Presentation on theme: "Unusual Failures in Hydrogen Production Sheldon W. Dean Dean Corrosion Technology, Inc. Allentown, PA May 17,2005."— Presentation transcript:
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. -NO X 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: CH 4 + 2H 2 O CO 2 + 4H 2 Typical conditions: 1650ºF (900 ºC) 600psi with excess H 2 O. If higher hydrocarbons are present: C x H y + (2X - ½Y)H 2 XCH 4 This step is run before reforming.
Typical Reforming Process 1.Pre-reforming (hydrogenation) 2.Steam introduction 3.Reform over catalyst 4.High temperature shift reactor (HTS) 5.Low temperature shift reactor (LTS) 6.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: CH 4 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 1¼Cr,1/2Mo To HTS 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.
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: K 2 CO 3 + H 2 0 2KOH + CO 2 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 H 2 added in some tests. K 2 CO 3 added in some tests to 90%.
Results of Lab Study 304L specimens did not fail until 50 psi H 2 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% K 2 CO 3 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.
BOILER WATER PREHEATER Control Valve By-pass Line Concentric Cone Heads Expander Location of cracks 315°F 220ºF
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