NC 3658.3 provides a flange leakage evaluation method based on the NC code for flanges using high strength bolts. Caesar II software has a nice module in its input spreadsheet to use this method efficiently with ease. In the following paragraphs, I will describe the methods of flange leakage evaluation by NC 3658.3 method using Caesar II.
Criteria for NC 3658.3 Method Application
NC 3658.3 Method for flange leakage evaluation can only be applied if the following two conditions are met:
- Flanges, bolts, and Gaskets used are designed based on as specified in ASME B 16.5a and
- Bolting material must have an allowable stress value at 100°F(38°C) >=20000 psi (138 MPa) (High Strength Bolting)
Governing Equations as Per NC 3683.5 method
By NC 3658.3 method the generated external moment (Mfs) is limited to a value as provided by the below-mentioned equation:
Mfs<=3125(Sy/36000)C X Ab (in U.S. Unit);
- Sy=Yield strength of flange material at the temperature and
- Ab= Bolt area The ratio of Sy/36000 has to be taken less than unity.
So it is obvious from the above equation that we have to enter the bolt area and yield strength as input into Caesar II Spreadsheet.
Flange Evaluation steps followed in Caesar II
- Select Flange Node (From/To/Both) and Calculation Type (NC-3658.3) as shown in Fig. 1.
- Input Bolt circle diameter from ASME B 16.5.
- Input Yield Strengths at the temperature from ASME BPVC code Section II Part D Table Y1.
- Calculate the Bolt Area (Ab) as shown below and input in required place:
Bolt area, Ab is the total cross-sectional area of the bolts at the root of thread or section of least diameter under stress.
Bolt Area Calculation for NC method
Calculate the bolt area as mentioned below
Droot = D1bsc – 2*0.21650635P & D1bsc = Dbsc – 2*h
- Droot = Root diameter of the bolt
- Dbsc = Basic major (nominal) diameter of bolt (as per ASME B1.1, para 8.3)
- D1bsc = Basic Minor diameter (as per ASME B1.1, para 8.3)
- h = Height of thread (as per ASME B1.1, Table 5)
- P = Pitch of Bolt (as per ASME B1.1, Table 1)
Root Area of bolt (Ab) = n x [(pi/4)* Droot* Droot)]
Where n = No. of bolts as per flange pressure rating class
- Now go to the Load Case Option Editor (Fig.2) and select flange analysis temperature based on operating/design conditions and click the Run button for analysis.
- Select the Load Case(s) for which Flange Leakage Check has to be performed and see the results. If the ratio of Flange Stresses to Allowable Stress is less than 100 % (See Fig. 2), Then the flange is within the allowable limit and should not produce leakage. If it is more than 100%, Then try to reduce moments at the flange by re-analysis of the system and recheck using the above method.
Allowable Stress for NC 3658.3 method as per Caesar II
Caesar II Allows the user to check the NC flange leakage check at 9 temperature case conditions (operating condition ), Hydrotest, and Sustained condition at a time.
In CAESAR II, an Equivalent flange Stress S is calculated based on the following formulas, and then the calculated stress is compared to Sy (or 2*Sy for occasional load cases), in the following manner:
S = 36,000* Mfs / (CAb * 3125) ≤ Min(Sy, 36000) (non-Occ)
S = 36,000 * Mfd / (CAb * 3125) ≤ 2.0 * Min(Sy, 36000) (Occ)
Note that both the above equation are in the FPS unit. For metric or other unit systems, the Caesar II software converts the 36,000 values in the above equations to the appropriate set of units.
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