Hi all,

My project specifies the rule for flange leakage check as follows:

Step 1: check the bending stress at the weld neck point of the flange and compare with a given allowable stress table which is defined by specification for each class. If the bending stress smaller than allowable stress. It's OK.

Step 2: if the Step 1 is failed. Pressure equivalent method shall be applied to check.

Sometime step 1 have been passed, but the flange will be failed by using step 2. Which can we trust even the project specification specified the rule?

Regards,

Hi,

I suggest to follow the Project Specification/Procedure - you are "covered", since, as you said, the Bending Stress at Flange-Pipe joint node is bellow the prescribed allowed limit.
I met several times on past projects this kind of approach for flanged joint load check.

You should be aware that Equivalent Pressure Method is generally conservative. If your calculations by this method indicate flange overload not exceeding 25...30%, then most probable, the detailed calculation as per ASME VIII Division 1 BPV Code (Caesar II flange module) will yield to acceptable results.

You may do also a check with "flange leakage" key words on this Forum, there are several interesting discussions about this subject.

Another important issue: Which is the relation between your DESIGN PRESSURE and ALLOWABLE WORK PRESSURE AS PER ASME B16.5 (OR B16.47 as aplicable)? Generally, a good and sound engineering approach should limit DESIGN PRESSURE at maximum 75...80% from ASME B16.5 WORKING PRESSURE values (according to your Material class and Design Temperature). In this way, a part pf the flange loading capacity remains available to withstand the external piping loads (bending moment and axial force).

If your DESIGN PRESSURE is equal to ASME B16.5 WORKING PRESSURE value, then you are wasting your time trying to apply Equivalent Pressure Method, all calculations will indicate overload. I've met several times this case, when people from Process Discipline did not understand these correlations and prescribed the Design Pressure equal to ASME B16.5 Pressure Rate (e.g. for them, the external piping loads doesn't matter...)

Best regards,

Hi Dorin,

Thank you for your reply,

I can believe design pressure from process, and of course ASME section VIII shall be applied if PEQ is failed.

However, my big question, as mentioned in 1st post, is why sometime step 1 (bending stress check) have been passed, but the flange will be failed by using step 2 (PEQ)? I'm understanding now that bending stress check should be the most conservative method, the second should be PEQ (more precision) method logically by the rule which have been defined by project specification ... Still be confused

Since I cannot contact with the guy who prepare the specification, so I want to ask the matter in this forum.

Regards,

Maybe we can have an opinion knowing details about "the given allowable stress table which is defined by specification for each class"; otherwise it is difficult to understand what step 1 represents.

For me it is difficult also to understand your concerns; I understand you have a specification with clear rules and you need to follow it.