Flange Leakage Checking in Caesar II Using ASME Section VIII Method

Methods for checking flange leakage Introduction: 

Leakage is a function of the relative stiffnesses of the flange, gasket and bolting.

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Flanges are designed to remain leak free under hydrostatic test pressure when cold and under operating pressure when hot.

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The design of flanges (ASME B 16.5) does not take into account the bending moment in the pipe. This generates wire drawing effect on the mating surface of the flange. Hence, additional flexibility is to be provided when a flange joint is located near a point of high bending moment. So, Leakage checking is required.

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Process Piping Flanges are designed in accordance with BPV code, Section VIII, Division 1, appendix 2, using allowable stress and temperature limits of ASME B 31.3.

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

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Flanges with rating 300 and size greater than 24 inch

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Pipe flanges carrying caterogy M fluid service

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Pipe flanges carrying Hydrogen or other flammable fluid

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PSV lines with NPS 6 inch or more

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Flanges in Jacketed Piping

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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 bo6 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

Flange Leakage checking in Caesar II using ASME Section VIII method My last two articles on flange leakage explains the basic theory behind flange leakage checking and methods for performing flange leakage checking using pressure equivalent method. In this article I will explain the steps taken to perform flange leakage check using Caesar II by ASME Sec VIII method. Normally this method is performed when the analysis in pressure equivalent method shows failure in Caesar II. Pressure equivalent method being too conservative does not provide us the actual realistic results. Hence this method is widely accepted where pressure equivalent method shows a failure. The steps involved are mentioned below: 

After performing static analysis click the “Analysis” and then click “Flanges” in CAESAR II as shown below.

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Provide a file name and select the proper Flange Type in the section as mentioned in the PMS. Input the flange parameters as shown below:

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Press the key “Ctrl+F” to select the concerning Size and Rating. Few data will automatically filled from Caesar II database. However it is good practise to confirm the data from governing code.

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Flange Class is the flange pressure rating and is required for checking P-T Rating of Equivalent Pressure.

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Flange Grade: Select Material Group No. as per ASME B 16.5 Table 1A list of material specification. Following table can be used as reference:

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Refer to the applicable flange code ASME 16.5 or ASME B 16.47 and update automatically filled flange data. Take special care while inputting for ‘Small End Hub Thickness’& ‘Large End Hub Thickness’. ‘Small End Hub Thickness’ must be equal to the Pipe Thickness. Large End Hub Thickness must be equal to (X-B) where X is Hub Diameter and B Flange ID.

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Enter hub Length h=(Y-tf) where Y is length through Hub and tf is flange thickness.

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Now it is the turn to enter the Bolts and gasket parameters. Take parameters from ASME B 16.20 or ASME B 16.21 as shown below:

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Check Bolt Circle Diameter, Number of Bolts and Bolt Diameter, suggested by CAESAR II in reference with ASME B 16.47, 16.5.

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Correctly input for Gasket information in conjunction with ASME B 16.20 or 16.21 as applicable.

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For Effective Gasket Modulus, press Shift+? Key and then follow “High End” in Help statement. Leak Pressure Ratio m, Gasket Seating Stress y shall be in accordance with ASME SECTION VIII Div.1. Values for Caesar II help can be taken by pressing Shift+?.

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For Nubbin Width in the right side, Leave blank for Welding Neck Type and input Ring joint width as per the ring no. as described on ASME B 16.20 Table 3 TYPE R RING GASKET DIMENSIONS AND TOLERANCES for ring type joint.

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Facing Sketch/ Facing Column can be inputted as follows mentioned below:

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Now input the material data as shown below:

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Select the material of Flange and bolt, and Input design temperature.

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Flange Allowable & Bolt Allowable at design and ambient temp can be taken from CAESAR II database for particular flange and bolt material.

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Flange Modulus of Elasticity shall be taken from table C-6 of ASME B-31.3.which is given in millions of psi convert it to consistent unit

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Use 1.0 for Flange Allowable Stress Multiplier and 2.0 for Bolt Allowable Stress Multiplier. Use project specific data if available.

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Now Input Design Pressure, Axial Force, Bending Moment from Static analysis of the Caesar file as shown below:

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Now run the analysis to study the output results. Normally If safety Factor is less than one then joint failure is predicted and in output this is shown by a * mark.

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If result shows failure then try to reduce the bending moment or axial force and reanalyse. If that is not possible check if the flange rating can be increased.