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….