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Question:

I have noticed some API RTJ flanges have raised faces, and some do not. When and why should an API flange have a raised face?

Answer:

API Spec 6A flanges fall into two categories designated as 6B flanges and 6BX flanges. You can easily distinguish between them by determining the seal ring used to seal the flange, 6B flanges seal with R-RX ring gaskets, and 6BX flanges seal with BX ring gaskets.

API 6B flanges, open face or studded, may or may not have raised faces. Raised faces on 6B flanges serve no functional purpose. Connected 6B flanges sealing with R or RX gaskets always have face to face stand-off.

Currently most manufacturers of wellhead equipment make all their 6B flanged or studded equipment with flat, or what most people call, full faces (without raised faces). In the past when wellhead manufacturers used castings for wellheads, many 6B flanges had the permitted raised faces to save a little steel. Raised faces still show up from time to time on new loose 6B flanges when manufacturers forge the flanges in closed dies, and forge the raised face in place on the flange forging to save steel.

API Spec 6A specifies that 6BX open face flanges shall always have raised faces with at least a 1/8" height. Studded face 6BX flange faces may have the raised face omitted.

Currently, most manufacturers of wellhead equipment make all their 6BX studded equipment with flat (full faces).

Several reasons exist for at least one of the 6BX mating faces to have a raised face.

API Spec 6A offers this general statement: "Type 6BX flanges are of the ring joint type and are designed with a raised face. Depending on tolerances, the connection make-up bolting force may react on the raised face of the flange when the gasket has been properly seated. This support prevents damage to the flange or gasket from excessive bolt torque. Therefore one of the flanges in a 6BX connection shall have a raised face. The type 6BX flange shall be of the through-bolted or studded design. NOTE Face-to-face contact is not necessary for the proper functioning of type 6BX flanges."

API fails to mention the practical reasons that become apparent in the field. Because the grooves and gaskets have dimensions that may allow face to face contact, one of the flanges must have a raised face to assure the bolting exerts the proper force against the gasket. Flat flange faces may have corner damage, or the face may have accumulated debris or excessive paint and not allow the bolts to properly compress and coin the BX ring gasket. Also the dimensional requirements for the full flat face of a flange references no tolerance for flatness, and small variations from flatness may occur because of wear on manufacturing machine tools. The presence of the raised face on 6BX flanges resolves all of these issues.

You can find more information about flange make-up at http://www.woodcousa.com/flg_make-up.htm.

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