BS806 - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

For BS806, the maximum hot stress case is considered to be the operating load case. Operating load case allowables are only given as per BS806 when the creep rupture strength governs the stress range allowable. See BS806 sect 4.11.2.

Stress Intensification Factors (SIFs) using BS806

BS806 SIFs printed are labeled fti and fto for bends, and Bi and Bo for intersections.

Calculate pressure stiffening using BS806

Pressure stiffening is not defined by default in the code. You can include pressure stiffening on bends in the analysis by including the Use Pressure Stiffening option in the setup file.

Pad thickness using BS806

There is no limit in BS806 for the beneficial effect of the pad on an intersection. Most codes limit the pad thickness to 1.5 times the header thickness. For BS806, CAESAR II does not limit the pad thickness.

Flanged end modifications using BS806

The code permits modifications due to flanged ends for all bend types. This includes closely and widely spaced mitered bends.

BS806 girth butt welds default value

The default SIF value for a girth butt weld is 1.0. This is also Markl’s original basis for SIFs.

Calculate the BS806 allowable stress limits

Use the equations below to calculate the stress allowables.

Expansion Allowable = lesser of (H)(Sc)+(H)(Sh) <or> (H)(Sc)+F

Sustained Allowable = Sy

Occasional Allowable = (Sy)(Occ)

Operating Allowable = S avg rupture at design temperature

Where:

H = Multiplication Factor 0.9 or 1.0 from CAESAR II

Sc = 0.2% Proof Stress at Room Temperature

Sh = 0.2% Proof Stress at Design Temperature

F = Mean Stress to Failure in Design Life at Design Temperature

Occ = Occasional Load Factor Default is 1.0

Calculate pressure at intersections using BS806

The pressure calculation at the intersections is made as required in BS806 4.8.5.1 Eq. (17). The pressure stress as per Eq. (17) is computed and then combined with the bending and torsional moment at each of the intersection ends 1, 2 and 3 respectively. The m factor is computed as required with a value of n=1, in other words, for non-interacting intersections. BS806 does not address reducers for SIF calculations.

Other BS806 Notes

When there is more than one thermal case to evaluate, read the following note carefully concerning CAESAR II and the application of BS806.

Regarding BS806 4.11.3.1 paragraph 2, for sectionalized systems: CAESAR II only makes the moment summation on a load case by load case basis, and does not take the largest moments for an axis for any combination of load cases. The CAESAR II method is designed to enable you to set up and combine the effects of each of the load transients that the piping system undergoes. This method, for the most part is used in the B31/ASME piping codes. The BS806 method is conservative in that it uses what is basically a shakedown approach and computes a single worst-case moment difference. The CAESAR II method satisfies the shakedown theory but, also computes the moment range for each different load traversed. The BS806 method of combining the maximum moment range is more conservative. The BS806 method also eliminates the need to know where on the pipe the stress is the highest. Use the moment tables in Appendix F, to get the moment difference between any two load cases. However, you cannot use the moment tables to get the maximum moment difference for any of the three moment axes as requested by the sectionalized piping rules. To satisfying 4.11.3.1(a) CAESAR II uses the moment difference between the cold and the hot case to compute the stress.

You can only enter a single modulus of elasticity for a single element in each job. Different elements can have different moduli of elasticity, but you cannot vary that modulus between load cases in the same run. Also, you cannot use a cold and a hot moduli of elasticity in the same run.

For BS806 in 4.11.5.2 the value of n is 1.0.for all branches of the non-interacting type. See the fourth paragraph 4.11.4.2 for the definition of n for interacting branches.

The CAESAR II equation modeling of the BS806 SIF curves for bends displays in the following plots.