Flange leakage checking is a very important activity in pipe stress analysis. Flange leakage checking ensures that there will be less chances of flange leakage. There are various methods available for flange leakage checking in conventional stress analysis software packages like Caesar II. NC 3658.3 method is one such popular flange leakage checking method. In this article, we will discuss the step-by-step methods for performing flange leakage analysis using NC 3658.3 method in software Caesar II.
NC 3658.3 provides a flange leakage evaluation method based on the NC code for ASME B16.5 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 it in the 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-analyzing the system and then re-check 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.
Is NC 3658.3 method applicable for ASME B16.47 Flanges?
The specific clause of NC3658.3 clearly states that the method is only applicable for ASME B16.5 flanges. So, in general, the method is applicable for flanges up to 24″ sizes. But, piping engineers frequently encounter piping systems having sizes greater than 24 inches means flanges from ASME B16.47 standard. Several studies have already been done regarding the applicability of the NC 3658.3 flange leakage checking method for ASME B16.47 flanges.
One such published paper with the title “Use of NC3658 flange design rules for ASME B16.47 flanges- An analytical and finite element based study” by Anindya Bhattacharya & Michael P Cross concludes that the simple method of checking a flanged joint based on NC3658.3 can be extended to B16.47 flanges, but there are some exceptions as mentioned below:
- NC 3658 flange design rules are not applicable for flanges of size 60 inches 600 Lb.
- Also, for sizes >=36 inch, 600 Lb. and higher flange rating, as the operating stress on bolts, are higher than twice the material allowable stress, the NC 3658 rules are not applicable.
For more details, you can refer to the paper from the following site: https://asmedigitalcollection.asme.org/ESDA/proceedings-abstract/ESDA2014/45837/V001T05A009/232054
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