Creep/Stress Rupture | Materials and Corrosion Control
| Damage Mechanism | Creep/Stress Rupture |
| Damage Description
|
· At high temperatures, deformation of stressed components under loads below yield stress.
· Deformation leads to damage that may eventually lead to a rupture. |
| Affected Materials | · All metals and alloys. |
| Control Methodology | · Generally, an increase of about 25°F (12°C) or an increase of 15% on stress can cut the remaining life in half or more, depending on the alloy.
· Heater tubes in fired heaters are especially susceptible as well as tube supports, hangers and other furnace internals. · Creep damage is not reversible. Once damage or cracking is detected much of the life of the component has been used up and typically the options are to repair or replace the damaged component. · For Fired Heater Tubes: o Alloys with improved creep resistance may be required for longer life. o Heaters should be designed and operated to minimize hot spots and localized overheating o Visual inspection followed by thickness measurements and or diametral measurements may be required to assess remaining life of heater tubes in accordance with API RP 579. o Minimizing process side fouling/deposits and fire side deposits/scaling can maximize tube life. |
| Monitoring Techniques | · A combination of techniques (Ultrasonic Testing, Radiographic Testing, Eddy Current Testing, dimensional measurements and replication) should be employed. · Destructive sampling and metallographic examination are used to confirm damage and conduct testing to determine degree of creep damage and available remaining life · For pressure vessels, inspection should focus on welds of CrMo alloys operating in the creep range. Most inspections are performed visually and followed by Penetrant Testing or Wet Fluorescent Magnetic Particle Testing at every T&I or as advised by CSD Metallurgical Specialist or local corrosion engineer in consultation with CSD Metallurgical Specialist. · Fired heater tubes should be inspected for evidence of overheating, corrosion, and erosion as follows: o Tubes should be VisualTesting examined for bulging, blistering, cracking, sagging, bowing & diametral expansion. o Wall thickness measurements of selected heater tubes should be made where wall losses are most likely to occur. o Calculation life assessment per API STD 530 taking into account both creep and metal loss o H-Scan or Combined Eddy Current/Diametral/ Thickness/Ultrasonic Testing or Laser scanning coupled with probabilistic life assessment for hydrogen or steam reformers (high-alloy material) |
| Inspection/Assessment Frequency | Most inspections are performed visually and followed by Penetrant Testing or Wet Fluorescent Magnetic Particle Testing at every T&I or as advised by CSD Metallurgical Specialist or local corrosion engineer in consultation with CSD Metallurgical Specialist |
| KPIs | · Number of visual inspections carried out
· Number of NDT inspections carried out · Number of metallographic replication and hardness checks carried out · Number of life assessments per API STD 530 carried out · Life fraction consumed · Ratio service life/design life · H-Scan or Combined Eddy Current/Diametral/Thickness/ Ultrasonic Testing or Laser scanning coupled with probabilistic life assessment for hydrogen or steam reformers · Tube diametral expansions limited to 4% for hydrogen or steam reformers (high-alloy material) and 6% for crude, vacuum and platformer/rheniformer (carbon or low-alloy steels) |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #3)
· API STD 530 · API RP 579 Fitness-For-Service · API STD 660, Shell and Tube Heat Exchangers for General Refinery Service |