High Temperature Effects on Vessel Integrity. Marc Levin, Ayman Cheta Mary Kay O Connor Process Safety Center 2009 International Symposium

Transcription

1 High Temperature Effects on Vessel Integrity Marc Levin, Ayman Cheta Mary Kay O Connor Process Safety Center 2009 International Symposium

2 Outline Motivation Basics / Basis for Pressure Vessel Design Conditions Mechanical & Metallurgical Failure Mechanisms Corrosion Failure Mechanisms Examples References Summary 2

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7 Motivation Where pressure rise is modest, but temperature rise is significant, the impact of temperature on vessel integrity becomes more important. To determine the temperature when a instrumented barrier should activate, understanding of the damage potential to the vessel vs. temperature is needed. 7

8 Motivation Some uncontrolled reactions can cause a temperature excursion without an increase in pressure Methanation Hydrogenation/Saturation Hydrocracking Some Decomposition Reactions In such systems, vessel safeguarding is not accomplished thru pressure relief devices Cannot expect pressure relief devices to open Rely on other barriers, e.g., instrumented systems with temperature sensing combined with emergency depressuring (manual or automatic) 8

9 Motivation Potential vessel failure is still a concern because vessel integrity deteriorates at high temperature Exceeding the vessel ultimate tensile strength is only 1 of many potential failure mechanisms Message: Determining the temperature where vessel damage could occur is complex; evaluating mechanical failure, such as excessive hoop stress alone, is not sufficient 9

10 Basics The ability of a vessel to maintain integrity at a given pressure also depends on the temperature - Design pressure has a coincident design temperature - Maximum Allowable Working Pressure (MAWP) has a coincident temperature rating (note: there is no MAWT) Sometimes, design temperature is based on target operating conditions, not what the vessel can take Documentation might not be readily available. Thus, it might require some digging to find the 10

11 Basis for Vessel Design Conditions Mechanical Damage to vessel condition/properties Metallurgical Changes in metal properties as a result of conditions Corrosion - Chemical or electrochemical attack as a result of its reaction with the environment Target operating conditions If design is based primarily on target operating temperature, then look for the appropriate design temperature for safeguarding vessel integrity 11

12 Additional Considerations Vessel Constituents Shell Heads Nozzles Welds If one is determining the temperature and pressure a vessel can withstand, each of these needs to be examined. 12

13 A Sampling of Failure Mechanisms Mechanical Plastic deformation (non-reversible) Damage (some common mechanisms) Chemical/Electrochemical attack - corrosion Creep - stress induced time-dependent deformation under load Erosion Fatigue repeated / fluctuating stresses, max < mat l tensile strength Fracture Embrittlement microstructural changes at high temp, H2 Thermal stresses non-uniform temperature distribution/differing thermal expansion coefficients 13

14 API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry - Section

15 Mechanical Failure Hoop (circumferential) stress Longitudinal stress Stresses on nozzles & welds 15

16 Mechanical Failure (cont d) 16

17 Metallurgical Failure Mechanisms: Selected High Temperature Cases Failure Mechanism Graphitization Spheroidization 885 F Embrittlement Sigma phase Embrittlement Creep Rupture Mat l Affected Carbon steel, 1/2Mo steel Carbon steel, low allow steels 400 series SS, Duplex SS 300 series SS, 400 series SS, Duplex SS All metals & alloys Temp Range [F] F F F F 700+ F Description Microstructure change after long-term, high temp. operation; carbide phases can decompose into graphite nodules Microstructure change where carbide phases change from normal, plate-like form to a spheroidal form; or agglomerate Metallurgical change in alloys with ferrite phase leading to loss of toughness Formation of sigma metallurgical phase leading to loss of toughness Metal components slowly and continuously deform under load (< yield stress) that can lead to rupture 17

18 Metallurgical Failure Mechanisms (cont d): Selected High Temperature Cases Failure Mechanism Thermal Fatigue Short Term Overheating Stress Rupture Dissimilar Metal Weld Cracking Mat l Affected All mat ls of construction All common mat ls of construction Ferritic (CS/low alloy) + Austenitic (300 series SS) Temp Range [F] T 200 F 510+ F Description Cyclic stresses caused by variations in temperature that can lead to cracking where movement/expansion is constrained Permanent deformation at relatively low stress levels from localized overheating, leading to bulging and rupture Coefficients of thermal expansion between ferritic steels and 300 Series SS differ by 30% or more, leading to high stress at the heat affected zone on the ferritic side. 18

19 Corrosion Failure Mechanisms: Selected Moderate-High Temperature Cases Failure Mechanism Chloride Stress Corrosion Cracking Caustic SCC High Temp. Hydrogen Attack Carburization Decarburization Oxidation Mat l Affected 300 Series SS, Ni alloys Carbon steel, Low alloy steels, 300 Series SS Carbon steel, Various alloys Carbon steel, Fe or Ni alloys Carbon steel, low allow steels Carbon steel, Fe or Ni alloys Temp Range [F] 140+ F 120+ F 450+ F F F Description Surface-initiated cracks on exposure to tensile stress, elevated temperature, and aqueous chloride Surface-initiated cracks on exposure to tensile stress, elevated temperature, and caustic H2 reacts with carbides in steel to form methane (which remains trapped) leading to cracks causing loss of strength Contact with carbonaceous mat l leads to absorption of carbon into metal Removal of carbon/carbides from steel at high temperature, leaving an iron matrix and causing loss of strength Metal converted to metal oxide Sulfidation Fe, Ni, or Cu 500+ F Reaction of metal with sulfur 19

20 Example 1: Elastic and Ultimate Tensile Stresses (API Std 530) 20

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22 Example 2: Hoop Stress vs. Creep Life 22

23 Example 3: High Temperature Hydrogen Attack Nelson Curves (API RP 941) 23

24 Example 4: Chloride Stress Corrosion Cracking Leaks in APTAC Magnedrive housing (Fall 2007 DIERS UG Presentation) Pinhole Leaks Bushings Spacer 24

25 Examination Pits Found on the ID of APTAC Magnedrive Housing Chloride SCC 25

26 Example 5: Caustic Stress Corrosion Cracking Refinery Example Caustic Wash Tower Post-weld Heat Treatment not done (temperature <150 F) Process upset 200 F Every weld in the tower cracked 26

27 Failure Mechanism Temperature Regimes 27

28 Considerations Will the vessel become permanently deformed or fail catastrophically? Some key mechanical properties, such as modulus of elasticity, yield strength, and tensile strength, reduce at higher temperatures. Will the vessel material be subjected to creep damage? See API 530 Will the vessel see any other damage (accelerated corrosion, environmental cracking,...etc.)? A materials/corrosion specialist should be consulted on a case-bycase basis. API 571 is very helpful and informative. 28

29 References API RP 571 (Dec. 2003) Damage Mechanisms Affecting Fixed Equipment in the Refining Industry API Std 579 (June 2007) Fitness-for-Service API Std 530 (Sep. 2008) Calculation of Heater Tube Thickness in Petroleum Refineries API RP 941 (Aug. 2008) Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants ASME Section II (July 2007) Boiler and Pressure Vessel Code Materials ASME Section VIII ( ) Boiler and Pressure Vessel Code Rules for Construction of Pressure Vessels 29

30 Summary: High Temperature Effects on Vessel Integrity When evaluating the impact of high temperature, note that there are many failure mechanisms that could be relevant Mechanical strength (plastic deformation) is only one aspect of vessel integrity Consult a pressure equipment integrity expert (mechanical/metallurgical/corrosion) to evaluate the effect of high temperature on a vessel 30

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