Steam Reforming - Tube Design

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Reformer Tube design principles
- Larsen Miller Plot
- Larsen Miller & Tube Design
- Design Margins - Stress Data Used
- Max Allowable & Design Temperature
- Tube Life
- Effect of Temperature on Life
- Material Types
HK40: 25 Cr / 20 Ni
HP Modified: 25 Cr / 35 Ni + Nb
Microalloy: 25 Cr / 35 Ni + Nb + Ti
- Alloy Developments
- Comparison of Alloys
Manufacturing Technology
- Welds
Failure mechanisms
- Failure Mechanisms - Creep
- Creep Propagation
- Common Failure Modes
- Uncommon Failure Modes
- Failure by Creep
- Creep Rupture - Cross Section
- Failure at Weld
Actions to Take if Tube Fails
- Pigtail Nipping
Inspection techniques
Classification of Problems
- Visual Examination
- Girth Measurement
- Ultrasonic Attenuation
- Radiography
Eddy Current Measurement
LOTIS Tube Inspection
LOTIS Compared to External Inspection

Published in: Technology, Business

Steam Reforming - Tube Design

  1. 1. Steam Reforming: Tube Design Gerard B. Hawkins Managing Director
  2. 2.  The aim of this presentation is to • Give an understanding of ◦ Tube design principles ◦ Tube manufacture ◦ Failure mechanisms ◦ Inspection techniques WWW.GBHENTERPRISES.COM
  3. 3. • Based on predicted creep life of material • Laboratory short-term test are performed for each material ◦ time to rupture is evaluated for a range of temperatures at constant stress ◦ a range of different stresses done • All of the data for a given material can be represented in one diagram by defining the Larson-Miller parameter, P, as a function of time (t) and temperature (T) • Data is analysed statistically and extrapolated to longer time-scales WWW.GBHENTERPRISES.COM
  4. 4. P (Larson-Miller Parameter) RuptureStress(psi) 100,000 50,000 10,000 5,000 1,000 16 17 18 19 20 21 22 23 24 25 26 P = T (log (t) + K) 1000 where T = temperature t = time K = constant WWW.GBHENTERPRISES.COM
  5. 5. • Process pressure (stress) is defined • Get P from Larson-Miller curve for a given metallurgy • From P, assuming a desired life (t) of typically 100,000 hours, a maximum allowable temperature (T) is defined • Repeat calculation until satisfactory design achieved • Do include some margin ◦ Use 80% of the average stress ◦ Allow for 25°C difference between design temperature and maximum allowable operating temperature WWW.GBHENTERPRISES.COM
  6. 6. Average Reported Stress Design Curve 80% of Average Reported Stress Temperature Stress WWW.GBHENTERPRISES.COM
  7. 7. Temperature Stress Design Curve 80% of Average Reported Stress Average Reported Stress Design Temperature Maximum Allowable Operating Temperature WWW.GBHENTERPRISES.COM
  8. 8. • Tube life is usually 100,000 hours • In reality statistics have been used • Should expect 2% failure before 100,000 hours • Provided tubes are operated at Maximum Allowable Operating Temperature WWW.GBHENTERPRISES.COM
  9. 9. 850 900 950 1000 1050 1100 5 10 20 50 100 200 MeanTubeLife(Hoursx1000) +20 Deg C (1560) (1650) (1740) (1830) (1920) Temperature °C or °F (2010) (+36 Deg F) HK40 tubes 38 barg (550 psig) pressure 95 mm (3.75") bore 13.46 mm (0.53") wall thickness 15.3 N/mm2 (2218 psi) stress WWW.GBHENTERPRISES.COM
  10. 10. HK40 Alloy HK40 20% Ni 25% Cr IN519 Alloy IN519 24% Ni 24% Cr 1% Nb 36X Manaurite 36X (Pompey) 33% Ni 25% Cr 1% Nb 800H Incoloy 800H 31% Ni 21% Cr 600 Incoloy 600 72% Ni 15% Cr 1% Mn H39W Alloy H39W (APV) 33% Ni 25% Cr 1% Nb H39WM Paralloy H39WM 35% Ni 25% Cr 1% Nb + Ti XM Manaurite XM 33% Ni 25% Cr 1% Nb + Ti KHR35CT Kubota Heat Resistant 35% Ni 25% Cr 1% Nb + Ti 0.45%C A304 Stainless Steel 8% Ni 18% Cr 800H and 600 are for GHR tubes A304 is only suitable for Bayonet tubes. WWW.GBHENTERPRISES.COM
  11. 11. 700 720 740 760 780 800 820 840 860 880 900 920 940 960 980 1000 2 5 10 20 50 100 200 Temperature °C Allowablestress(MN/m²) hk40 in519 h39w 36x xm WWW.GBHENTERPRISES.COM
  12. 12.  Development of wrought stainless steel • Historically “standard” material for the last 30 years • Generally available • Served industry well (reliable) WWW.GBHENTERPRISES.COM
  13. 13. • Available for the last 30 years • More expensive than HK40 • Choice of thinner tubes at same price, or longer lives • Typical names include H39W, 36X WWW.GBHENTERPRISES.COM
  14. 14. • Most recent development • Twice as strong as HK40 • Cost effective (not twice the price) • Offers options of higher heat flux, increased catalyst volume, fewer tubes, improved efficiency or longer tube life • Requires skill to produce • Typical brands include H39WM, XM, KHR35CT WWW.GBHENTERPRISES.COM
  15. 15. Low Carbon Stainless Wrought Pipes Add Ni, Cr, C Add Nb Improved Carbides Add Microalloy Additions Improved Carbides 1960 1975 1985 25/20 Cr/Ni 25/35/1 Cr/Ni/Nb HP Mod TUBES MADE BY CENTRIFUGAL CASTINGS (High Carbon 0.4%) 25/35/1 plus Cr/Ni/Nb additions CreepStrength HK40 Microalloys WWW.GBHENTERPRISES.COM
  16. 16. 0 5 10 15 20 25 30 35 Tube Material RuptureStrength(N/mm2) 0 5 10 15 20 Tube Material MinimumSoundWallThickness(mm) HK40 IN 519 HP Nb Mod HP Microalloy 0 0.002 0.004 0.006 0.008 0.01 0.012 Tube Material CatalystVolume(m3/m) Calculated to API RP 530 100,000 hour life at 900 Deg C (1650 Deg F) Based on 125.2mm (4.93") OD tube, 35.7 kg/cm2 (508psi) pressure
  17. 17. Pouring Cup Liquid Alloy In Internal Coating Liquid Stream Drive Rollers Solidified Tube End Plate Steel Mould 5-6 metres long (Spinning at high speed) Hollow Liquid Tube formed by Centrifugal Forces WWW.GBHENTERPRISES.COM
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  19. 19. • Welds of different metallurgies are a source of weakness • Tube material developments with resultant higher stresses put more demands on welds • PAW and EBW now increasingly available – narrow welds – no shrinkage – flexibility in tube metallurgy (no consumable required) • With HK40 welds weakest point • Therefore placed welds away from peak heat flux WWW.GBHENTERPRISES.COM
  20. 20. • Slow, sustained increase in length/diameter as a result of stress at elevated temperature • Culminates in rupture WWW.GBHENTERPRISES.COM
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  22. 22. • Normal “end-of-life” failures – creep rupture – weld cracking due to creep • Overheating accelerates normal “end-of-life” – over-firing – flame impingement • Thermal cycling also accelerates normal “end-of- life” WWW.GBHENTERPRISES.COM
  23. 23. • Thermal gradients • Thermal shock • Stress corrosion cracking • Dissimilar weld cracking • Tube support system WWW.GBHENTERPRISES.COM
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  27. 27. • If leak is small with no impingement on neighbouring tube, continue running! ◦ But monitor regularly • Replace tube • Nip pigtails (but consider effect on remaining tubes) WWW.GBHENTERPRISES.COM
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  29. 29.  NDT –visual examination –tube diameter (or circumference) measurement –ultrasonic attenuation –radiography –metallurgical examination –LOTISTM WWW.GBHENTERPRISES.COM
  30. 30. Exposure Time CreepStrain Damage Corresponding Parameter Action in Plant A - observe B - observe, fix inspection intervals C - limited service until replacement D - plan immediate replacement C D Rupture A B I, II, III: Creep Ranges WWW.GBHENTERPRISES.COM
  31. 31. • Prior to shut-down –hot tubes, hot spots, leaks • Bulges, distortion, scale, color, staining –can indicate overheating –adequate access (scaffolding) needed • Use TV camera to look at bore –cracking often starts in bore WWW.GBHENTERPRISES.COM
  32. 32. • A useful, often undervalued method • Tube diameters as cast can vary by up to 3 mm • 1% growth (around 1 mm (40 thou)) significant ◦ HK40 - Bulge to 2-3% then fail ◦ HP Alloys - Bulge to 5-7% (less data) then fail • Must have base-line readings • Need to measure at same locations ◦ hot spot and max temp areas • Tubes can go oval • Need staging for access WWW.GBHENTERPRISES.COM
  33. 33. 10 5 4 2 6 3 6 1 7 8 9 Sketch of the inspection system 1 Inspected tube 6 Water chamber 2 Emitting probe 7 Ultrasonic pulser 3 Receiving probe 8 Amplifier 4 Probe assembly 9 Analog gate 5 Water feed 10 Recorder X1 X2 WWW.GBHENTERPRISES.COM
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  35. 35. • Excellent in principle • Poor track record in practice – tends to fail sound tubes • Difficult to calibrate • Best to use repeat tests – look for deterioration • Manufacturers recommend radiography of suspect areas • Scaffolding not needed WWW.GBHENTERPRISES.COM
  36. 36. • Use in suspect areas – hot spots and bulges • Main benefit in butt weld inspection • Time - consuming ◦ area sterilisation • Limited to sampling • Sensitivity ◦ accurate alignment • catalyst removal • Staging needed WWW.GBHENTERPRISES.COM
  37. 37. • Eddy current measurement ◦ Similar crawler to ultrasound device ◦ No contact, uses AC coil/sensing coil • Baseline readings recommended • Issues ◦ Magnetic permeability variation in HP alloy ◦ Depth of penetration through wall less sensitive to inner wall cracks • Can also include OD measurement WWW.GBHENTERPRISES.COM
  38. 38. • Capable of obtaining measurements within 0.002” (0.05mm), allowing tube diameters to be determined within 0.05% • Tubes can be scanned quickly - typically 3 minutes per tube • Well proven and reliable equipment ◦ Used by the US military for over 20 years ◦ Proven in methanol plant reformers over 15 years WWW.GBHENTERPRISES.COM
  39. 39. • GBHE experience from design and operation of reformers can be used to interpret LOTIS creep measurement data • Assessment of remaining tube life • Recommendations for adjusting process conditions to optimise performance and life • Recommendations for adjusting firing pattern to compensate for differential creep WWW.GBHENTERPRISES.COM
  40. 40. 3.5 4 4.5 5 5.5 Axial Position (In) TubeDiameter(In) Good Tube Tube with Creep Damage WWW.GBHENTERPRISES.COM
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  42. 42.  Set up takes less than 30 minutes  LOTIS can be used on horizontal tubes prior to installation  No couplants (water or gel) required & no damage to the tube  Typically used on new tubes as a quality control check and to establish a baseline  Used at each catalyst change (4-5 years) to assess damage and collect data for allow tube life prediction and reformer tuning  Can be used on aged tubes to compare creep with baseline of top section  Used on failed tubes to assess actual creep strain WWW.GBHENTERPRISES.COM
  43. 43.  External inspection can be confused by rough tube exterior  Tube bowing can restrict access to external tube crawlers  Refractory can restrict access to external inspection  External inspection tends to rely on careful interpretation, which may be subjective  LOTIS gives a precise measure of diameter WWW.GBHENTERPRISES.COM

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