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Piping Flange Fundamentals
Flange Defining Factors
Flanges are categorised based upon certain criteria, and these categories are usually defined by relevant piping standards
and specifications (discussed later). A flange is defined by:
•
Type – the geometry of the flange as a whole. Welding neck, slip-on, and socket weld, are examples of different
flange types.
•
Face – the sealing area of the flange. Flat face, raised face, and ring type joint, are examples of different flange
faces.
•
Standards and Specification – flanges are manufactured to comply with given standards and specifications.
Standards and specifications dictate the dimensions, geometry, schedule, and material, of a given flange (to
name a few factors).
•
Dimensions – the dimensions of a flange’s hub, face, blade etc. Dimensions depend upon nominal pipe size (NPS)
and the pressure class required for a given application.
•
Nominal Pipe Size (NPS) – a dimensionless unit of measurement defining the size of the item (pipe, fitting etc.)
that connects to the flange.
•
Pressure Class – the pressure-temperature rating of the flange for a given material. Despite the name ‘pressure
class’, this factor is material and temperature dependent.
•
Material – the material from which the flange is manufactured e.g. cast iron, carbon steel, stainless steel etc.
•
Schedule (SCH) – a pipe’s thickness/schedule. The schedule of a pipe is relevant only for welding neck and lap-
joint flanges because the schedule of these flanges must match the associated pipe schedule to which they are
connected. The other flange types either slide partly into, screw into, or penetrate through, their associated flange,
thus the flange schedule does not need to match the pipe schedule. The schedule is relevant for swivel-ring
flanges, but these have limited application and will not be discussed further.
All of the aforementioned bullet points will be discussed in a logical order in the coming sections. For now, it’s important to
realise that flanges are not unique items. Flanges are manufactured for a specific purpose, with many design factors already
considered. Should a flange ever fail, the exact same flange can -theoretically- be ordered to replace its predecessor6; this
has significant real-world benefits, which will be discussed later in the Standardisation section.
6 A root cause analysis (RCA) should be conducted on any flange that has failed unexpectedly. If the cause of failure is not determined, the same failure may occur
again even with a new flange.