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Environmental Noise Assessment And Control For Compressor Stations.

Increasingly, environmental noise emission is being classified and treated by regulators as a contaminant or pollutant, similar to other types of emission, such as [NO.sub.X], odor or particulate. Tighter restrictions on environmental noise place real demands (with non-trivial cost ramifications) on compressor operators to reduce noise emissions.

By virtue of the extreme sound power levels associated with compression equipment, there is virtually no such thing as a compressor installation that is free of environmental noise concerns. Typically, the sound power produced by an unsilenced gas turbine, for example, is on the order of a thousand times greater than other common types of industrial sources of sound. The potential radius of environmental noise impact of a compressor station, depending on the degree of noise control included in the design, can range up to several miles. Thus, even for the most remote compressor installations, there are usually one or more sound sensitive receptors within earshot. Accurate noise assessment and effective noise control are essential in allowing a compression facility to co-exist in harmony with surrounding land uses.

Environmental Sound Emission Limits

Acceptability limits for environmental noise emissions vary, from jurisdiction to jurisdiction. In general though, the principle upon which environmental noise limits are premised is that of avoiding disturbance or annoyance to neighboring residents. Whereas limits for worker noise exposure are based on minimizing risk of hearing loss, the limits for environmental noise impacting a residence are set at much lower sound levels. The onset of adverse noise impact, as evidenced by demonstrable loss of enjoyment of property and concerted community reaction to noise, occur at much lower sound levels (e.g., often on the order of 40 to 50 decibels) than the onset of the risk of hearing loss (80 to 90 decibels).

Sound level limits are often based on the amount of background sound occurring in the area surrounding the sound source under assessment. In areas having an essentially urban acoustic environment with a background sound comprised of man-made activities such as traffic and other industry, the appropriate sound level limit is higher than would be applicable cable for a quieter, rural area. In some jurisdictions, determining the applicable limits for environmental sound emission involves measuring the background sound (without the operation of the source under assessment).

In some ways, the limits for noise emissions are less abstract than those for other types of emissions, since they are based upon the potential for the industrial sound source to be audible or disturbing at a neighboring residence. When the limits are significantly exceeded, vehement noise complaints can be expected. A preponderance of complaints over time may bring the industry under the scrutiny of the regulatory authorities, or of the company's own corporate directors who wind up fielding the telephone calls from angry residents.

Even in the rare cases where a new compressor site is to be located remotely enough that there are no nearby noise sensitive receptors, regulations in some jurisdictions now include `pristine environment' provisions. These regulations may require the proponent to assess the environmental noise impact at a predetermined radius, to address the possibility of future encroachment of other land uses, such as residences. Realizing that a remote area may not always be remote in future is a wise perspective at design time, since remedial noise control retro-fitting is often prohibitively costly, and not always possible.

These factors emphasize the importance of considering noise impact early in the design process of a new compressor facility, or a contemplated expansion. The need for an environmental noise assessment of an existing facility may be triggered by:

* requirements for a baseline study in anticipation of an expansion,

* in response to complaints over existing noise emissions,

* as an acoustic performance qualification with regard to contractual responsibilities for newly installed equipment, or

* to satisfy regulatory requirements for periodic acoustical auditing.

Effective assessment and control of environmental noise emissions requires accurate identification and quantification of the significant noise sources, and an understanding of the available means of noise mitigation.

Sound Sources And Mitigation

Compared to general industrial facilities, the noise sources associated with a compressor station are well understood. Unless the compressor is electrically driven. the dominant sound source is usually the engine. In order of significance, the engine sound is comprised of the exhaust noise, the intake noise (in the case of a turbine engine) and the casing radiated noise. Since most compression equipment is housed indoors or inside some type of enclosure, the sound radiated by the engine casing is typically an environmental issue only insofar as the sound transmits through the walls of the enclosure and through ventilation openings.

Similarly, the sound radiated by the casing of the compressor itself is often contained within a building or enclosure, and is an issue of transmission to the outdoors. Compressor sound is also carried out through the suction and discharge piping and will re-radiate through the walls of any above-ground piping. For several reasons discussed here, pipe-radiated sound is the dominant source of emission for many modern compressor stations.

Other secondary sources of sound include noise radiated from exposed exhaust and intake ductwork, and ancillary equipment such as oil coolers and gas after-coolers, and occasional activities such as blow-downs, purges, and bleed-valve venting during startup.

In our work conducting baseline sound studies, design assistance for new Facilities, acoustical performance qualification of new equipment, complaint investigations and acoustical audits of dozens of compressor stations of differing vintages over the past ten years, it is interesting to note that great progress has been made in controlling environmental noise from compression facilities. There have been many installations where tight limits on environmental noise emissions have been successfully met. Several technological advances have made this success possible.

Improved Measurement Techniques. The first of these advances is improvement in acoustical measurement techniques. The ability to design effective noise control and meet emission limits depends upon the accuracy of the source sound level information available for the major equipment. Unfortunately, since the components of a compressor facility cannot be run in isolation of one another, particularly when measuring under full-load conditions, there is always interference introduced from other sources not under test. This is particularly true when measuring the performance of noise control hardware such as exhaust silencers. To the degree that such hardware is functioning properly, the emitted sound levels will be low with respect to the multirode of other sources on site (such as piping radiated noise). In this regard sound intensity measurement techniques, which have an inherent ability to reject interference from background sounds, have proven indispensable in improving the accuracy of source sound level measurements [1]. Sound intensity methods are discussed in greater derail in the following section.

Improved Exhaust Silencers. Another major advance in controlling noise emission from compressor facilities has been a growing understanding of the acoustical characteristics of exhaust silencers, particularly those for gas turbines. Specifically, since the late 1980's and early 90's there has been valuable research published about the effects of high temperatures on the acoustical performance of absorptive silencers (the parallel splitter baffle type of silencers, which are filled with acoustical media, such as mineral wool). [2], [3], [4]. While it has been long recognized that the elevated temperatures in a gas turbine exhaust stack result in longer wavelengths of sound than at room temperature (due to the increased speed of sound), it is only recently that work has been done to understand the effects of the high temperatures on the acoustical properties of the sound absorbing media. The results of this work have revealed that, at higher temperatures, absorptive silencers generally provide less low frequency absorption, and more high frequency absorption than at room temperature. It is important to know about this phenomenon, and account for it in designing and selecting silencers for hot exhaust stacks.

Quieter Cooling Fans. With regard to ancillary equipment incorporating axial fans, such as cooling towers, oil coolers and gas after coolers, there have been recent improvements in fan blade design that have allowed significant reductions in fan generated noise emissions.[4] Aerodynamically designed `high-solidity' fan blades offer significant increases in airflow efficiency, such that the necessary rotational speed of the fan to move a given quantity of air can be reduced. Since the sound generation of an axial fan varies as the 5th power of the blade tip speed, these lower speed fans generate considerably less noise.

Optimized Acoustical Pipe Lagging. Positive recent efforts have been made in understanding the methods of reducing pipe-radiated compressor noise. The most common method of controlling pipe-radiated noise is acoustical wrapping of the pipe, termed `acoustical lagging.' The Pipeline Research Committee of the American Gas Association commissioned a study in 1998[5] to investigate thoroughly and optimize the acoustical performance of pipe lagging systems. The final report from this study is available for purchase from the AGA. Some of the interesting parameters quantified in the study include:

* The fact that, at low frequencies (below about 200 Hz), wrapping or lagging actually amplifies the sound radiated by the pipe, for virtually all pipe diameters and lagging configurations. This means that lagging is only effective for controlling high frequency sound, such as compressor whine, but not low frequency rumble such as turbulent flow noise.

* The noise reduction possible through acoustical lagging is limited by the Fact that the wrapping makes contact with the pipe surface. If more than about 30 decibels of noise reduction is required (overall, A-weighted), some other means of mitigation may need to be considered (e.g., enclosing the piping with a structure that does not touch the pipe surface, or burying the pipe).

Because of these and other issues, piping radiated sound is the primary residual source of environmental noise emission at many otherwise well-silenced compressor installations. In this regard, it is important to recognize that the sound typically radiated from aboveground piping is tonal in nature (composed of an identifiable pitch, or whine related to the rotational speed of the compressor and the number of blades on the impeller), and is therefore generally more audible and disturbing to offsite receptors than bland, `broadband' sounds, such as that of the engine exhaust.

The degree to which pipe-radiated sound is a problem for offsite receptors may not be intuitively obvious to someone conducting a casual walk-through listening test at a compressor facility. This is true because of the relatively immense radiating surface area associated with a typical piping run, compared to the other on site sources. Even though, qualitatively, the perceived sound level near any given section of pipe may not seem excessive, when considered in conjunction with the size of the radiating surface, this apparently modest sound level can represent a large amount of total radiated sound power.

This `perceived-sound-level-versus-radiating-area' is only one of several ways in which the significance of on-site noise sources at a compressor facility is not intuitively obvious from site observations. Some sources which are located high above ground level, or which have upwardly directive characteristics, such as the exhaust stack, may tend to sound less significant at ground level than other essentially innocuous sources. Often, pieces of equipment which the facility operators feel are noisy culprits turn out to be insignificant at the offsite receptors, which may also be shielded from the ground level sources by intervening topography, buildings or bush, for example. Again, these considerations emphasize the importance of accurate measurements of the total emitted sound power of the significant sources onsite.

Modern Acoustical Measurement And Analysis

Standing amidst a multitude of sources onsite at a compressor installation, it may be difficult or impossible to determine the significance of each source, either qualitatively using one's ears, or quantitatively using traditional (sound pressure) measurement methods. At any point onsite, the sound pressure level represents an `acoustical soup' with contributions from all of the major sources. Even offsite, while a sound pressure measurement is of use in determining whether the overall station sound level meets a sound level limit at a receptor, it does not assist in determining which of the station components contribute most to the overall sound level.

The development and application of sound intensity measurement methods to compressor station noise assessment over the last ten years has dramatically improved the ability to quantify individual component sound power levels. The term, `sound intensity', refers to the flow of acoustic energy outwards from a sound source. Unlike a simple microphone, such as those connected to traditional sound level meters, a sound intensity probe measures both the magnitude and the direction of the propagating sound. By defining an appropriate enclosing measurement surface, it is possible to capture only the sound originating from the source under test, and reject extraneous sounds from other sources.

Sound intensity methods are an indispensable tool in conducting a modern environmental noise assessment for a compressor station, because the significant sound sources are located close together and cannot be operated in isolation from one another. For instance, the AGA/PRC study on pipe lagging mentioned earlier was based entirely on sound intensity measurement techniques. Virtually all major manufactures of acoustical instrumentation now offer sound intensity analyzers and probes. More information on this powerful measurement technique is available on the Internet, at the websites of several of the acoustical instrumentation manufacturers, and at www.SoundIntensity.com.

Once the sound power levels of all of the major sound sources have been accurately quantified, the measurement data can be used in conjunction with predictive modeling methods to determine the contribution of each source to the overall sound levels at the residences. The International Standards Organization (ISO) has recently published a standard prediction method for modeling outdoor sound propagation, knowing the sound power levels of the significant sources, the topographical information between the sources and the receptor locations, and the prevailing meteorological conditions in the vicinity.[6] The prediction models are generally calibrated against measured offsite sound pressure levels, after which they serve as a useful tool in designing noise control measures and making decisions toward developing an environmental noise control plan.

Summary

The technology exists, both in regard to measurement techniques and mitigation methods, to control environmental noise emissions effectively from gas compression facilities. The relative significance of the various sources at a compressor installation are not intuitive, nor or the acoustical characteristics of the available noise control methods. Without proper noise control measures, the considerable sound power associated with compression equipment has the potential to create adverse environmental impact at a significant radius, of up to several miles. Proper utilization of the available means of noise assessment and control are critical in responsible operation of a modern compressor installation.

REFERENCES:

[1] B. Gastmeier and R. Stevens. "Applying Sound Intensity Methods In-situ to Measure Exhuast Sound Power Levels and Estimate Silencer Performance," in Proceedings of Spring Environmental Noise Conference, Alberta Energy & Utilities Board, April 1996.

[2] R.B. Tatge and D. Ozgur. "Gas Turbine Exhaust System Silencing Design," Proceedings Noise-Con 91, pp. 223-30, 1991.

[3] R. Stevens and R. Ramakrishnan. "High Temperature Effects on the Insertion Loss of Absorptive Duct Silencers," Canadian Acoustics Journal, Vol. 12, No.3, (Sept. 1993).

[4] L.L. Beranek and I. Ver. Noise and Vibration Control Engineering, Ch.10 (John Wiley & Sons, Inc., 1992).

[5] PRG/AGA. "Acoustical Pipe Lagging Systems Design and Performance," AGA catalog No. L51795, 1998.

[6] H.F. van der Speck. "Advanced Low Noise Air Cooling Fans," Paper presented at the 3rd Biennial Spring Conference on Environmental & Occupational Noise, Alberta Energy and Utilities Board, April 2000.

[7] Internation Standards Organization. "Acoustics--Attenuation of sound during propagation outdoors -- Part 2: General method of calculation," Standard ISO 9613-1, First edition 1996-12-15.

Robert Stevens is one of four principals of HGC Engineering, a consulting engineering firms specializing exclusively in noise, vibration and acoustics. Over the last ten years he has conducted acoustical assessments, surveys and qualification tests of compressor installations across the continent including most of the stations operated by TransCanada Pipelines in Canada. Stevens has conducted research into methods of low frequency noise attenuation in ducts, effects of high temperatures on absorptive silencers, resonant dissipative silencers, and the use of sound intensity measurement methods in new applications. For more information, visit www.hgcengineering.com or www.SoundIntensity.com.
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Comment:Environmental Noise Assessment And Control For Compressor Stations.
Author:Stevens, Robert
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Jun 1, 2001
Words:2696
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