An Acoustic Solution for Dangerous Noise
The noise recorded coming from the dry cooling system at the Severn Power CCGT Power Station at Uskmouth B was 130dB-135dB, a full 50dB above the acceptable levels described in the Noise at Work Regulations.
To make matters worse, the noise generated had a very low-frequency bias. Low-frequency noise is the most difficult to treat from a soundproofing perspective due to the excessive length of the wave cycle.
Solutions for low-frequency noise issues typically involve wrapping the problem in significant quantities of acoustic insulation, with many standard solutions being as deep as 500mm to 700mm. The sheer volume of lagging required for an insulation-based approach to a project like Uskmouth, with a daunting 8,000m2 of ducting to be covered, would be expensive, time-consuming to install and prohibitively disruptive.
What’s more, there were areas around the ducting at Uskmouth which simply wouldn’t have been able to accommodate such an excessive construction height of soundproofing material.
Paul Absolon, Technical Director, responded to the challenge of creating a ‘thinner’ soundproofing system that would meet the necessary Noise at Work Regulations requirements but would be cost-effective and efficient to install.
In order to minimise disruption to Uskmouth, an off-site simulation was created near Burton on Trent, using a large section of identical ducting with a ‘door’ sealing up either end. Within the duct were several very powerful speakers. For testing, highly sensitive microphones were placed in strategic positions along the outside of the duct to measure any ‘leakage’. Acoustic insulation solutions were conceived, implemented and assessed in this controlled environment.
Exploring a wide range of acoustic materials from SIG’s industrial acoustics range, it was clear that a single product was not going to be able to solve the problem on its own, so Paul opted for a combination of products working in concert, layer upon layer.
The first layer consisted of CMS HT1B elastomeric isolation pads, constructed from a polyurethane-bound rubber granulate specifically formulated to dampen and/or isolate noise and vibrations at source and independently tested by the Institute of Structural Dynamics at the Technical University of Dresden.
The 50mm thick pads were bonded to the surface of the duct at a rate of nine per square meter, creating 300mm spacings; so, as well as the dampening effects of the material itself, the construction benefited from large, evenly distributed airspaces in its foundations. Sound waves move less effectively through dead air.
The second layer consisted of 50mm-thick QuietSlab SVX3, a high-performance, mineral-fibre acoustic lagging. MIMA (the Mineral Wool Insulation Manufacturers Association) describe the acoustic insulation properties of mineral wool (and mineral fibres more generally): “Porous materials such as mineral wool work to control and reduce noise by allowing air movement into the fabric of the material. The fluctuations of air molecules – which form sound waves – move into the body of the mineral wool, where friction between the air particles and the material’s narrow airways cause sound energy to be dissipated as heat.”
The third layer comprised CMS WBBKT Acoustic Barrier, a high-density, barium-sulphate-loaded thermoplastic polymer, which is thin, flexible and easy to work with. Whereas the QuietSlab SVX3 layer is designed to absorb and dissipate noise, this dense acoustic barrier is designed to resist the passage of noise and is particularly adept at preventing the passage of low-frequency noise.
The forth layer duplicated the second. The fifth layer duplicated the third. The sixth and final layer consisted of a corrosion-resistant Aluzinc casing.
By alternating between thick noise-absorbent layers and thin-but-dense noise-resistant layers, Paul was able to create a soundproofing solution with a depth of just 170mm – between 66% and 76% thinner than a 500mm to 700mm standard solution. However, the successful reduction of the construction height would mean nothing at all if it failed to deliver the necessary levels of noise reduction. The proof would be in the testing.
Personnel from Siemens attended the test. They were standing in relatively close proximity to the simulated duct whilst technicians from Muller-BBM set up their equipment. As always with these situations, there were delays, so the Siemens team were standing around for quite some time. Naturally, they were a little impatient and asked when the test was going to commence. They were told the test had been running for the last 10 minutes. The speakers within the ducts had been generating noise levels of 130-140dB and no-one had noticed. Only when the lagging protecting the ‘door’ to the duct was removed could the true extent of the racket within be appreciated.
The testing, once complete, revealed that Paul’s solution had reduced the noise generated by 39%, to just 82-83dB(A), under the 85dB required by the Noise at Work Regulations.
Peter Ullrich, project director at Siemens Energy, commented: “Effectively controlling noise and reducing sound emissions was a top priority for us in the Uskmouth project. Not only was it essential that the dry cooling system satisfied all the legal acoustic obligations and regulations but just as important was that neighbouring properties were not disturbed by additional noise levels.”
Paul Absolon said, “It was a real challenge working on the Uskmouth project, the kind of challenge we relish. We avoid a one-size-fits-all approach, taking a fresh approach to each project. This ethos has seen us apply our knowledge and experience to everything from schools, theatres and churches to oil rigs, refineries and, now, power stations.”