Impending Mine Safety and Health Administration (MSHA) noise regulations that will require the implementation of feasible engineering and administrative controls has brought about the reassessment of current noise levels and attenuation technology available to cement plants. This article summarizes literature on the topics of cement plant noise, tools for assessing noise levels, engineering control options, and case studies of successful attenuation.

Cement plant noise Literature concerning the area of cement plant noise is found primarily in international publications. Researchers have identified numerous sources that may contribute to high noise levels, including grinding mills, compressors, fans, blowers, coal rail car unloaders, material handlers, power transmission equipment, and crushers. Similarly, outdoor noise pollution also has been noted at some plants, thus requiring noise attenuating technologies for noise reduction out doors.

Part of the development of a successful noise control strategy with engineering controls is dependent on understanding the regulations that are currently being promulgated. In the case of the cement industry, the proposed MSHA Noise Rule (Department of Labor, Mine Safety and Health Administration: Health Standards for Occupational Noise Exposure in Coal, Metal, and Nonmetal Mines; Proposed Rule. Federal Register. 30 CFR Parts 56, 57, 62, 70, and 71. 1996.) has rules that influence the establishment of noise controls.

Tools for assessing noise levels A successful noise control program that focuses on engineering control of noise requires the institution of a hearing conservation plan and the use of proper monitoring equipment, surveys, maps, and modeling.

A thorough hearing conservation plan should be established where noise exposure exceeds a 85-dBA time weighted average for eight hours. A good program consists of the following components:

* Noise measurement and analysis;

* Engineering control of noise sources where feasible;

* Administrative controls and personal protection where noise control is not feasible;

* Audiometric testing;

* Employee training and education;

* Record keeping; and

* Evaluation

Noise sources are evaluated through the use of dosimeters, sound level meters, and octave band analyzers. Evaluation of noise in a plant environment requires a well-constructed sound survey. Numerous sources provide useful criteria for surveys, as well as commonly used calculations. Some industrial facilities have developed sound level contour maps through noise modeling programs. These models are developed by importing noise data, noise emitter statistics, and information related to natural and artificial obstacles that can be drawn from zoning and topographical maps. Currently, a number of software programs are available for simulating noise emissions.

Engineering noise controls Investigation of a noise problem requires consideration of as many potential solutions as possible. Many options for control exist. The highest tier of engineering control is to guide the design of quiet machinery or to make design changes in an existing plant. The development of engineering controls at the design stages of a plant avoids retrofitting, which can be impractical on technical and economic grounds.

Ultimately, if a quieter process cannot be substituted, then the noise source can be modified for attenuation. Control at the source may consist of the reduction of the driving force, reduction of radiating surface response to driving forces, or reduction in radiation efficiency of sound- generating surfaces.

One of the primary causes of noise in a manufacturing plant is the vibration produced by process machinery. By focusing on this noise source, many problems may be solved. In many cases, damping has been effectively utilized to attenuate vibration problems. Elastic materials, such as rubber and plastic, are used to dissipate energy from vibrating surfaces with a resulting reduction in amplitude and structural stress.

When noise reduction cannot be achieved at the source, it may be useful to modify the noise propagation path. Measures including full or partial enclosures; sound barriers; room absorption; active control; and changes in duct geometry, fan type, and flow characteristics can be applied to a typical plant to aid in noise control.

Enclosures for noise sources or for employees in a noisy environment can significantly decrease noise levels reaching the employee. The effectiveness of an enclosure is dependent on the transmission loss of the enclosure. Acoustic performance may be affected by leaks, use of varying noise absorbent and damping materials, and rigidity of the underlying structure of the enclosure.

Barriers work effectively when placed between a noise source and a receiver. These specially designed walls reflect sound waves away from the receiver resulting in a noise shadow behind the barrier. They work most effectively when they are close to the noise source and are as high as possible within the plant. Important design elements that must be considered for effective noise attenuation include the effective height of the barrier, the wavelength of the noise, and the angle of deflection.

Placement of sound-absorbing materials on the walls of a room diminishes the reverberation of sound waves off of hard surfaces. Typically, acoustic absorbers are soft, porous materials that absorb sound waves and are described in terms of an absorption coefficient. Material composition and thickness, mounting style, and sound frequency all effect the absorbent coefficient. When noise is estimated on a subjective scale in the form of an annoyance scale, reduction of reverberation time was perceived to be more effective than noise source reduction. Improvements in speech intelligibility, decreased risk of accidents, and improved control of machinery failure were some of the benefits of this technology.

Active noise control uses the principle of superposition of waves to attenuate noise. Active noise control can be achieved by processing the original offensive sound and subsequently injecting it back into the sound field, but at 1800 out of phase, thus generating a canceling wave form. Active noise control has been applied to one-dimensional field and duct noise, enclosed spaces and interior noise, noise in some three-dimensional spaces, and in personal hearing protection. The biggest successes have come in the area of piping and duct noise attenuation. Active noise control has had more limited applications to broad-band noise, or noise in three-dimensional spaces, because of the principles of noncausality, stability, spatial mismatch, and the infinite gain controller requirement.

Flow-related noise concerns, such as HVAC and piping noise, conveyor noise, and compressor noise, require other noise control measures, as well as some of the previously mentioned technologies. HVAC-related noise can be difficult to attenuate, especially considering the wide range of sizes of fans, ducts, and stacks.

Some basic considerations for a quiet design of HVAC systems include: minimizing pressure loss within ducts, selecting of fans that operate at maximum efficiency on performance curves while providing necessary ventilation, and designing ductwork leading to fans to minimize turbulence.

Retrofitting ventilation with silencers, insulation within ducts, and active noise control also has been successful for noise abatement. Piping has been attenuated with pipe lagging modified with the addition of fiberglass layers, decoupling layers of insulation, and more efficient damping. Conveyors have been improved through substitution of quieter operating drives, damping of metal parts, and reductions in operating speeds. Compressor station noise has been reduced by various measures, including silencers, pipe lagging, and fan changes, depending on the specifics of the process.

Case Studies Retrofitting or redesigning a manufacturing plant for noise control can be a difficult endeavor. Proper noise control requires the teamwork of management, process engineers, mechanics, and outside vendors and acoustic consultants. Some success has been achieved by plants in a number of noisy industries, including can manufacturing, recycling, mining, and stamp manufacturing.

The area of compressor noise also has received a lot of attention in literature on case studies showing noise control through enclosures, silencers, and acoustical blankets. Blankets with thermal and noise insulation capabilities have been utilized in the power industry. Some plants with outdoor noise problems affecting residential neighborhoods have used noise barriers, absorptive panels, acoustic blankets and curtains, and silencers for noise attenuation.

The APCA Occupational Health and Safety Committee is sponsoring an engineering noise control tutorial at the IEEE/PCA Cement Industry Technical Conference in Roanoke, Va., on Monday, April 12, 1999.

The title of the tutorial is "Noise Control Strategies for the Cement Industries. " Subjects to be covered during the tutorial include: developing noise control solutions; establishing noise control priorities; identifying and selecting optimum products for retrofitting equipment; working effectively with design engineers for proactive noise control; predicting the impact of new equipment on the in-plant and community noise environment; and qualifying new manufacturing or process equipment to determine whether it satisfies stated purchase specifications. Each attendee will receive a diskette with spreadsheet programs for noise control.

This course is recommended for plant or process engineers, equipment designers and manufacturers, health and safety professionals, and others charged with noise control for their companies. For more information, call PCA at 847-966-6200, ext. 358.