Gas, O&M, Plant Optimization, Reciprocating Engines, Retrofits & Upgrades

Wall of sound: Managing acoustics around reciprocating engines

Noise has become a hot topic for nearly every industry, and power generation is no exception. There are evolving regulations and interpretations to consider as well as the community response to noise.

Anymore, just meeting the limits doesn’t mean you’re done. Unique challenges can arise from community response to your project and/or others in your industry that can affect your design.

Addressing those challenges becomes a balancing act to optimize acoustical footprint while meeting the overall project budget, maintaining construction and maintenance flexibility, and achieving contractual operational requirements. Whether it’s overall sound levels, low-frequency noise issues, or some distinctive local situation, there are options to satisfy all stakeholders.

Reciprocating engine projects introduce even more variables that need to be addressed in design and development. The intermittent operation, sound profile, low-frequency component, and proximity to residences are some of these variables. The application and characteristics of RICE units requires a different way of thinking about noise mitigation than in other power generation applications.

Most regulations in North America are focused on A-weighted sound levels (dBA). The A-weighting system is developed to represent the relative loudness perceived by the human ear and is less sensitive to low-frequency noise. Many jurisdictions also include penalties for tonal noise, for example the limit may be 5 dBA less if there is a tonal noise present. Unfortunately, most regulations do not provide numeric limits for low-frequency noise. In some regulations a C-weighted limit is provided. These limits are designed to protect the community from low-frequency noise and possible noise-induced vibrations.

The major noise sources on a reciprocating engine power plant include the engine hall, charge air inlets, exhaust systems and stacks, cooling radiators, and transformers. The plants typically generate a significant amount of low-frequency noise, between 31 and 100 Hz, from the engine bodies and exhaust, as well as broadband and higher frequency sound levels from the radiators, charge air inlets, and transformers. 

The reciprocating engines are designed to operate within a building without a sound enclosure around the engine. This creates a sound environment often in excess of 110 dBA within the building. Interior sound levels this high require double hearing protection and there are workplace safety limits for how much time a worker can spend in the engine hall while the units are operating.

Due to the large size of the engine room, the fundamental modal resonance of the building is at a low frequency (i.e. 31 Hz) creating an environment where low-frequency noise can resonate within the engine room and cause the building shell to be excited.

Typically, the engine hall walls are constructed of insulated metal panels, generally with a Sound Transmission Class (STC) of approximately 25 to 30. The STC rating of a partition is a widely used, convenient single value metric used to describe the acoustical performance of the partition. It is calculated from the measured transmission loss values between 125 Hz and 4,000 Hz. The acoustical properties of the partitions for low frequency sound are not defined by the STC and should be derived from field measurements of similar installations. The overall transmission loss of low frequency bands as well as interior sound absorption need to be carefully analyzed to properly evaluate the performance of building. The STC rating of the engine hall walls will change depending on project needs.

Commercially available mitigation options can achieve significant reductions to operational sound levels. Design of mitigation options is highly specific to the project’s location, site dimensions, proximity of neighbors or other sensitive receivers, regulatory criteria, public sensitivity, etc. and there is no one-size-fits-all or silver bullet for mitigation.

A “standard” mitigation package for a single-engine site would include various options. Typically, the engine manufacturer or vendor will supply the radiators with minimal to no sound mitigation. The stack and charge air inlets associated with the engine generally come with nominal silencers.

Some projects require considerably more mitigation than others. A project that required significant mitigation included concrete precast wall panels (STC55), ultra-low-noise radiators, significant additional stack silencing, and engine hall ridge-vent silencing. To provide a general understanding of the standard unmitigated engine package vs the custom designed mitigation package detailed above, sound levels were modeled for both a single-engine project and a three-engine project.

Sound levels in dBA and dBC are provided for both designs at 500 feet in Table 1. The model assumed flat terrain over grassland for this analysis. Many factors can influence sound levels offsite and each facility should be modeled to determine predicted impacts at the property line and neighboring noise sensitive areas.

Table 1: Relative Order of Unmitigated vs. Significantly Mitigated Impacts1

Operating Scenario Unmitigated dBA @500 ft Unmitigated dBC @500 ft Mitigated dBA @500 ft Mitigated dBC @500 ft
One Reciprocating Engine 65 90 50 75
Three Reciprocating Engines 70 95 55 80

1. Approximate sound levels based on general design and mitigation assumptions. Facility sound levels would change slightly based on design and equipment installed.

Sound contours for the single engine facility were generated by the modeling software which shows a two-dimensional representation of how sound moves away from the source. Figure 1 shows the sound contours for the standard unmitigated engine package, and Figure 2 (both shown at end of story) shows the sound contours for the custom designed mitigation package detailed above. The contours represent sound transmission over flat terrain for each scenario. Significant elevation changes would alter how sound moves away from the source.

Custom mitigation to address specific issues like low-frequency noise, tonal noises, etc. can be designed and applied. However, cost vs. performance degradation should be considered unless the mitigation is required for regulatory compliance.

Figure 1: Single Reciprocating Engine Plant without Mitigation
Figure 2: Single Reciprocating Engine Plant with Mitigation
Weger

About the authors: Gabriel Weger is an Environmental Engineer at Burns & McDonnell with 7 years of experience working in the acoustics and noise control field, and is an elected member of the Institute of Noise Control Engineering. He has worked with numerous industrial and generation clients to mitigate noise emissions to onsite employees and neighboring sensitive areas.

Brewe

Ian Brewe is a Board Certified Member of the Institute of Noise Control Engineering who has worked in the noise control industry his entire career as both an acoustical consultant at Burns & McDonnell and as manager of a noise control equipment company. He has designed and supplied acoustical solutions for power plants around the globe. 

Howell

Chris Howell is a Project Manager in the Environmental Services division at Burns & McDonnell, specializing in permitting of traditional and renewable power generation, transmission, and distribution facilities. He has performed predictive and compliance noise studies throughout the world, and is an elected member of the Institute of Noise Control Engineering.