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STRESS INTENSITY FACTOR (SIF) FOR SPECIAL GEOMETRIES IN PIPING STRESS ANALYISIS

Stress Intensity Factor (SIF) according to ASME B31J for special geometries not covered in ASME B31.1 and ASME B31.3 codes

Piping Design is a critical component of engineering projects and process plants.  In order to get safety pipe systems, the flexibility analysis (stress analysis) is essential, because the pipe stress analysis predicts stresses in the pipeline due to thermal gradients, weights and pressures. 

Stress intensification Factors (SIF) are  parameters that allow the designer to estimate the maximum stresses in the pipe line and the fatigue failure in a piping component or joints. Following paragraphs explain the determination of the stress intensification factors (SIFs) for special geometries not covered by ASME B31.1 or B31.3 codes.

WHAT IS THE STRESS INTENSIFICATION FACTOR?

The stress Intensification Factor (SIF) is a multiplier factor on nominal stress for typically bends and intersection components so that the effect of geometry and welding can be considered in a beam analysis (as for example calculations performed with CAESAR II software). The stress intensification factor is used in a pipe stress analysis as follow:

                       (Beam stress)· (SIF) ≤ Allowable Stress

ASME piping codes (ASME B31.1 and ASME B31.3) stablish the formulation to obtain the Stress Intensification “I” and flexibility factor “k” for fitting geometries according to ASME B16.9.

Table D300 - ASME B31.3 - Examples of Stress Intensification Factor formulation for some geometries.

STRESS INTENSIFICATION FACTOR (SIF) FOR SPECIAL FITTING GEOMETRIES

For special geometries (such as valves, strainers, anchor rings, bands, etc.) or modified geometries from ASME B16.9  codes (as for instance trimmed elbows), it is necessary to obtain a valid stress intensification factor in order to avoid problems due to flexibility analysis. These values of SIF are out of the scope of design codes as ASME B31.1 or ASME B31.3. However, ASME B31.J code provides the standard method for determining the stress intensification and flexibility factor required.

According to ASME B31.J, there are two ways in order to obtain SIFs:

  • Real test specimen with the real geometry.
  • Virtual test specimen, FEM analysis.

The advantage of the virtual test specimen (FEM analysis) is obvious because a real test specimen (especially for larger diameters) can be very expensive compared with the finite element analysis.

ANALYSIS METHODOLOGY - STRESS INTENSIFICATION FACTOR FEM ANALYSIS

FEM analysis will allow to simulate the standard test method for determining stress intensification factors (i-factors) for metallic components as per ASME B31J and stress-life (S-N) fatigue model per Markl fatigue test curve.

Following steps summarizes analysis methodology to determinate stress intensification factor “i”:

Steps to obtain the stress intensification factor

CADE STUDY CASE - FEM SIF CALCULATIONS ACCORDING TO ASME B31.J AND PIPE FLEXIBILITY CALCULATIONS

CADE is pleased to have participated in many relevant worldwide projects where the bi-directional calculations (between FEA analysis and pipe flexibility analysis with CAESAR II software) are required. A correct evaluation of the stress intensification factors in special geometries reduces the risk of failure in the pipeline, therefore, this part of the calculation must be essential in the flexibility stress analysis. Nowadays, most of c piping specifications require this calculation.

Following images show an example of project conducted by CADE, which results obtained in the determination of the stress intensity factor (SIF) in a modified geometry (trimmed elbow) was used in the flexibility calculations performed with CAESAR II software.

Trimmed elbow test specimen results. FEM analysis.
Table. SIF Comparison Trimmed vs Untrimmed elbow.
Caesar II Pipe flexibility analysis with the SIF obtained in the trimmed elbow.

SUMMARY AND CONCLUSIONS

  • Stress Intensification Factor (SIF) is a multiplier factor on nominal stress obtained in a beam analysis (as for instance, calculations performed with CAESAR II software).
  • ASME B31.1 and ASME B31.3 codes stablished the stress intensification factor (SIF) for geometries show in ASME B16.9 code (normal geometries).
  • For special or modified geometries (such as valves, strainers, anchor rings, bands, trimmed elbows, etc.) the stress intensification factor shall be calculated according to a specimen show in ASME B31.J code which stablish two ways in order to obtain the stress intensification factor: real test specimen in a laboratory or virtual test specimen with a FEM analysis.
  • FEM (Finite Element analysis) is the cheapest option to simulate the test specimen mandatory in ASME B31.J in order to obtain a real SIF.
  • CADE engineering is the best option to perform these types of multidisciplinary analyses, because it has the engineering knowledge (with more than a thousand projects in more than 15 years of activity), highly skilled and committed team of engineers, together with a solid business management, which make CADE an independent leading engineering and consultancy company around technology, equipment and plants within oil&power industries.