Creep Loading - CAESAR II - Help

Creep is the slow, permanent deformation of a solid material under mechanical stresses. Creep occurs at elevated temperatures during long-term exposure to high levels of constant stress below the yield strength of the material. Power boiler piping is an example of the type of piping subjected to creep loading.

Materials

At elevated temperatures, creep governs the allowable stress properties for a material. Allowable values controlled by creep, also called time-dependent allowables, are a function of duration of loads.

Default material allowables for ASME B31.3 and ASME B31.1 are based on 100,000 hours.

Creep life varies for EN-13480 materials, as shown in the following examples. CAESAR II includes creep life in the material name.

For material 1.0345S-16-100 (material number 406 in CAESAR II):

• 16 - Indicates a maximum thickness of 16 mm.

• 100 - Indicates the allowable is for a load duration of 100,000 hours, which is the default value for CAESAR II.

For material 1.0345S-16-200 (material number 468 in CAESAR II):

• 16 - Indicates a maximum thickness of 16 mm.

• The allowable is for a load duration of 200,000 hours.

If the material allowable corresponding to the load duration you need is not available, then you can create a new custom material with the required data.

Calculations

The software calculates creep stresses according to EN-13480 equation 12.3.5-1:

s5 = Pcdo/4en + 0.75iMA/Z + 0.75iMC/3Z £ fCR

where:

Pc = Calculation pressure (SUS)

MA = Resultant moment from weight and other sustained mechanical loadings (SUS)

MC = Resultant moment from thermal expansion and alternating loadings (EXP)

fCR = Hot allowable stress

In CAESAR II, creep stress (CRP) is a scalar combination of one sustained case (SUS) and one expansion case (EXP). The first two terms of the equation are the sustained stress components and the third term is the expansion stress component.

You do not need to specify additional load multipliers to implement creep according to the equation, as shown in the following Load Case Editor example. If you specify additional load multipliers, the software applies these as additional scale factors.

The software implements EN-13480 creep methodology for many of the other supported piping codes.

SIF Methodology

CAESAR II considers the single-SIF and double-SIF methodologies for EN-13480 creep. For the double-SIF method:

SbA = [(iiMi)2+(ioMo)2]1/2/Z. (due to primary loads from the sustained load case)

SbC is defined as SbA, except that it uses the range of resultant moments from the thermal expansion load case.

s5 = Pcdo/4en + SbA + SbC/3 < fCR

Load Case Editor

CAESAR II does not recommend creep load cases, so you must manually create a creep load case using the CRP stress type. You can define a creep stress range for each strain range as needed.

The following load set example contains two pressures (P1 and P2) and two temperatures (T1 and T2). T2 is in the creep range.

• Two creep load cases (L9 and L10) correspond to P1 and P2 operating conditions at T2.

• The final load case (L11) is the maximum combination case from all the CRP cases to obtain maximum stress or loads in all the creep cases.