Consideration of Flanged Elbow or Flanged Bend in Stress Analysis

What is a Flanged Elbow?

Flanged pipe bends or flanged elbows are piping elbows connected with flange fitting to fitting. This means there is no spool or pipe piece in between the elbow endpoint( weld point) and flange. Flanged elbows are widely used for piping connections requiring compact pipe routing. Flanged elbows are purchased as a fitting. On the other hand, flanged bends are prepared when a flange (weld neck or any other type) is directly welded to a piping elbow.

Types of Flanged Elbows

Based on the number of flanges connected to the pipe elbow fitting, flanged bends are of two types:

  • Single Flanged Elbow and
  • Double Flanged Elbow

When one side of an elbow is connected to a flange but the other side is connected to pipe it is called single flanged elbow. Again when there are flanges on both sides of the elbows, it is called double flanged elbow. Refer to Fig. 1 for typical example.

Single and Double Flanged Elbow
Fig. 1: Single and Double Flanged Elbow

Effect of Flanged Elbow or Flanged Bend

When the flanges are attached near elbows or bends (Near Control Valve assembly and equipment nozzles); they exert a severe restraining force to the flexibility of the bend, thus reducing the flexibility of the bend and increasing the force and moment in the nearby support or nozzle. ASME B 31.3 Code provides two correction factors C1 and C2 to take care of the same effect. So basically the single flange and double flange option provides the following impacts in a piping system:

  • Stress Intensification and Bend Flexibility changes
  • Load values in the nearby support changes
  • Calculated Force and Moment values at nearby nozzles increases

Flanged Bend Modeling in Caesar II

In Caesar II software the effect of the flanged bend can be easily taken care of by modeling flanged elbows as mentioned below.

Sometimes dummy attachments at elbow also provide a similar effect that is why a few organizations have the practice of using flanged elbow while modeling the trunnions from the elbow.

Single or Double Flange Option should be applied to Stress Analysis if there is any flanges or valves (heavy/rigid body) within two diameters of the ending weld point of the bend as shown in Fig. 2.

Criteria for using Flanged Bend in Caesar II
Fig. 2: Criteria for using Flanged Bend in Caesar II

The modeling steps for the flanged bend are shown in Fig. 3. Single flanged bend is indicated by selecting 1-Single Flange as shown in Fig. 3. Similarly Double Flanged Bend can be indicated by selecting 2-Double Flange option.

Caesar model showing the flanged bend application criteria
Fig. 3: Caesar model showing the flanged bend application criteria
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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