Introduction:
- 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 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.
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
- Pressure Equivalent method based on ASME B 16.5 pressure temperature table and
- 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
Hey i must say that you have done a super work by sharing method for checking the flange leakage and if it is stainless steel flange bolts then after fixing forgot for years.
dowel pins |clevis pins | DIN 7
Thank you very much…
Very good article. Congratulations.
Good , usefull basic one shd remember
Hi Anup, this website is pretty helpful for the stress engineer, i give you the thanks for sharing your Knowledge. I have a question, according to the Section VIII Div 1 Sec. 2. I have analyzed several flange studies and almost always I find out that the Bolting Stress is “Overstress” for flanges of 150#, specially those ones between 6″ & 20″.I have read on the COADE forums that this is a mistake on the Standard, because several flanges have been done and they didn’t have problems during it’s operational cycle. So I want to asked to you if you found out this too. And why this happens. Thanks.
JAB
Yes I also find the same results..In 90% of situations 150 rating flanges are failing in Caesar II…I asked Coade about the same..but they could not provide any satisfactory reply…
Hi,
Just to add to your discussion, the standard flanges of ASME B16.5 have been taken from some old standards (i dont remember the exact one) which was based on practical testing rather than analysis.
The analysis in ASME Sec VIII Div 1 was developed much later. Hence most of the standard flanges as per B16.5 do not pass in the analysis as per Div 1 App 2 but are still in use because they have been proved effective through practical use over the years.
Dear,
I hope you are doing fine buddy, I would like to congratulate that way you are continuing sharing knowledge here.
As a general remark, I would like to add to this discussion that usually we have 150# flanges in lines where pressures are low. However I have encountered the same problem.
As a practice I’ve started to shift the flange connection i.e., break-up flanges/ valve assembly etc near to support location (if possible) in coordination with layout group. This hugely decreases the bending stresses at connections & design becomes safe.
However, another solution is to switch the class from 150# to 300# at those points where Peq failure is more that 200-250%, as a general note, most PMS allows one rating higher flanges available.
Do I need to follow the client comment in running the flange leakage for 150# lines with temp. Of 50 deg C with Fuel oil product that does not fall on flammable fluid?
Client requirements are the final requirements. So you have to perform leakage checking
Could you describe wire drawing effect specifically? I can understand overall theory, however i can’t understand what is wire drawing effect.
Hi all,
Thank you for this nice article.
I would like to add that under ASME Sec VIII Div 1 App2 method, in addition to calculation of induced stresses, flange rigidity check should also be done and checked if Flange rigidity <=1 for both operating & seating condition so that leak tightness is ensured.
Even after all calculations are performed, the actual bolt tightening done on the flange also affects the joint integrity and leak tightness. For this ASME PCC-1 App O gives guidelines using joint component approach.
Though it is not mandatory, it can also serve for ensuring leak tightness of the joint.
Thanks.
interesting article!