Seals

Fig. 1. Front end seal of T-295/335-23,5 High Pressure Cylinder (HPC) turbine with honeycomb O-rings

Honeycomb Seals

Performed on the basis of recommendations developed in 2005, the first application of honeycomb O-rings for the rear end seal of MPC-2 (medium-pressure cylinder) turbine T-250/300-240 led to positive results: the danger of interference in the end seal caused by close to the limit value of negative Relative Rotor Expansion (RRE) (−4.5 mm) in the rear end seal was eliminated, the steam stopped to blow on oil seal and the risk of oil flooding has been reduced. The increase in the efficiency of the gland exhauster from the seal was indirectly noted.

Subsequently, honeycomb O-rings were installed in the end seals of the low-pressure cylinder (LPC) of T-250/300-240 turbine, and then honeycomb O-rings were used for shroud, diaphragm and end seals of T-100/120-130 and T-250/300-240 turbines.

As the practice of long-term operation of turbines shows, there were practically no cases of damage of the rotor areas opposite to honeycomb surfaces at installation of honeycomb O-rings. Honeycomb rings are subject to wear in case of interference, but in the absence of foreign objects hitting the seals, neither the rotor nor the sealing joints surface of the moving blades bands are damaged.

The results of the work were studied by the Moscow Branch of ORGRES. It was shown that in comparison with the axial-radial shroud honeycomb seals increased efficiency of the flow part of the T-110/120-130 HP (high pressure) turbine by more than 1 %, and for the water passage of the HP turbine T-250/300-240 the increase was almost 1.7 %.

The use of honeycomb O-rings in diaphragm seals and end seals has excluded the presence of protrusions and hollows on the rotor shaft—mandatory design features of labyrinth seals and unavoidable radius blends (fillets) in them with radius, nominally equal to 0.5 mm, which are structurally unavoidable concentrators of dangerous thermal fatigue stresses in diaphragm seals of the second stage of HPC) (and MPC-1 for T-250-240 turbine) and high-temperature areas of end and intermediate seals.

Based on the research findings of change of density of flow parts in connection with application of honeycomb seals, conducted from 2005 to 2014 by several organizations, it is possible to expect confidently increase of efficiency of a flow part of turbines as a result of application of honeycomb sealing surfaces in shroud, end and diaphragm seals on ~1.3...1.6 % (Fig. 3).

Fig. 3 Change of relative internal efficiency of the HPC of the T-110/120-130 turbine, before (1, 2) and after (3, 4) installation of honeycomb shroud seals: 1, 3 – modes with HPH (high-pressure heaters) off; 2, 4 – modes with HPH on.

The generalized feasibility study shows that the payback period of measures on installation of honeycomb seals at modernization of the operated turbines does not exceed two years.

Durability of operation of a set of honeycomb seals, either delivered as part of a turbine unit or installed during modernization of the turbine flow part is also analyzed. Plenty of turbines upgraded by installing honeycomb seals with an operating time of 37,500 to 44,500 hours (and one turbine with an operating time of over 59,000 hours) were inspected and tested. The condition of the honeycomb sealing surfaces with traces of interference after five years of operation is shown in the Figure 4.

Fig. 4 Wear of the honeycomb sealing surface after five and ten years of operation: a – allowable wear; b – unacceptable wear.

Fig. 5 Wear of honeycomb seals after five years of operation: a – shroud seal of a flowing part of the HPC of K-200-130 turbine after interferences; b – shroud radial seal of a flowing part of the HPC of K-200-130 turbine after interferences; c – end honeycomb seal of the LPC of T-250/300-240 turbine.

The results of the research show that in comparison with axial-radial seals, honeycomb shroud seals increase the efficiency of the flow part of HP in turbines of high and supercritical parameters and maintain the density of the flow part after five or more years of operation even in the case when as a result of interferences, the samples of 1...2 mm deep on the honeycomb surface are formed. Pictures (Fig. 5) show the honeycomb diaphragm seal rings with traces of interferences after five years of operation. Permissible honeycomb surface wear for continued operation of the rings without replacement follows from the above mentioned tests. At the same time, inspections and tests of honeycomb O-rings after 10 or 11 years of operation clearly indicate that they must be replaced.

The studies carried out (see above) confirm that the dynamic stability of the shaft line, in particular the HPH (high-pressure hose), is maintained. A number of bench works on comparative estimation of power and flow characteristics of honeycomb, radial and axial-radial shroud seals were carried out. The comparison is made at the standard parameters, which are the same for all types of seals under study.