There are three types of flange facings commonly found on a plant.
The surface finish of the facings is specified in the Flange
Standards. Note that they are refined and superseded by BP
Engineering GS 142-4 - Pipe Flanges and Fittings. A section on
surface finish on the different flange facings is in this book extracted
from GS 142-4.
a) Ring Type Joint (RTJ)
Typically found on the most severe duties, for example high
pressure gas pipework. Ring type metal gaskets must be used
on this type of flange facing.
- RTJs to API 6A Type B, BS 1560 and ANSI B16.5:
The seal is made by metal-to-metal contact between the
gasket and the flange groove. The faces of the two
opposing flanges do not come into contact and a gap is
maintained by the presence of the gasket. Such RTJ
flanges will normally have raised faces but flat faces may
equally be used or specified.
- RTJs to API 6A Type BX:
API 6A Type BX flanges seal by the combined effect of
gasket compression and flange face-to-face contact and
will therefore always have raised faces. The flanges also
use special metal ring joints. A Type BX flange joint which
does not achieve face-to-face contact will not seal and
should not be put into service.
b) Raised Face (RF)
Sealing on a RF flange is by a flat non-metallic gasket (or a flat
metallic gasket for special applications), which fits within the
bolts of the flange. The facing on a RF flange has a concentric
or phonographic groove with a controlled surface finish. If the
grooves are too deep (or a rough surface finish), then high
compression is required to flow the relatively soft gasket material
into the grooves. Too shallow (exceptionally smooth surface
finish) and again high compression is required as a leak path
then becomes more possible. It is important to always check
the flange surface finish for imperfections which would make
sealing difficult. A radial groove for example is virtually impossible
to seal against.
Note that the surface finish on the flange facing depends on
the type of gasket being used.
Further details are given in Section 3.8 (Spiral Wound Gaskets)
and 3.9 (Sheet Gaskets).
c) Flat Face (FF)
Sealing is also by compression of a flat non-metallic gasket
(very rarely a flat metallic gasket), between the phonographic/
concentric grooved surfaces of the mating FF flanges.
The gasket fits over the entire face of the flange.
FF flanges are normally used on the least arduous of duties
such as low pressure water drains and in particular when using
cast iron, cunifer or bronze alloy, where the large gasket
contact area spreads the flange loading and reduces flange
bending.
NOTE: Both ANSI B16.5 and BS 1560 specify Flat Face
Flanges and Raised Face Flanges as well as RTJ Flanges. API
6A is specific to RTJ flanges only.