Table of Contents

## 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.

## Analysis Methodology

**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
- https://docs.hexagonppm.com/reader/O04dxcwMfibZIK1cyksZvQ/libWcNN9xB96GAfpXDAXlg

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!

thanks a lot nice article

Just wanted to share that the European standard, EN-1591, which is a very detailed flange leakage assessment is now supported in CAESAR II 2019 (V 11) as well. It is geared to use on very specific and the most critical of flange assemblies.

Could you explain the PCC-1 calculation

Hello, talking about all the methods for checking flange leakage, have you ever analyzed this paper: Robert G. Blick: “Bending Moments and Leakage at Flanged Joints” (published in 1950 as “A Gulf Company Publishing Publication, Sun Valley, California – Petroleum Refiner” in 3 parts: Vol.29 – No.2, February (Part I); No.5, May (Part II); No.6, June (Part III))

Max. allowable bending moment: M = Min (M1, M2)

M1 = Criterion for sufficient gasket pressure = …

M2 = Criterion for excessive gasket pressure = …

I’ve tried to apply it, but the results seem very instable. I’ve never seen this applied practically, but one of my clients had this as a possible method.

Thanks for your time.

Hi Anup,

Just want to know one thing how to calculate the moment & forces.