Caesar II - Methods for flange leakage checking by Pressure Equivalent Method

Methods for flange leakage checking by Pressure Equivalent Method using Caesar II

Caesar II methodology for Pressure Equivalent method:

• Model the stress system from stress isometric. After modelling is complete select the flange which need checking as per pressure equivalent method. In Fig. 1, node 20-30 and node 30-40 denotes the flange assembly. And flange leakage checking is required for node 30.
• Now select the Flange Node(From/To/Both) and Calculation Type as shown in Fig. 2.
• Select Flange Class/Grade through ‘Read from File’ button and refer to ASME B 16.5/ ASME B 16.47 material tables.
• Required data will automatically be filled. By default the value of G will be taken as the mean gasket diameter. User to update the value of G as per ASME B 16.20 & ASME Sec. VIII Div. 1, Appendix 2, Table 2-5-2. based on following equation:
• b0 = 1/4, G = Mean diameter of the gasket contact face
• b0 > 1/4, G = Outside diameter of gasket contact face less 2b, b=basic gasket width from code. check the above mentioned link for details.
• Now go to the load case editor and select the temperature at which flange leakage checking to be performed as shown in Fig. 3. Now run the analysis to check the results. In case the generated stress exceeds the allowable results will be shown in red colour. Check Fig. 4 for output results.

Analysis Criteria:

The criteria regarding when flange leakage checking is required should be mentioned in the ITB documents or project specs. But as a general practice the following can be used:

• Flanges with rating 600 or more
• Flanges with rating 300 and size greater than 24 inch
• Pipe flanges carrying caterogy M fluid service
• Pipe flanges carrying Hydrogen or other flammable fluid
• PSV lines with NPS 6 inch or more
• Flanges in Jacketed Piping
• Flanges where stress engineer finds a very high bending moment

Analysis Methodology:

Two widely used methods are practiced in prevalent Process Industry. These are

1. Pressure Equivalent method based on ASME B 16.5 pressure temperature table and
2. ASME BPVC Sec VIII Div 1 Appendix 2 method.

Pressure Equivalent Method:

In this method the axial force (F) and bending moment (M) on the flange is converted into equivalent pressure (Pe) using following equations.

Equivalent Pressure for Axial force, Pe1=4F/ΠG^2

Equivalent Pressure for bending moment, Pe2=16M/ΠG^3

Here G=diameter at location of gasket load reaction =(Gasket OD+ID)/2 when bo<=6 mm

                                                                               =(Gasket OD-2b) when bo>6 mm.

Here bo=basic gasket seating width as given in table 2-5.2  of ASME sec VIII:

Now add these two equivalent pressures with pipe design pressure (Pd) to find total pressure (Pt=Pd+Pe1+Pe2) and enter into the ASME B 16.5 pressure temperature rating table associated with flange material. If Pt is less than the allowed pressure on the rating table corresponding to the associated temperature then flange will not leak.

ASME BPVC Sec VIII Div 1 Appendix 2 method:

In this method flange stresses (longitudinal hub stress, radial flange stress and tangential flange stress) are calculated based on code provided equations/formulas. These calculated stresses are then compared with allowable stresses as given in BPVC code Sec VIII Div 1 Appendix 2, Clause 2-8.

For calculating flange stresses one need to calculate the flange moment which is dependent on bolt load. Bolt load has to be calculated for two design conditions; operating & gasket seating, and the most severe will govern. For more details of the equations and calculation methodology the above mentioned code can be referred. Click on the following title if you are interested in Caesar II methods for flange leakage analysis using a) Pressure Equivalent Method and b) ASME Section VIII method