When designing pressure vessels, one of the critical factors engineers must consider is the creep behavior of materials. Creep is the tendency of a material to deform permanently under the influence of constant stress at elevated temperatures over extended periods. This phenomenon is especially important in high-temperature applications, such as those involving steam, gas, or other industrial processes that operate at temperatures above the material’s normal design limits. Ignoring creep can lead to premature failure of pressure equipment, making it crucial for engineers to account for it during the detailed phase of pressure vessel design.
Various international codes provide guidelines on how to incorporate creep considerations into pressure vessel evaluations. In this article, we will explore how ASME VIII Division 1, EN 13445, PD 5500, and AD 2000 address the creep behavior of materials and the methods employed to ensure long-term integrity.
What is Creep?
Creep is the time-dependent deformation of materials under stress. At elevated temperatures, materials experience a steady increase in strain, even under constant load, which is different from the elastic deformation that occurs at lower temperatures. This deformation is most pronounced at temperatures greater than approximately 30% of the material’s melting point. For materials used in pressure vessels, creep becomes especially important when they are subjected to sustained high temperatures for long periods, as is common in industries like power generation and petrochemical processing.
Creep Considerations in Pressure Vessel Codes
Different pressure vessel codes have varying approaches to how creep is factored into the design of a pressure vessel. Let’s take a closer look at how four major codes – ASME VIII Division 1, EN 13445, PD 5500, and AD 2000 – deal with creep.
1. ASME Boiler and Pressure Vessel Code (ASME VIII Division 1)
The ASME Boiler and Pressure Vessel Code (Section VIII Division 1) is one of the most widely used standards for the design of pressure vessels. It provides a comprehensive set of rules for material selection, design, and testing. While ASME VIII Division 1 does not explicitly address creep in its general design rules, it does provide some mechanisms for incorporating creep in design.
Creep in Material Allowable Stress Tables: In ASME Section II, Part D, material stress data is provided for various materials. When allowable stresses are presented in italics, it indicates that the material’s creep rupture strength has been taken into account for 100,000 hours (about 11.4 years) of operation at a given temperature. This allows engineers to assume that creep behavior is inherently accounted for in the material’s stress limits for the specified operational time and temperature.
2. EN 13445 (European Standard for Unfired Pressure Vessels)
The EN 13445 standard is the European equivalent of ASME VIII and provides specific guidelines for unfired pressure vessels. This standard takes a more explicit approach to creep behavior than ASME.
Creep Allowable Stresses Formation: EN 13445 provides detailed creep rupture curves and allowable stress formulas for a variety of materials. These curves represent the creep rupture strength of materials at various temperatures and times. The designer must use these curves to calculate the creep allowable stresses for the material, based on both the expected temperature and the operating time of the pressure vessel.
3. PD 5500 (British Standard for Pressure Vessels)
The PD 5500 standard, which is widely used in the UK, addresses creep in a similar way to EN 13445, but with some differences.
Creep Allowable Stresses Tables for US Materials: When using US materials listed in PD 5500, the creep allowable stresses are typically predefined based on material specifications and service life. The code provides these values for various materials, and the designer can use them directly without needing to calculate the creep effects manually. The creep stresses are defined for expected operating life durations (such as 100,000 hours).
Creep Allowable Stresses Formation for European Materials: For European materials, however, PD 5500 requires designers to calculate the creep allowable stresses using creep rupture curves and material-specific data similarly to EN13445. This ensures that the material selected is appropriate for the expected service conditions, particularly for high-temperature applications where creep is a significant concern.
4. AD 2000 (German Pressure Vessel Code)
The AD 2000 standard, commonly used in Germany and other parts of Europe, provides another approach for managing creep considerations in static equipment design.
Creep Allowable Stresses Formation: AD 2000 provides creep rupture limits and material-specific data, which are used to calculate the creep allowable stresses. Like EN 13445 and PD 5500 (European materials), AD 2000 requires a detailed creep analysis for materials exposed to high temperatures over extended periods.
Conclusion
Creep is a crucial consideration in the design of pressure vessels subjected to high-temperature environments. As pressure vessels are often used in industries where elevated temperatures are commonplace, engineers must ensure that the materials selected can withstand the stresses imposed by creep deformation over the vessel’s service life. Different design codes such as ASME VIII Division 1, EN 13445, PD 5500, and AD 2000 offer varying approaches for factoring in creep into design of pressure vessels.
ASME VIII Division 1 makes it easier for designers by assuming that creep behavior is automatically considered for 100,000 hours of operation in certain materials.
EN 13445, PD 5500, and AD 2000 provide more explicit guidelines for calculating creep allowable stresses, especially when the expected service life exceeds 100,000 hours or involves complex high-temperature conditions.
By understanding how each code approaches creep and following the appropriate guidelines, engineers can design safer and more efficient pressure vessels that will maintain their integrity over the long term. A new software for pressure vessel analysis called VCLAVIS.com supports engineers in correctly calculating the creep allowable stresses, especially in the European Codes, and provides proper guidance via a step-by-step procedure, ensuring accurate and compliant designs tailored to the specific operational conditions of each vessel.
