Mills

Application example: Sealing tube ball mill for coal grinding in a power plant

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.

Application example: Espey WD500 for ball mills in the cement industry

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.