The control of noise emissions from on-site power plants relies on a thorough understanding of issues and concerns around industrial noise generally. Here, Elden F. Ray goes back to basics.
by Elden F. Ray
Power plant noise control and the principles of acoustic engineering apply universally to virtually every type of facility. The approach is all the same. Noise control becomes a driving factor when having to meet some regulatory or environmental noise requirement. In critical facilities and control rooms it is imperative to have effective communications, which requires a low-noise environment; noisy environments can be quite fatiguing and adversely affect production.
Sound level and criteria
Some basics about acoustical terminology are introduced here. The term ‘sound level’ is understood to mean an A-weighted sound level (LA) unless otherwise noted. (For instance, LA 85 dB is read as an A-weighted sound level of 85 decibels.) The A-weighted sound level is generally used to assess community response to noise; C-weighting for assessing low-frequency or possible infra-sound impacts. These electronic weightings are used in sound meters to synthesize how the ear receives sound which is a function of sound level and frequency. An A- or C-weighted measurement (LA or LC) encompasses all the sound energy from 10 Hertz (Hz) through 20 kHz as a single comprehensive sound level for simplicity and are widely used for determining compliance with ordinances and specifications.
The use of dBA and dBC as sound level descriptors is no longer promulgated but are expected to be around for many decades because of the slow nature of how changes are effected. Today, it is still common to come across acoustical standards that were changed in 1962 but are still being cited; if not careful, this can result in a 10 dB error. So always make sure you are working with the most recent standards.
When dealing with increases or changes in sound level in a community, the issues can be complex because people all react differently and it is difficult to make everyone happy. It is very important to have good community relations at the beginning of any new project and to minimize impacts on the community. ‘Community’ not only means residential and business communities in the area but your industrial neighbour as well. Table 1 presents expected community responses to changes in environmental sound levels to give an indication of potential reactions.
Table 1. Perception of and reaction to environmental sound level changes
In the US there are no federal standards or laws with regard to limiting noise from power generation facilities and industrial sites with the exception of energy transportation facilities (gas compressor stations), where the Federal Energy Regulatory Commission (FERC) mandates sound levels not to exceed a day-night level (DNL) of 55 dB at the nearest residential receptor; this is equivalent to producing 48.6 dB on a 24-hour basis. Noise regulations vary greatly across states, regions and communities, where the regulations range from well written, to well meaning, to woefully inadequate, to non-existent. In some states or regions the regulatory or permitting board imposes some noise limit at the property line or into the nearby community after study and having hearings. The complexity of facilities and noise regulations can be daunting and each facility should retain the services of a well qualified professional noise control engineer to assist with those issues.
Power plant noise
There are numerous main sources of noise within a power plant and only a few of the major components are addressed here. The two types of plants most frequently planned today are those using combustion turbines or diesel engines as prime drivers for turning generators; and frequently, the waste heat is used to generate steam for auxiliary services. Each main driver can be broken into three principal sources of noise: inlet, casing and exhaust.
The sound power level from equipment can range from about 120 dB to well over 155 dB depending on the size and type of machine. There is no easy method to generally categorize one unit as being noisier than another: a 15 MW turbine can have sound levels as high as a 160 MW unit. However, diesel engines are more predictable, and generally the higher the horsepower or kilowatt rating, the higher the noise level because more cylinders and fuel are needed. The casing noise is usually not an issue because most units have their own enclosures. However, some of the very large units, particularly diesel units, may be free-standing inside a large building. This can create problems in limiting employee OSHA (US Occupational Safety & Health Administration) noise exposures. OSHA regulates the employee time-weighted exposure to sound levels, and does not limit the sound level itself, so near-field and facility sound levels can be well in excess of 85 dB without penalty.
Here is a shopping list of some basic concerns for noise:
- combustion turbine (CT) inlet emits significant blade passing tones and harmonics
- large CTs may produce very low-frequency tones, causing infrasound problems in communities; of particular concern is when a heat recovery steam generator is connected.
- diesel engine firing rate produces very strong, low-frequency tones, and multiple units can create a beating phenomenon when two tones come in and out of phase with each other
- superchargers on diesels create tones similar to that of a CT inlet (same mechanism)
- steam generators, regulators, by-pass and control valves and piping
- generators spinning at 50 or 60 Hz and harmonics may produce unpleasant low-frequency noise for the community
- air-cooled condensers (ACC) can have significant steam-condensing noise
- cooling towers produce fan and gearbox noises that can travel large distances
- fuel forwarding or pumping stations remote from the main facility can be overlooked for noise
- fuel gas pressure regulating, metering and valve stations
- main step-up transformers produce significant tones at harmonics of the line frequency
- condensate pumps, condenser units and associated piping
- openings in enclosures and barrier walls for piping and electrical penetrations not sealed.
- doors and windows left open on enclosures and main engine building.
- piping and pipe hangers not acoustically isolated from structures
- blow-off and venting processes
- remote water-pumping stations.
The sound emissions from power equipment can be mitigated to comply with regulatory limits by applying noise control devices or specifying low-noise equipment. How much mitigation depends on the distance to the nearest property line or noise sensitive receptor. Noise-sensitive receptors are generally residential areas, schools, hospitals and parks, but also of concern are any nearby industrial facilities operating precision machinery that could be affected by low-frequency sound energy (infrasound). These low-frequency sound waves can couple with building structures, causing induced vibration and secondary noises.
In order to predict or model noise from equipment, we need to understand the sound field - that is, how the sound will propagate from the equipment or the sources of noise. Near field, far field, free field and reverberant field are frequently mentioned. These are regions that describe certain characteristics of sound propagation as illustrated in Figure 1.
The near-field region is probably the most difficult to understand as this describes the region where noise propagation is neither well developed nor can be accurately measured because of the spatial variation of sound levels. Complex structures and equipment arrangements also add to the variability.
Figure 1. Definition of sound fields
The far field starts where the sound field is more stable and propagation is fairly uniform. This location is dependent on frequency (wavelength) and is usually two to four major source dimensions away from the noise source.
The free field describes where the sound level decreases approximately six decibels for every doubling of distance. For a ‘line source’ such as a long pipe or duct, the lateral decrease is three decibels per doubling of distance. The ‘decibel decrease per doubling of distance’ rule is theoretical, so field measurements will be close but not exact for the most part.
The reverberant field occurs where freely propagating sound waves are reflected back from a wall, a ceiling, or other surfaces, causing variations in sound levels as illustrated. The sound field really becomes complicated when the near and reverberant fields overlap; that is, there is no free field. This is frequently the case inside industrial facilities and buildings, which can cause issues in regard to suppliers claiming their equipment emits LA 85 decibels at 3 feet (0.9 metres).
Acoustic modelling methodology
The principal method for predicting sound levels is based on the following physical parameters: the power of the sound source (LW), the path (A) over which the sound travels to a receptor resulting in a sound pressure level (Lp) in decibels, which is heard and measured by a sound level meter at that compliance location. Basically, this is mathematically calculated by:
Lp = LW - A
As an analogy, think of a light bulb that has a power level of watts (LW) but the brightness of the light (Lp) is determined by how close or how far away you are and if an object blocks it or if the light is absorbed (A). Sound behaves in a similar fashion, and this basic equation is used for each significant source of noise. The total noise at a receptor location is the cumulative addition of all the noise sources as adjusted by ‘A’ to account for the unique sound path geometry between each source of noise and each receptor location. This equation is applied to each of the nine octave bands as each frequency has unique values for both the source and path of sound.
Figure 2. Noise signatures are needed to effect noise control solutions
To fully model a complex site, each source of noise is modelled on a master three-dimensional co-ordinate grid and geometrically defined as to its shape, size, and sound power level. There are sophisticated computer programmes available specifically designed for modelling and analysing industrial noise.
Accuracy is largely controlled by the variability of the source’s sound power level and the mitigation applied at the source. Most commercial equipment is mass-produced, and manufacturing tolerances and assembly variances can cause significant variances in the sound power level for a class of equipment, easily by ±3 dB. This is not the fault of the manufacturer but inherent in most manufacturing processes due to tolerances. Even field erection and installation of the equipment can cause significant changes in noise as a result of foundation design and fabrication, equipment mounting and connections, openings in walls and piping penetrations. In addition, many field or erection changes occur without consideration of possible noise impact. Piping connections and supports are frequently disregarded in the design for low-noise performance and end up compromising performance because of inadequate isolation or lagging. Beware that most acoustical treatments are also thermally insulating, and polymers and plastics cannot be used in high-temperature environments because of out-gassing and fire potential.
Fundamentals of noise control
In mechanical systems, known or definable mechanical motions are termed ‘forcing frequencies’ - that is, motions or vibrations generated by the machine or device itself. A motor turning at 58 Hz with a six-bladed fan has a blade passing frequency (bpf) of 358 Hz. A gear having 200-teeth spinning at 180 rpm has a frequency of 600 Hz. A transformer may buzz at 480 Hz. (And we have not looked at harmonics yet.) Figure 2 shows the noise signature of a compressor inlet having a fundamental blade passing frequency at 2100 Hz with multiple harmonics. Just knowing the principal forcing frequency may not be sufficient in effecting a solution to a noise problem.
Noise control or mitigation involves several steps, and the amount of noise reduction is driven by having to meet an environmental noise limit or some regulatory limit. Developing a simple model using the classical approach to noise control (shown below) allows the examination of the options for effectively and economically reducing noise:
source of noise → path of noise →received noise
Applying noise control involves affecting one of these three elements. Most often it is the ‘path of noise’ that is controlled by use of acoustic enclosures, barrier walls, duct silencers and other similar noise control treatments. This method is the most widely used as the degree of noise control can be tailored depending upon the noise requirements and generally it is the more economical approach. Reducing the noise at the ‘source of noise’ can be expensive because most equipment manufacturers assemble their product using commodity parts that are economically produced for the industrial market. Reducing noise at the source may require a complete redesign and retooling process which takes time and money.
Frequently, more silencing is needed than what can be described as the bare minimum in order to account for noise from other equipment or sources that all combine to create a total sound level. Thus, a balance of plant or total noise analysis must be performed to adequately account for all possible sources of noise, including those out of scope.
The reduction of significant sources of noise frequently results in what were once obscure sources of noise now becoming important when having to meet a low-noise requirement. Grouping smaller sources together can be beneficial in that a common noise barrier or enclosure can solve a lot of small problems. For new installations, examine the plant arrangement and locate the noisiest equipment or operations away from noise-sensitive areas.
Do your homework
Industrial noise control can be as simple as adding an enclosure or barrier wall around some equipment or as complex as a facility upgrade or designing a facility from scratch. It is important to do the homework and define all the variables and issues, including regulatory and permitting requirements, and to have good proactive community relations for future growth.
Elden Ray is an acoustic/noise control expert with Universal Silencer, Stoughton, Wisconsin, US.
Stand-by power at London’s Stock Exchange
The London Stock Exchange installed three emergency diesel-powered generators on the rooftop of the exchange, in a densely populated area of the business district with high-rise buildings located directly across from the generators. The noise from the generator exhausts were found to be intrusive and caused complaints from the nearby office buildings.
The optimized generator enclosure on the rooftop of the London Stock Exchange
Field measurements identified the key problem frequency, which occurred at three times the rotational frequency, to be an unusual aerodynamic occurrence from an engine that the standard reactive mufflers did not adequately attenuate. This low-frequency tone needed approximately a 15 dB reduction to have the noise compatible with the existing business environment.
The building was already completed, and limited space and weight allowances made incorporating a solution a real challenge. The critical situation called for special analysis with scale modelling to arrive at a solution that would fit into the existing exhaust system while minimizing back pressure on the engines. Our analysis determined the optimized reactive silencer design, and laboratory modelling at the University of Southampton, UK, confirmed the approach and further refined and tuned the design to achieve the required reduction.
Power generation in the Canary Islands
The local power utility Endesa expanded an existing Canary Islands power plant by adding two MAN 18.9 MW 48/60 18-cylinder diesel engines. The plant is located on the coastline with local residences and resort hotels nearby across a small bay. The performance requirements for the exhaust system were quite aggressive because the operator did not want to create higher noise emissions into the community but demanded very good fuel efficiency as well.
Measurements verified that the silencers were performing as expected; however, there was too much aerodynamic pressure loss, causing higher-than-expected fuel consumption. The challenge was to reduce the pressure losses while maintaining acoustical performance. Each exhaust system had a two-stage silencer installed, and field diagnostic measurements and analysis determined both units had to be modified to achieve the required aerodynamic performance.
A one-twelfth model of the silencer was made. It was determined that the solution was the elimination of a center chamber wall, and other internal modifications would achieve the desired results for reducing the pressure drop. A parallel analysis was performed, and the removal of the center chamber wall provided the space needed for an acoustical pack section.
The field modifications were carried out and the pressure losses were reduced from 49 o 30 mbars, some 6 mbars below the criterion. The acoustic performance was improved by reducing the sound level at the stack exit from 73 dB to 68 dB and there was no increase in engine firing rate sound pressure levels. The owner/operator was very pleased with the performance.
Modifications to a Canary Islands power plant reduced both noise emissions and aerodynamic losses