The approaches to noise control are designed to address noise problems in the metal products industries in a specific manner.
In designing a system for reducing noise, in metal products industry, engineering controls should be used wherever applicable. These controls can be effective if used properly. Systems, however, are much more complicated and must be given much more consideration.
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Instead of just using engineering controls to quiet noise, the placing of the machines is also a vital factor. If machines are layed out too closely together, the operator may be exposed to an excessively high dB level, whereas if spaced adequately apart, noise levels will be within acceptable limits.
By reducing noises, the operation is usually increased and machines run much more efficiently and workers actually work at greater productivity with less noise.
A great deal can be done to cut down the amount of noise generated by plant, machinery or equipment at source. The correct use of attenuators will greatly reduce the level of noise transmitted through a building by means of ducting, trunking or other noise transmission media.
However, where the initial source of noise is a particularly troublesome one, so that even the most careful selection of equipment and mounting techniques fails to produce the amount of noise reduction required, other measures must be considered. There may also be instances where the degree of noise reduction obtained by these methods, though sufficient for normal purposes, is not enough for an area where exceptionally quiet conditions are required for special reasons. In such cases, the additional steps which can be taken include the use of absorbent materials or the erection of special sound barriers and enclosures to control the transmission of noise from source.
Another practical method of reducing noise in metal products industry is by increasing the distance between the source and work vicinity. Machines, processes and work areas which are approximately equally noisy should be located together. Areas that are particularly noisy should be segregated from quiet areas.
The office space in a factory should be as far as possible segregated from the production area and preferably be located in a separate building. If a common wall is unavoidable it should be heavy with minimum connecting doors and no permanent openings.
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It must be emphasised that noise reduction at the source always enjoys top priority, but with a systematic approach and careful attention to design detail, enclosures can be one of the most powerful noise reduction measures available to the acoustical engineer.
Of all the machine tools, the power press or punch press has over the years presented one of the most formidable challenges to the acoustical engineer. There is no single noise source present but rather many noise sources. Some of these noise mechanisms can be traced to easily identifiable singular causes, while others result from a complex combination or sequence of events.
Finally, there is the metal-to-metal clatter associated with material handling which cannot usually be ignored. Here we have part-to-die-set impact at load, part-to-part impact or part-to-bin impact after ejection and finally scrap collection.
Mechanical noise can be contributed by belts and motors and if the fan casings are inadequate, they may drum or rattle. The fan itself, of course, should always be isolated from the system and from the fabric of the building, to prevent physical conduction.
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Any acoustical study of steel mills and metal fabrication plants will reveal many of the industries inherent noise sources to be present in most plants. The noise of these operations can be reduced significantly.
The following major classes of noise sources are found in most metal industry operations:
1. Impact machines.
2. Pneumatic equipment.
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3. Furnaces.
4. Machine tools.
5. Welding.
6. Material handling systems.
7. Mechanical equipment.
1. Presses:
In terms of the work force exposed to press noise and the difficulty of silencing, presses probably could be ranked as the number one noise problem in the United States and other countries.
Presses may be classified into two types:
1. Automatic.
2. Manual.
Typically, manual presses operate at speeds of 20-40 strokes per minute, while the automatic presses may operate up to 1500 strokes per minute. Capacities may range up to 1600 tonnes. Some presses are horizontal or inclined, but the majority is vertical. They use single-step dies or progressive dies and process metals from steel to aluminium to brass of various thicknesses.
Sound levels of press operations may be generated by various mechanisms and may be classified as follows:
1. Vibration of the press structures, induced by the impact forces.
2. Mechanism noise (clutches gears, etc.).
3. Material handling (ejectors, conveyors, etc.).
The particular noise problem of any press will depend upon its design, speed and metal cutting or forming operation. The mechanisms of noise generation of any specific press may generally be identified by a brief visual auditory inspection. In some cases, detailed acoustical analysis may be required.
The following sections present approaches to virtually every press noise problem which may be encountered and is based on information compiled from a complete literature search and experience with noise abatement programmes involving several thousand presses.
Open back inclined, OBI, presses are generally less rigid than straight side presses and are therefore inherently noisier. OBI presses utilise tie rods to increase rigidity; these must be well maintained to provide minimum noise levels. Also, it is often found that vibrations induced into the tie rods radiate considerable noise and require vibration damping or lagging.
The noise of a press is generally higher as the actual load approaches the normal capacity of the press. Presses should therefore be selected with 50 per cent 100 per cent excess capacity.
In operations involving shearing, blanking and punching using punch presses, the large impact forces exerted by the descending punch on the plate placed upon the die and the shearing action take place simultaneously.
A plate to be blanked is placed upon a stationary die and is engaged by a descending punch whose width is slightly less than the clearance between these paced parts of the die. The portion of the plate between these spaced parts of the die is, thus, sheared from the body of the plate and moved downward ahead of the punch.
If the punch is designed, all the shearing action occurs simultaneously. If the lower face of the punch is slightly inclined, the shearing action is distributed over a greater part of the stroke.
The maximum force then endures for a shorter period and the duration of the force is increased, as shown by the shape of the shaded area. A further increase in the angle of the lower face of the punch brings about a pronounced decrease in the duration of the force.
In an operation involving the punching of several holes at one stroke of a press, a similar reduction of total force can be attained by stepped punches. The punching of successive holes occurs progressively and not all the punches are operative at a given time. Where pierced blanks are required, punching and blanking operations frequently are performed in multistage dies.
Perforations are first made in the material; the material is then moved the length of one stage and the piece is blanked around the perforations while first-stage operations are being performed on the following increment of material. It is desirable, where possible, to equalise the work between stages and to keep the strokes in the several stages out of phase to maintain the individual impulses as small as possible.
Shinaishin reported an 8 dBA reduction with a slanted die. While slanted or inclined dies and stepped punches are used in many plants, it should be pointed out that this is generally not a complete solution to noise control. Even with these die modifications, sound levels may be above 90 dBA. Also, such die modifications are not applicable to all parts. However, any improvements in noise level will come by experiment and testing results.
The obvious solution to the problem of noise emanating from an automatic machine is to enclose the unit in a sound-attenuating structure. A large maker of die sets for mechanical power presses usually adopts this approach to quiet the presses in its shop. One of the presses, a straight side, 20 tonne model fitted with a slide feed, normally operates at 600 rpm.
To prevent excessive heat build-up louvers are cut into the top of the press enclosure. Flat panels are located beneath the louvers to act as baffles. Exact locations of the baffles are determined by trial and error. Slight shifting of the baffles makes a great difference in the reduction of noise. A small exhaust fan is mounted in one of the louvers to further reduce heat build-up.
This press, running under high-speed conditions, now has a noise level in front of the press of 86 dBA scales. In the rear, the reading is now 85 dBA. This reduction brings the press noise well within Occupational Health and Safety Act requirements.
Power presses with automatic feed devices can also be enclosed in noise barriers made from leaded vinyl. These floor-to-ceiling curtains can be used to contain either single presses or banks of units. A press attendant can go in and out of the area to load additional stock in feed devices and to adjust press malfunctions.
Total exposure time within the area can be adjusted to the time permitted by the noise level. If fail-safe electronic sensors are used on the presses, they can be adapted to activate alarms when a press needs attention for a malfunction or loading.
Most noise produced by a mechanical power press is from the impact produced by the die set closing on a piece part. In such cases, a bellows constructed of leaded vinyl attached to the press slide can be timed to enclose the impact area when the press is stroked.
The balance of the upper part of the press, with attendant gears or belts and motor, can be enclosed with a plywood or lead or lead/foam structure. If such an arrangement does not reduce the noise level to permissible limits, a second structure can be built around the lower section of the press.
High tonnage presses, drop hammers and other high impact machines create vibrations in addition to airborne noise. These vibrations, unless short-circuited, can be transmitted throughout an entire plant and even into office areas, where they may be translated into audible noise. To prevent this machine must be isolated from their foundations. One highly, efficient method of accomplishing this isolation is through the use of lead anti-vibration pads.
Harder materials require greater force, thus producing higher noise levels. Metal working operations involving stainless steel are noisier than those involving cast steel; operations on brass and aluminium are relatively quiet. Shinaishin reported a 14 dB reduction with an experimental substitution of work stock material from steel to a lead-steel composition.
The sound level radiated by a structure is proportional to the vibration level induced in the structure and it’s radiating surface area. In general, heavy structural members transmit vibration, while lightweight structural elements often vibrate excessively and generate noise. Thus, large light weight parts such as flywheel guards, frames and legs are often the major noise problems of a press.
The first approach to vibration analysis is the identification of the specific structural elements generating noise. Both octave band sound pressure levels in the vicinity of the press and octave band vibration levels of the press structural elements must be measured.
Then:
1. Peak frequencies in the sound pressure level spectra may be correlated to peak vibration frequencies of the press structure.
2. Measured vibration levels may be correlated to expected (theoretical) sound pressure levels.
The (theoretical) near field sound pressure level above the coincidence frequency of a structure may be computed from the measured or predicted vibration levels by the following relationship –
Lp = Lv – 20 log f + 150
Where,
Lp = sound pressure level, dB
Lv = vibration level, dB re 1.0 gram
f = frequency, Hz.
The possible use of vibration damping treatment for press noise reduction was investigated by Stewart, Bailey and Daggerhart.
Their investigation concluded the following:
For a single degree-of-freedom system, it can be shown that when the ratio of the pulse duration to the system natural period is much less than one, the maximum response can be reduced by increasing the mass of the structure. On the other hand, when the ratio of the pulse duration to the system natural period is much greater than one, i.e., the force is applied slowly, the maximum response occurs while the force is acting.
In the latter case, the response is inversely proportional to stiffness, i.e., increasing the stiffness should reduce response and hence, reduce noise. When the duration of the force is equal to one-half the natural period of the system, a pseudo resonance exists.
Control of resonant response can be achieved by detuning the system or adding damping. As pointed out by Harris and Crede, however, a tenfold increase in the fraction of critical damping produces a decrease in maximum response of only about nine per cent. Thus, damping has little potential for reducing punch press noise.
This limitation applies only to heavy press structural elements. Damping, however, is effective when resonant vibrations are identified in lightweight machine components, such as flywheel guards.
Air ejection systems are commonly used to eject small parts or scraps from press dies and are sources of high noise levels. Reduction of noise levels can be obtained either by changes in the methods of handling material or by silencing the air system.
The following specific approaches may be considered:
1. A thrust silencer may be used.
2. Noise caused by high air velocity can be reduced by decreasing the linear flow velocity by increasing the nozzle opening, for same air mass flow. If the diameter of the nozzle is doubled, in a constant volume velocity system, flow velocity is reduced to one-fourth and noise is reduced nearly 30 dB (noise of air jet varies approximately as fifth power of velocity).
However, thrust would also be reduced to one-fourth of original value. For proper ejection, the nozzle must be aimed more accurately and more efficiently toward the target. If the distance from the nozzle to the target were reduced 50 per cent, a 30 per cent velocity reduction would give the same thrust. Experiments must be conducted to determine the maximum thrust required for minimum noise.
3. Air used for parts ejection should be controlled by a reducing valve to minimum pressure and should be regulated to be on only when required for ejection. These measured should also reduce energy consumption considerably.
4. Make the nozzle an integral part of the die set; better, of the die (mount air coupling in die set; possibly of the quick-acting type). The best solution consists of eliminating nozzles completely. A few strategically drilled holes, connected with a common conduit and ending in a quick- connect coupling ejects the parts practically noiselessly.
5. The turbulence and consequently, the noise, depend not only on the shape of the nozzle, but also on obstacles in the jet. When a jet hits a slot of the kind that is common in shearing tool, the noise may increase by more than 10 dB, compared to the noise emitted from the same jet impinging on a flat surface. This noise may be minimised by careful aiming of the air jet.
6. An excellent means of practically noiseless part removal consists of using compressed air for creating vacuum. Commercially available devices give highly satisfactory performances.
7. Even with these air noise reduction measures, sound levels may still exceed 90 dBA. A partial enclosure over the die space may be required for additional noise reduction.
8. For stamping ejection first preference should be given to ‘push-through’ evacuation. The stampings should simply fall on the press-bed (bolster plant) and from there they are pushed out (mechanically), unless the press is inclined or the press-bed and the bolster plate have sufficiently large openings for direct evacuation.
9. If for some important reason (secondary operations, part size and/or configuration, etc.) gravity is impractical (or impossible) then use positive action mechanical knock-outs. These may be driven by linkages, air cylinders, hydraulic cylinders, etc.
Stock from metal rolls is often positioned using a clamp stock indexer which induces noise-producing impacts into the stock. The best method for noise control is to replace the impact indexer with a new mechanical roll feed.
As an alternative to noise abatement, the indexer may be enclosed and vibration damping pads may be applied to the affected stock. The pneumatic exhaust of indexers should also be muffled. An acoustic infeed tunnel is another approach.
The noise produced by parts rattling on conveyors exiting the press may be reduced by the application of a vibration damping material to the chute. The design in Fig. 16.8 utilising cardboard provided a 10 dBA noise attenuation. An even greater noise reduction would be achieved with a visco-elastic damping layer.
A manually operated press cycle begins with the release of the break and the engagement of a clutch. In mechanical clutches, this involves the impact of a metal pin.
Recent research identified clutch noise as a primary contributor to press noise. The pin type clutch of manual presses was found to be loudest, with 124 dBA peak levels and decay times up to 0.18 seconds. In the analysis of one press, the clutch provided 66 per cent of the acoustical energy.
Clutches and breaks should be periodically maintained to insure minimum noise levels. In some cases, the impacts may be damped or cushioned; however, satisfactory designs for non-metallic pin clutches have not been developed.
The use of air clutches, which employ pneumatic mechanisms in lieu of positively acting clutches, is the recognised solution to noise control for this problem. Air clutches are also somewhat noisy, but may be quieted by applying a damping to the matching surfaces.
The installation of a barrier is another solution to clutch noise problems. This approach may also shield the noise from other press mechanisms from the operator.
In some presses the metal-to-metal impacts of stripper plates contribute significantly to overall press noise. Where this noise is identified, the plate may be damped or a non-metallic contact surface may be used. This may result in noise reduction of up to 10 dBA.
In large blanking presses the ram is hollow. The forming die runs in grooves on the side of the press and completely closes off the end of the hollow ram. There are slots in the ram that are used normally when the press is used in blanking operation to extricate the work from the die. This is similar to removal of a cookie from a cookie-cutter.
These slots are in the side of the ram. When the press is being used in the forming mode, these slots are not required and when the die ‘snaps through’, it cuts off the work. This gives rise to high noise levels.
This noise may be abated by simply plugging the slots. The plugs may be removed to reconvert to a blanking operation. It is reported that the noise from one press was reduced from 94 dBA to 88 dBA by this method, using a plywood plate cover with a Neoprene gasket.
The most common and least effective technique of press noise control is the use of vibration mounts. Vibration isolation will reduce vibration transmitted to the floor; however, floor vibrations seldom are an industrial noise problem.
One recent study indicated a 15 dB reduction in the vibration level of press legs; however, it is unlikely that vibrations of other press members would be reduced significantly. It should be pointed out that press mounts do have advantages other than noise reduction and their use is not discouraged. Large noise reduction should not be expected, however.
Air noise due to pneumatic control exhausts may be a major noise source of presses.
Punch presses frequency creates unnecessary noise because of double impact. The metalworking operation is performed as the moving head, generally carrying the punch, approaches its lowermost position. The mass of the moving head tends to accelerate the crank, causing it to get ahead of the flywheel and thereby take up any lost motion in the clutch.
One impact results when the punch hits the work and a second impact occurs in the clutch mechanism an instant later as the flywheel catches up with the crank. A properly equipped punch press is provided with a brake and air cylinder or a counterbalance to retard the downward motion of the moving head.
By preventing the crank from getting ahead of the flywheel, it is possible to eliminate the second impact. Care in adjusting and maintaining these features or presses will eliminate unnecessary impacts. This double impact also subjects bearings, gears and clutch parts to extra wear, with subsequent increase in maintenance and cost.
At the end of the cutting action, when the slug or the blank snaps out suddenly from the sheet metal, all the clearances in the bearing and the elastic strains in the press body structure are suddenly released and a high intensity noise is created. This can be reduced, sometimes even totally eliminated by counterbalances (press or die counterbalances).
Simple mechanical devices or more sophisticated hydraulic or pneumatic devices may be employed for this purpose. Counterbalances are more effective in case of heavy stock; in case of thin stock their adjustment becomes too delicate. Such devices should be installed by the press manufacturer;
In delicate cutting and forming tools it is customary to provide bumper blocks for limiting positively the shut height of the dies. In order to eliminate the noise created by the impact, put a resilient shock- absorbing plastic ring or disc on the bumper blocks.
Best results are obtained with comparatively thin layers, about 1/16″ Drastic noise reductions are sometimes achieved in this way. Such bumper blocks are very effective in controlling punch penetration in case of cutting operations. Hold punch penetration to a minimum in order to keep noise intensity within reasonable limits.
Gears are frequently identified as major contributors of press noise.
Guards:
Flywheel guards should be constructed of damped metal or open mesh.
Enclosure:
The installation of acoustical enclosures to reduce press noise may be considered. With the noise being radiated from the localised area of the die and plunger, a localised enclosure would provide significant noise reduction. Assuming the use of heavy materials and nearly airtight construction, a noise reduction of 10-15 dBA may be expected.
Three typical press enclosure applications are described as follows:
1. A partial enclosure of a press is shown in Fig. 16.11, achieved a noise reduction from 108 dBA to 89 dBA.
2. Partial enclosures were constructed to supplement noise reduction achieved by a parts ejection silencer. The enclosures resemble a box shaped around the die with the far side and bottom missing and also serve as safety guards. Noise reductions from 106 to 85 dBA were achieved with a Summit (large size) press and from 99.5 to 82.5 dBA for a Benchmaster (small size) press.
3. Allen and Ison reported a partial enclosure of ram, die, infeed and ejection on a 50 tonne test press. A reduction of 13 dBA was obtained for an enclosure. The model enclosure was made of cardboard, ½ 1b/sq ft, lined with 1″ of polyurethane foam. Later a steel enclosure was installed for durability. Totally enclosures may also be used for press noise reduction.
The practical limitations of such enclosures, however, are ‘Enclosures are good solutions when few machines are to be treated and the spacing allows their use’. Petrie states that, ‘If frequent access is not required the simplest solution is the complete enclosure around the source’. In general, enclosures are applicable only where a limited number of presses are involved and accessibility constraints are not present.
The sound levels of multi-slide machines typically range from 90 to 120 dBA. The most common solution for noise reduction of these machines is acoustical enclosures.
The primary noise of most multi-slides may be identified as air used for parts ejection. Air mufflers may be used for reduction of this noise. On some machines, noise may also be generated by a cam follower. This noise may be treated with an impact plate.
It should be recognised that it is not feasible to reduce the noise of all press operations to below the OSHA limits.
The following are examples of cases where sufficient noise reduction may not be possible:
1. For manually operated presses where die noise is dominant and a die enclosure is not possible due to accessibility requirements.
2. For press rooms where several presses in an area create ambient sound levels above 90 dBA, even with silencing.
3. For operations involving hundreds of presses where economic hardships caused by production decreases, accessibility problems and installation of enclosures are prohibitive.
4. Where visibility of the die is required and splashing of lubricant precludes the use of an enclosure.
5. Presses with lightweight frames which are operated at near their minimum design capacity are inherently noisy due to excessive structural vibrations. Noise control may often be achievable by replacement of the press with a new unit.
2. Steel Mills:
The employee noise exposure within a steel mill is generally lower than would be recognised from a brief overview of mill operations.
Several factors inherently minimise employee noise exposure:
1. Many employees perform all or a portion of their tasks from enclosures or booths. While many of these enclosures were installed for other production purposes, they also serve as very effective measures to reduce noise exposure.
2. The primary function of many employees is to correct problems and perform adjustments in the mill operation. Thus, they are not engaged in noise-associated activities for a major portion of their work day.
3. Some work areas complete a day’s tasks in less than an eight hours shift, limiting noise exposure significantly. Other employees rotate jobs because of heat exposure.
As an initial noise control effort in any mill, the use of employee enclosures should be considered. Approaches to noise reduction for many common noise sources found in steel mills.
The following section present noise control approaches for various other mill operations:
Steel couplings are located between the motor-gear drive and mill stands to provide torque transfer and to insure system alignment. The mill stands cannot operate with an uncoupled shaft. The couplings are relatively quiet when under load or with a bar in the mill. Noise is generated by vibrations introduced into the coupling when rattling in the unloaded condition.
The A-weighted sound level often exceeds 100 dBA at 3′. The noise level of any coupling system is dependent upon its tightness; however, this is not a feasible parameter when considering noise abatement, since all couplings are periodically tightened and gradually loosen under operational loads.
The only technology known for reducing the coupling noise is a nylon insert type wobbler box. The manufacturer could not estimate the noise reduction to be expected for these couplings. The experience of many mills indicates that to use these couplings successfully, everything must be in very good alignment and maintenance must be excellent.
They are easily stripped during a cobble and are sensitive to heat. It is questionable if any mill other than a new facility could operate with anything requiring such close tolerances.
Since the couplings are noisy only under no-load conditions, it has been considered to create a continuous false load condition for the couplings.
The use of a constrained layer damping treatment to the coupling box may provide noise reduction. Localised enclosures may also be tried.
The sound levels in finishing areas may range up to 110 dBA and are due to several specific noise sources, including:
1. Vibration of the stock on the pull-up rolls.
2. Unscrambling of the stock from piles.
3. Vibration of stock being sheared.
4. Structural vibration of shear due to cutting impact.
5. Dropping of parts into bin or onto stack.
6. Stock hitting the metal stops.
Noise problems in finishing areas of steel mills have received considerable industry-wide attention for several years without much success in achieving effective noise reduction.
Potential noise control solutions were investigated for each of the noise sources and will be discussed here. Since finishing involves operators working directly with the steel product, the use of employee enclosures is not practical.
While minor items of noise control are sometimes possible for finishing areas, these would generally have no influence due to the overriding effect of noise from more severe sources. The most dominant noise source is vibration of stock on the pull-up rolls.
Vibration of the stock is the primary source of noise on pull-up roll conveyors. Thus, vibration damping of the rolls would not be effective. The potential design solution of coating the exterior of the rolls with a material to cushion the metal-to-metal impacts between the stock and pull-up rolls was investigated.
This material must meet the following requirements:
1. Must have a high module of elasticity to provide impact isolation.
2. Must provide high wear characteristics.
Conventional plastic and rubber materials which would be ideal as impact isolators would not withstand the wear requirements. Many mills have maintained an ongoing effort to locate a wear resistant roller coating for several years. None of these efforts has resulted in finding a successful material.
The wear problem is inherent with conventional resilient materials. The angle of incidence of the steel bar upon the pull-up roll ranges between 0° and 20°, as shown in Fig. 16.15. A wear curve as shown in Fig. 16.16 illustrates that compliant materials cannot be successfully used in this application.
3. Cooling Beds:
Cooling beds which involve sliding of steel bars across rails can generate noise levels in excess of 110 dBA.
The noise generated by the steel motion on the bed is due to frictional vibrations induced by the microscopic roughness of the bar and cooling bed rails. The frictional vibration forces are directly proportional to the coefficient of friction between the surfaces and independent of the velocity of motion.
The high temperature (over 1000°F) of the bar severely limits potential modification to the rails for the purpose of reducing friction forces. The coefficient of friction between steel surfaces is 0.45-0.55. This value may be reduced to 0.03-0.1 by application of a lubricant to the metal interface.
This has been attempted at one mill and was found to be impractical for the following reasons:
1. The lubricant caught fire.
2. The lubricant wore off very rapidly.
3. The lubricant could not be cleaned off the bar for painting.
Polishing the bed rails to a micro finish would reduce friction forces and resulting noise levels. It is questionable, however, whether such a finish could be maintained under production conditions for any extended period of time and feasibility of this approach cannot be established.
In view of the limited acoustical effectiveness and probable production interference of other design approaches, the use of employee enclosures is the best approach for noise control of this operation.
3. Foundries:
With OSHA’s National Emphasis Programme, noise control in foundries has received more attention than ever. Noise control, however, is not new for foundries. Practical solutions now exist for most foundry noise problems.
The following sections discuss other noise problems found in foundries:
Castings are removed from sand moulds by means of the shakeout which dislodges the casting by impacting the flask on a metal deck. Sound levels may range up to 110 dBA.
In automated foundries, it is possible to reduce shakeout noise by means of an acoustical enclosure or partial enclosure. Satisfactory results achieved by such enclosures have been reported by several investigators.
In a jobbing shop, where castings of many sizes are handled, it may not be feasible to enclose the shakeout.
Two alternative solutions may be considered:
1. Rubber bumper bars (80 durometer, 1″ thick and 1½” wide) may be bonded to a ½” metal bar which is spot welded to the shakeout deck. When the shakeout grates vibrate, these bumper blocks repeatedly strike the red hot castings, but they do not stay in contact with them long enough to burn. This design has been successfully employed in foundries and noise reductions of 7 dB are reported. A service life of up to 9 months has been reported.
2. In another foundry, steel chains with links 1½ inches in diameter were threaded through automobile radiator-hose material. Lengths of chain threaded hose were placed longitudinally across the shakeout grate and secured with sufficient slack at appropriate intervals to the sides of the shakeout deck. This eliminated the impact of metal against metal, preventing casting damage and reduced noise. Maintenance costs were not high and the hose lasted about a month in that particular plant.
2. Core Oven Flame Adjustment:
It has been investigated that core oven burners in a foundry were found to vary from 88 to 94 dBA. It was learned that the burners were purposely out of adjustment so that they could be heard. The operators of the ovens were afraid of a flameout and to assure them of a flame, the operators permitted the burners to run with a low-pitched roar that is characteristic of combustion noise. The solution here was to use an electronic flame detector which will alarm and shut off the gas rather than depending on the noise of the burners to indicate a flame.
It was experimentally determined that operators in small three-sided, open top cleaning (grinding or chipping) booths experience a sound level buildup of approximately 8 dBA due to sound reflection from the booth walls. The sound level during normal grinding operations for an experimental set-up was 99 dBA. This sound level was reduced to 91 dBA by lining the booth walls with a 4″ layer of glass fibre thermal insulation.
To provide effective noise reduction within grinding booths, interior wall surfaces may be lined with a 1″ minimum thickness of glass fibre or open cell polyurethane foam faced with a protective layer of 24 GA perforated sheet metal with 20 per cent open area or heavy wire mesh screen.
Noise reductions from above 110 dBA to below 90 dBA are reported as a result of lining casting tumblers with resilient wear retardant rubber linings.
4. Gas Furnaces:
Gas furnaces are inherently noisy due to combustion noise. Combustion noise is very similar to air noise from a jet. Combustion creates an additional low frequency component. Whereas jet noise intensity is proportional to approximately 7 times the jet diameter, combustion noise is proportional to approximately 50 times the burner diameter.
Combustion noise is basically derived from variations in the heat release rate.
There are two general types of combustion noise, which are:
1. Turbulent combustion noise.
2. Combustion oscillations.
Turbulent combustion noise occurs when there are random fluctuations in the rate of combustion in the combustion reaction zone. This type of noise manifests itself as a ‘roar’. Practically all common fuel-burning systems have turbulent reaction zones which produce turbulent combustion noise. For satisfactory combustion, high mean flow velocities with high turbulence intensities are required. For quietness, velocities and turbulence levels should be kept low.
Combustion oscillations are a phenomenon that arises in a small percentage of gas burners. However, when they occur, the oscillations are noticed because of the rumbling, trumpeting or screeching that is emitted by the flame. Rayleigh stated that oscillations will occur when the periodic heat release is in phase with the maximum pressure variations.
Occasionally combustion oscillations will manifest in the form of toroidal vortices. When these vortices interact with the flame front, they give rise to discreet tone generation.
Due to the complexities of combustion oscillations, there is no definite solution to the problem.
The following list of remedies is not necessarily in order of effectiveness, cost or etc.:
1. Altering the air and fuel rates to the burner can often remove oscillations. This is cheap and simple, but frequently either cannot be done at all or can be done within very narrow limits.
2. Changing the type of burner is frequently effective. A completely different design of burner should be chosen; this modification is rather drastic and often out of the question for political or economic reasons.
3. Altering the configuration of the supply pipework can occasionally solve oscillation problems. This can be done where acoustic standing waves are thought to exist upstream of the burner. This modification is not one which is commonly found to be effective, however.
4. Modifying the combustion chamber geometry can remove oscillations. This can be done by placing a refractory wall part way across the combustion chamber or by altering the gas flow path (and hence changing the acoustic; an example of this is the partitioning of the first ‘reversal chamber’ in a shell boiler with two combustion chambers exhausting into a common flue). This modification may not be possible for reasons involving heat transfer or static pressure in the system.
5. Providing ‘pressure release’ orifices at various points in the ‘hot’ part of the system. These are often placed near to the burner and need be no more than ½ inch in diameter in many cases. Again, these may be impractical because, for instance, of pressure considerations.
6. Modification of the burner head design may alter the flame transfer function so that oscillation disappears. This must, of course, be done so as to retain proper flame stability and combustion quality.
7. Fitting tunable acoustic filters such as quarter-wave tubes or Helmholtz resonators can ‘tune out’ oscillations. These consist simply either of a side-branch tube connected, for instance, to the combustion chamber and fitted with a piston or a cavity resonator similarly situated, with a variable volume.
These are usually successful if fitted in the correct position (near to the burner is often very effective) but can be unsightly. Their chief disadvantage is that they are so sharply tuned that if the frequency of the oscillation wanders (as it frequently does), they go ‘off resonance’ and cease to be effective.
8. A reactive expansion chamber type of flue stack silencer can be effective if the amplitude of oscillation is not too large. This remedy can be quite expensive where a large silencer is required and in any case does not tackle the problem at its source.
9. In certain situations, it is possible to change the composition of the fuel supplied to the burner. This may be the case where, for instance, a factory manufactures its own gas. The effect of this would be principally to alter the transfer function of the flame. Alternatively, it may be possible to change from liquid to gaseous fuel or vice versa.
10. Increasing the pressure drop across the burner can prove effective in curing oscillations; this would normally be done in conjunction with a burner head modification. It would, however, usually be accompanied by an increase in the level of broadband turbulent combustion noise, but in many situations this may be unimportant.
11. Altering conditions on the flue stack would change the transfer function of the combustion chamber and may prove to be an effective remedy; the provision of a flue break, for instance, has proven to be successful in the past.
5. Electric Arc Furnaces:
Electric arc furnaces are bowl-shaped metal housings with a refractory brick lining in which scrap steel is melted. The melting of the steel is achieved by an arc struck between three graphite electrodes and the steel. The arc potential and current are approximately 450 volts and 60000 amperes respectively.
An internally luminous column is formed between two hot spots, one on the electrode tip and one on the melt. The hot spot and plasma column have temperatures from 3600-4000°C and 5000-6000°C respectively.
The major portion of the electrical energy used is converted into thermal energy which melts steel. Some of the electrical energy is converted into other forms of energy, such as chemical, mechanical and electromagnetic. These energy forms are manifested by undesirable secondary effects such as exhaust gases, vibration, noise, side wall heating and light.
Sound levels of up to 115 dBA are generated by the furnace. The noise has two major components, which are electric arc noise, present at all harmonics of 60 Hz and mechanical noise from scrap movement within the furnace.
The noise time history of a melt follows this general pattern:
1. Noisy bore-in.
2. Quiet pool.
3. Noisy melt-down.
(a) Delta cave-ins.
(b) Wall cave-ins.
4. Quiet flat-bath.
Since the furnace does not require constant attention during the melt cycle, employees may usually spend most of the time when high noise levels are being generated in an acoustical enclosure as a means of achieving OSHA compliance. Employees in adjacent areas engaged in other activities, however, may also be exposed to excessive noise levels.
The only approach to noise abatement for these employees would be to isolate them by means of a wall or to relocate their work area. This is often not possible, since foundry and steel mill operations often require large work spaces with crane access.