+ economic + + flexible + + innovative + BENSON Boiler

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1 + economic + + flexible + + innovative + BENSON Boiler Research & Development at the BENSON Test Rig by Siemens AG Power Generation (PG)

2 designs and constructs fossil fired power plants manufactures steam and gas turbines, generators, electrical equipment and I&C is licenser for BENSON boilers develops an improved concept with vertically tubed water walls for BENSON boilers This booklet should remind you of the exhibition in the monitoring room of the BENSON test rig in Erlangen, Germany, where the fundamental research and development of Siemens/PG is performed on: heat transfer in boiler tubes smooth vertical, inclined, horizontal rifled pressure loss in boiler tubes thermoelastic design of water walls feedwater treatment erosion corrosion The BENSON know-how allows for reliable design and ensures customer s benefit via validated codes based on extensive investigations. Siemens AG Power Generation PG BENSON license BENSON test rig Department PG W7 Freyeslebenstraße 1 D Erlangen Germany Tel: Fax: joachim.franke@erl11.siemens.de Framatome ANP GmbH (A Framatome and Siemens company) Department FANP NT31 Freyeslebenstraße 1 D Erlangen Germany Tel: Fax: holger.schmidt@framatome-anp.de 2

3 Evaporator systems for boilers by Siemens/PG Principle Natural circulation (drum) BENSON with superposed circulation BENSON (once-through) Superheater Evaporator Economizer Operating pressure Water wall tubing bar bar bar vertical vertical spiral or vertical BENSON Boilers are the world-wide most often built once-through boilers with approx units: steam pressure up to 310 bar steam temperatures up to 650 C steam capacities up to 1232 kg/s (4435 t/h) 3

4 Advantages of BENSON boilers Highest efficiency of power plants Use of worldwide and difficult coals Enthalpy Critical point Pressure (load) Modes of operation Suitable for subcritical and supercritical pressure Wide scope in design (oversized combustion chamber, slag tap furnace) Economical and low-stress operation Temperature 545 C Flexible operation mode Load 4-6 %/min Load Main steam temperature independent of fuel and degree of fouling. Low-stress start-up Time Rapid load changes with sliding pressure operation Improved concept with vertical tubed water walls based on R&D by Siemens/PG with additional advantages: Simple design and easy maintenance of water walls similar to drum boilers Low part-load of 20% with high steam temperatures Simple start-up system without recirculation pump Optimized flow chracteristic of water wall tubes (see next page) 4

5 Advantages of BENSON boilers with vertically tubed water walls Once-through characteristic at high mass flux (approx kg/m 2 s) Pressure drop at constant mass flux Friction Hydrostatic Hydrostatic System of parallel tubes Due to equal pressure drop in all parallel tubes: Mass flow decreases in the excessively heated tube Nominal heated tube Excessively heated tube Optimized flow characteristic in case of excessive heat input of water wall tubes due to low mass flux Natural circulation characteristic at low mass flux (approx kg/m 2 s) Pressure drop at constant mass flux Friction Hydrostatic System of parallel tubes Due to equal pressure drop in all parallel tubes: Mass flow increases in the excessively heated tube Nominal heated tube Excessively heated tube 5

6 Milestones in the field of BENSON boilers 1924 Siemens buys the BENSON Patent from Mark Benson 1926 Siemens manufactures to three BENSON boilers 1929 (30 t/h to 125 t/h) 1933 Siemens introduces variablepressure operation 1933 Siemens awards licences to several boiler manufacturers 1949 The world s first once-through boiler with high steam conditions (175 bar/610 C) 1954 The first BENSON boiler with supercritical pressure (300 bar/605 C) 1963 The world s first spiral-tubed water walls in membrane design 1987 First hard-coal-fired boiler >900 MW with spiral-tubed water walls 2000 About 1000 BENSON boilers with > t/h sold in total 2000 First order of a BENSON boiler with vertical tubed water walls in low mass flux design BENSON boilers licence since state: Steinmüller 1939 Austrian Energy 1950 Deutsche Babcock 1951 Mitsui Babcock 1954 Babcock & Wilcox 1954 Burmeister & Wain 1954 Kawasaki 1960 Babcock-Hitachi 1995 Ansaldo 1996 Foster Wheeler 1999 Bharat Heavy Electricals Ltd. (BHEL) 6

7 BENSON boiler Boiler activities by Siemens/PG Boiler concepts Arrangement of heating surfaces Thermal hydraulic design Start-up systems Control concepts Water chemistry Interaction of boiler and turbine R&D Computer programs New water wall/evaporator design Vertical tubed water walls with optimized rifled tubes BENSON Boiler with superposed circulation Horizontal evaporator tubes for advanced power plants with fluidized bed combustion or coal gasification Increase of availability Reliable design based on extensive knowledge of heat transfer and flow stability Material preservation by thermal elastic component design Prevention of pipe wall thinning and resulting failures Reduction of operating cost Low pressure loss and steady-state flow condition in evaporator zones and separators Optimized feedwater chemistry 7

8 BENSON test rig and range of parameters investigated Test section Pressurizer Spray condenser Main heater Preheater Feedwater tank Circulation pump Trickle cooler Dosing pump Piston pump Technical data: System pressure 330 bar Temperature 600 C Mass flow 28 kg/s Heat capacity 2000 kw Reduction valve Main cooler Tube Geometry Number of measurements > > Heating uniform one-side uniform one-side vertical Tube orientation inclined horizontal Test matrix for heat tansfer and pressure drop investigations Test parameter Pressure 25 p 280 bar Mass flux 100 m 2500 kg/m 2 s Heat flux 0 q 950 kw/m 2 Tube inner diameter 8 d 50 mm 8

9 Heat transfer and pressure drop in boiler tubes Schematic course of wall temperature and pressure loss in an uniformly heated vertical smooth evaporator tube Heat transfer region Convective heat transfer to steam flow Post -CHF region/ Post-dryout region Steam quality 1.0 Steam Boiling crisis 0.8 Convective heat transfer through water film p L Wall temperature 0.6 Fluid temperature 0.4 Saturated nucleate boiling 0.2 Subcooled boiling 0 Convective heat transfer to water flow Pressure loss gradient Temperature Water 9

10 Heat transfer in boiler tubes Effect of gravity on heat transfer in inclined and horizontal smooth tubes Inner wall temperature ( C) Calculation with WATHUN Inclined tube Pressure 50 bar Mass flux 1000 kg/m 2 s Heat flux 400 kw/m 2 Tube inner diameter 24.3 mm Steam quality 15 Inner wall temperature ( C) Fluid Horizontal tube Pressure 100 bar Mass flux 500 kg/m 2 s Heat flux 300 kw/m 2 Tube inner diameter 24.3 mm Steam quality 10

11 Heat transfer in boiler tubes Improvement in heat transfer by rifled tubes Wall temperature in smooth and rifled tubes Pressure Mass flux Heat flux 150 bar 500 kg/m 2 s 300 kw/m 2 Steam quality Fluid Rifled tube Smooth tube Rifled tube Inner wall temperature ( C) Smooth tube 11

12 Heat transfer in boiler tubes Wall temperatures in vertical rifled tubes at different loads Inner wall temperature ( C) 400 High load Pressure 212 bar Mass flux 770 kg/m 2 s Peak heat flux 310 kw/m Fluid Calculation with WATHUN Low load Pressure 100 bar Mass flux 250 kg/m 2 s Peak heat flux 200 kw/m Fluid Fluid enthalpy (kj/kg) 12

13 Heat transfer in boiler tubes Optimized rifled tubes reduce wall temperatures or allow mass flux reduction Inner wall temperature ( C) Smooth tube Mass flux 1000 kg/m 2 s Standard rifled tube Mass flux 1000 kg/m 2 s Optimized rifled tube Mass flux 1000kg/m 2 s 360 Calculation with WATHUN Pressure 212 bar Peak heat flux 310 kw/m Standard rifled tube Mass flux 1000 kg/m 2 s Optimized rifled tube Mass flux 770 kg/m 2 s Smooth tube Mass flux 1500 kg/m 2 s Fluid enthalpy (kj/kg) 13

14 Pressure loss in smooth and rifled boiler tubes Pressure 100 bar Mass flux 1000 kg/m 2 s Heat flux 100 kw/m 2 Tube inner diameter ca. 13 mm Related pressure loss 20 Location of boiling crisis p wet steam p water Rifled tube Calculation with DRUBEN 4 Wetted surface Smooth tube Unwetted surface Steam quality Smooth tube Water Rifled tube Steam 14

15 Thermoelastic design of water walls increases flexibility (1) Rack plate ϑ 1 σ ,5 ϑ 2 σ ,37 4,91 0,45-4,01 8,47 8,44 12,9 13, , ,01-8,47 Temperature field ϑ [ C] Stress field σ [N/mm 2 ] Measured values Temperature and stress fields in a rack plate at a gradient of 10 K/min, quasi-steady-state conditions 15

16 Thermoelastic design of water walls increases flexibility (2) Firing 500 C Stress analysis with WATHAN based on R&D increases reliability of water walls 380 C WATHAN Input data: pressure, temperature, mass flux, steam quality, heat flux, geometry WATHUN-calculation (heat transfer coefficients) Stress analysis FEM-calculation Temperature field Thermal stress Mechanical stress Stress assessment Primary stress < S m Primary and secondary stress < 3 S m Fatigue analysis (for p/p k < 1) FEM-calculation Temperature field Thermal stress differences (Wetted and unwetted tube) Service life assessment Thermal stress Permissible differences range of stress Height [m] 60 σ ax,t+p Position q. q max σ ax,w T F T W σ ef σ al Nomenclature. q Average heat flux. qmax Max. loc. heat flux TF Fluid temperature TW Wall temperature σef Effective stress σax,t+p Axial stress (T+P) σ1,t+p Princ. stress1 (T+P) σ2,t+p Princ. stress2 (T+P) σax,w Axial stress (weight) σal Allowable stress 10 σ 1,T+P σ 2,T+P Heat flux [kw/m 2 ] Temperature [ C] Stress [N/mm 2 ] 16

17 Feedwater treatment. Erosion-corrosion (1) Appearance Parameters of influence Material (Cr-, Mo-, Cu-contents) Geometry (pipe, bend, etc.) Fluid velocity Temperature Steam quality Feedwater chemistry (ph, O 2 ) Exposure time Mechanism Fe 3 O 4 Oxide layer (magnetite) protects against erosion-corrosion Fe OH + Metal loss caused by erosion-corrosion (mass transfer) Fe (OH) 2 Velocity profile Steel Wall adjacent turbulent layer Flow core 17

18 Feedwater treatment. Erosion-corrosion (2) Effect of material composition ph = 7; O 2 = < 5ppb ph = 9,5; O 2 = < 5ppb ph = 7; O 2 = 500ppb Wall thinning mm/a 10 St Mo NiCuMoNb 5 Ferritic steel CrMo CrMo 9 10 X10CrNiTi 18 9 Austenitic steel St µm-Metco 33- coating T = 180 C v = 20 m/s t = 200 h

19 Feedwater treatment. Erosion-corrosion (3) Effect of thermal hydraulic and water chemistry parameters Measurement T =180 C ph = 7 O 2 5 ppb t =200 h Carbon steel Measurement v = 35 m/s ph = 7 O 2 5 ppb t =200 h Carbon steel Measurement T = 180 C v = 39 m/s O 2 5 ppb t = h Carbon steel Measurement T = 120 C v = 35 m/s ph 5 ppb t =200 h Carbon steel Wall thinning mm/a , , Calculation with WATHEC 0, Fluid velocity (m/s) Water temperature ( C) ph Oxygen concentration (ppb) 19

20 Research & development by Siemens/PG allows reliable design of BENSON boilers based on computer programs as: WATHUN DRUBEN STADE DEFA/DEFOS DYNASTAB WATHAN WATHEC/COMSY Heat transfer Pressure drop Flow distribution in parallel tube systems Design of boilers Dynamic stability Material strength Erosion-corrosion Printed by and copyright (2001): Siemens Power Generation Freyeslebenstaße 1 D Erlangen Germany Siemens Aktiengesellschaft Subject to change without prior notice