Pressure design of Miter Bend (17th Session of ASME B31 3)

🌟 Welcome to the 17th Session of ASME B31.3 Course! 🌟
👋 Hello everyone! I'm Ali, and I hope you're all doing well. Today, we're diving into the pressure design of miter bends. Let's break down the details!
🔧 What is a Miter Bend?
Miter bends are used to change pipe direction by fabricating multiple pipe segments.
Though not popular in process piping designed per ASME B31.3, there are some common misconceptions about their usage.
📘 Clarifying Miter Bend Usage
It's often said that miter bends are not allowed or only used in utility services. This isn't entirely accurate.
Always verify claims in piping design by referring to the relevant Code, standard, and specification.
📏 ASME B31.3 Requirements
According to paragraph 304.2.3, there are specific requirements for using miter bends.
Fig. 304.2.3 provides the nomenclature for calculating the maximum allowable internal pressure (Pm) for miter bends.
🔍 Key Definitions
Single Miter Bend: Made from 2 segments (1 cut).
Multiple Miter Bend: Made from 3 or more segments (2 or more cuts). We'll focus on multiple miter bends.
🧮 Calculating Maximum Allowable Pressure (Pm)
Based on Clause (a) of Para. 304.2.3, calculate Pm using Eqs. (4a) and (4b). The lesser value is used for comparison with the design pressure.
If Pm is less than the design pressure, consider:
Increasing the number of cuts.
Increasing the radius of the bend.
Increasing the pipe thickness.
Using an elbow instead of a miter bend.
🔧 Example Calculation
For a 90° multiple miter bend with Sch. No. 80 pipe, θ = (90/2)/2 = 22.5°, making our calculations valid.
Using Excel, we find Pm = 919 psi for a miter with 2 cuts.
📉 Challenges with Miter Bends
The calculated Pm (919 psi) is significantly lower than the design pressure (1380 psi).
Increasing the number of cuts to 8 makes fabrication difficult and costly.
Alternatively, using thicker pipe is less efficient compared to factory-made long radius elbows.
🔎 Practical Insights
It's often more practical to use long radius elbows due to ease of fabrication and cost efficiency.
Skilled welders and precise calculations are essential for miter bend fabrication.
Miter bends can cause higher pressure drop, turbulence, and corrosion risks, which must be carefully considered by designers.
🎓 Conclusion
Miter bends have limited usage due to pressure design constraints.
In our next session, we'll explore the strength of branch connections based on ASME B31.3.
Thank you for joining this session! Stay tuned for more insights in our next episode. 🙌
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