A flange is specified by the following information:
a) Type and Facing: i.e. whether the flange is for example “Weld Neck, RTJ” or “Socket Weld, RF”.
b) Nominal Pipe Size: required for all flanges, usually in inches.
c) Flange Pressure Class: required for all flanges, e.g. Class 150, 300, 900, 1500, 2500 etc.
d) Standard: i.e. ANSI B16.5, BS 1560, API 6A, API 605 or ASME B16.47.
e) Material: a material specification must be stated and will be as quoted in the piping specification.
f) Pipe Schedule: only for Weld Neck, Socket Weld, Composite Lap Joint and Swivel Ring flanges where the flange bore must match that of the pipe, e.g. Schedule 10, 30, 40, ,80, 120, 160, etc.
Normally, the flange specification will be hard stamped on the flange. In the case of existing plant, the information may not be legible and it is then necessary to identify the flange by visual observation and physical measurement.
a) Visual Observation: required to identify the type of flange and type of gasket used.
b) Physical Measurement: required to identify the nominal bore and the class of the flange. Check the number of studbolts, studbolt diameter, studbolt Pitch Circle Diameter (PCD) and the flange thickness. Compare these figures with standard flange data as found.
Pipe Flanges - Do's and Don'ts
a) Always visually inspect the surface finish of the flange for injurious defects. It should be clean, degreased and free of any defects, nicks or burrs. The permissible imperfections in flange surface finish specified in ASME B16.5, Table 3 are not allowed for critical applications. Raised Face and Flat Face flanges should also be checked for flatness with a straight edge.
b) All rust and burrs must be removed, small scratches should be removed by polishing, other defects may mean component replacement.
c) Check the phonographic or concentric groove on the face of Raised Face and Flat Face flanges. Any radial defects for example will be virtually impossible to seal against.
d) RTJ grooves must be kept scrupulously clean, corrosion free and undamaged.
a) When cleaning a flange face, never use a tool which may damage the surface finish. A soft wire brush is recommended.
b) Do not use unnecessary force, say by tirforing or pulling, to bring flanges which are clearly misaligned together. This may overstress adjacent pipework and will make sealing of the flanged joint difficult and unreliable. Instead, report the situation.
c) Do not mate flanges manufactured to different standards unless specified in an approved design.
Flange Surface Finish and Flange Distortion
To create a seal, gasket has to fill up the voids in the flange surface present because of surface finish and any flange rotation (or relative distortion) between the two surfaces.
The flange surface will thus give a broad indication of which gasket materials are likely to be suitable. Finishes of standard raised face flanges usually fall within the range 3.2 to 12.5mm, but this may be expressed in micro inch or roughness number.
a) Sheet Jointing
The recommended surface finish for the compressed fibre jointing (above a thickness of 1 mm) is 3.2mm to 12.5mm Ra (125m in 500min. CLA). These values are also used for graphite laminate (above a thickness of 0.8mm). For tongue and groove flange facings or for very thin gaskets (0.4mm or below), a surface finish of 1.6mm to 6.3mm Ra (63 to 200 in. CLA) is possible.
Surface finishes below 1.6mm are not recommended due to their negative effect on creep resistance of the gasket.
b) Spiral Wound Gasketing
This type of gasket requires a range of surface finishes dependent upon the application:
- General - 3.2mm to 5.1mm Ra (125min. to 200min. CLA)
- Critical - 3.2mm Ra (125min. CLA).
- Vacuum applications - 2.0mm Ra (80min. CLA)
c) Solid Flat Metal
A surface finish in the order of 1.6mm Ra is acceptable but for more critical conditions, a finish no more coarse than 0.8mm Ra is preferred. Again for optimum performance, the smoother the flange surface finish, the better the performance.
d) Metallic Ring Joint Gaskets
The angled surfaces (typically 23°) of both grooves and octagonal gaskets and the contact faces of oval gaskets should have a surface finish no rougher than 1.6mm Ra.
e) Machining of Flange Faces
Under no circumstances should flange seating surfaces be machined in a manner that tool marks extend radially across the seating surface. Such tool marks are practically impossible to seal regardless of the type of gasket being used.