By Peter Bulkley, David Brown, UK
A coal fired power plant’s boiler accepts its fuel in the form of a fine powder that is mixed with air and blown into the boiler. This mixture ignites instantly in the boiler to create the intense heat that converts the water that is pumped into tubes inside the boiler into steam, that in turn drives the power plant’s turbine. The pulverization by mechanical means allows the coal to be transported directly into the boiler and burned like a gas to give efficient combustion. But a malfunction of the pulverizer will make the power plant unavailable, and experience tells us that outages such as this occur from time to time, accounting for around one per cent of total capacity, so improved plant reliability is highly desirable.
David Brown won a contract in 2005 to partner Drax Power in maintaining 65 coal pulverizing mill gearboxes at the Drax power station in the United Kingdom
Recent research has shown around half of pulverizer problems occur when a drive train fails. These failures occur mainly because of contamination of the train’s lubricant and its excessive leakage. Four types of pulverization equipment are common in use: ball tube mills, impact mills, vertical roller mills, and ring race and ball-race mills, all of which are expected to function under trying conditions. The selection of high-quality materials and components in the design of drives for pulverizing mills, especially the currently popular vertical planetary drive, is one way to counter these difficult operating conditions. But what does this mean in practice?
The potential of the Asian and North American markets for coal-fired power plants is encouraging designers and manufacturers to build on decades of experience in the field and to introduce interesting innovations. It is widely acknowledged that one of the most highly desirable characteristics of vertical planetary drives is the use of a heavy-duty, single piece, cast spheroidal graphite (SG), iron gear case because it is able to transmit the enormous thrust loads involved in pulverization.
The great weight of this type of structure has a vibration-deadening effect that reduces noise and the risk of oil leakage that might otherwise compromise the operation of the unit and lead to expensive downtime. In essence, the heavier the webbed gear case (webbing makes optimum use of expensive materials) the better, and while there is clearly a trade-off between capital and operational costs, the benefits of the higher initial investment in heavy-duty designs are not lost on the current market. Increasingly finite element analysis of each gear case is employed to ensure a design matched to the customer to ensure performance under load.
It is also important to note that major benefits are derived from designing the annulus of the drive so that it is installed solidly within the primary gear casing, although some manufacturers have adopted the solution of a separate annulus that forms a sandwich between the bottom gear case and the top coupling. As the very nature of the equipment leads to both wear material and fine coal particulate matter finding its way into the lubrication oil, reduction of potential leakage/ingress points offers a distinct advantage.
Employing a separate annulus means that overall dimensions can be smaller, thus saving money, but its use also increases the potential for lubricant contamination and subsequent oil leaks, and may not offer the strength and integrity of a single-piece casing. A single piece gear case completely encloses and affords better protection to the annulus itself and internal gears, as well as being of higher integrity itself an important consideration for transmitting the axial mill loads safely to the mill foundations.
The interior of the casing of a mill gearbox unit during refurbishment
Until now, the one disadvantage of a single-piece casing has been that by its very nature it has impeded servicing of the inside of the drive. To address this challenge, David Brown’s latest VMD range of planetary units employs all the best practice described above as well as incorporating removable inspection covers in the frame that facilitate inspection and maintenance of the drive components.
A further development presented in this range is the inclusion of sectional four-piece covers that can be removed if temperature sensors in the thrust pads, under the output coupling, signal overheating. This arrangement permits the inspection and, if necessary, the removal of the thrust pad without having to disconnect the output coupling. This has been identified as a significant benefit by many end users, enabling a much faster turnaround for inspection, maintenance or repair of this critical element.
David Brown employed general design principles in its VMD range that are a result of many years of application experience. In addition to the benefits of a heavy-duty, single-piece, SG, iron, cast casing, the use of case-hardened and profile-ground gears is a major contribution to accuracy, durability and noise reduction. PTFE thrust pad bearings support the main output coupling to transmit the enormous thrust loads to the mill foundations and reduce start-up loading. Thanks to an interference fit, the output coupling can be simply installed or decoupled using an oil-injection method.
David Brown’s VMD planetary units incorporate removable covers in the frame that facilitate inspection and maintenance of the drive components
A self-aligning sun gear and planet gear system, connected to the main bevel shaft through a double-jointed ‘button’ plate coupling, ensures effective load sharing across the full face width of the gears and durability of the unit is further increased by using heavy-duty labyrinth-type seals on the input shaft and output couplings.
The company has recently launched a range that includes 11 models. It caters for maximum static thrust loads from 1100 to 6500 kN for the largest units and maximum absorbed power ratings from 450 to 1600 kW. In each case, however, the model can be tailored to match exact customer requirements to make them suitable for new-build projects as well as drop-in replacements, thanks to the company’s applications experience and problem-solving approach. Customization might typically involve modification of the basic design to suit floor-mounting positions, vary the input shaft centre height or accommodate specific output couplings.