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. In this section I will describe the methods of flange leakage evaluation by NC 3658.3 method using Caesar II.
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
- Boting material must have a allowable stress value at 100°F(38°C) >=20000 psi (138 MPa) (High Strength Bolting)
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)
Here Sy=Yield strength of flange material at 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.
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 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.
Calculate the bolt area as mentioned below :
Droot = D1bsc – 2*0.21650635P &
D1bsc = Dbsc – 2*h
Droot = Root diameter of bolt
Dbsc = Basic major (nominal) dia 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
Fig.1: Input Spreadsheet from Caesar II for flange leakage check by NC 3658.3 method
- 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 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.
Fig.2: A typical Caesar II flange leakage analysis report