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Essentials of Stress Intensification Factor (SIF)

by Engineering Geek Expert in industrial engineering including piping,
The design and engineering of a typical piping system is a primary concern for engineers. As part of it, they need to consider the Stress Intensification Factor (SIF) - a crucial aspect in designing the piping systems as per the American Society of Mechanical Engineers (ASME) Section III Code. 

However, most of the analysis methods related to SIFs are conservative. It results in an expensive inspection and modification of a piping system design.

Read on as this article would highlight the essentials about SIF, including its background, formula, checklist, and more.

Stress Intensification Factor in Piping Systems

A typical piping system of any facility contains the combination of pipes and different fitting components. Such components include tees, bends, O’lets, and more that connect two different pipes as per the requirement.

When engineers perform piping stress analysis, they can use simple beam theories for straight pipes. However, such methods do not help to reflect the behavior of the piping fittings as they vary in thickness, shape, cross-sections, and other terms. Hence, the Stress Intensification Factor came into consideration to evaluate the stress limits of such piping systems.

SIF Formula

SIF is the basis of most piping stress analysis processes. Engineers can calculate SIF using different methods, including FEM (Finite Element Methods) techniques, Analytical Methods defined by Piping Codes, and more.

Stress Intensification Factor is:

SIF = Actual Peak Stress in Piping Component Part / Nominal Stress in Piping Component Part

Let’s take an example to measure SIF.

We consider an unreinforced fabricated Tee junction. We have the below data:

Header outside diameter (Dh)= 12.75”
Header nominal thickness (T)= 0.375”
Branch outside diameter (Db)= 10.625”
Branch nominal thickness (t)= 0.365”
Mean radius of the header (r2)= 6.1875”

We have to calculate both in-plane SIF and out-plane SIF here. However, before that, we have to analyze flexibility characteristics (h) and flexibility factor (k) as per the ASME B31 code.

h= T/r2 = 0.375/6.1875= 0.06
k= 1 (as per ASME B31 code)
Out-plane SIF i0= 0.9 / h2/3 (as per ASME B31 code) = 0.9/0.062/3 = 5.83
In-plane SIF i1= 0.75 * i0 + ¼ = 0.75 * 5.83 + 0.25 = 4.62

SIF Checklist

Piping designers and engineers should calculate the accurate Stress Intensification Factor in certain situations. Below is the checklist, which contains a combination of such circumstances.

  1. If there is a leak in the contained media, a risk of significant problem arises.
  2. When you use the existing rules, the calculated stress at intersection results, 40% more than the allowed.
  3. The design cycles are more than 1000, and the process fluid is extremely corrosive.
  4. SIF used from B31 codes can be more conservative when the ratio of branch diameter (d) to head diameter (D) is less than 0.5.
  5. The combination of pressure cycling with external load cycling is significant.
  6. Poor weld quality of single side, along with reinforced pads and unreinforced welded intersections replacing welding tees.
  7. If the header diameter (D) to thickness (T) ratio is higher than 50, hillsides or laterals are used.

Conclusion

The SIF calculation is crucial in any project that contains piping stress analysis. It can be obtained through a straightforward methodology. You should compare the use of SIF and flexibility with the original model. If the unique model is more conservative, you should validate a more accurate solution.

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About Engineering Geek Junior   Expert in industrial engineering including piping,

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Joined APSense since, November 20th, 2019, From Seattle, United States.

Created on Feb 19th 2020 00:38. Viewed 435 times.

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