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Tutuka Power Station was put into full commercial operation in June 1990. Tutuka - a Zulu word meaning progress - is a 3,600 MW thermal power station located in the Mpumalanga province of South Africa. Each of Tutuka’s six turbine-generators runs separately with its own fuel and water supply, output transformer and instrumentation. Each turbine generates 609 MW at full load.
The power station is owned and operated by Eskom, a state-owned utility that supplies 95 percent of South Africa’s electricity, as well as exports power to neighboring countries. Eskom owns and operates 24 power stations with a nominal capacity of 39,872 MW (equaling more than half of the electricity generated in continental Africa), making it one of the top utilities in the world.
During the plant’s first year of operation, management decided to look more closely at plant availability and reliability. One of the targets identified was plant lubrication. The proper selection and management of lubricants became a priority to improve machine reliability and reduce lubricant consumption.
The first task involved the acquisition of lubricants and filters in use at the plant. This entailed removing all duplicate lubricant grades and lubricants not provided by the contracted supplier, as well as ensuring that all filters were in accordance with OEM recommendations. The next step was to establish a basic lubricant condition-monitoring program focused on the plant’s lubrication challenges.
To supplement the onsite lubricant tests for basic lubricant parameters, a suitable independent oil analysis laboratory was contracted to provide a more thorough analysis of the oil samples.
Plant sampling frequencies were determined by plant running hours or based on machine criticality. Also considered were past machine reliability and plant-specific lubricant specifications and limits established for each lubricant monitored.
Coal Pulverizor Savings
The first major success was with the boiler milling plant. There are six mills feeding each boiler; five on load at any given time with one mill on standby. The mills consist of a rotating tube that utilizes steel balls to pulverize the coal. Each mill is equipped with a lubricating system for main bearings, drive motor bearings and main drive gearing. Initially, reliability problems occurred because of contamination. The lubricant supplying the main bearings to the mill contained fine coal dust and fly ash. This was corrected by modifying the existing shaft sealing arrangements and by eliminating points of contaminant ingress. The original equipment supplier suggested oil change intervals every 4,000 running hours, meaning each mill would require a full lubricant service at least twice per year. With the implementation of condition monitoring, plant improvements and better plant housekeeping, the current average lubricant replacement interval is 27,968 running hours. To complete a full lubricant change-out on a boiler mill, 1,985 liters (L) (525 gal) of oil is drained and replaced.
Excluding lubricant top-ups, annual savings of approximately 43,684 L (11,541 gal) of lubricating oil have been achieved.
To achieve these savings, the following processes were implemented:
Since the program was implemented, no machine or plant breakdowns caused by lubricant failures have been recorded, although on occasion, the plant has been unavailable for use because of severe lubricant contamination by coal dust or water.
The philosophy outlined above and successfully implemented on the boiler mills has been used elsewhere within the plant, achieving similar success. The boiler draft group has six large-capacity fans and one air heater. Previously, the lube oil systems were drained and the lubricants were replaced during each major plant outage or shutdown, occurring approximately every two years for a particular turbine-generator set. Current practice is to drain and clean the lube oil tanks and filter the oil using a portable offline filtration unit prior to returning the lubricant to the oil tank. If test results indicate the lubricant is unfit for further use, it is replaced. The volume of oil reused in this manner is 5,940 L (1,569 gal).
|Table 1. Estimated Savings for Each Boiler Mill|
Volume of Oil Required per Change-Out
Recommended Service Interval (Hours)
Current Average Interval Between Services (Hours)
Volume of Oil Saved per Boiler Mill
Volume of Oil Saved for 36 Boiler Mills
EHC Hydraulic Reliability
At the turbine plant, the turbine governor and stop valves are hydraulically operated using a phosphate ester fire-resistant hydraulic fluid (electrohydraulic control system or EHC). Each turbine has a hydraulic fluid reservoir and an offline fluid-conditioning unit to remove moisture, excess acidity and particulate contamination from the fluid. Fuller’s earth filter elements are used to remove excess acidity. When the Fuller’s earth filter elements were replaced, fine particles of the filter media were released into the hydraulic system, resulting in numerous filter blockages in the servo turbine control valves. Samples taken downstream of the filter over a period of four months indicated an average ISO Cleanliness Code of 18/15 (ISO 4406). The recommended level for a hydraulic system containing servo valves is an ISO Cleanliness Code of 16/13, because high levels of contamination can result in damage to close-tolerance servo valves due to “silting” and the formation of adherent metallic salts. An additional trap was installed downstream of the filter and its fines filter. The result: an average ISO Cleanliness Code of 15/12 (ISO 4406), as well as the virtual elimination of filter blockages in the servo valves.
Moisture and particulate contamination of the hydraulic control fluid (EHC) was detected on the low-pressure turbine steam bypass valves causing slow and unreliable valve response. Corrective action was taken to minimize moisture ingress into the fluid reservoirs, and all nonoriginal (retrofit) oil filters were removed and replaced with original specified filter elements. The servo valve blocks are overhauled at every plant outage, during which all spools are carefully inspected and measured. At this time, all O-rings, seals and oil mesh screens are replaced. All servo valve blocks tested on a test stand prior to installation at the plant. These measures have resulted in improved fluid cleanliness from an average ISO Cleanliness Level of 18/15 (ISO 4406) to 16/13.
Turbine Lube-Oil Optimization
Incorrect lube oil purifier operation and maintenance were responsible for high lubricating oil consumption on the turbine lubricating system. By optimizing plant operation and ensuring that proper maintenance and operating procedures are used, lubricant losses have been reduced by 58 percent.
The implementation of monitoring and optimization programs has been successful - not only in lubricant consumption savings, but also savings of manpower, reduced outage intervals, consumables usage and breakdowns.
Lubricant consumption can be divided into two clear categories: nonconsumable and consumable lubricant consumption. Nonconsumable lubricants are all lubricating and control fluids, which are replaced on-condition. Consumable lubricants are greases and fluids replaced routinely (per schedule).
From 1990 until the end of 2000, nonconsumable lubricant consumption decreased 79 percent. One way to monitor this is to calculate the consumption ratio for the lubricant consumed using the following formula: Consumption Ratio = Annual Nonconsumable Lubricant Purchases / Total Machine Charge Volume (Installed Volume).
The consumption ratio for the Tutuka Power Station has reduced from 0.35 in 1990 to 0.074 in 2000.
Furthermore, nonconsumable lubricant consumption can be divided into lubricant services and lubricant lost. Lubricant services is defined as all lubricants drained and replaced on-condition. Lubricant lost is the volume of lubricant replaced due to leakages, spillages and unnecessary lube services. Although the volume of lubricant lost is not easy to reduce, because it is not always possible to repair system leaks and prevent fluid losses, the lubricant lost volume has been reduced by 75 percent over the same 10-year period.
Consumable Lubricant Savings
A reduction in consumable lubricants consumption (greases and mill gear lubricants) is not always possible because plant lubrication requires a minimum lubricant application rate for proper operation. The use of alternative lubricants for girth gears on the boiler mills has resulted in major savings in consumable lubricant consumption. These lubricants may cost more per unit (liter or kg), but require lower application rates, resulting in lower overall costs, better plant performance and lower consumption. The cost per unit of the replacement lubricant is 2.2 times that of the original product, while the application rate is reduced by 77 percent. Thus for a given period of 2,000 plant running hours, for example, the savings in terms of lubricant cost is 97 percent.
General plant greasing of pump, motor and fan bearings is a contentious issue. The majority of bearings were being overgreased, resulting in excessive lubricant consumption and premature bearing failure. Working in conjunction with OEM bearing suppliers, plant maintenance and operating personnel and lubricant suppliers, plant grease consumption reduced from a monthly average of approximately 618 kg in 1994 to 227 kg in 2000 without any adverse effects on plant reliability and availability.
The overall lubricant consumption index is a measure of the total cost to lubricate the power station per unit of electricity sold. Because of factors beyond the control of Tutuka Power Station management, output from the station reduced by approximately 39 percent over the past number of years and lubricant unit costs have increased by approximately 235 percent over the same period mainly due to the inflationary effects of rising oil prices and a weak local currency. Therefore, in spite of reduced production and increased lubricant costs, the overall lubricant consumption index (Rand/GWh) has improved by 2.66 percent over the past 10 years.