Steam Turbine Efficiency

 

Steam turbine efficiency of ES_Rankine may be input by User or calculated by the software automatically.

If User selects the items with "(Efficiency by User)" in the "Cycle Type" combo box of "Start" tab of DESIGN window, User has to input steam turbine efficiency for himself.   If User selects the items without "(Efficiency by User)", steam turbine efficiency is automatically calculated by the software.

Input method of Last Stage Exhaust Loss can be selected in the "Exhaust Loss Application" frame of "Last Stage" tab of DESIGN window.   If the Exhaust Loss was selected as input by User, then Design Annulus Velocity should be input, too.   The Design Annulus Velocity input shall be used as reference velocity to read the exhaust loss of Operation Heat Balance from a built-in, standard exhaust loss curve.

Input method of BFP Driving Turbine can be selected in the "BFP Driving Turbine exhausting to Condenser" frame of "Aux(1)" tab or "Start" tab of DESIGN window.

 

Steam Turbine Efficiency

The steam turbine efficiency automatically calculated by the software is calculated based on the method described in the Reference Document [1] and corrected in order to reflect practical efficiencies applied currently in the industry.

The BFP Driving Turbine efficiency calculated automatically is predicted by a method developed by ENGSOFT.

In case of User input, the steam turbine efficiency input is assumed as Turbine Wheel Efficiency, by which UEEP(Used Energy End Point) is calculated.   And, turbine internal efficiency that ELEP(Expansion Line End Point) is calculated by, is calculated automatically by applying Last Stage Exhaust Loss.

Same is true for BFP Driving Turbine, but internal efficiency as well as ELEP is not calculated because of no need to read turbine steam expansion line for bleeding.

 

Last Stage Exhaust Loss

The last stage exhaust loss automatically calculated by the software is calculated based on the method described in the Reference Document [1] and corrected in order to reflect practical losses applied currently in the industry.

ES_Rankine uses a built-in, standard exhaust loss curve instead of applying an individual curve for each last stage blade.   Exhaust loss is specific to each last stage blade design and different from manufacturer to manufacturer even if blade length and pitch diameter are same.   Therefore, it is meaningless to apply different exhaust loss curve in engineering-purpose heat balance software.   The built-in, standard exhaust loss curve of ES_Rankine has the lowest exhaust loss of approximately 19.5 kJ/kg at 165 m/s annulus velocity.

Actual exhaust loss curves provided by manufacturers have similar parabolic shapes.   Prominent difference among the curves is relative magnitude of exhaust loss to annulus velocity.   Therefore, if User wants a specific exhaust loss at a specific annulus velocity, please use User input of exhaust loss with input of Design Annulus Velocity, which will lead to a specifically designed exhaust loss curve.

If User selects User input of exhaust loss with input of Design Annulus Velocity, ES_Rankine applies the User-input exhaust loss at it is for Design Heat Balance, and the exhaust loss for Operation Heat Balance is calculated according the equation below.

PLoadEL = DgnEL * PLoadEL_Std / DgnEL_Std

wherein :

 

PLoadEL

: Exhaust loss of Operation Heat Balance

DgnEL

: Exhaust loss input by User for Design Heat Balance

PLoadEL_Std

: Exhaust loss for Operation Heat Balance, read from the built-in, standard exhaust loss curve by the annulus velocity calculated in Operation Heat Balance.

DgnEL_Std

: Exhaust loss read from the built-in, standard exhaust loss curve by the User input Design Annulus Velocity.

 In order to calculate annulus velocity, the blade length and pitch diameter of the last stage blade should be known.   The blade length and pitch diameter can be input by User directly, or let them input automatically when User selects a reference blade in the Reference Design combo box of "Last Stage" tab in DESIGN window.   Pitch diameter is diameter of the circle connecting the center position of blade.

It is recommended to design the last stage blade to have the annulus velocity between 200 m/s ~ 300 m/s, so that exhaust loss at part load becomes less and overall efficiency of turbine at part load does not become far less than design point efficiency.   If last stage blade is designed to have the least exhaust loss at design point, the exhaust loss at part load increases dramatically due to its parabolic shape and it results in less efficient turbine cycle in view of entire load range.

Last stage blade is not easy to manufacture due to its long blade length.   Therefore, turbine manufacturers use one of proven design they have used previously, so that exhaust loss of last stage blade varies project by project depending on exhaust volume flow rate.   Effect of exhaust loss to overall turbine efficiency is considerable so that right selection of last stage blade is important.    

When User purchases steam turbine, he should evaluate the exhaust loss and annulus velocity of last stage blade at design point.   If the exhaust loss at design point is the minimum, purchaser should accept that efficiency at part load decreases dramatically.   It is recommended to specify annulus velocity at design point in purchase inquiry, so that overall turbine efficiencies of bidders can be based on same operation profile.

 

Turbine Expansion Line Read

The turbine expansion line read for bleeding is in accordance with the method described in the Reference Document [1].

 

Reference Document :

1. GER-2007C, A Method for Predicting the Performance of Steam Turbine-Generators... 16,500 kW and Larger, Revised July 1974 by R. C. Spencer, K. C. Cotton and C. N. Cannon, General Electric Company


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