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Reformer Tube Metallurgy: Design Considerations; Failure
Mechanisms; Inspection Methods
Gerard B. Hawkins Managing Director
Contents Design principles Recent developments in metallurgy Failure mechanisms Monitoring and inspection techniques
From an operator's viewpoint -how can you maximize tube life ?
-- what can you do if a tube fails ?
Introduction
The steam reformer is one of the most important and costly parts of the plant
Tubes operate at limits of temperature and pressure
Tube replacement is expensive
Re-tube cost for a "typical" 50 mmscfd H2 plant is ~10% of installed plant cost
Reformer Tube Design
Based on predicted creep life of material Laboratory short-term test are performed for each
material • Time to rupture is evaluated for • 1) a range of temperatures at constant stress • 2) at a range of different stresses
Stre
ss, σ
Temperatures, T T1 T2 T3 T4
Time, t
Larson - Miller Curve
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 analyzed statistically and extrapolated to longer time-scales • tests are normally 100’s to 1000’s hrs long
Larson-Miller Diagram - Results of 170 Rupture Tests on Typical HK40
P (Larson-Miller Parameter)
Rup
ture
Str
ess
(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
Effect of Temperature on Tube Life
Temperature oC 850 900 950 1000 1050 1100 5
10
20
50
100
200
Mea
n Tu
be L
ife (H
ours
x 1
000)
+20°C (+36°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
Effect of temperature on tube life
Deg F Deg C1580 860 10 years1616 880 5 years1652 900 2.5 years1697 925 11 Months1742 950 4.5 month1787 975 2 Months1832 1000 4 weeks1922 1050 5.5 days2012 1100 1 day2102 1150 8 hours2192 1200 2 Hours
Methodology
Calculate expired life fraction, F, for each tube: • F = n1/N1 + n2/N2 + n3/N3 + ……...
Where • ni = actual time at temperature i • Ni = mean life at temperature i
Calculate ni from thermal history Calculate Ni from Larson-Miller
Calculation of Ni
Temp Range
Temp Range
Time Spent
(Hours)
Time Spent
(Hours) (oC) (oF) ni
Tube A ni
Tube B
850-860 1562-1580 2000 0
860-870 1580-1598 7800 3950
870-880 1598-1616 1300 350
880-890 1616-1634 1800 1150
890-900 1634-1652 0 1850
900-910 1652-1670 0 5600
12900 12900
0 2000 4000 6000 8000 10000 12000 850
860
870
880
890
900
Time on-line (hours)
Max
imum
Tub
e W
all T
empe
ratu
re
TUBE B
TUBE A
oC oF
1652
1634
1616
1598
1580
Contents
Design principles Recent developments in metallurgy Failure mechanisms Monitoring and inspection techniques
From an operator's viewpoint -how can you maximize tube life ?
-- what can you do if a tube fails ?
Metallurgical Developments
HK40 25 Cr / 20 Ni • Development of wrought stainless steel • Historically “standard” material for the last 40
years • Generally available • Served industry well (reliable)
Metallurgical Developments
HP Modified 25 Cr / 35 Ni + Nb • Available for the last 30 years • More expensive than HK40 • Choice of thinner tubes at same price, or
longer lives
Metallurgical Developments
Microalloy 25 Cr / 35 Ni + Nb + Ti • 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
Development of Steam Reformer Tube Alloys
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 C
reep
Str
engt
h
HK40 Microalloys
Year
Comparison of Alloy Strength, Tube Thickness and Tube Volume
HK40 IN 519 HP Nb Mod HP Microalloy
0
5
10
15
20
25
30
35
Tube Material
Rup
ture
Str
engt
h (N
/mm
2 )
0
5
10
15
20
Tube Material
Min
imum
Sou
nd W
all T
hick
ness
(mm
)
0
0.002
0.004
0.006
0.008
0.01
0.012
Tube Material C
atal
yst V
olum
e (m
3 /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
Centrifugal Casting Process for Tubes
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
Photo of Casting Process
Tube microstructure (as cast)
Light oxides on inner wall (machined away)
Tube microstructure
As-Cast condition Network of primary carbides
Aged condition Secondary carbides precipitation
Austentite grains
Primary carbides Fine secondary carbides (Precipitate)
0.1mm
Fabrication
Welds of different metallurgy 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) Weld failures rare nowadays
Contents
Design principles Recent developments in metallurgy Failure mechanisms Monitoring and inspection techniques
From an operator's viewpoint -how can you maximise tube life ?
-- what can you do if a tube fails ?
Creep Damage
Slow, sustained increase in length/diameter as a result of stress at elevated temperature
Culminates in rupture Dominant damage mechanism
Categories for Classification of Creep Damage
Exposure Time
Cre
ep S
trai
n
Rupture
I, II, III: Creep Ranges
Isolated creep voids (x 250) Micro-fissures (x 250) Creep cracking (x 100)
Creep Crack Development
Creep Crack Development Through The Tube Wall
Start : Cracks 30% from inner wall Growth : Cracks grow to break inner bore Failure : Cracks progress to outer wall
Most Common Steam Reformer Tube Failure Mechanisms
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”
Less Common Steam Reformer Tube Failure Mechanisms
Thermal gradients Others
• Thermal shock • Stress corrosion cracking • Dissimilar weld cracking • Tube support system
Creep Rupture
Creep Rupture - Tube Section
Tube Failure at a Weld
Weld Failure - Detail
Contents
Design principles Recent developments in metallurgy Failure mechanisms Monitoring and inspection techniques
From an operator's viewpoint -how can you maximise tube life ?
-- what can you do if a tube fails ?
Inspection Methods and Monitoring Techniques
NDT • Visual examination • Tube diameter (or circumference) measurement • Ultrasonic attenuation • Radiography • Metallurgical examination
Combination of methods needed
Visual Examination
Prior to shut-down • Hot tubes, hot spots, leaks
Bulges, distortion, scale, colour, staining • Can indicate overheating • Adequate access (scaffolding) needed
Use TV camera to look at bore • Cracking often starts in bore
Tube Diameter Measurement
Measure diameter - often undervalued method Tube diameters (as cast) vary by up to 3 mm 1% growth (around 1 mm) significant
• HK40 - 1 %; HP Alloys - ~4-5% Must really have base-line readings Limited locations only really reliable at welds
• Won’t be max temp areas Tubes can go oval Need staging for access
Ultrasonic Testing
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
10
5 4
2
6
3
6
1
7 8 9
X1 X2
Ultrasonic Attenuation
Categories for Classification of Creep Damage
Exposure Time
Cre
ep S
trai
n
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
Ultrasonic Attenuation
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
Radiography 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
Radiography - Weld Crack
Eddy Current Measurement 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
Metallurgical Examination
Selective “early retirement” of tubes for metallurgical investigation
Concern about validity of sample • How representative is sample? • Statistical significance of sample size
Accelerated creep tests or elapsed life tests of no value • Life of a tube? • first failure mean life last failure • 6 years 52 years 242 years
Other Inspection Methods Surface replication
• Time consuming • Spot result on surface, means creep damage
is through wall Conventional ultrasonic inspection of dissimilar
welds is recommended New - Laser mapping of tube bore
• Extremely high accuracy
LOTIS - Laser Optical Tube Inspection System
Highly accurate creep strain measure over entire tube length
Creep damage can be characterised by increases in the reformer tube diameter
Spinning laser measures tube ID Available only through licensing.
LOTIS Laser Mapping Probe
General Theory of Optical Triangulation
IMAGED SPOT
IMAGING LENS
TARGET SURFACE
OBJECT SPOT
INSPECTIONRANGE
FOCUSING LENS
DIODE LASER
PHOTODETECTOR
LOTIS Field System
LOTIS Application Method and Output
LOTIS Tube Inspection System
Capable of obtaining measurements within 0.002” (0.05mm) • Measures tube diameters within 0.05%
Tubes can be scanned quickly - typically 3 minutes per tube
Well proven and reliable equipment - used in power plant for over 14 years
Proven in reformers for over 8 years
3D Modeling of Creep Damage in Reformer
3D Modeling of Creep Damage in Reformer
LOTIS Inspection of Reformer Tubes
NDT Technique Capabilities
LOTIS Limitations
Only inspects inside surface Requires tubes to be empty of
catalyst Probe cannot be submerged in water
Options following Single Tube Failure
If leak is small with no impingement on neighboring tube, continue running!
Replace tube Nip pigtails (but consider effect on remaining
tubes)
Pigtail Nipping
Leak
Impingement on refractory and other tubes
Before nip
After nip
Inlet Pigtail
Header
Header
Outlet Pigtail
X X
X X
X – X = Nip poisitons
Row of Steam Reformer Tubes
Pinched Tube
Pinched Tube in Steam Reformer
Conclusions
Tube life can be maximized by • Use of improved metallurgy • Good temperature control
Tube life can be monitored by a combination of NDT and TWT measurement
Example of remaining tube life prediction given Pigtail nipping increases options following a tube
failure
Conclusions
The future Tube metallurgy improvements have reached a
plateau • Nothing new on the horizon
Future improvements are more likely to be in smart coatings to improve heat transfer
TUBE LIFEOUTPUT
PRODUCTION MAINTENANCE
The Eternal Dilemma….