What is Flange Leakage?
Flange leakage is a serious problem in the Piping Industry. It has a tremendous potential to cause a severe hazard in plants. Hence, the possibility of leakage needs to be investigated during the design stage to reduce leakage possibility during operation.
- Basically, Flange Leakage is a function of the relative stiffnesses of the flange, gasket, and bolting.
- Flanges are designed to remain leak-free under hydrostatic test pressure when cold and under operating pressure when hot.
- The design of flanges (ASME B 16.5) does not take into account the bending moment in the pipe. This generates a 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.
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.
Flange Analysis Criteria
The criteria regarding when flange leakage checking is required should be mentioned in the ITB (Invitation To Bid) 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 16 inch
- Pipe flanges carrying category M fluid service
- Pipe flanges carrying Hydrogen or other flammable fluid
- PSV lines with NPS 4 inch or more
- All Flanges in Jacketed Piping
- Flanges where stress engineer finds a very high bending moment
This list is not exhaustive. Always refer to your stress or project guidelines for more details about flange leakage criteria.
Three widely used methods are practiced in the prevalent Process or Power Piping Industry. These are
- Pressure Equivalent method based on ASME B 16.5 pressure temperature table and
- ASME BPVC Sec VIII Div 1 Appendix 2 method.
- NC 3658.3 method
Pressure Equivalent Method
In this method, the axial force (F) and bending moment (M) on the flange are converted into equivalent pressure (Pe) using the 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 needs 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.
Some more ready references for you:
Flange Selection Guidelines
Pressure Equivalent Method in Caesar II
Flange leakage calculation ASME Section VIII in Caesar II
Flange leakage calculation NC 3658.3 method in Caesar II
Procedure for Flange Bolt Tightening of Various Sizes of Flanges
NC 3658.3 Method
In this method, the flanges are evaluated using the ASME BPVC Section III Subsection NC-3658.3 method. The calculated flange moments are compared to some limited values as calculated from code equations. For more details click here.
References (External Link):
- ASME B16.5
- ASME BPVC SEC VIII