About the Inexpediency of Widely Using Honeycomb Seals in Steam Turbine Flow Paths

The article presents the results from an incisive analysis of using end and internal honeycomb seals (HS) in two T-60/73-7.8/0.04 steam turbines produced by the Kaluga Turbine Works (in Russian, KTZ) intended for being used at the Ufa TETs-5 combined heat and power plant. The honeycomb seals were installed in the flow path of these turbines on the customer’s request and have been applied at KTZ for the first time. Expectedly, the use of HSs could decrease parasitic steam leaks, thereby resulting in improvement of the turbine efficiency. According to assessments made by the ARMS research and production enterprise and expectations of KTZ specialists, the replacement of conventionally designed labyrinth seals by seals with honeycomb inserts would make it possible to increase the turbine efficiency by approximately 1% due to essentially narrower radial gaps in the end, diaphragm, and shroud seals. Experimental investigations of HSs during their operation under model test bench conditions have shown that parasitic leaks of working fluid may indeed be reduced using these seals. However, this is only possible for a narrow range of optimal ratios between the physical design parameters of the seal honeycomb structure and the gap between the stator and rotor elements. The ideas of applying HSs in steam turbines were borrowed from the experience gained from operation of gas turbine engines. In steam turbines whose supports are made with plain bearings, the rotor rotates over an intricate trajectory with respect to the steam turbine casing longitudinal axis. With vibration that takes place under steam turbine real operating conditions, the use of HSs is not expedient, at least in turbines with a flexible shaft. It is shown, taking a particular example, that the narrow gaps (0.15–0.30 mm) between the steam turbine stator and rotor elements as recommended by the HS manufacturers, with which the useful effect from application of HSs manifests itself, cannot be implemented under field conditions. Such statement stems from the results of acceptance tests of two T‑60/73-7.8/0.04 steam turbines produced by KTZ that were carried out at the factory’s test bench. It has been found after opening the turbine and examining its parts after the thermal tests on the bench that all HSs made with narrow radial gaps were destructed. The commissioning team involving the customer’s representatives has recognized that further operation of the steam turbines with such seals is impossible. A decision was made at KTZ to fit the steam turbines with new seals with increased radial gaps. Had the turbines not been opened at the factory, the destructions of their HSs would have been revealed in the course of steam turbine overhaul after 6 years of their operation.