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Under guidance of:Mr. P. Kaiser
Senior manager
BAF/ECL/SPM
TATA STEEL
May – June 2013
11/1/2013

1
Steel Making process:-

11/1/2013

2
Steel making is done mainly in three stages:Iron making process
2) Conversion to steel
3) Rolling process
Steel is manufac...
PROCESS FLOW:continuous casting is done once the iron has converted into

steel and billet and ingots are send to various...
HSM line sends hot rolled coils thickness 2-6 mm width 800-1500 mm to CRM.
First the hot coils are passed through the pic...
M
11/1/2013

HSM.mp4

6
:-BAF:What is BAF??
 After cold rolling, annealing is done to soften the sheet for obtaining superior

mechanical propert...
11/1/2013

8
11/1/2013

9
11/1/2013

10
11/1/2013

11
Skin Pass Mill (SPM)

It is necessary to impart a small amount of deformation (typically

between 0.5 to 1.5%, depending o...
For CRM complex a skin pass mill of 1 mt.pa capacity has been selected
having following facility.
a. 4 hi single stand mil...
Annealing curve time Vs temperature
soaking

Heating hood
cooling

Cooling hood
cooling

Control heating

Rapid/bypass
coo...
For the increased production rate time spend during the cooling or say the time
taken by bypass cooling should be minimiz...
BUT: Entire design or existing design can not be changed as it would require
constructional changes to the plant which is...
11/1/2013

17
Bill of material
Component

Quantity

Standard

Round bar

2

IS-2062

Angles

2

IS-2062

Square bar

1

IS-2062

Rivets
...
Selection of tube material
 It should have good thermal conductivity
 Should be able to withstand thermal stress and hig...
Design analysis
Given:Di = 18 mm

Tube Material = Mild Steel

Do = 22 mm

Fin Material = Mild Steel

L tot = 1210 mm

Fin ...
Properties:
Mild Steel: k = 55 W/m-K
Hydrogen (T̅ = 590 K): µ = 1.377 x 10-5 Ns/m2
C p = 14.54 KJ/Kg K
k = 0.32 W/m K
Pr =...
Neglecting fouling & η0,c = 0 (since inside of tube is not finned),
1/UA = 1/h c A c + R w + 1/η0,h hhAh
To calculate wate...
1.Flow Across bank of Tubes
For staggered configuration, the maximum velocity may occur at either the transverse plane A1
...
V max will occur at A1, which is given by
V max = {ST/(ST – D)}.V
V max =10.16 m/s
Re D, max = ρV max D/μ = 0.072×10.16×0....
2.Compact Heat Exchanger

The overall heat transfer coefficient based on the gas-side surface area is given by:

1/Uh = 1/...
Methods of improving effectiveness:1. BY USING CORRUGATED FINS.
Initially, we considered both Aluminium and Copper as the ...
Failure of heat exchanger:-

General corrosion

11/1/2013

Cogging due to calcium and magnesium ions

27
Suspected reasons for heat exchanger tube failure
I. Pipe and tubing imperfections
II. Welding faults
III. Fabrication iss...
Problems:Scale, in addition to causing physical blockage of piping, equipment, and the cooling
tower, also reduces heat tr...
Literature show that the fouling of heat exchangers can occur by any of the following ways
1. Sedimentation fouling– due t...
Erosion corrosion
Problem:-

The tubes of BAF bypass heat exchanger at CRM have failed in service. The tubes are
getting p...
Sample details:Department

Cold rolling mill

Component

Heat exchanger tubes

Equipment

BAF bypass heat exchanger

Life ...
Industrial cooling water is flowing through the tubes and cools the hot hydrogen
gas hydrogen gases passing over the finn...
Corrosion of pipe due to use of
ICW

Tube Failure of Finned
shell and tube type heat
exchanger

11/1/2013

34
The cleaning agent used for the tube bundle was a solution under the trade name of ‘Alkali’, a

combination of compounds,...
Due to poor heat treatment and improper matrix formation, bainite structure having

less pearlite supports brittleness an...
Zinc was completely absent from the residue, although galvanized steel is expected to
have zinc at the surface and hence ...
Suggested alternate cleaning agents/methods:EDTA is a versatile chelating agent. It can form four or six

bonds with a me...
conclusion
Effectiveness improvement
 Use of corrugated fins
 Using inserts
 Using corrugated tubes
 Fin material copp...
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BAF heat exchanger

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Effectiveness improvement and failure prevention of BATCH ANNEALING FURNACE heat exchanger

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BAF heat exchanger

  1. 1. Under guidance of:Mr. P. Kaiser Senior manager BAF/ECL/SPM TATA STEEL May – June 2013 11/1/2013 1
  2. 2. Steel Making process:- 11/1/2013 2
  3. 3. Steel making is done mainly in three stages:Iron making process 2) Conversion to steel 3) Rolling process Steel is manufacture by 1) Injecting oxygen to molten Iron at supersonic speed in a Special type of vessel i.e. LD (Linz Donnavitz) vessel.  Two lines of manufacturing is used known as LD1 and LD2 . 11/1/2013 3
  4. 4. PROCESS FLOW:continuous casting is done once the iron has converted into steel and billet and ingots are send to various shops after the various rinsing and cleaning operations like argon rinsing station, ladle refining etc. Hot strip rolling of ingots done at HSM shop to make sheets and it is transported to CRM Dept. where mainly 3 operations carried out on strips. ii. BAF line operation CGL1 iii. CGL2 i. 11/1/2013 4
  5. 5. HSM line sends hot rolled coils thickness 2-6 mm width 800-1500 mm to CRM. First the hot coils are passed through the pickling line containing HCl in order to remove the scales making them ready for cold rolling. After the pickling and trimming coils are feed into the tandem mills( 6hi-5stand mills) and reduces the thickness of strip to 0.25mm-3.2 mm according to requirement. Part of coils which are sent through BAF + SPM, are first processed in ECL. About 2/3rd of CR goes to the Batch Annealing Furnaces (BAF) where the coil are stacked, covered and heated in closed atmosphere in a 100% hydrogen environment. This process improves the mechanical properties of the strip and helps in stress relieving. These coils are then processed through Skin Pass Mill (SPM). Skin passing removes Luder bands, impart desired surface finish, and improve flatness. 11/1/2013 5
  6. 6. M 11/1/2013 HSM.mp4 6
  7. 7. :-BAF:What is BAF??  After cold rolling, annealing is done to soften the sheet for obtaining superior mechanical properties for deep drawing application. Annealing process restore ductility increases internal stresses and restores formability of the cold rolled strips.  In the tata steel plant 100% batch annealing has been chosen for the annealing route because it gives some additional advantages over other processes like uniformity in the product for which tata steel is known worldwide. Some advantages of using BAF route over others are following:a. Flexibility in input raw material b. Process simplicity c. Economic lot size d. Flexibility in capacity addition. 11/1/2013 7
  8. 8. 11/1/2013 8
  9. 9. 11/1/2013 9
  10. 10. 11/1/2013 10
  11. 11. 11/1/2013 11
  12. 12. Skin Pass Mill (SPM) It is necessary to impart a small amount of deformation (typically between 0.5 to 1.5%, depending on the grade) to annealed steel strips for primarily a. Preventing stretcher -strains marks during subsequent forming application b. Imparting the desired surface texture on the strips. 11/1/2013 12
  13. 13. For CRM complex a skin pass mill of 1 mt.pa capacity has been selected having following facility. a. 4 hi single stand mill with roll bending b. Wet type skin passing c. Work hard texturing using the electrode discharge texturing(EDT) method. Why use hydrogen instead of any other gas for annealing??? The hydrogen with oxygen makes a highly explosive mixture and nitrogen can be used at place of hydrogen but one major disadvantage behind using nitrogen in BAF is that it will take approximately 150 hours for the same amount of annealing for which hydrogen only takes 50 hours, hence more production at CRM. 11/1/2013 13
  14. 14. Annealing curve time Vs temperature soaking Heating hood cooling Cooling hood cooling Control heating Rapid/bypass cooling Ramp of heating 11/1/2013 14
  15. 15. For the increased production rate time spend during the cooling or say the time taken by bypass cooling should be minimized, hence a heat exchanger is used below the BAF through which hot hydrogen circulates and fast cooling is obtained added with cooling hood cooling. Project outline:Main aim of the project was to increase the effectiveness and prevent the failure. This can be fulfilled by many ways like I. Changing the entire design of heat exchanger II. Modifying the existing dimensions III. Changing flow rates of fluid. 11/1/2013 15
  16. 16. BUT: Entire design or existing design can not be changed as it would require constructional changes to the plant which is not feasible  Flow rate can not be changed because it would require replacement of pumping system There was 14 heat exchanger failure within 5 months – Effectiveness improvement Design point of view 11/1/2013 Failure point of vied 16
  17. 17. 11/1/2013 17
  18. 18. Bill of material Component Quantity Standard Round bar 2 IS-2062 Angles 2 IS-2062 Square bar 1 IS-2062 Rivets 4 SS Name plate bracket 1 IS-2062 Name plate 1 IS-2062 Angles 4 IS-2062 Plate (LHS Channel) 1 IS-2062 Plate (LHS Channel) 1 IS-2062 Cover plate 1 IS-2062 RHS tube sheet 1 IS-2062 LHS tube sheet 1 IS-2062 Tubes 207 IS-2062 Stay bar 14 IS-2062 LHS partition plate 2 IS-2062 RHS partition plate 3 IS-2062 Sealing angle ᶲCS 18 IS-2062 Bolts and nuts 42 IS-2062 Gasket 1 IS-2062 Cover plate 1 IS-2062 Channel flange 1 IS-2062 Channel plate 2 IS-2062 Channel plate 2 IS-2062 11/1/2013 18
  19. 19. Selection of tube material  It should have good thermal conductivity  Should be able to withstand thermal stress and high pressure of fluids  Should be compatible to tube and shell fluids based on these consideration a BAF heat exchanger tube material should be copper alloy, stainless steel or galvanized carbon steel. It must be taken consideration while using the galvanized tubes that the fluids temperature are under the melting point temperature of zinc since zinc has very low melting point temperature.  Poor choice of material may lead to thermal fatigue, leak, cracks. 11/1/2013 19
  20. 20. Design analysis Given:Di = 18 mm Tube Material = Mild Steel Do = 22 mm Fin Material = Mild Steel L tot = 1210 mm Fin Pitch = 8 fins/inch L fin = 1140 mm Number of Tubes = 207 t = 26G = 0.405 mm Number of Tube Passes = 6 Pin, water = 3.5 bar Tube Configuration = Staggered Tin, water = 30° C Number of Shell passes = 1 Tout, water = 58° C Height of Tube Bundle, H = 605 mm Tin, H2 = 550° C Width of Tube Bundle, W = 810 mm V̇ water = 35 m3/h Transverse Pitch of the Tubes ST = 50 mm V̇H2 = 15000 m3/h Longitudinal Pitch of the Tubes SL = 44 mm ρH2 = 0.072 Kg/m3 Average no. of tubes per pass, n = 34.5 11/1/2013 20
  21. 21. Properties: Mild Steel: k = 55 W/m-K Hydrogen (T̅ = 590 K): µ = 1.377 x 10-5 Ns/m2 C p = 14.54 KJ/Kg K k = 0.32 W/m K Pr = 0.68 Water (T̅ = 315 K): µ = 6.31 x 10-4 Ns/m2 Cp = 4.179 KJ/Kg K K = 0.634 W/m K Pr = 4.16 ρ = 991.08 Kg/m3 To calculate effectiveness of the heat exchanger we need to calculate the overall heat transfer coefficient. 1/UA = 1/(η0hA)c + R f ,c”/(η0A)c + R w + R f, h”/(η0A)h + 1/(η0hA)h 11/1/2013 21
  22. 22. Neglecting fouling & η0,c = 0 (since inside of tube is not finned), 1/UA = 1/h c A c + R w + 1/η0,h hhAh To calculate water side convection coefficient h c u = V̇/An= 35×4/(3600×π×34.5×0.0182) =1.107m/s Re = ρ u D/μ = 991.08×1.107×0.018/6.31×10-4 = 31296.767 Therefore, flow is turbulent. For turbulent flow in circular tubes, Dittus – Boelter equation is used, N u D = 0.023 Re4/5Prn Where n = 0.4 for heating & n = 0.3 for cooling To calculate gas side convection coefficient hh , we will consider two different models, one the flow across banks of tubes & other that of compact heat exchanger. 11/1/2013 22
  23. 23. 1.Flow Across bank of Tubes For staggered configuration, the maximum velocity may occur at either the transverse plane A1 or diagonal plane A2. 11/1/2013 23
  24. 24. V max will occur at A1, which is given by V max = {ST/(ST – D)}.V V max =10.16 m/s Re D, max = ρV max D/μ = 0.072×10.16×0.022/1.377×10-5 = 1168.73 For this range of Re, it is appropriate to use the results of Zukauskas, Hh =N u D k/D = 21.05×0.32/0.022 = 306.25 W/m2K 11/1/2013 24
  25. 25. 2.Compact Heat Exchanger The overall heat transfer coefficient based on the gas-side surface area is given by: 1/Uh = 1/h c(Ac/Ah) + A h R w + 1/η0,hhh If fin thickness is assumed negligible, A f, h =3.074×10-3 m2 Re = G Dh/μ = 0.6417 × 0.7715/1.377×10-5 = 35932.91 Cmin = Ch = 4362 W/K q = Cc(Tc,o – Tc,i) = Cc(Tout,water – Tin,water) = 40266.75(58° - 30°) = 1127.469 kW qmax = Cmin(Th,I – Tc,i) = Cmin(Tin,H2 – Tin,water) = 4362(550° - 30°) = 2268.24 Kw ε = q/qmax =1127.469/2268.24 = 0.497 11/1/2013 25
  26. 26. Methods of improving effectiveness:1. BY USING CORRUGATED FINS. Initially, we considered both Aluminium and Copper as the substitutes but the temperature range of the hot side (≈550) prevented us from using Aluminium as it is very near to its melting temperature. So, we have considered copper as a substitute. 2. USING INSERTS :3. USING CORRRUGATED TUBES:- 11/1/2013 26
  27. 27. Failure of heat exchanger:- General corrosion 11/1/2013 Cogging due to calcium and magnesium ions 27
  28. 28. Suspected reasons for heat exchanger tube failure I. Pipe and tubing imperfections II. Welding faults III. Fabrication issues IV. Improper design specification and material V. Pitting VI. Stress corrosion cracking VII. Corrosion fatigue VIII.Crevice corrosion IX. Selective leaching or de alloying X. Thermal fatigue XI. fouling Fouling :-If there is a gradual decline in heat transfer, fouling may be the culprit. Scale is a hard deposit of predominantly inorganic material on heating transfer surfaces caused by the precipitation of mineral particles in water. 11/1/2013 28
  29. 29. Problems:Scale, in addition to causing physical blockage of piping, equipment, and the cooling tower, also reduces heat transfer and increases the energy use. For example, the thermal conductivity BTU/ [hr (ft2) (F/in)] of copper is 2674, while the common cooling water scale calcium carbonate has a thermal conductivity of 6.4 BTU/ [hr (ft2) (F/in)]. A calcium carbonate scale of just 1.5 mm thickness is estimated to decrease thermal efficiency by 12.5 %. In compression refrigeration systems, scale translates into higher head pressures, hence an increase in power requirements and costs. For example, 1/8" of scale in a 100 ton refrigeration unit represents an increase of 22% in electrical energy compared to the same size unit free of scale. 11/1/2013 29
  30. 30. Literature show that the fouling of heat exchangers can occur by any of the following ways 1. Sedimentation fouling– due to suspended particles. 2. Inverse solubility fouling—due to calcium and Mg salts 3. Chemical reaction of fluid and tube material--Chemical reaction due to exposure to hot heat transfer surface n degradation of one of the process stream resulting in carbonaceous deposits, resulting in chemical reaction fouling. 4. Corrosion product fouling 5. Biological fouling –due to mixing of oil, small organisms etc Due to mixing of oil in the water, biological fouling takes place and organism generate and deposit at a faster rate, if and tube is cracked at PL-TCM section heat exchanger then from there oil mixes to the ICW which is carried to the water tank of ICW and circulated to BAF heat exchanger. This is also a major reason for the biological fouling. 11/1/2013 30
  31. 31. Erosion corrosion Problem:- The tubes of BAF bypass heat exchanger at CRM have failed in service. The tubes are getting punctured at random locations along their length resulting in leakage. ICW which flow through the tubes is used to cool the hot hydrogen gas flowing over the tubes:11/1/2013 31
  32. 32. Sample details:Department Cold rolling mill Component Heat exchanger tubes Equipment BAF bypass heat exchanger Life expected 5 years Life obtained 2-3 years Material IS:2062 Grade B UMC 0566A2378 Heat exchanger type Shell and tube type heat exchanger 11/1/2013 32
  33. 33. Industrial cooling water is flowing through the tubes and cools the hot hydrogen gas hydrogen gases passing over the finned tubes. The heat exchange is comprised of a casing and several tubes which are covered with the fins. The fins were found ruptured in multiple locations. Most are the tubes were fouled and ions of magnesium and calcium along with the other impurities were deposited (sticking in inner surface) in the tubes causing the blockage of flow and, poor convective heat transfer between cold water and metallic tube hence reducing the overall effectiveness of the heat exchanger. Once it has been reported that within 5 span of months 14 such heat exchanger assemblies has been affected by same issues of serious fouling. 11/1/2013 33
  34. 34. Corrosion of pipe due to use of ICW Tube Failure of Finned shell and tube type heat exchanger 11/1/2013 34
  35. 35. The cleaning agent used for the tube bundle was a solution under the trade name of ‘Alkali’, a combination of compounds, including Na2CO3 , Silicates and mostly(>80%) NaOH. The solution is primarily used in ECL (Electrolytic Cleaning), but was found to be suitable for cleaning tube bundles in BAF Heat Exchanger. At any concentration, sodium hydroxide should not be stored in aluminum or zinc (galvanized) containers or allowed to come in contact with magnesium. The most common materials of construction for the storage of sodium hydroxide are carbon steel and polyethylene. Sodium Hydroxide being one of the strongest bases commercially available will react with these metals. Zn + 2NaOH + 2H2O  Na2 [Zn (OH)4] + H2(g) Zinc react with sodium hydroxide to form sodium Zincate which dissolve in water. Eventually all the zinc will dissolve in the water and iron will loose the protection of zinc and rusting will start. 11/1/2013 35
  36. 36. Due to poor heat treatment and improper matrix formation, bainite structure having less pearlite supports brittleness and corrosive properties. Hence due to high hardness and abundant bainite structure, weld section becomes brittle and corrosion starts from welding joint. This can be prevented using high pearlite matrix better heat treatment of weld sections. Evidence from Lab Test of failed samples:The lab test of eroded and punctured tubes presented us with a few facts Iron oxides were the most common material found in the residue apart from compounds of Calcium and Magnesium, although water samples didn’t indicate high iron content in the inlet. This indicates that iron was rusted. 11/1/2013 36
  37. 37. Zinc was completely absent from the residue, although galvanized steel is expected to have zinc at the surface and hence as part of the residue. This indicates that zinc was completely worn out. Both the points support the theory that zinc was dissolved and iron in carbon steel began to rust. 11/1/2013 37
  38. 38. Suggested alternate cleaning agents/methods:EDTA is a versatile chelating agent. It can form four or six bonds with a metal ion, and it forms chelates with both transition-metal ions and main-group ions. EDTA is frequently used in soaps and detergents, because it forms complexes with calcium and magnesium ions. In other applications, EDTA dissolves the CaCO3 scale Ethylene diamine tetra acetic acid (EDTA) deposited from hard water without the use of corrosive acid. It is a better option, compared to other chemicals such as NaOH or HCl, as it is unlikely to react with the tube surface, but will remove the scale formed due to Calcium and Magnesium 11/1/2013 38
  39. 39. conclusion Effectiveness improvement  Use of corrugated fins  Using inserts  Using corrugated tubes  Fin material copper based alloy Failure prevention  Use of EDTA instead of alkali for galvanized tubes  Suitable amount of bainite-pearlite matrix ration and proper heat treatment of weld sections References www.tatasteel.com www.sciencedirect.com www.wikipedia.com www.gooogle.com/images www.tema.org Fundamental of heat and mass transfer by Incropera/ bergman/ lavine Heat and mass transfer by J.P.Holman 11/1/2013 39

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