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Piping Flange Fundamentals
When selecting a flange, the material is chosen to meet process requirements first, whilst the temperature and pressure
requirements are then met based on the material chosen.
Bolted, Threaded and Welded Joints
Flanges are a type of bolted joint. Other common types of joint include threaded joints and welded joints.
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A bolted joint requires a flange and fasteners (nuts, bolts, or studs).
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A threaded joint requires a male and female screw thread, the male thread screws into the female thread.
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A welded joint is made using a weld (the process of melting/fusing metal by applying heat).
Bolted Joint
The type of joint used depends on many factors, including pressure, temperature, type of process fluid, operating
characteristics of the system, and the surrounding environment. A bolted joint may be used if:
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Other types of joint are not suitable e.g. welding may not be possible within areas that pose a fire or explosion
risk (Ex areas); this is mostly a concern for an already operational piping system, not one that is under construction.
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A machinery item must be disconnected from the service line in order that maintenance or replacement of the
machine can occur.
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Quick field assembly is required using only basic hand tools.
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The item (e.g. tank, pipe, machine) to which the flange is connected must be frequently maintained; it is quick and
easy to disassemble and assemble a flange, but not a weld.
Some of the main disadvantages associated with a bolted joint include:
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Insulating a bolted joint (thermal insulation) costs more than insulating a threaded or welded joint.
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Bolted joints require more physical space than threaded or welded joints.
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Each bolted joint represents an additional leakage point (even if assembled correctly).
As a general rule, threaded joints are suitable for lower pressure and temperature applications only, whilst bolted and
welded joints are suitable for higher pressure and higher temperature applications. If a threaded joint must be leak tight,
and leakage cannot be tolerated, it can be seal welded. The seal welding technique is only used for higher service pressure
conditions and is not an ideal solution because it creates a stress concentration point which will be prone to fatigue failure.
The advantage with welded joints is that the weld can be proved using non-destructive testing (NDT) techniques e.g.
penetrant testing, ultrasonic testing, magnetic particle testing, hydrostatic pressure testing etc.; proving a flange -and flange
gasket- is more difficult.