In this solution, an abradable coating, normally of a metal alloy composition for steam turbines, is applied by spray coating to the standard design seal segments. This reduces leakage flow by reducing the tip clearances. As shown in figure 1, the abradable coating reduces the clearance between the hard metal parts of the rotor and seal segment base material. The abradable coating can be applied to various types of seal segments, especially for applications in the balance piston and dummy piston section of high pressure turbines.

Figure 1 below shows the typical configuration for a Abradable Seal.

FIGURE 1- TYPICAL ABRADABLE SEAL ARRANGEMENT

Operational benefits in the event of a rub, are that the sharp edge of the rotor seals, which are harder that the abradable coating material, will have minimal damage or wear during contact with the coating. Figure 2 below demonstrates this benefit between a hard rub (metal to metal compared to a rub with coating).

   Hard Rub                                                                         Rub with Coating

FIGURE 2- NORMAL SEAL (HARD RUB) AND ABRADABLE SEAL RUBBING COMPARISON

Based on scope of implementation, these generally provide 0.1 to 0.2% performance improvement depending on application.