Part 7 - The basics of Shell and Tube Heat Exchanger

Understanding Thickness - Tmin, Tselect and TMAWP

Author

Syed Mohd Idrus
October 20, 2023

Part 7 - The basics of Shell and Tube Heat Exchanger

Understanding the difference between Tmin, Tselect, TMAWP. Before we move forward on the rest of the calculations of the heat exchangers, let us understand the difference between Tmin, Tselect, TMAWP

  • Tmin and its relation with Design pressure, DP

  • Tselect and its relation with Maximum Allowable Pressure, PMAP

  • TMAWP and its relation with Maximum Allowable Working Pressure, PMAWP

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TODAY'S PERSPECTIVE

Tmin and its relation with Design pressure, DP

  • What Is It? Think of Tmin as the thickness required as per design pressure without considering corrosion allowance. Therefore Tmin is smaller than actual thickness of the plate that we are going to purchase. Check here for calculation example.

  • Why It Matters: Engineer needs to understand that we don’t usually run our equipment at Design Pressure (We operate it at Operating Pressure), the calculated thickness (at Design Pressure) is a safe margin.

  • When it comes to procurement of the plate though, remember to take into account the Corrosion allowance and Mill allowance.

Tselect and its relation with Maximum Allowable Pressure, PMAP

  • What Is It? Tselect is thickness of the plate that has taken into consideration the Corrosion allowance and mill allowance. Therefore it will be thicker.

  • Why It Matters: Engineers need to understand that once we purchase the plate (at Tselect), we will need to recalculate the Pressure that the thickness can withstand. And this pressure is what we called as Maximum Allowable Pressure.

TMAWP and its relation with Maximum Allowable Working Pressure, PMAWP

  • What Is It? TMAWP is also called as Thickness at corroded condition. Therefore TMAWP = Tselect - Corrosion Allowance - Mill Allowance. Recalculate the pressure will give us Maximum Allowable Working Pressure.

  • Why It Matters: This is significant because, when our heat exchangers is running at a pressure that is equivalent to Maximum Allowable Working Pressure (higher than Design), our comfort level will rely upon answering the question — Is our corrosion allowance still intact? If yes, we are OK. If not, we will probably need to Derate.

To conclude, Tselect is higher that TMAWP is higher than Tmin. Therefore this would also mean PMAP is higher than PMAWP is higher than Pdesign.

That's all!

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