High levels of HFAE can cause fatigue failures at welded features such as pipe supports and small bore connections. National Grid has experienced fatigue failures as a result of this phenomenon at a number of compressor sites including Carnforth Units A/B (failure of a blank weldolet) and Moffat Unit B (failure of the recycle line due to a longitudinal crack through a main weld).

The conventional approach for the control of vibrations is to avoid resonance caused by the coincidence of the excitation frequency, and any natural frequency of a system. This is not always practical, particularly if vibrations occur over a wide range of frequencies. Constrained layer damping (CLD) is a passive treatment using a three-layered sandwich system whereby viscoelastic layers (materials having both viscous and elastic properties) are sandwiched between constraining layers of stiff materials such as steel, aluminium, titanium or carbon fibre composite. The increased availability of polymeric materials, which have viscoelastic properties and high damping capabilities has made it possible to control the structural vibrations. The polymers are commonly used in two basically different configurations to dissipate the mechanical energy of vibrations as heat. Generally, the constrained layer damping system is used for very stiff structures, across industries such as aircraft, spacecraft, appliance, and computer industries.

Increased understanding of HFAE leading to efficient and effective attenuation of the resonant frequencies will lead to improved mitigation methods and therefore improved maintenance and inspection routines. Hence National Grid initially commenced investigation into the issue under project - “Develop novel mitigation method for high frequency main pipework vibration” NIA_NGGT0008. Project NGGT0008 is continuing to develop a grouted ring solution however the potential of CLD is such that it could offer improved performance both in parallel with and combined with the grouted ring. There is no duplication between NGGT0008 and this project, both will be complimentary in delivering the most optimal solution for each installation.

Benefits

The development of the constrained layer damping technique as a low cost solution for high frequency acoustic excitation.