Vibrations in piping systems can cause a myriad of issues ranging from equipment damage to fatigue failure. While there are some causes of vibration that are observable through normal operation and inspection, Acoustic-Induced Vibration (AIV) is different as it is caused by acoustic waves occurring at high frequencies. As these high-frequency sound waves travel through a piping system, they excite circumferential vibrations, leading to dynamic stresses on the pipes. Unaddressed, these dynamic stresses can lead to damaged or ruptured pipes. Source: Price, Stephen M. and Donald R. Smith. “Sources And Remedies Of High-Frequency Piping Vibration And Noise.” (1999). There are generally two methods for addressing Acoustic-Induced Vibrations: Extensional-Layer Damping and Constrained-Layer Damping. Extensional-Layer Damping involves the installation of an outer shell that reduces acoustic stress on a pipe. Constrained-Layer Damping combines a strong outer shell with an additional more flexible thermoplastic inner layer. Both options can help reduce vibrational and acoustic stress, but there are differences between the two in terms of overall levels of stress relief as well as cost considerations. Extensional-Layer Damping: A Straight Shell One popular method for Extensional-Layer Damping consists of a multi-layer outer shell of carbon fiber composite that can be installed around an existing piping system to reduce vibrational stresses. This method can easily be applied prior to installation or in the field after pipes are installed, even if they’re in service. The number of layers of composite and the orientation of the fibers directly affect the overall stress reduction. Additional layers can be added to increase the amount of stress reduced, but research has shown that after 5 or 6 layers, reduction experiences diminishing returns. Source: Price, Stephen M. and Donald R. Smith. “Sources And Remedies Of High-Frequency Piping Vibration And Noise.” (1999). Number of Layers Maximum Stresses (MPa) Reduction in stress Minimum Stress in the Pipe (MPa) 1 35.711 – 34.825 2 27.953 21.7% 27.004 3 22.952 35.7% 22.075 4 19.466 45.5% 18.675 5 16.898 52.7% 16.184 6 14.923 58.2% 14.232 7 13.367 62.6% 12.781 8 12.101 66.2% 11.567 Extensional-Layer Damping works best for preventing damage to pipes that are subject to intermittent or moderate levels of vibrational stresses during operation. It is cost-effective for a field-installed system and is versatile as the design can be customized to match the needs of the specific piping system. Extensional-Layer Damping is not a viable solution, however, for piping systems that are subject to higher levels of AIV. Constrained-Layer Damping: A Viscoelastic Layer The Constrained-Layer Damping method adds a viscoelastic layer inside of the barrier layer that absorbs additional vibrations and increases the effectiveness of the system. Many of the common viscoelastic materials used for the inner layer are commercially available rubbers. Research shows that the high-strength barrier layer forces the inner viscoelastic layer to deform in shear when the pipe wall is deformed. Below is a table of the shear modulus and loss factor of common viscoelastic materials at a frequency of 28.19 Hz. Source: Price, Stephen M. and Donald R. Smith. “Sources And Remedies Of High-Frequency Piping Vibration And Noise.” (1999). Material Frequency (Hz) Shear Modulus (MPa) Loss Factor Dyad 601 28.19 5.000 0.600 Vinyl 70A 28.19 6.508 0.544 Silicone 50A 28.19 0.962 0.377 Silicone 30A 28.19 0.318 1.431 Buna-N/Nitrle 60A 28.19 4.028 0.649 Buna-N/Nitrle 40A 28.19 1.549 1.035 Cyclic Loading Tests 28.19 4.194 0.237 Butyl 28.19 2.908 0.433 While Constrained-Layer Damping offers higher levels of stress relief from AIV, there are cost and application implications. Many of the common materials used for the inner layer do not easily adhere to steel pipes. To provide better adhesion, elastomeric polymers like urethanes and epoxies can be used but require a skilled application team. The temperature of the piping system also plays a critical role in the ability of the constrained-layer system to reduce stress from AIV and must be taken into consideration when designing an optimal system. Advanced FRP Solutions Are your critical assets facing Acoustic-Induced Vibration (AIV)? Contact Advanced FRP Systems to speak with a knowledgeable partner who will take the time to diagnose the causes of AIV and design the best solution for your piping system and process.