BHEL Steam Generators Material Selection Overview by Veerendar Aluvala DM/BHEL/Trichy

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1 BHEL Steam Generators Material Selection Overview by Veerendar Aluvala DM/BHEL/Trichy

2 Presentation Summary Pulverised Coal Combustion (PCC) Design Aspects Boiler Parameters Drum Type Boilers OTSC Boilers Fuel Analysis Site Conditions Material Selection

3 Technology Developments Pulverised Coal Combustion (PCC) Fluidised Bed Combustion (FBC) Circulating Fluidised Bed Combustion (CFBC) Combined Cycle Gas Turbine/Co-generation Plants Integrated Gasification Combined Cycle (IGCC)

4 Pulverised Coal? Fineness of pulverised coal not being less than 70% thru 200 mesh (75 microns) and 98% thru 50 mesh (300 microns). Pulveriser is required Gas velocities m/s

5 Tower Type Basic Boiler Configurations Conventional Two Pass

6 Boiler Parameters

7 Heat Balance Diagram (Typ.)

8 Boiler Parameters define heat duty of the boiler Heat fired = (SH + RH) heat duty Boiler Efficiency Coal fired Air quantity = Heat fired / HHV of coal = kg. of air/kg. of fuel x Qty. of coal fired Gas quantity = Coal + Air Ash in coal

9 Increase of Plant Cycle Efficiency due to Steam Parameters

10 Implications of Steam Parameters on Boiler Design Boiler type Materials Reliability and Availability

11 Types of boilers Drum type Once-through type

12 Drum type boiler Steam generation takes place in furnace water walls Fixed evaporation end point - the drum Steam -water separation takes place in the drum Separated water mixed with incoming feed water

13 Drum Type Boiler Natural Circulation Boiler Circulation thru water walls by thermo-siphon effect Controlled Circulation Boiler Thermo-siphon effect supplemented by pumps

14 Types of Circulation

16 Types of Circulation

17 Supercritical: Pressure > Supercritical (221 bar) Ultra Supercritical: Supercritical with SHO temperature > 593 deg.c Adavnced Supercritical: Ultra Supercritical with SHO temperature > 700 deg.c

18 Once Through Boiler Once -through flow through all sections of boiler (economiser, water walls & superheater) Feed pump provides the driving head Suitable for sub critical & super critical pressures

19 Operating Pressure Range

20 Once-thru Boiler Major differences from Drum type boiler : Evaporator system Low load circulation system Separator

21 Once -thru Boiler-Furnace Wall

22 Spiral Tube Arrangement Features Reduced number of tubes with pitch. Increased mass flow. Mass flow rate can be selected by number of tubes.

23 Evaporator heat absorption

24 Enhancing Capacity Empowering Nation SPIRAL TO VERTICAL TRANSITION Presentation on Supercritical Boilers 21 th Nov. 2014

25 Transition Area Vertical Wall Tubes Vertical Tube Forgings Spiral Wall Tubes Spiral Tube Forgings

26 Spiral Furnace Windbox Panel

27 Supercritical boiler Spiral Furnace Wall Assembly

28 Spiral Wall Burner panel (Sliding Pressure Supercritical Design)

29 Sliding Pressure Supercritical Design Spiral to Vertical Transition Area - Load Transfer Support Fingers SPIRAL WALL SUPPORT

30 Vertical Wall Windbox Only a Few Bends at the Top and Bottom Straight Tubes

Enhancing Capacity Empowering Nation FRONT WALL RIFLED TUBING SIDE WALL RIFLED TUBING Vertical Wall Sliding Pressure Supercritical Design SCREEN TUBES SMOOTH TUBING HANGER TUBES SMOOTH

31 Enhancing Capacity Empowering Nation FRONT WALL RIFLED TUBING SIDE WALL RIFLED TUBING Vertical Wall Sliding Pressure Supercritical Design SCREEN TUBES SMOOTH TUBING HANGER TUBES SMOOTH TUBING ARCH RIFLED TUBING SIDE WALL RIFLED TUBING FRONT WALL RIFLED TUBING REAR WALL RIFLED TUBING SMOOTH TUBING FROM THIS ELEVATION ALL WALLS Presentation on Supercritical Boilers 21 th Nov. 2014

32 Spiral vs. Vertical Wall Comparison Spiral Furnace System Applicable for all size units Benefits from averaging of lateral heat absorption variation (each tube forms a part of each furnace wall) Simplified inlet header arrangement Large number of operating units Use of smooth bore tubing throughout entire furnace wall system One material utilized throughout entire waterwall system No individual tube orifices Less maintenance & pluggage potential Vertical Furnace Wall System Limited to larger capacity units (>600 MW depending on fuel) Less complicated windbox openings Traditional furnace water wall support system Elimination of intermediate furnace wall transition header Less welding in the lower furnace wall system Easier to identify and repair tubes leaks Lower water wall system pressure drop thereby reducing required feed pump power

33 Once -thru Boiler Low load circulation system : At part loads once -thru flow not adequate to cool the tubes To maintain required mass velocities boiler operates on circulating mode at low loads Excess flow supplied by feed pump or a dedicated circulating pump

34 ONCE - THROUGH OPERATING RANGE

35 Once - thru Boiler Low load circulation system : The excess flow over the once-thru flow separated in separator and Returned to the condenser thru a heat exchanger or Recirculated back to the boiler directly by the dedicated circulating pump

36 Separator : Once -thru Boiler Separates steam and water during the circulating mode operation Runs dry during once-thru flow mode Smaller in size compared to drum in a drum type boiler

37 Separator Separator vessel for supercritical steam generator

38 SH Start-up System with Circulating pump WW ECO Separator C HWL Flash Tank C MEFCV BRP To Condenser Mixing Sphere Deaerator HPH BFP

39 SUB CRITICAL Vs SUPER CRTICAL Particulars Sub critical Super critical Type Drum type Once through Once through Operating pressure Below bar Below bar Above bar Steam generation process Boiling (Two phase heat transfer) Boiling (Two phase heat transfer) No boiling process. Phase change is gradual. Thick walled component Start up and load change rates Water wall construction Circulation Thick walled drum is provided Base Vertical Circulation at all loads Smaller vertical separators provided Better start up and load change rates Generally spiral wall. Vertical wall for higher capacity Higher loads: Once through Lower loads : Circulation Water quality Base More stringent requirements

40 Materials used in various Pressure Parts of Subcritical Boilers Area of application Drum Water walls, Economizer SH and RH Material Carbon steel / Low Alloy steel Carbon Steel Tubes ASME specification Pipes - SA 299 SA192 SA210 Gr.A1 SA210 Gr.C SA106 Gr.B SA106 Gr.C 1 ¼ Cr ½ Mo SA213 T11 SA335 P11 2¼ Cr 1 Mo SA213 T22 SA335 P22 9 Cr 1 Mo ¼V SA213 T91 SA335 P91 18 Cr 8 Ni SA213 TP304 H - 18 Cr 10 Ni Cb SA213 TP347 H -

41 Materials Section Supercritical Boilers Economiser SA 210 Gr C Water wall SA 213 T11 / T22 / T23 Superheater & Reheater - Tubing SA 213 T11 / T22 / T23 / T91 / T92 / S304H / TP347H Superheater & Reheater - Headers & Piping SA 106 Gr B / Gr C, SA 335 P11 / P22 / P91 / P92

42 Materials in typical OTSC Boiler

43 Furnace design Furnace for Burning Pulverised Coal are Designed To prevent formation of slag deposits To allow complete combustion by providing adequate retention time. Temperature at entry to closely spaced SH/ RH sections is brought down well below the lowest of the ash softening temperatures. Adequate mass flow through water wall tubes so that tube metal temperatures are well within allowable limits. Ensuring the above in design stage will lead to high boiler availability

44 Furnace Design ( contd..) The furnaces of BHEL boilers are conservatively sized with low heat release rates to ensure maximum output all the times. The possibility of slagging and fouling of heat transfer surfaces is minimised even while firing worst type of fuels. Furnace design accommodates wider range of coals normally experienced by Indian utilities.

45 Furnace Selection Criteria Cross Sectional Area Heat Release Rate - NHI / PA Effective Projected Radiating Surface Loading - NHI/ EPRS Volumetric Heat Release Rate - Q Fired / Volume Burner Zone Heat Release Rate Q Fired / FZS Furnace Residence Time Distance Between Furnace Bottom-hopper & Lower Most Fuel Nozzle Distance Between Upper Most Fuel - Nozzle & Bottom of SH Furnace Exit Gas Temperature (FEGT) Furnace Bottom Opening

46 Design criteria SH / RH / ECONOMISER Adequate heating surface to get rated SHO / RHO temperature over control load for the range of coals. Low gas velocity to minimise pressure part erosion Proper tube spacing to prevent plugging and fouling of heat transfer surfaces Proper pressure part support and alignments

47 APPLICABILITY OF CODE REGULATIONS IBR: Boiler means any closed vessel exceeding litres (five gallons) in capacity which is used expressly for generating steam under pressure and includes any mounting or other fitting attached to such vessel, which is wholly or partly under pressure when steam is shut off ASME Sec. I: Applicable to boilers in which the steam or any other vapour is generated at a pressure more than 15 PSI (g).

48 MAXIMUM ALLOWABLE WORKING PRESSURE IBR: (Reg. 2) It is the working pressure of any component of the boiler. Calculation Pressure, in relation to a boiler, means the design pressure of any part adjusted to take into account the pressure drops corresponding to the most severe conditions of pressure drop and hydraulic head; IBR: (Reg. 2) "Design Pressure" means:- (i) in relation to a natural or assisted circulation boiler, the maximum allowable working pressure in the steam drum of the boiler; (ii) in relation to a once through forced-circulation boiler, the maximum allowable working pressure at the final superheater steam outlet;

49 DESIGN PRESSURE Operating Pressure Design Pressure

50 SAFETY VALVE IBR Reg. 621 Each boiler shall be equipped with two safety valves for relieving the steam pressure. The diameter of the valve shall not be less than 19 mm. The minimum relieving capacity of the safety valve shall be sufficient to discharge all the steam that can be generated by the boiler without allowing the pressure to rise more than 10% above maximum allowable working pressure.

51 DESIGN PRESSURE ASME Sec. I: (PG 21.1) No boiler, except a forced-flow steam generator with no fixed steam and water line that meets the special provisions of PG-67, shall be operated at a pressure higher than the maximum allowable working pressure except when the pressure relief valve or valves are discharging, at which time the maximum allowable working pressure shall not be exceeded by more than 6%. (PG 21.2) In a forced-flow steam generator with no fixed steam and waterline it is permissible to design the pressure parts for different pressure levels along the path of water-steam flow. The maximum allowable working pressure of any part shall be not less than that required by the rules of Part PG for the expected maximum sustained conditions of pressure and temperature to which that part is subjected except when one or more of the overpressure protection devices covered by PG-67.4 is in operation.

52 Requirement of Overpressure Protection

53 DESIGN TEMPERATURES - ALLOWANCES Area IBR ASME Radiant SH/RH 50 o C Actual Metal Convective SH/RH 39 o C Temperature for Gas Economiser 11 o C Touched Portion Furnace & Boiler 28 o C or 371 C (Min) Tubes Gas touched Headers 28 o C

54 DESIGN - CALCULATION OF THICKNESS REQUIRED Description IBR (Reg. 338, 350, 270) ASME (PG 27) Minimum Tube thickness PD C 2S + P PD D + f 2S + P Minimum Pipe/Shell thickness Where, PD SE P C = 0.75 for P 70 kg/cm² C = 0 for P > 70 kg/cm² P = Design Pressure D = OD for Tubes, ID for Pipes/Shell S = Allowable Stress E = Ligament Efficiency PD C (or) 2SE+2yP PR C SE (1 y) P A-317 Formula: D ( 1- e^(-p/se))/2 + C + f C = Threading and Structural stability allowance P = Design Pressure D = Outside diameter S = Allowable Stress E = Ligament Efficiency f = factor for expanded tube ends y = Temperature coefficient (0.4 to 0.7) R = Inside Radius of cylinder

55 ALLOWABLE MAXIMUM FLUID TEMPERATURES AND METAL TEMPERATURES FOR DIFFERENT MATERIALS (Reg. 47)

56 Material Carbon steel Method of manufacture Maximum permissible working pressure Cold drawn seamless No restriction 454 Maximum permissible temp, C Hot finished seamless Do 454 Do Butt welded [Max. nominal bore allowable 102 mm. (4 in)] 21 kg/cm² Form 260 Do Straights, bends or fittings. Electric resistance welded No restriction 454 Straights Cast steel Castings No restriction 454 Molybdenu m steel Chromium molybdenu m Steel Copper THE PRESSURE AND TEMPERATURE LIMITS WITHIN WHICH PIPES, TEES, BRANCHES, ETC., (REG. 349 ) Cold drawn seamless and castings. Cold drawn seamless and hot finished seamless Solid drawn up to and including 12.6 kg/cm² 127 mm.(5 in.) dia. No restriction 524 Do No restriction 621 Do Straights, bends or fittings Not allowed Straights and for SH steam bends.

57 ALLOWABLE MAXIMUM FLUID TEMPERATURES AND METAL TEMPERATURES FOR DIFFERENT MATERIALS Material ASME Alloy Oxidation Limit, Deg. C IBR Allowable, Deg. C Carbon steel SA -210 Gr A SA-106-C/B 427 SA -299 (drums) SA-515, 70 1 Cr-1/2 Mo SA-213, T SA-335, P12 1-1/4 Cr-1/2 Mo SA-213, T11 SA-335, P11 2-1/4 Cr-1Mo SA-213, T SA-335, P22 2-1/4 Cr-1.6W-V-Cb SA-213, T23 SA-335, P23 9 Cr-1 Mo. V SA-213, T SA-335, P91 9 Cr-2W SA-213, T92 SA-335, P92 18 Cr-10 Ni Cb SA-213, TP347H Cr-9 Ni -3Cu-Cb-N SA-213, Super 304H

A CASE STUDY ON BOILER TUBE FAILURE AT JPL

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