Dear All,

Can any one tell me that How I can perform flange leakage check for API-6A flange (Rating above 2500#)?

Regards,
Shamalia Bhavin

My take is thus:

The flange rating charts in API 6A, 6AF1, 6AF2, etc. are constructed from the convenience of SES (the group who did the work) from a standpoint of FEA and experiment.

It's not constructed from the convenience point of a stress analyst.

My recommendation is thus:

1. Place a CNODE anchor at every flange pair to check.
2. Place a node name at these points that include the size (if you have multiple sizes) and api pressure class (if you have multiple pressure classes).
3. Export from CAESAR a local restraint summary, isolating your flanges.
4. For each size + pressure class, find the maximum Fx from the local restraint summary.
5. This Fx is the axial load. Note: Careful construction of your CAESAR model should allow you to discern whether this is compression or tension, but I tend to assume compression = tension for conservatism.
6. Round up to the next tension level that's listed on the API 6A chart.
7. At your design pressure rating, find the correlating bending moment.
8. This tension and bending moment combination will serve as a "recommended" combined limit.
9. Calculate sqrt(My**2+Mz**2) for all flanges to be checked. This is your bending moment.

Notes:
1. If any flange exceeds bending moment or tension limits on the chart outright, obviously you will need to reroute to fix this.
2. The "recommended" combined limit is a soft limit. If any flanges have higher bending moments but much lower tensions, then check these outliers manually against the chart.

Hi Michael,

We are also required to check flange leakage as per API 6A.

I have one doubt regarding axial load to be considered. In case if axial load is in compression what is suggested as the charts provided mention axial loads in TENSION?

The flange connection consists of two flanges, bolts, and a gasket. We know the dimensions of the gasket, and we're specifying the bolt tension. Therefore, we can estimate the pre-load stress in the gasket. If we add compression on top of that, we could potentially deform the gasket permanently. Now, when we slack off on the flange compression, we can introduce a leak point.

If you converted bending moment to compression on one end and tension on the other, add the compression from the bolts, and then add straight compression, and then you exceed yield, you will therefore have a permanently deformed gasket that may allow leaks on the first try.

If you exceed the gaskets allowable stress, then you'll potentially see leakage at some point in the 20 years following.