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)

100,000

50,000

10,000

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

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

ife (H

+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

Temperature °C

/m²) hk40

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

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

HK40 Microalloys

Tube Material

HK40 IN 519 HP Nb Mod HP Microalloy

Tube Material

(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)

–visual examination

– tube diameter (or circumference) measurement

–ultrasonic attenuation

– radiography

–metallurgical examination

–LOTISTM

Exposure Time

Damage Corresponding

Parameter Action in Plant A - observe B - observe, fix inspection intervals C - limited service until replacement D - plan immediate replacement

Rupture

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

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

• 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

Axial Position (In)

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

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