Flexible coupling definition. NFPA 13 (2007, section 3.5.4) defines a flexible coupling as one that allows axial displacement. The coupling must allow at least 1 degree of angular movement without damaging the pipe. For sizes 8 inch and larger, angular movement is less than 1 degree but not less than 0.5 degree. For sprinkler systems the standard calls for flexible connections against earthquake damage and gives some example applications. Always check the current edition of the standard for each project.
Typical applications
- Flexible couplings on main risers and branch line risers
- Flexible coupling on the horizontal part of the connection
- Absorbing ground settlement or movement around tanks
- Absorbing inter-storey drift
- Correcting misalignment (with two flexible couplings)
- Curved line design
- Absorbing thermal stress
- Seismic separation and sway bracing
Inter-storey drift example
In tall buildings the risers move laterally during an earthquake. Take storey height H = 4 m and a drift ratio of 1/150: the drift per storey is 4000 × 1/150 = 27 mm. If a 200 mm (8") coupling allows 1.67 degrees, the movement a 4 m pipe can take up is about 116 mm. So one flexible coupling at each floor is enough to absorb this drift.
Thermal expansion
For steel pipe, expansion is found with μ = α × L × ΔT (α = 1.2 × 10⁻⁵ for steel). Example: a standard 5.5 m length with a 40 °C temperature difference gives 1.2 × 10⁻⁵ × 5500 × 40 = 2.64 mm. The permitted gap between pipe ends of a 100 mm (4") coupling is 3.2 mm, so using one coupling per 5.5 m of pipe absorbs this expansion. For an anchored system, leave a safety margin of N × E × 1/2 when calculating the number of couplings (N). Use an expansion joint where larger movement is expected.
Expansion joint (Model 500 / 501)
A sliding-type joint providing 0–76 mm (0–3") of axial movement. The standard unit consists of XGQT2 or 1212 flexible couplings and Sch. 40 grooved pipe nipples. Components are coated with RAL 3000 red epoxy. An integrated safety feature prevents over-travel and the grooved parts from pulling out.
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