Document G6zyYD4oYQq3wJvDmYgr0aYnv

UNIT 6 - OTHER ATTENUATING METHODS At this point, it would be worth your time to again review Unit 3, "Overview of Noise Attenuation," First, have the obvious and easy to fix noise situations, such as listed below, been identified? A ready solution is often self-evident. -worn gears -worn bearings -replace other worn -out equipment that produces excess noise -metal belts -vibrating fan shroud or other panels -simple absorption methods, cover equipment, etc. -cavitation -noisy valve, wrong size valve, etc. -fan speed too high, fan too large, fan out of balance, fan duct transmitting noise. Other noise reduction techniques to be discussed now include: mufflers, damping, vibrations isolation, lagging, control valve noise attenuation, high velocity gas stream noise, resonating mufflers, high fluid velocity noise (lagging), fans and ventilation system noise. On 7?-;s9 conf :FNT7a[ I. VIBRATION ISOLATION A solid piece of equipment that vibrates is a source of sound energy that can be transmitted through a structure to a person. Breaking the path leading from the source to the person is an effective means to remove the noise. Let's consider first a homey and yet real problem. You flush a toilet, and it is heard in every room that is quiet. Was the noise air-born? Most likely not as the primary path. A brief S-P-R diagram may help.. SOURCE PATH RECEIVER *Easy to Break Path C O N F T D F N T IA l KRMOUUeC) TYPICAL INDUSTRIAL SITUATION OLOOP \eLEC, V> SV\K>K2&X\V\S^ f ^RUVPVAe^T t>PRY**e> \S^LM0R6 1p_:,^-_^-0 *. *a- - _3I fItrr-- --"H4IU^.-.i........... X w\<s.w pp*6*, OIL OKt \U^RVU.v PtfuMDATlON Vv&CiE^VC?H fAO D 077991 rnNFTDFNTIa, &-V I In the industrial situation shown on the previous page, the design of a system and selection of the spring isolators, inertia block size and weight and also the proper vibration pad is very complex. The two primary choices that exist to reduce noise transmission are: (1) to insolate the vibrating equipment (as was done in the above), and (2) stiffen so the vibration is overcome. Various types of vibration isolators are shown below. Sleeve Type DO 07799? donftdfntial Some dB reductions possible when jsing flexible connectors to Isolate piping are shown below. 50 ---------- 1--1--1--1--1--1--1--1--1-- 45 ilTj 20in 3QHWOQD > 40 -- m n XMUU COWOUUTID 'J w 35 HUK* CUJNI.CSS JTCQ. " JOINT..................... Sl2hl2M u Z\ 30 25 ALL JONTWrm 20 mmcft caskcts t*HMACLTA1 HLHHMOSLEOOta> --/w / > 4 s -- 1_ % 15 10 -j 5 -~P _A 0 5o0 so n too iso too 3oo <oo too too aoo moo moo < too ISO too 300 400 00 too ttoo MOO MOO 31004000 OCTAVE BANDS IN HERTZ II. DAMPING Reduction in pipe-wall vibration levels by various types of commercial flexible connector*. These measurements were made at atmospheric pressure. Another method to reduce the response of vibrating surfaces is through damping. While damping is complex it is easily demonstrated merely by hitting a resonating metal panel with and without a damping material being present. Common damping materials Include; elastomeric sheeting, sheet lead and damping felt. Often, damping material is used along with additional plate. This is shown below. Vibrating panel as vibrating panel This combination of stiffening and damping is generally very effective. Also, of course, if the vibrating panel can be reduced in size, vibration is reduced and noise is thus reduced. DO 077993 OONFIDFNTTAL III. GAS VELOCITY A moving gas stream results in noise in three primary ways: 1. A fan, car engine, or any reciprocating mechanism can pump a regular stream of energy pulses into moving air or gases. The result is noise. 2. A jet of high velocity gas or steam has so much energy that it shears the surrounding air and creates an unstable region which moves at a rate which the ear recognizes as noise. 3. A gas stream directed into an obstacle which vibrates and thus creates noise (i.e., wind blowing on a telephone wire or air blowing over a sharp metal edge). Jets High pressure plant air is often released, for example; (1) to move some object or (2) the air has been used and it now needs to be released and cleared away. The noise made when high pressure air is released can be a serious problem. For example, release of 75 psi air to the atmosphere can result in noise of 110 dB. In example 1 above, the need is to maintain the thrust of the air and yet reduce the noise. Devices for doing this are pictured below. /t 1 ' v -throST I* DO 077994 OONFTDFNTTAL DO 07 7995; CONFTDFNTTAl Ip- These are cheap ----- less than $10. They reduce the nolee (18 - 20 dB) and also lower the use of compressed air. In example 2, where the air Is released to the atmosphere, numerous mufflers are available at low cost. They are so cheap that making them yourself Is a waste of time. One example are the mufflers installed on pneumatic tools that result in excellent attenuation. Steam released to the atmosphere is often the cause of noise. With steam, a new set of problems are encountered. Steam keeps expanding, so it is hard to reduce the velocity. Also, steam condenses, and the . water is corrosive. There are special mufflers for steam. Often a good solution is to pipe the steam away from the area so that people are not exposed to the noise. IV. REASONATING MUFFLER Resonant-type mufflers also absorb noise. They are very effective over a narrow frequency range. The design of these is very complex. A pneumatic conveying jystem handling synthetic fiber fluff discharged into a filter bag-type separator as shown by the sketch at right. Blower noise traveled with the air and product and was objectionable where it discharged in the filter area. An absorbing-type muffler was not attractive because of the possibility of snagging and plugging. A resonant-type muffler was supplied by Universal Silencer Corp. which provided the noise reduction shown by the table below. Octave Band Center Frequency-Hi Noise Reduction--dB 63 125 250 500 1000 2000 4000 8000 12 23 13 11 10 X X X --Application of resonating typa muffler. V. CONTROL VALVES Valves that control flow and also have a. high pressure drop can contribute substantial noise. The two basic approaches to noise reduction DO 077996 confidential u-' I used with control valves include: Cl)source treatment and, (2) path treatment. Noise from control valves comes from: 1. Mechanical Vibration. Lateral motion of the valve plug or the trim parts resonating cause such noise. 2. Hydrodynamic Noise. Cavitation is the major cause of such noise. 3. Aerodynamic Noise. The flow of compressible fluids (gases) is undoubtable the most common source of valve noise. Noise results because of the conversion of the mechanical energy of flow to sound energy as the fluid passes through the valve restriction. ' Turbulence is the cause of this noise. Sources of turbulence include: Obstructions in the flow path, rapid expansion, and direction changes. Hydrodynamic Noise As stated, cavitation is the major cause of such noise in control valves. Cavitation can contribute 10 or more dB of noise. With the upstream and downstream pressures above the liquid vapor pressure and if at some point within the valve the pressure is lower, cavitation results. Flow conditions that accurately preduct cavitation exist. Special cage valve bodies are used that prevent cavitation. Aerodynamic Noise The noise level in a control valve results from the flow path through the valve. 100 dB or more noise can result when the pressure drop is excessive, and high turbulence results as the fluid goes across the valve. Special cages with multiple openings reduce turbulence. These can drop noise level by 16 - 18 dB where the pressure drop ratio does not exceed 0.65. Where the pressure drop ratio exceeds 0.65, the special cages take part of the pressure drop; and downstream of this, a diffuser takes the remaining pressure drop, and up to 20 dB drop can be expected. Where large flows and high pressure drop ratios exist, further DO 077997 CONFTDFNTTA1 1^ - modifications in the cage result in 25 - 30 JB reductions. Instrument personnel and also suppliers can relay the various valve modifications that have been used and what else can be done. The special cages for valves that reduce the noise also reduce the flow capacity by about 20%. Thus, if you are pushing at the maximum flow rate for the valve and/or line in use, these special cages cannot be used to attenuate noise. In such situations, you are forced to use path treatment. With hydrodynamic noise, cavitation is occurring. The physical damage this does should not be permitted to go on. Thus, path treatment without elimination of cavitation is not wise. Special cage valve bodies that eliminate cavitation are needed; and after this, path treatment can take place. Normally, noise is transmitted down the pipe, and path treatment is an important method to reduce the noise that evolves downstream. Path treatment can be used to control emission of aerodynamic noise, as well as hydrodynamic noise. Such path treatment methods include: 1. Heavier walled pipe (schedule 80 vs. 40 gives 5-6 dB drop) 2. Accoustical insulation (10-15 dB drop) 3. Thermal insulation (7-8 dB drop) (The above do not reduce the noise in the fluid stream; they mask it. Therefore, where treatment is discontinued (for example, at the end of 50 feet of lagged pipe), noise again reappears . 4. Silencer (noise attenuation of 20-25 dB possible - very low pressure drop exists across silencers. They are used if velocity at outlet does not exceed 0.3 Mach, or 240 ft/second). The Sigma computer in Engineering has a valve noise prediction program. Conditions can be varied (l.e., flow, valve size, line size, pipe thickness, etc.) and the Impact on noise can be calculated. A DO 077998 CONFTDFNTTaL, (> > special entry number Is needed to use the program. Help from various engineering personnel Is available to do this. VI. HIGH FLUID VELOCITY High fluid velocities in pipes can generate sound. Generally, sound is generated by high fluid velocities in piping components (valves or sharp bends or turns, branch discharge, contractions and expansions, water hammer). Often, it is not the velocity increase that results in noise, but an abrupt change in velocity that results in turbulence. In one case, just merely reversing a valve reduced the turbulence; and 7-9 dB reduction was realized. In domestic piping, the noise radiated by pipe is small compared to what is transmitted to and radiated by supporting structure (walls, joists, etc.), Here, as was discussed earlier, vibration isolation is important. Industrial piping is larger, and sound levels may often be high. Here, appreciable noise is radiated by the pipe walls. Also, the pipe support structure is heavy and massive; and therefore normally radiate little noise. Here, lagging or pipe covering results in more noise reduction than does isolation. Lagging or covering the pipe with a two-inch layer of low-density glass wool is effective at frequencies ahove 1000Hz - - 10-20 dB reduction is possible. Thick layers of thermal or accoustical material (four to six inches) even provides some noise reduction (7-10 dB) at the lower frequencies. Such thicknesses theoretically will give 30 dB attenuation at higher frequencies. Such high dB reduction is difficult to attain with lagging since leaks occur in the acoustical material that limit practical effectiveness to 12-15 dB. Much care - - using several layers and overlapping to cover cracks and joints - is n cessary to achieve high dB reductions. DO 077999 conftdfntiai (0-1 VII. FANS, BLOWERS. VENTILATIONS SYSTEMS Noise generated by fans and blowers has been discussed in part in Unit 3. Speed and balance of the fan contribute noise. Lowering the speed or balancing the fan can reduce noise. Lined ducts or a plenum chamber also attenuate noise. It is best to buy the fan/blower you need that gives the permitted noise or less. Large fans and blowerB should only be purchased on the say-so of fan experts. The below steps are useful when purchasing small blowers and fans to ensure quiet equipment. 1. Design so the required air flow has the lowest static pressure. Thus, the fan can operate at low tip speed (low noise level). 2. Select a fan that operates near maximum static efficiency related to the required air flow and pressure drop across the system. 3. The best fan (selected from the fans that meet the above requirements) is the one with the lowest sound pressure level. Request this data from the manufacturers. 4. Avoid any fan that has sharp peaks in the one-third octave band sound power spectrum. A peak means one component is contributing this noise. Other factors: 1. Speed faster - - more noise 2. More blades - - more noise 3. Dampers increase noise 4. Abrupt turns increase noise 5. Vibration (of the fan, duct, or shroud) increase noise 6. High-static pressure increases noise 7. Increased air volume increases noise 8. Diffusers increase noise 9. Offset flexible ducts are noisy DO 078000 CONFIDENTIAL (/"/ m 10. Room etc. noise can be transmitted through ducts 11. Design bends and elbows to give the least amount of turbulence 12. Install absorbers in bends/elbows 13. Use fan guide vane to reduce turbulence 14. Mount fans on inertia block and use flexible collar to connect to duct to reduce noise 15. Noise travels upstream and downstream, and silencers may be needed at intake and discharge 00 O 78001 CONF T qFMTT Al (T/ 7 Ducts transmit sound. For example, a far with six blades turns at 1160 rpm. A blade passes the lip of the scrolls 116 times per second. Each en counter produces a puff of energy in the exhaust stream. We can change the fan speed, dampen or stiffen the scroll, lag the fan body, etc. None of these measures will do anything about the noise in the air stream Inside the duct. Placing an absorber in the duct will help. Also, incorporation of a duct si lencer will reduce the noise. Duct Silencers One significant consideration in the use of duct silencers in the amount of pressure drop the silencer introduces. To maintain fan effectiveness, this needs to be as low as possible. People who supply these have much experience, and the use of their catalog data to select a silencer is mandatory. The Industrial Acoustics catalog is available. Absorbers in Ducts Ventilation ducts are often important noise transmission paths. They can be lined to attenuate; by absorbing the sound that travels along them. The noise reduction obtained by lining Is calculated (with fair accuracy) using the below: NR = XI dB/foot A P - perimeter of the duct in inches A cross-sectional area of duct in square inches 0( Of Liner 0.50 0.55 0.60 0.70 0.80 0.85 0.90 X 4.78 5.46 6.16 7.65 9.16 10.02 10.87 DO 07800? C'.ONF T DFNT T F SOUND ABSORPTION ALONG A TRANSMISSION PATH IN DUCTS