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
2. The aim of this presentation is to
• Give an understanding of
◦ Tube design principles
◦ Tube manufacture
◦ Failure mechanisms
◦ Inspection techniques
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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
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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
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7. Temperature
Stress
Design Curve
80% of Average
Reported Stress
Average Reported
Stress
Design
Temperature
Maximum
Allowable
Operating
Temperature
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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
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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
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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.
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12. Development of wrought stainless steel
• Historically “standard” material for the last 30
years
• Generally available
• Served industry well (reliable)
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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
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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
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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
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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
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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
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20. • Slow, sustained increase in length/diameter as
a result of stress at elevated temperature
• Culminates in rupture
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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