Flange leakage is a serious problem in the piping industry. It has a tremendous potential to cause severe hazards to operating 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 B16.5) 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.
Flanges Used in Oil & Gas Processing Plants
Process Piping Flanges are designed in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Appendix 2, using allowable stress and temperature limits of ASME B31.3.
Reasons for Flange Leakage
Numerous possible reasons may cause a flange assembly to leak. Some of the common causes of flange leakage problems are:
Excessive Piping System Loads:
Forces and bending moments: can loosen bolts or distort flanges.
Causes include insufficient flexibility, excessive mechanical force, and poor support placement.
Incorrect Gasket Size or Material:
Wrong size: noticeable during installation.
Wrong material: This may cause issues like corrosion or blowouts later.
Vibration Levels:
Excessive vibration can loosen bolts, resulting in leaks.
Thermal Shock:
Rapid temperature changes can deform flanges and cause leaks, exacerbated by varying thermal expansions.
Improper Gasket Installation:
Off-center gasket: This leads to uneven compression and potential leaks.
Fastener Issues:
Too tight or not tight enough: uneven bolt stress due to improper tightening or loss of bolt tension over time.
Overly tight fasteners: excessive pressure on gaskets or fatigue on equipment, especially in high-temperature conditions.
Improper Flange Alignment:
Misalignment causes uneven gasket compression and potential leaks.
Flange Facing and Surface Finish:
Deeper serrations: Can prevent proper gasket seating, leading to leakage.
Damaged Flange Faces:
Corrosion pitting: Creates leakage paths.
Contaminants: Dirt, scale, scratches, protrusions, and weld spatter can lead to uneven gasket compression and leakage.
Most of the reasons mentioned above are construction-related, which can be eliminated by following proper best practices during construction activity. However, the major design cause that could result in flange leakage is excessive forces and moments that can be controlled during the design phase. The following section provides flange analysis methodologies to check the suitability of flanges against high forces, which may cause excessive stresses.
Flange Leakage 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 a rating of 600 or more
Flanges with a rating of 300 and size greater than 16 inch
Flange Leakage Checking by Pressure Equivalent Method
In this method, the generated axial force (F) and bending moment (M) on the piping/pipeline flange are converted into equivalent pressure (Pe) using the following equations.
Equivalent Pressure for Axial force, Pe1=4F/ΠG2
Equivalent Pressure for bending moment, Pe2=16M/ΠG3
Here G=diameter at the 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 the 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 the flange will not leak.
Flange Leakage Checking by 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 ASME code-provided equations/formulas. These calculated stresses are then compared with allowable stresses as given in ASME 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 on the equations and calculation methodology, the above-mentioned code can be referred to.
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 the code equations. For more details, click here.
The EN 1591-1 calculation code offers a comprehensive method for analyzing flange leakage by considering the behavior of all components involved—flanges, bolts, and gaskets. Unlike simpler methods, EN 1591-1 accounts for factors such as gasket thickness reduction due to flange stress and changes in gasket elasticity with temperature variations. This advanced approach provides not only an allowable stress check on the components but also an indication of the expected leak tightness of the flange assembly. It is applicable to both regular piping flanges and custom-designed body flanges for equipment. The code incorporates gasket characteristics based on EN 13555, including maximum allowable surface pressures, modulus of elasticity, and minimum seating pressure for various tightness classes. Additionally, it factors in the coefficient of thermal expansion of flange and bolt materials
I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.
22 thoughts on “Methods for Checking Flange Leakage | Flange Leakage Calculation | Flange Leakage Analysis”
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.
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.
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.
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?
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.
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.
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.
There is an app that taken into account of external force and moment together with ASME section VIII method,
https://apps.apple.com/us/app/flange-leakage/id1573350477?ign-mpt=uo%3D2
Sir, i have read this article.it’s very useful to me. But I have doubt regarding wire drawing effect in flanges.. what is meaning of wire drawing effect as you have mentioned in starting that “The design of flanges (ASME B16.5) does not take into account the bending moment in the pipe. This generates a *wire drawing effect* on the mating surface of the flange.”
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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.
Hi , can you share for me if you have any guideline for the European standard, EN-1591 methods ?
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.
There is an app that taken into account of external force and moment together with ASME section VIII method,
https://apps.apple.com/us/app/flange-leakage/id1573350477?ign-mpt=uo%3D2
Sir, i have read this article.it’s very useful to me. But I have doubt regarding wire drawing effect in flanges.. what is meaning of wire drawing effect as you have mentioned in starting that “The design of flanges (ASME B16.5) does not take into account the bending moment in the pipe. This generates a *wire drawing effect* on the mating surface of the flange.”
Good for sharing.