Pipework Failure Mechanism – 4 Key Steps

Small Bore Connections, Tech Library
  1. The majority of all Pipework Vibration Failures occur at Small Bore Connections (SBCs).
  2. Frequently, small (normal or abnormal operational) vibration levels on a main line can excite increased levels of vibration of vulnerable SBCs.
  3. Poor geometry / location of the SBC can result in damaging stress levels at the welded interconnection between the SBC and the Main Line.
  4. Over time, these operational stress cycles can result in through-wall fatigue fractures and resulting leakage from the pipework (HCR).

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Large Scale Deformations

Small Bore Tubing, Tech Library

Small Bore Tubing (SBT) runs must be designed to accommodate Large Scale Deformations (e.g. due to Main Line thermal expansion). Traditionally, this type of movement was always enabled by incorporating ‘pigtails’ (multiple circular coils of tubing) within the SBT runs One of the most radical recommendations within the new guidance notes is the replacement of ‘Pigtails’ with ‘Rectoils’.

Laboratory testing by Xodus has shown that Pigtails are not the most effective method of reducing SBT stresses caused by large scale deformations. In contrast, the rectangular coil design of the ‘Rectoil’, when applied in the correct way and with the correct geometry can offer superior performance.

The design of the Rectoils is covered in great detail in the guidance notes and is very prescriptive, including:
  • Precise minimum lengths for all Rectoil sections R1, R2 & R3
  • Precise minimum lengths for all pipe straight sections A, B & C
  • Precise orientation (short side of Rectoil must be in 1st pipe run (line A)
  • Precise spacing between the coils of the Rectoil
  • Precise orientation of Rectoil such that the direction of expected largest deformation will cause separation of coils within the Rectoil.

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Structure Mounted Arrangement

Small Bore Tubing, Tech Library

This relates to Small Bore Tubing (SBT) which spans between a Main Line pipe and a fixed point on a nearby structure.

  • SBT pipe run must contain 3 bends
  • Guidelines specify the minimum length of each straight section
  • Bends 1 & 2 must be in different planes
  • Structure support must be > minimum distance from Bend 3 
  • Pipe pends must be > minimum distance from fittings
  • Pipe must support no additional components

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