The applicability of two-tier low pressure turbine in high-power condensing steam turbines

Development of powerful and super-powerful steam turbines with minimal specific metal consumption highlights the problem of Low Pressure Turbines (LPT) with maximal single circuit flow into the condenser. This problem may be solved by extension of the last stages blade length, or by application of Bauman stages at constant maximal blade length in the last stages, or by application of two-tier LPT that are free from the Bauman stage shortages. This study analyses these three solutions in terms of the final LPT efficiency. The tip and root parts of the blades with over 1000 mm length operate at very low efficiency which results in lower LPT total efficiency. Thus the condensing turbine LPT steam flow increase by extension of the last stages maximal blade length inevitably hurts the LPT efficiency. The alternative solution is the application of two-tier LPTs. In this case the last stages have widely used 1200 mm long blades and the 50% LPT flow capacity increase is reached without the LPT efficiency reduction.