Steam Reforming - Tube Design

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

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

• 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

Average Reported
Stress
Design Curve
80% of Average
Reported Stress
Temperature
Stress

Temperature
Stress
Design Curve
80% of Average
Reported Stress
Average Reported
Stress
Design
Temperature
Maximum
Allowable
Operating
Temperature

• 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

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

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.

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

 Development of wrought stainless steel
• Historically “standard” material for the last 30
years
• Generally available
• Served industry well (reliable)

• 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

• 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

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

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

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

• 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

• Slow, sustained increase in length/diameter as
a result of stress at elevated temperature
• Culminates in rupture

• 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”

• Thermal gradients
• Thermal shock
• Stress corrosion cracking
• Dissimilar weld cracking
• Tube support system

• 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)

 NDT
–visual examination
–tube diameter (or circumference)
measurement
–ultrasonic attenuation
–radiography
–metallurgical examination
–LOTISTM

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

• 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

• 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

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

• 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

• 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

• 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

• 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

• 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

3.5
4
4.5
5
5.5
Axial Position (In)
TubeDiameter(In)
Good Tube Tube with Creep Damage

 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

 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

Steam Reforming - Tube Design

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