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ABOUT FLANGES

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Double Studded Adapters and
Spacer Flanges

 

Except for Crossover Connections, Double Studded Adapters (DSA's) serve as the shortest method to join flanges of different sizes or pressure ratings. The DSA has a body resembling a flange with a different seal size or bolt pattern on each side. The connecting bolts for each side, called "Tap End Studs" thread into tapped holes in the body, conforming to the same size bolts and bolt circle as nominal for the specified flange each side shall fit. The internal bore will not exceed that specified for the smaller side, and if nominally different on each side, shall transcend by a taper, (see illustration below).   API specifies no thickness requirement for DSA's. DSA's usually have a minimum thickness equal to the thickness of a flange of equal size and pressure to the largest and highest pressure side of the DSA, and may have any amount of additional thickness as necessary to satisfy design requirements or allow for lock screws.

BOLT CIRCLE LAYOUT, DOUBLE STUDDED ADAPTER

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DSA's shall have their bolt circles oriented so that equipment assemblies will maintain consistent orientation throughout the equipment stack-up. All API conventional flange bolt circles have a number of evenly spaced holes, divisible by four. All API equipment shall have bolt circles with two holes of each bolt circle equally straddling a common centerline.


SPACER FLANGE

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In the event an equipment assembly needs a small amount of extension, a Spacer Flange may have the appearance of a DSA yet have the same connection geometry on both sides. Such a spacer flange may have its bolt circle drilled through and attach within the equipment stack using extra-long flange bolts.  Spacer flanges may have any length (thickness) not constrained by design requirements. Studded spacer flanges must have sufficient length (thickness) to accommodate tapped holes that accept full thread engagement of the tap end studs.  Spacer flanges with drilled through holes may have a much shorter length (thickness), so long as the root thickness between opposite ring grooves remains adequate to the stresses necessary to coin the ring gaskets to seal (the drilled through spacer flange experiences only compression and pressure loading). Bolts produce no stress in drilled through spacer flanges. As a general rule, a drilled through spacer flange may have a minimum thickness of 4 times its ring groove depth.

11" 5M x 11" 5M SPACER FLANGE ILLUSTRATION

DSA's and spacer flanges may or may not have lock-down screws to secure internal bore wear bushings or sealing devices.  

DSA's having a different number of bolts on each side may not allow enough space between tapped holes for drilling lock-screw holes. Adding thickness to the DSA design, if equipment stack-up height permits, may remove this limitation.

For more about DSA minimum thickness, see Q & A 9. 


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