Transcript Unusual Failures in Hydrogen Production

Unusual Failures in
Hydrogen Production
Sheldon W. Dean
Dean Corrosion Technology, Inc.
Allentown, PA
May 17,2005
Hydrogen is New Opportunity
• Ideal fuel for fuel cells.
- Exhaust only water.
- Energy conversion is efficient.
• Fuel for engines.
- No hydrocarbons or CO in exhaust.
- NOX can be minimized.
• Other uses:
- Removal of S, N, etc. from fuel.
Steam-Hydrocarbon
Reforming
• Largest source of hydrogen today.
• Methane (natural gas) feed.
• Higher hydrocarbons can be used.
• High temperature-catalytic process.
• Used for more than 50 years.
Steam-Hydrocarbon
Reforming
• Reforming reaction:
CH4 + 2H2O  CO2 + 4H2
Typical conditions: 1650ºF (900 ºC) 600psi with
excess H2O.
• If higher hydrocarbons are present:
CxHy + (2X - ½Y)H2  XCH4
This step is run before reforming.
Typical Reforming Process
1.
2.
3.
4.
5.
6.
Pre-reforming (hydrogenation)
Steam introduction
Reform over catalyst
High temperature shift reactor (HTS)
Low temperature shift reactor (LTS)
Cooling and water condensation
Materials Problems
Involving Corrosion
• HTS inlet piping: Cracks in bypass:
(KOH SCC ).
• LTS exit piping: Cracks in mixing tee
(corrosion fatigue).
HTS Inlet Bypass Cracking
BACKGROUND
• Large steam hydrocarbon reformer.
• Feed: CH4 and some higher HCs.
• Potassium promoted reforming catalyst.
• Start-ups run with nitrogen then steam.
• HC feed added when plant temp. OK.
Background Continued
• HTS designed with bypass for start-up
acceleration.
• HTS piping: 1¼% Cr, ½% Mo steel.
• Bypass: 304 SS because of possible
condensation during operation (carbonic acid
corrosion concern).
• Operating temp: 850 ºF (455 ºC).
HTS Cracking Incident
• Many start-ups in first 6 months (>10).
• Then 2 months continuous operation.
• HTS by-pass split open and tore off suddenly!
• Shock wave and fire resulted.
• Explosion heard 30 miles away.
• Repairs required 3 months to finish.
Diagram of By-pass
To HTS
1¼Cr,1/2Mo
By-pass 304L
Location of cracks
Failure Analysis
HTS By-pass
• Many cracks found on interior of 304L.
• Pipe interior has black oxide.
• Cracks show white halos.
• Samples of pipe with crack taken.
• Metallographic sections examined.
Photo of Pipe Interior
Metallographic Section
Failure Analysis Continued
• Cracked section broken open.
• SEM examination of fracture surface.
• Energy dispersive X-ray analyses of fracture
surfaces.
Crack Fractograph
EDS Analysis
K
Results of Failure Analysis
• Cracks are transgranular and branched.
• Cracks are typical of caustic SCC.
• No evidence of sodium present but potassium
is widespread.
• Residual stress in pipe from cold forming.
Conclusion
• Pipe failure caused by KOH SCC.
• Surprising because SCC usually causes leak
before break in 304L; Catastrophic SCC
failures are rare!
• Questions: Where did KOH come from?
• How did it survive in by-pass but did not crack
1¼% Cr, ½% Mo steel pipe?
Investigation of
Source of KOH
• Transfer line refractory not source.
• Waste heat boiler not source.
• HTS catalyst not source.
• Reforming catalyst is source:
– Potassium promoted catalyst.
– Depletion of K found in catalyst.
Source of KOH
• Potassium carbonate added to catalyst.
• Carbonate decomposes in hot steam:
K2CO3 + H20  2KOH + CO2
• KOH vaporizes during start-up when hot:
– 1788ºF (976ºC) V.P. 40 torr
• KOH deposits on cool bypass.
Laboratory Study
• Purpose to define when KOH SCC occurs.
• Use ASTM G 129 Slow strain rate method
(SSR).
• Because water partial pressure fixed only one
variable (temperature) available.
KOH Lab Study
• Temperatures: 370, 420, 550ºF (188, 216,
288ºC).
• 304L, 1¼%Cr, ½%Mo steel specimens.
• KOH concentrations calculated for each
temperature.
• Head space had H2 added in some tests.
• K2CO3 added in some tests to 90%.
Results of Lab Study
• 304L specimens did not fail until 50 psi H2 or
CO added to head space.
• 304L OK at 370ºF, failed at higher
temperatures.
• 1¼% Cr, ½% Mo steel failed at
370
º,420 º but not at 550 ºF.
• 90% K2CO3 caused no change.
Recommendations
• Redesign by-pass with free draining 1¼% Cr,
½% Mo steel material.
• Questions for further research:
Why is hydrogen necessary for SCC ?
Is there an upper temp limit for 304L?
Are other alloys better for piping?
Mixing Tee Cracking
Background
• Large hydrocarbon reforming plant.
• Feed: refinery off gas (mainly methane).
• Condenser used as boiler water pre-heater.
•By-pass used to prevent boiling in BWPH.
Mixing Tee Incident
• Plant operated for about 3 years.
• Leak noted in mixing tee at exit of BWPH.
• Plant shut down and inspected internally.
• Extensive cracking found in tee and down
stream, also in expander.
315°F
Concentric Cone
Heads
220ºF
BOILER WATER PREHEATER
Control Valve
By-pass Line
Location of cracks
Expander
Failure Analysis
• Most cracks associated with welds.
• Metallography: some cracks straight, others
branched.
• Some cracks showed beach marks.
• Fracture surface quasi-cleavage.
• EDS shows faint Cl and S on fracture surface.
Investigation
• Several older plants had similar cracks.
• Found in hydrocarbon reformers (not natural
gas feed plants).
• Upgrade to 316L or Alloy 20 not successful.
•Upgrade to 625 successful.
Laboratory Study
• ASTM G 129 SSR tests.
• Cyclic straining tests:
– 0.3% offset pre-strain,40.7 to 13.7ksi stress range.
– 2000 cycles @ 1 Hz.
– Potential control, Ag/AgCl ref. electrode.
• 304L, 2205, 625 specimens.
Results: 304L
• No cracking with SSR tests.
• No cracking with cyclic straining w/o
electrochemical polarization.
• Cracking >200mV, 10ppm Cl-, 150ºC.
• Cracking >200mV, 1ppm Cl-, 1ppm NaCNS,
150ºC.
Results: 625 & 2205
• No cracking in cyclic straining tests up to
+600mV.
• No cracking in SSR tests.
Conclusions
• 304L shows synergistic cracking in water with
Cl- and potential > 200mV.
• SCN- aggravates chloride cracking.
• Both 625 and 2205 resist cracking.
Recommendations for
Plant Design
• Minimize thermal cycling of equipment.
• Use resistant materials (2205 or 625) where
thermal cycling occurs.
• Avoid concentric cone heads on heat
exchangers where condensation occurs.
Issues for Further Study
• What are the effects of other impurities in hot
water on corrosion fatigue?
• What role does corrosion play in the cracking
process?
• What other alloys resist corrosion fatigue?
• Can we predict cracking susceptibility?
THANK YOU!
Acknowledgement
• Air Products and Chemicals, Inc.
• Intercorr International Inc.
• W. R. Watkins
• K.L. Baumert
• J. W. Slusser