Marc Levin, Ayman Cheta
Mary Kay O’Connor Process Safety
Center
2009 International Symposium

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

3

4

5

6

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

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

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

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 b i f th t t d “d i t t ”
10

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

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

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

API 571 – Damage
Mechanisms
Affecting Fixed
Equipment in the
Refining Industry -
Section 4.0
14

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

Mechanical Failure (cont’d)
16

Metallurgical Failure Mechanisms:
Selected High Temperature Cases
Failure
Mechanism
Mat’l
Affected
Graphitization Carbon steel,
1/2Mo steel
Spheroidization Carbon steel, low allow steels
Temp
Range [F]
Description
800-1100°F Microstructure change after long-term, high temp. operation; carbide phases can decompose into graphite nodules
850-1400°F Microstructure change where carbide phases change from normal, plate-like form to a spheroidal form; or agglomerate
600-1000°F Metallurgical change in alloys with ferrite phase leading to loss of toughness
885 F
Embrittlement
Sigma phase
Embrittlement
400 series
SS, Duplex
SS
300 series
SS,
400 series
SS,
Duplex SS
Creep Rupture All metals & alloys
1000-1750°F Formation of “sigma” metallurgical phase leading to loss of toughness
700+°F Metal components slowly and continuously deform under load (< yield stress) that can lead to rupture
17

Metallurgical Failure Mechanisms (cont’d):
Selected High Temperature Cases
Failure
Mechanism
Thermal
Fatigue
Mat’l
Affected
All mat’ls of construction
Short Term
Overheating –
Stress Rupture
Dissimilar
Metal Weld
Cracking
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

Corrosion Failure Mechanisms:
Selected Moderate-High Temperature Cases
Failure
Mechanism
Mat’l
Affected
Temp
Range
[F]
140+°F
Description
Chloride Stress
Corrosion
Cracking
Caustic SCC
300 Series SS,
Ni alloys
Surface-initiated cracks on exposure to tensile stress, elevated temperature, and aqueous chloride
Carbon steel,
Low alloy steels, 300
Series SS
Carbon steel,
Various alloys
120+°F Surface-initiated cracks on exposure to tensile stress, elevated temperature, and caustic
High Temp.
Hydrogen
Attack
Carburization Carbon steel,
Fe or Ni alloys
Decarburization Carbon steel, low allow steels
Oxidation Carbon steel,
Fe or Ni alloys
Sulfidation
450+°F H2 reacts with carbides in steel to form methane (which remains trapped) leading to cracks causing loss of strength
1100+°F Contact with carbonaceous mat’l leads to absorption of carbon into metal
1000+°F
Fe, Ni, or Cu 500+°F
Removal of carbon/carbides from steel at high temperature, leaving an iron matrix and causing loss of strength
Metal converted to metal oxide
19
Reaction of metal with sulfur

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

21

Example 2: Hoop Stress vs. Creep Life
22

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

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

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

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

Failure Mechanism Temperature Regimes
27

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-by- case basis. API 571 is very helpful and informative.
28

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

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

31

32

33