenance manuals

coal pulverizer maintenance improves boiler combustion

coal pulverizer maintenance improves boiler combustion

Coal pulverizers are the heart of a pulverized coal-fueled boiler. Often, the root causes of nonoptimized combustion lie with the pulverizers. Capacity; reliability; and environmental issues such as slagging, fouling, and higher-than-desired CO or NOx emissions; overheated superheater and reheater tube metals; and cinder fouling of selective catalytic reduction catalyst and air heaters have all, at times, been linked to poor pulverizer performance.

It is common in our experience to find pulverizers that are performing poorly, yet the degree to which unit reliability, efficiency, capacity, and environmental emissions are affected by them is often underappreciated. However, there are steps that can be taken to measure, quantify, and monitor pulverizer operation so that changes can be made to improve performance.

Obtaining representative samples of coal fineness and fuel distribution is the first step. The best method we have found to do this is by using an isokinetic coal sampler. All fuel lines must be sampled and the fineness sieved from each coal pipe separately. The fuel mass flow to each burner must also be measured.

An isokinetic sampler similar to the one shown in Figure 1 can be used with a dirty-air velocity probe to establish the proper sample extraction rate. The fuel line velocities that are measured are used to compute the primary airflow and air/fuel (A/F) ratios of each coal pipe. The velocities and A/F ratios are valuable for diagnosing combustion issues.

Tuning improvements can only be implemented after the true current performance is measured. Sampling single pipes, or sampling at a single location, is totally unacceptable in our experience. All fuel pipes must be sampled and sieved individually for best accuracy.

The fuel lines must be tested/sampled under normal operating conditions. Often during testing, we have observed that operating conditions are changed. For example, we have seen primary airflow reduced, classifiers reset for best fineness, and fuel flow brought back to mill design fuel flow rates. In other words, the assessment is not representative of normal operation. Testing under special conditions proves nothing. Only testing under normal operational conditions enables a useful diagnosis.

Acceptable standards for best low-NOx burner performance are coal fineness of 75% passing a 200-mesh sieve and less than 0.1% remaining on a 50-mesh sieve. Fuel balance should be within the range of plus or minus 10%. However, in our experience, it is common to find fuel fineness that is well below 65% passing through a 200-mesh sieve and more than 1% remaining on a 50-mesh sieve. Furthermore, it is common to see fuel imbalances that exceed plus or minus 30% to the burners.

2. Lopsided fuel distribution. This test data shows pulverizer fuel flow rates measured during an actual test. The fuel distribution is poor in this case. It should be balanced within plus or minus 10%. Source: Storm Technologies

Once the data are compiled, out-of-specification readings must be investigated. An internal inspection should be completed to check the wear of grinding elements and classifier housings, vanes, and other internal components. Also, check for foreign matter that might be blocking fuel flow paths. Any problems identified should be corrected.

Achieving best fuel balance is done by first balancing the system resistance in all of the fuel lines using orifices and then increasing fuel fineness. Figure 3 shows the typical results of this approach to fuel balancing. Of course, internal pulverizer blue printing to best mechanical tolerances and optimizing an accurate and repeatable air/fuel ration is also important.

There are various adjustments and mechanical tuning measures that can be completed to improve the performance of a modern coal pulverizer. Locations identified on Figure 4 are keyed to the following improvements (journal pressures listed are for a #943-size pulverizer):

Install synchronized straight vane coarse particle guide blades (A). The retrofit lengthens the classifier blades, improves material to 3/8-inch-thick AR400 or better, and implements critical synchronization of the classifier blades for fuel/air two-phase mixture homogenization.

Install orifice housings (E) to support future balancing efforts. The change offers two advantages: It is easier for maintenance personnel to change out orifice plates, and it speeds testing and balancing efforts.

Verify that roll-to-ring clearances (I) are absolutely no greater than 1/4-inch over the full grinding length of the rolls and that the clearance is parallel to the bowl for the full width of the rolls.

Additionally, ensure that venturi sensing lines, connections, and transmitters are all in good condition. Tempering air dampers should be stroked and corrections made to ensure that they close at least 99%. This should also be done for hot air dampers.

All internal mill surfaces must be smooth so that the swirl of the coal/air mixture may enter and leave the classifier without spoiling or turbulence caused by double layer tiles, welded pad eyes, or other internal surface discontinuities. This, combined with precise primary airflow measurement and control, is important for uniform fuel distribution at the classifier outlet. All internal dimensions should be verified and technically directed by a qualified service engineer during installation of performance parts and before closing the mill.

Overhauling Stock gravimetric feeders can also be worthwhile. The refurbishment should include calibrating load cells properly, installing modern microprocessors, adjusting belt tension appropriately, and completing accurate speed calibration.

Another pulverizer performance monitoring technique is to observe the drive motor power input in correlation with the coal feed rate. The relationship of ton/hour to kWh power input is a very helpful leading indicator (Figure 5). A reduction in the power used by a coal pulverizer does not usually result in an improved heat rate. Instead, more grinding power nearly always correlates with better coal fineness. The only exception is with a ball tube mill.

Pulverizer capacity is not simply a measure of coal throughput; capacity refers to a certain coal throughput at a given fineness, raw coal sizing, HGI (Hardgrove Grindability Index), and moisture. Often, if the desired coal throughput or load response is not achieved, the primary airflow will be elevated to higher flow rates than are best for capacity. However, increased throughput achieved in this way sacrifices fuel fineness (Figure 6).

6. A negative correlation. The three main factors that constitute pulverized capacity are Hardgrove Grindability Index (HGI), fineness, and coal throughput. Increasing throughput will adversely affect fineness. Source: Storm Technologies

This is very common. When the primary airflow is higher than optimum, it creates entrainment of larger-than-desired coal particles leaving the mills, promotes poor fuel distribution, lengthens flames, and impairs low-NOx burner performance. We have found that targeting an A/F ratio around 1.8 lb of air per lb of fuel is best. For some pulverizer types, such as ball tube mills and high-speed attrition mills, often a 1.6 A/F ratio is optimum. Never have we observed good combustion conditions or good mill performance with A/F ratios of 2.5 or greater. However, it is common to find A/F ratios of 2.2 to 2.5 during baseline testing.

Results of as-found airflow to fuel flow testing from a sample plant are shown in Figure 7. In this particular case, the A/F ratios tested were well above the desired A/F ramp. When operators bias the primary airflow up, above the optimum, it may improve wet-coal drying, load response, and reduce coal spillage from the grinding zone, but it is not good for the furnace burner belt performance.

7. Missing the mark. The air-to-fuel (A/F) flows shown in this graph are much higher than optimum. Installing properly sized rotating throats is often required to achieve targeted A/F ratios. Source: Storm Technologies

All combustion airflow inputs should be measured and controlled, if possible. We prefer to use the tried and proven venturi or flow nozzles for this purpose because they are rugged, reliable, offer repeatable results, and are less prone to impulse line plugging.

Several components can be retrofitted to improve the performance of MPS mills. The changes may cost a significant amount of money, but the work will usually pay for itself through improved heat rate. One 450-MW coal-fired unit in the Midwest spent $750,000 on testing, changes, and tuning, but calculated that it saved millions by improving heat rate and by allowing higher-slagging fuel to be used at a reduced cost, which greatly increased its market competitivness.

8. Extending component lives. Getting 8,000 hours per year performance requires condition-based maintenance utilizing periodic isokinetic coal sampling and venturi hot K testing and calibration. Source: Storm Technologies

Cold air has a different density than hot air, which can result in a variance in measured velocity at similar mass flow rates. Because the K-factor will vary, we prefer to conduct Hot-K airflow calibrations that use typical operational air or gas density when developing an average K-factor. That information is important when developing a pulverizer primary airflow curve and when measuring all combustion airflows.

Most instrumentation technicians can calibrate and check using the Hot-K method to verify calibration accuracy. As previously mentioned, high primary airflows are one of the most common root causes of poor pulverizer performance, in our experience, so obtaining accurate and representative measurements is very important.

The goal is to obtain the best possible burner belt combustion because it improves heat rate, reduces slagging/fouling, lowers emissions, and reduces cost. All of the following actions can help improve burner belt combustion:

mill maintenance methods | | miller magazine

mill maintenance methods | | miller magazine

Maintenance, repair and technical service are significant since they affect lifetime of milling machines and ensure them to work non-stop at desired levels. If regular and good maintenance is not performed, machines cannot work for long years. Predictive maintenance monitors active machines and equipment in order to detect possible breakdowns. Thus, it prevents downtimes for machines because of unplanned maintenance as well as loss of production and unnecessary part replacements. As a result, maintenance time and downtime stemming from breakdowns decrease up to %25-30. It enables eliminating minor flaws before letting them to evolve into more serious problems.

Long time ago, mills were giving a break for production at weekends and maintenance teams were performing their duties until noon on Mondays to allow the mill to be back into action. Now most mills are active 7 days and 24 hours. The downs for maintenance are now at least every 10 weeks. Maintenance can be defined as all kinds of activities like repairs, replacements, inspections etc. in order to maintain working of buildings and equipment during their desired life expectancy.

Maintenance, repair and technical service are significant since they affect lifetime of milling machines and ensure them to work non-stop at desired levels. If regular and good maintenance is not performed, machines cannot work for long years. Regular maintenance is one of the most important factors that affect product quality. Regular maintenance prevents downtimes for machines because of unplanned maintenance and it enables to decrease costly part replacements and dependency to third party repairers.

INSUFFICIENT MAINTENANCE Insufficient maintenance results in shutdowns for the mill and causes a decrease in capacity and inadequate production. Accelerated amortizations for machines, increase in mineral oils and product costs are also among results of insufficient maintenance. And the ultimate results may be loss of prestige and bankruptcy because of poor quality products.

Expenditures on maintenance cannot show direct returns, but they are very important for guaranteeing the overall return from the operation. Small amounts of costs for maintenance can prevent the mill from far reaching breakdowns and a huge financial damage. Besides, regular maintenance on machines also decrease spare part and service costs. We can diagnose many problems in the mill at early stages thanks to maintenance. One of the first things to examine when a mill is experiencing poor results is the maintenance.

MAINTENANCE METHODS Adequate maintenance program can be achieved by the use of three different methods: Unscheduled maintenance, preventive maintenance and predictive maintenance (Fig. 1). Management and inspectors, can choose and arrange maintenance systems according to specific needs and local conditions by making use of those methods.

2-SCHEDULED MAINTENANCE The aim of Planned or Preventive maintenance is to minimize the need for emergency maintenance. It is performed by either one of the following sub-methods: Periodical Maintenance (Preventive Maintenance) Predictive Maintenance

A-Periodical Maintenance (Preventive Maintenance) The aim of this method is to prevent components of machines or equipment from failures and to improve their strength before any breakdowns. All components of machines or equipment are regularly monitored and failures or possible failure sources are determined and eliminated.

In order to perform the periodic maintenance, a maintenance schedule consisting of check lists is prepared. The maintenance schedule is prepared and kept by the engineer responsible from the maintenance. But it is performed by mainteFig 1. Maintenance Methods nance technicians.

Periodic maintenance includes four main group of activities for component of equipment: cleaning adjusting (e.g. belt tension), replacing (e.g. oil, filters etc. ) inspection and examination. (Please refer sources at the end of the article for further information about necessary factors to take into consideration for periodic maintenance.)

b- Predictive Maintenance This is a relatively new approach as a result of latest technological developments. This concept was first introduced in Turkey in 1988 by R. Kubilay Kse at Middle East Technical Universitys National Machine Design and Manufacturing Congress. He chose to use the term Kestirimci Bakm for Predictive Maintenance.

When performing predictive maintenance, some of the parameters are measured when machines and equipment are still working. Some measuring devices are used for this. Results are recorded at a specific time interval and results are evaluated according to some statistical methods and tendency analysis. Results show some information about machines and equipment. Changes in those results are tracked and possible defects at machines and equipment may be detected before they actually occur and maintenance can be performed on those machines accurately and adequately. In short, predictive maintenance is based on the performance of the machine.

This method enables to monitor machines or equipment in order to detect possible breakdowns. As a result, unscheduled shutdown of machines and equipment, loss of production and unnecessary part replacements are minimized.

Advantages of Predictive Maintenance Expected life time and productivity of equipment increases Enables corrective maintenance for critical components. Machines that are in good condition are not stopped unnecessarily and this saves time. Maintenance time and downtime for machine breakdowns decrease 25-30%. Work-load and labor costs decrease. Maintenance costs decrease significantly. It is 8-12% more profitable than preventive maintenance. More energy saving. Prevents minor flaws to become source of bigger problems in time. Possibility of significant breakdowns on machines decrease to the lowest level. Production loss decreases considerably. Production levels go up by 20-25% Breakdowns decrease by 35-45%.

PREDICTIVE MAINTENANCE PHASES 1. Measurement: Critical points to be analyzed and measured are identified. Necessary measurements are performed during working conditions of the machine. Predictive maintenance usually involves non-destructive tests like oil analysis, temperature analysis (infrared thermography measurement), ultrasonic test, vibration measurement and visual inspection. 2. Analysis: Results of measurements are analyzed. The source of the failure is analyzed. 3. Repair: The detected fault is evaluated according to working schedule of the mill and necessary repair works are done.

Maintenance leader maybe a mechanical engineer or another qualified technician who studied engineering or technical disciplines. Maintenance must be performed by well trained personnel who have technical knowledge. This training will also include safety issues and detailed information about special machines.

If we do not have enough personnel that have necessary and adequate technical experience or if we need machines to work more efficient, we have to seek technical support from manufacturer of machine or equipment to perform maintenance. This can be done with service contracts.So that, manufacturers will replace failed parts with original and high quality ones and warrant coverage will continue.

Personnel at the production floor are at the first level of maintenance. Maintenance workers on the spot are the second level of maintenance, and equipment engineers constitute the third level of maintenance.

Predictive maintenance is generally performed by a contracted or expert technician. Predictive maintenance team must be qualified and well trained about cutting edge technology. They must have measurement devices and know how to use them. And they must also know how to analyze data from former measurements and report them properly.

ORGANIZING MAINTENANCE ACTIVITIES In order to evaluate equipment which maintenance was performed on, maintenance schedule and an inspection schedule are needed. Maintenance schedule has to be planned by maintenance leader in cooperation with management and he/she should complete a number of arrangements before maintenance program takes effect.

Responsible technicians have to draft a form for each machine and equipment of the mill. The forms have to include following information: Name of the machine Code of the machine Location Year of production Name of manufacturer Capacity Mechanical drawing Power of engines Serial number List of spare parts Supply source

Past maintenance activities and maintenance specifications from manufacturers must also be recorded and loaded into maintenance programs. All catalogues, maintenance and repair manuals, spare part catalogues etc. must be asked from manufacturers. This procedure must be performed for every machine at the plant.

Production manager of the mill must keep records of inspection results and part replacement during repair works. He/she must prepare timetables for preventive maintenance tasks. Maintenance leader must continuously track equipment conditions as part of decision process. This enables to change maintenance programs when it is feasible to do so.

CRITICAL SPARE PARTS MUST BE IN STOCKS It is important for the mill to have critical spare parts available in stocks against possible failures. Giving order for a spare part and stopping the whole shift will cause important losses for the plant. Millers should not evaluate critical stocks as a financial burden for them.

Spare part inventory of the mill may change according to maintenance activities in the mill. The management should decide on the components and their amount to stock. To do so they evaluate the location of the mill and they compare logistics cost and stock cost. Spare parts inventory must be tracked closely by management of the mill by means of a computer program.

COMPUTER ASSISTED MAINTENANCE MANAGEMENT SOFTWARE Records should be kept well for a reliable maintenance program. Newly developed software (Computerized Maintenance Management System CMMS) support record keeping, scheduling and cost control tasks significantly. Many software packages involve various interactive modules that share a common database. Each module is assigned to a specific maintenance task like preventive maintenance, inventory control, work orders, procurement, equipment history, job scheduling, backup schedule, human resources planning, budgeting, cost control etc. Maintenance software are designed to run on desktop or laptop computers.

Can you help me locate a manual for the Natasha Posho Mill, a small, one-person operated non-electric model? A family is operating one, but with cover, the amount of milling being done is minimal. Yet they thing they must have it maintained every 2 months. I would like to get an actual hours of operation to maintenance ratio. Thank you so much.

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