steamreforming-tubedesign-130924143050-phpapp01.pdf

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)

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


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 25C difference between design temperature

and maximum allowable operating temperature


Average Reported Stress

Design Curve 80% of Average Reported Stress

Temperature

Stre

ss


Temperature

Stre

ss

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 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


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.


700720

740760

780800

820840

860880

900920

940960

9801000

2

5

10

20

50

100

200

Temperature C

Allo

wab

le st

ress

(MN

/m) 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 C

reep

Str

engt

h

HK40 Microalloys


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


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

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


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)

Tube

Dia

met

er (I

n)

Good Tube Tube with Creep Damage WWW.GBHENTERPRISES.COM

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


AimReformer Tube DesignLarsen Miller PlotLarsen Miller & Tube DesignDesign Margins - Stress Data Used Max Allowable & Design TemperatureTube LifeEffect of Temperature on LifeMaterial TypesMaterial TypesHK40 : 25 Cr / 20 NiHP Modified : 25 Cr / 35 Ni + NbMicroalloy : 25 Cr / 35 Ni + Nb + TiAlloy DevelopmentsComparison of AlloysManufacturing MethodologyManufacturing MethodologyWeldsFailure Mechanisms - CreepCreep PropagationCommon Failure ModesUncommon Failure ModesFailure By CreepCreep Rupture - Cross SectionFailure at WeldActions to Take if Tube FailsPigtail NippingInspection TechniquesClassification of ProblemsVisual ExaminationGirth MeasurementUltrasonic AttenuationUltrasonic AttenuationUltrasonic AttenuationRadiographyEddy Current MeasurementLOTISTM Tube InspectionLOTISTM Tube InspectionLOTISTM Tube InspectionLOTISTM Tube InspectionLOTISTM Tube InspectionLOTIS Compared to External InspectionSlide Number 44

steamreforming-tubedesign-130924143050-phpapp01.pdf

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