7/26/2019 Piping Elbow or Bend SIF (Stress Intensification Factor) – What is Piping

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7/26/2019 Piping Elbow or Bend SIF (Stress Intensification Factor) – What is Piping

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Piping Elbow or Bend SIF (Stress

Intensification Factor)  April 28, 2016   1 Comment (http://www.whatispiping.com/bend-sif#comments)

 

The term SIF is tooconfusing for many

of the pipingengineers. Thatswhy the followingarticle is taken toexplain the bendSIFs in simplelanguage.Every pipingEngineer whopossess a little basicof Piping Stress

Analysis theorymust be aware of the term SIF orStressIntensification Factor. The term indicates a multiplier of Bending andTorsional stresses. This Intensifier acts local to a piping Component ( tees,elbows, bends, Olets ,etc) and Its value depends on component geometry.The minimum value of SIF is 1.0. It is widely used by piping stress engineersin places where the actual stress calculation is quite difficult due to itsdifficult geometry (Varying thickness, cross section, curvature etc) as unlikestraight Pipes the simple Beam theory is not applicable. So in this situation it

is required to assume additional stresses by suitably incorporating a SIF. Thefollowing article will provide an example of SIF calculation of piping elbow orpiping bends following process piping code ASME B31.3.

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7/26/2019 Piping Elbow or Bend SIF (Stress Intensification Factor) – What is Piping

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(http://www.whatispiping.com/wp-content/uploads/2013/09/SIF.jpg)Stress Intensification Factor for a Piping Bend/Elbow:

In layman’s language the SIF of a bend or elbow can be defined as theratio of bending stress of an elbow to that of straight pipe of samediameter and thickness when subjected to same bending moment.Whenever the same bending moment is applied to a bend because of ovalization the bending stress of the elbow will be much higher thanthat of strainght pipe. That is why the SIF value will always be greaterthan or equal to 1.0 (for straight pipe).

The process piping code ASME B 31.3 provides a simple formula tocalculate the SIF of a bend or elbow. As per that code

SIF in-plane = 0.9 / h^(2/3)SIF out-plane = 0.75 / h^(2/3)Here h=T R1 / r2^2h =Flexibility characteristics, dimensionless

T =Nominal wall thickness of bend, inR1 =Bend radius, inr2 =Mean radius of matching pipe, inThe inplane and outplane concept for a bend can be obtained from theattached figure from code or in layman’s language the same can beexplained as follows:The in-plane bending moment is the bending moment which causes elbowto close or open in the plane formed by two limbs of elbow.In a similar way the out plane bending moment can be defined as thebending moment which causes one limb of elbow to move out of the planekeeping other limb steady.

From the above mentioned equations the following can be interpreted:For the same pipe size and same pipe thickness1. A short radius elbow is having more SIF as compared to a long radius

elbow.2. With increase in bend radius the SIF decreases and finally reaches to

1.0 for straight pipe.3. The SIF for a 45 degree elbow and a 90 degree elbow is same as bend

radius is same.4. With increase in nominal pipe thickness or schedule the SIF of a bend

(90 degree) keeps on decreasing till its value is equal to 1.0.

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7/26/2019 Piping Elbow or Bend SIF (Stress Intensification Factor) – What is Piping

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piping stress (http://www.whatispiping.com/category/piping-stress)

piping stress analysis (http://www.whatispiping.com/category/piping-

stress-analysis)SIF (http://www.whatispiping.com/category/sif)

SIF of a bend (http://www.whatispiping.com/category/sif-of-a-bend)

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3. Major Stress related differences in Between 2012 edition and 2010edition of ASME B 31.3(http://www.whatispiping.com/differences-in-between-2012-edition-and-2010-edition-of-asme-b-31-3)

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