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Mills of many types, such as ball or beater mills, are used in almost every branch of industry, including for instance the cement and lime industries, power plant technology, and the mining industry. Some of these applications can place extremely high demands on the sealing system as a result of very large seal diameters, or of the large forces generated by the high kinetic energy of the impacts in hammer mills. Espey® carbon floating ring seals from EagleBurgmann Espey, often also fitted with injection inlets to feed the material to be ground, have been proven the decades in mill applications where they have demonstrated their robustness and ease of servicing.
Issue and challenge:
The steam generators of coal-fired power stations are fired by coal dust consisting of particles that are small enough to be transported by streams of air. It is made in tube ball mills by grinding and drying coal. The operators' coal-fired power station has zwei steam generators, each with a power of 600 MW. They are supplied with coal dust by 12 tube ball mills. The harsh operation and the high levels of dust generated by the grinding process in the grinding chamber place extreme stresses on the sealing system. It must be possible to mount the shaft seals easily without the need to dismantle the tube ball mill or its components. They are fitted to the inlet and outlet equipment between stationary and rotating components. Each mill needs four shaft seals: two serve as mill end seals for a shaft diameter of 1605 mm, while two serve as end bearing seals for a shaft diameter of 160 mm.
Solution from EagleBurgmann Espey:
Espey designed a combined Espey® WD200/500 with barrier gas connection and four sealing rings for the mill end. It can be fitted easily, thanks to the split casing. The internal diameter of the seal is 1605 mm (63.19 “). The barrier air seal for the end bearing is provided by an Espey WD200 with four seal rings. Both seals are designed for a temperature of 110 °C (230 °F), a pressure of 1.08 bar (15.66 PSI) and a rotation speed of 15.8 rpm. Both seals achieve full prevention of the escape of dust to the atmosphere by supplying air as barrier gas. No additional supply systems are therefore needed. The Espey sealing system easily copes with axial and, in particular, with radial shaft movements such as are found in ball mills as a result of the high mechanical stresses, as well as trapping the high levels of dust.
Issue and challenge:
Cement is manufactured in ball mills. Pre-treated limestone is crushed or ground by rotation of the mill cylinder and the balls contained within it. The temperature must not exceed a specified limit and is therefore continuously monitored. If necessary, the mill is cooled by water injection. Air injected in parallel keeps the water injection nozzles clean. The operator of the ball mill defined his expectations of the sealing system as follows: Ensuring the sealing function of the drive shaft to the mill cylinder, whilst at the same time connecting stationary supply lines to the rotating unit, rigid anchorage of the seal casing to the ground, along with easy assembly.
Solution from EagleBurgmann Espey:
As a solution to this, Espey designed the Espey WD500 Special with eight seal rings, and with connections for injecting water and air from stationary supply lines for a shaft diameter of 610 millimetres (24.02 “), a pressure of 6 bar (87.02 PSI), a temperature of 60 °C (140 °F) and a rotation speed of 60 rpm. The floating arrangement of the seal rings easily compensates for the radial movements of the shaft that arise from the heavy loading found in ball mills. Unlimited axial shaft movements can be absorbed by the Espey sealing system. The split construction of the housing allows the seal to be fitted without dismantling any of the ball mill's components or its drive and is therefore extremely easy to service even when repairs are needed. Because the drive shaft of the ball mill does not have suitable wear protection, Espey designed a two-piece shaft sleeve with a suitable coating to protect the shaft. The sealing system has been used without failure or repair since 2007.
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