By Jörg Theis
Senior Application Engineer, Intergraph Germany
CAESAR II 2017 features the ability to evaluate piping operating within the “creep” range. The basis for this evaluation is defined in the EN 13480 code standard, but it can be used with all piping code jobs as well.
What is creep and why should I use this?
Creep is the slow, permanent deformation of a solid material under mechanical stresses. Creep occurs during long-term exposure to high levels of constant stress below the yield strength of the material. If a piping system needs to work over a long time you should evaluate this condition.
For example, in higher temperatures when you use a standard steel, the creep range starts at about 750°F/400°C, and the material begins to creep over a longer time period. For some components, such as supports with a design temperature in the creep range, a time-dependent design is recommended.
The Equation from EN 13480 (Section 12.3.5-1) is:
The (p_c d_o)/(4e_n )+ (0.75iM_A)/Z section of the equation is the Sustained (SUS) stress, where MA refers to the resultant moment due to sustained loadings, just as in EN 13480, 12.3.2-1. This calculation is exactly the same as a SUS load case in CAESAR II when you specify piping code EN 13480.
The (0.75iM_C)/3Z section of the equation is the creep evalutation, where MC is the resultant moment due to thermal expansion and alternating loads, just as it is specified in EN-13480 (12.3.4-2 or 12.3.4-2, depending on the liberal stress setting). However, the 3Z denominator of this equation relates to the code standard:
“In equation (12.3.5-1), one-third of the moment MC shall be considered with respect to the material behaviour in the creep rupture stress range, unless confirmed otherwise by detailed inelastic analysis.”
From EN 13480: The permissible stress in the creep range (time-dependent design) “fCR” is defined in Section 5 [Eq. 5.3.2-1]: fCR = SR 200,000t / 1.25 with SR 200,000t as the mean creep rupture strength according to the material standard at calculation temperature t and an observed life of 200,000h.
To perform this analysis, CAESAR II 2017 now includes an additional load case stress type, CRP. CRP is a scalar combination of one SUS and one EXP case. CAESAR II defaults the Output Type with CRP to Stress. If you specify additional load multipliers, there are additional scale factors.
For a given material, you need the allowable creep stress range for operating conditions, which is defined by the life of the material. This is usually given as 0,2 %-proof strength at the temperature for a specific time.
For this creep evalutation, additional materials were added to the CAESAR II material database. For example, one of these new materials is 1.0345S-16-200. The “200” suffix shows that this is the material entry for creep with a lifespan of 200,000 hours. While the EN code provides some higher stress for a longer lifetime, CAESAR II only adds the conservative entries. Default material allowable stress values for ASME B31.3 and ASME B31.1 are based on 100,000 hours of life.
You can use this CRP stress type addition for high temperature conditions to define a creep stress range for each strain range you need.