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Piping Flange Fundamentals
Introduction
Flanges offer a mechanical means of joining pipes, fittings (elbows, tees etc.), and valves. Compared to welds, flanges
are a non-permanent type of joint that can be easily assembled and disassembled (ideal for systems that require
maintenance). Flanges are installed via welding, screwing, or lapping, and they are the second most popular joining
method after welding.
A flange assembly consists of:
•
Flange (blade, hub).
•
Gasket (metallic, composite, or non-metallic).
•
Fasteners (nuts, bolts, or studs).
Flange Assembly
Flange Terminology
Flange terminology and nomenclature can be confusing due to the similar terms, definitions, and phrases that are used.
To make the learning process easier, readers should clearly understand the following terms:
•
Flange types – refers to the flange design. Examples of flange types include the welding neck (weld neck), slip-
on, socket weld, threaded, blind flange and lap joint type flanges. Flange types are selected based on the
temperature and pressure requirements, and are identifiable by their geometry.
•
Flange faces – refers to the area used for sealing of the flange; a gasket is usually installed between the two
opposing flange faces. Examples of flange faces include the flat, raised, ring-type joint (RTJ), lap joint, tongue
and groove, and male and female designs.
•
Flange surfaces – refers to the condition of the flange face sealing surface. A flange face surface may be smooth,
or serrated1. The smoothness of a flange face surface is defined by its Roughness Average (Ra) or Arithmetic
Average Roughness Height (AARH).
All of the afore mentioned topics will be further discussed. It is important to realise that there are many aspects that
influence not only what flange type is chosen for a particular application, but also what face and what surface. For example:
•
Certain systems may require welded joints that can be easily inspected (this is not always possible with certain
flange types).
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Certain flange faces may not be suitable for higher pressure systems because the maximum sealing pressure is
too low (flat face designs).
•
Certain materials will tend to have poor finishes that yield a correspondingly rough sealing surface; these rough
surfaces require a gasket if a leak tight seal is to be achieved e.g. cast-iron flanges.
1 ‘Serrations’ are machined grooves cut into the surface of a flange’s face. Gasket material flows into the grooves, which results in a more reliable seal being obtained;
the grooves also help hold the gasket stationary.