For convenience operators using certain API flanges will request that those flanges have double drilled bolt circles.
Double drilled flanges have two complete bolt circles that mate with two flanges rated at two different working pressures. These flanges have the same size nominal bore and use the same ring gasket, or ring gaskets having the same pitch diameter.
The Most Common Double Drilled Flanges:
|Nominal Bore||Bolt Circles*||Ring Gasket|
**This discontinued 960 psi flange originally had a 20-1/4″ bore.
The following illustration demonstrates a typical double drilled flange:
7-1/16″ 2000/3000 DOUBLE DRILLED FLANGE
|THE 7-1/16″ 2000 BOLT CIRCLE DRILLED INSIDE THE 7-1/16″ 3000 BOLT CIRCLE REMOVES MUCH OF THE STRUCTURE OF A 7-1/16″ 3000 FLANGE, REDUCING ITS STRENGTH.|
|THE FLANGE NECK DIAMETER MUST CORRESPOND TO THE LOWEST PRESSURE (SMALLEST) BOLT CIRCLE, TO ALLOW WRENCH CLEARANCE, REDUCING THE NECK STRUCTURAL STRENGTH TO THAT OF A STANDARD FLANGE OF THE LOWEST PRESSURE RATED BOLT CIRCLE.|
Less common double drilled flanges do not use the same ring gasket as both flanges they join with, yet these joining flanges have identical ring groove pitch diameters. Joining these double drilled flanges requires a combination ring gasket which has a different profile on each side to mate with and seal on the different groove width and depth dimensions.
The following double drilled flanges use combination ring gaskets:
|Nominal Bore||Bolt Circles||Combination Ring Gasket|
The illustration below shows a cross-section of a combination ring gasket in place in a made-up connection between flanges, one double drilled, with ring grooves of differing width and depth but the same pitch diameter.
|LOWER FLANGE DOUBLE DRILLED, JOINED TO A SMALLER STANDARD FLANGE WITH A NARROWER RING GROOVE USING A COMBINATION RING GASKET.|
Usually makers of double drilled flanges drill the smaller bolt circle into a flange body that conforms to the dimensions of the higher pressure rating of the two bolt circles. The flange thickness usually remains the same as that required for the highest nominal rating while the flange neck diameter must conform to the lowest nominal pressure to allow nut and wrench clearance.
CAUTION: Equipment with double drilled integral flanges always present an orientation problem because the smaller (lowest pressure) bolt circle does not straddle the common centerline required for orientation of other connected equipment.
Double drilled flanges cannot bear the API Monogram, never-the-less operators may sometimes encounter or specify these unusual crossover combination flanges. Do not use these for pressures higher than the lowest pressure flange they will join.
In the field a few API 6B flanges or studded face connections have triple drilled bolt circles; the resulting flange, or tapped and studded face, resembles “swiss cheese” and presents a risk of failure when used for any pressure service, even that of the lowest nominal pressure bolt circle. WOODCO USA strongly recommends against the use of any triple drilled flanges.
Some API 6BX flanges of each nominal bore size with different pressure ratings share the same ring gasket, and on rare occasions operators have requested double drilled 6BX flanges. Because these flanges have not commonly seen service some field personnel might mistakenly believe a flange drilled to join with both 15,000 psi and 10,000 psi working pressure flanges has a working pressure equal to the flange it joins. A catastrophic failure may result from such a connection. WOODCO USA strongly recommends against the use of any 6BX double drilled flange for any application.
For more information about joining flanges of unequal pressure and the uses of combination ring gaskets, see Crossover Connections.