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Steam Reforming: Tube Design
Gerard B. Hawkins Managing Director
The aim of this presentation is to • Give an understanding of ◦ Tube design principles ◦ Tube manufacture ◦ Failure mechanisms ◦ Inspection techniques
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• 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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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
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• 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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Average Reported Stress
Design Curve 80% of Average Reported Stress
Temperature
Stre
ss
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Temperature
Stre
ss
Design Curve 80% of Average Reported Stress
Average Reported Stress
Design Temperature
Maximum Allowable Operating
Temperature
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• 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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850 900 950 1000 1050 11005
10
20
50
100
200
Mea
n Tu
be L
ife (H
ours
x 1
000)
+20 Deg C
(1560) (1650) (1740) (1830) (1920)Temperature °C or °F
(2010)
(+36 Deg F)
HK40 tubes38 barg (550 psig) pressure
95 mm (3.75") bore13.46 mm (0.53") wall thickness
15.3 N/mm2 (2218 psi) stress
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HK40 Alloy HK40 20% Ni 25% CrIN519 Alloy IN519 24% Ni 24% Cr 1% Nb36X Manaurite 36X (Pompey) 33% Ni 25% Cr 1% Nb800H Incoloy 800H 31% Ni 21% Cr600 Incoloy 600 72% Ni 15% Cr 1% MnH39W Alloy H39W (APV) 33% Ni 25% Cr 1% NbH39WM Paralloy H39WM 35% Ni 25% Cr 1% Nb + TiXM Manaurite XM 33% Ni 25% Cr 1% Nb + TiKHR35CT Kubota Heat Resistant 35% Ni 25% Cr 1% Nb + Ti 0.45%CA304 Stainless Steel 8% Ni 18% Cr
800H and 600 are for GHR tubesA304 is only suitable for Bayonet tubes.
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700720
740760
780800
820840
860880
900920
940960
9801000
2
5
10
20
50
100
200
Temperature °C
Allo
wab
le st
ress
(MN
/m²) hk40
in519
h39w
36x
xm
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Development of wrought stainless steel
• Historically “standard” material for the last 30 years
• Generally available
• Served industry well (reliable)
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• 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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• 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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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
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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
)
HK40 IN 519 HP Nb Mod HP Microalloy
0
0.002
0.004
0.006
0.008
0.01
0.012
Tube Material
Cat
alys
t Vol
ume
(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
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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• 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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• Slow, sustained increase in length/diameter as a result of stress at elevated temperature
• Culminates in rupture
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• 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”
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• Thermal gradients
• Thermal shock
• Stress corrosion cracking
• Dissimilar weld cracking
• Tube support system
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• 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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NDT
–visual examination
– tube diameter (or circumference) measurement
–ultrasonic attenuation
– radiography
–metallurgical examination
–LOTISTM
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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
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• 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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• 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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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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• 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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• 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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• 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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• 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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• 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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3.5
4
4.5
5
5.5
Axial Position (In)
Tube
Dia
met
er (I
n)
Good Tube Tube with Creep Damage WWW.GBHENTERPRISES.COM
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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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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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