Rated Flange Methodology

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Last updated: Dec 28, 2023 by Tiradej Bunyarattaphantu
5 min read

Last Updated: 2023.12


DesignCalcs supports ASME Off the Shelf flanges. In the software, these are referred to as rated flanges. This article will surface flange details and explore the methodology behind how the software handles the calculations for these flanges.

The Rated Flange designer uses the default settings to automatically create a passing flange when it is opened. The size is first determined and then the smallest passing class is selected.

Determine Reference

The reference is either determined from the Section VIII, Division 1 Table U-3 / Section VIII, Division 2 Table 1.1 or via direct reference:

If the reference is to be based on Table U-3 or Table 1.1, it is determined as follows:

  • Reference is B16.5 2003 - Size ≤ 24"; 2010 edition

  • Reference is B16.5 2009 - Size ≤ 24"; 2011 addenda or 2013 edition

  • Reference is B16.5 2013 - Size ≤ 24"; 2015 edition or 2017 edition or 2019 edition

  • Reference is B16.5 2017 - Size ≤ 24"; 2021 edition
  • Reference is B16.5 2020 - Size ≤ 24"; 2023 edition

  • Reference is B16.47 1996 - Size > 24"; 2010 edition

  • Reference is B16.47 2006 - Size > 24"; 2011 addenda

  • Reference is B16.47 2011 - Size > 24"; 2013 edition or 2015 edition or 2017 edition

  • Reference is B16.47 2017 - Size > 24"; 2019 edition or 2021 edition

  • Reference is B16.47 2020 - Size > 24"; 2023 edition

If the reference is to be based on direct reference, it is determined as follows:

  • Reference is B16.5 2003 - Size ≤ 24"; B16.5 Reference selection is 2003

  • Reference is B16.5 2009 - Size ≤ 24"; B16.5 Reference selection is 2009

  • Reference is B16.5 2013 - Size ≤ 24"; B16.5 Reference selection is 2013

  • Reference is B16.5 2017 - Size ≤ 24"; B16.5 Reference selection is 2017

  • Reference is B16.5 2020 - Size ≤ 24"; B16.5 Reference selection is 2020

  • Reference is B16.47 1996 - Size > 24"; B16.47 Reference selection is 1996

  • Reference is B16.47 2006 - Size > 24"; B16.47 Reference selection is 2006

  • Reference is B16.47 2011 - Size > 24"; B16.47 Reference selection is 2011

  • Reference is B16.47 2017 - Size > 24"; B16.47 Reference selection is 2017

  • Reference is B16.47 2020 - Size > 24"; B16.47 Reference selection is 2020

Assign Material Group

The material group is assigned based on the material selected and the reference.

Place Reference Notes

Based on the material selected, notes are provided in the Rated Flange window for your reference. These are currently not used to restrict the design; we strongly advise that you review them.

Determine Pressure Rating

The Pressure Rating is determined from the units of measure, reference, material group, rated flange class, and the temperature. The output is in PSI for customary units and MPa for metric units.

  1. Metric data is not included in the B16.47 1996 reference. If a metric design is used with this reference, the temperature will be converted from Celsius to Fahrenheit for the determination. The result will then be converted from PSI to MPa.
  2. If the temperature is above 1500°F (816°C), the Pressure Rating cannot be determined as the pressure/temperature ratings only go up to 1500°F (816°C).
  3. If the temperature is below 100°F (38°C), 100°F (38°C) will be used in the determination as this is the lowest temperature in the pressure/temperature ratings.
  4. Straight line linear interpolation is used between data points.
  5. It is possible that the combination of the reference, material group, and rated flange class will not have corresponding pressure/temperature ratings. If this is the case, the Pressure Rating cannot be determined.

Determine Effective Pressure

If external loads are not considered, then the Effective Pressure is determined as the sum of the Design Pressure and the Static Head.  If external loads are considered, the Effective Pressure is determined as the greater of the following (based on Code Case 2901 and UG-44(b) 2019 edition and later):

  • The sum of Design Pressure and Static Head
  • (16*ME + 4*FE*G + pi*G3*(Design Pressure and Static Head))/(pi*G3*(1 + FM))  - ME, FE and G are defined on the help page for Rated Flanges.  FM is determined from Code Case 2901 and UG-44(b) 2019 edition and later.

Compare Effective Pressure and Pressure Rating

The Effective Pressure must be less than or equal to the Pressure Rating in order for the rated flange design to pass. If the Effective Pressure is greater than the Pressure Rating, the Effective Pressure label at the bottom of the window will be displayed in red.

Reference Check

The size, class, and type are checked to make sure that the combination is included in the references. If the combination is not included, the Rated Flange will be placed in a failed status. In that case, the labels for these fields will be displayed in red. One exception to this is NPS 22 flanges; these were added in the 2017 version of B16.5. If the size is 22 and the flange otherwise passes; but the reference is earlier than 2017, it will be given a pass w/comment status. There are several combinations that are dual rated for two classes; these will be given a pass w/comment status assuming the Pressure Rating check passes.

Perform Weld Detail Calculations

This step is only performed for Slip-On and Socket Welding type rated flanges and only when the "Perform Weld Detail Calculations" checkbox is checked.  The detail requirements per ASME Section VIII, Division 1 UW-21 are used for both Division 1 and Division 2 designs at this time.  The requirements are in terms of "min" weld leg and groove depth depending on the detail chosen.  The details are currently only checked in the new condition.

Perform MDMT Calculations

If the material is UCS-23 and the UCS-66 calculations are being performed, tg is needed.  If the material is not UCS-23 or UCS-66 calculations are not being performed, then a "MDMT Exemption" must be selected or typed in manually or the flange will fail these checks.

First the the appropriate minimum thickness of the flange needs to be determined.  For Weld Neck, Weld Neck Series A, and Weld Neck Series B this is 0.5*(hub diameter - bore B).  For blind Flanges this is the thickness divided by 4. For Lap Joint, Slip-On, and Socket Weld Flanges this is 0.5*(hub diameter - bore min B).  For Threaded Flanges Class 150 this is 0.5*(hub diameter - bore min B. For Threaded Flanges Class 300 thru 2500 this is 0.5*(hub diameter - counter bore threaded).

For flanges that are not blind, The appropriate minimum flange thickness is compared to the host component thickness and the lesser is used as the governing thickness.  For blinds, the governing thickness is the appropriate minimum thickness.

The user can override the tg value with the value they wish to use to determine the base impact exempt MDMT.  The governing thickness, the units of measure and the curve are then used to get a base impact exempt MDMT per Table UCS-66.

If the UCS-66(b) reduction is taken, the MDMT Pressure for the flange is divided by the MAP of the flange at MDMT.  This is used as the coincident ratio to then determine the reduction per UCS-66(b). Note that the MAP is not reduced for external loads.

If the UCS-68(c) reduction is taken, the flat reduction in MDMT will be taken.

The base impact exempt MDMT is reduced by the UCS-66(b) and UCS-68(c) reductions as applicable.  The end result is the Computed Minimum Temperature.  This value must be the same or less than the MDMT of the flange for the flange to pass this check.