Flanged pipe joint

The essentials

A flanged pipe joint connects two pipes end to end by means of flanges, which are flat rings either cast integral with the pipes or fixed to their ends. The flanges are brought face to face with a gasket (a thin packing ring of rubber, fibre or similar material) between them and are clamped together by bolts and nuts passing through holes drilled on a circle. Such joints are strong, leak-proof and can be dismantled, so they are widely used in water, steam and oil pipelines.

Key points

• Parts: two pipes with flanges, gasket, bolts and nuts (with washers if required).
• Gasket: makes the joint leak-proof by filling small irregularities between the flange faces.
• Bolt holes lie on a pitch circle (bolt circle) and are equally spaced; the number of bolts is usually even (for example 4 or 6).
• Angular spacing of bolts = 360° ÷ number of bolts. Distance between adjacent bolt centres (chord) = pitch circle diameter × sin(180° ÷ number of bolts).
• Example: 6 bolts on a 200 mm pitch circle are 60° apart, with adjacent centres 100 mm apart.
• Flanges are usually thickened where they join the pipe, with a fillet, to strengthen the junction.
• Some designs use a spigot on one flange fitting into a recess on the other to keep the pipes aligned.

Exam tips

• Draw the side (end) view of the flange with the pitch circle as a thin chain line and the bolt holes equally spaced.
• In the sectional front view, hatch the flanges and pipes but not the bolts.

Common mistakes

• Omitting the gasket in the assembly.
• Spacing bolt holes unevenly or placing them off the pitch circle.