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Subject: 6BX Flange Connection Weepage

 

Question:

While recently pressure testing a BOP stack with a 18-3/4" 15M Studded Flange connection, weepage was seen on the exterior of the connection after bleeding the pressure off. I'm talking droplets here. The connection was bone dry with pressure applied. The ring grooves were verified to be in Spec with a newly inspected BX ring gasket installed. The pressure test charts were perfect straight lines. Do you have any thoughts on the cause and/or relevance of this "weepage" that we found?

Answer:

We can't tell if this test you mention was a shop test or a rig test, and if you inspected the ring grooves before or after the test. We also don't know how you measured the tightness of the connection, or if the raised face of the flange contacted the face of the studded connection.

If this was a shop test and no leakage occurred while at test pressure, and after disassembly the ring groove checked out OK, you have a good test and should accept it. See the following link for helpful tips,  http://www.woodcousa.com/test_rack_main.htm.

If this was a test at the rig prior to drilling operations through the BOP, you have a problem. You should retighten the bolts to specified tension or until the flanged raised face contacts the studded connection face, and test again. A field test should have no visible leakage during or after a test. Sometimes retightening a connection that has previously leaked will not stop the leak on a second test, and when this happens on a second attempt to test, don't waste your time attempting a third try. Disassemble the connection, dry all of the connection, reconfirm the condition of the ring grooves, install a new gasket and reassemble the connection and tighten to specified torque, then test again.

By way of explanation, leakage as you describe can occur as pressure rapidly declines when released after test.

If 6BX flange bolts have less than specified torque applied, test pressure may cause some flange face separation. The design of BX ring gaskets allows pressure to energize the gasket against the ring groove outside surfaces preventing a leak, even with some flange face separation occurring. In this case, rapid pressure release after test may allow the previously energized ring gasket to draw back from one or the other ring groove outside faces and momentarily allow water trapped below the ring gasket to escape to the flange surface outside of the gasket groove. For a shop pressure integrity test, such a "leak" results in no problem.

In the field, reliability of flange joints to remain leak free over time is critical, and any sort of leak cannot be permitted.

We believe that in the case you have described, you may not have tightened your bolts to the specified torque. 6BX flanges, usually torqued to 50% of bolt yield strength may also be torqued to 70% of bolt yield as specified for subsea service.

The use of an SBX ring gasket will usually aid in making leak free field connections. For more information about how SBX ring gaskets work, see http://www.woodcousa.com/SBX_Rings.htm.

You may find it helpful to review the following links for additional information.  

http://www.woodcousa.com/field_appr.htm

http://www.woodcousa.com/flange_chart.htm

http://www.woodcousa.com/bolt-sequence-index.htm

 

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