Document YjzwvdzDrVw5bj5BEKekL2Rx0

212 CHAPTER 14 Table 21 ....Typical Community land Traffic Activities Ratings Corresponding to the Background: Noise Curvesin Fig.' 21 - Condition Corrcqioadfag Cot* fa fig. 31 Nighttime rural; ho' nearby traffic of ooheern Daytime rural; no nearby traffic of concern . Nighttime suburban; no nearby traffic of concern Daytime suburban; no nearby traffic of concern Nighttime urban; no nearby traffic of concern -.Daytime urban; no nearby traffic of concern ., 300 to,1000 ft from intermittent lirfit traffic 300 to'1000 ft from continuous light'traffic Within 300 ft of intermittent light traffic 'Within 300 ft of continuous light traffic.1000 to 2000 ft from continuous medium density. , ' tihffiC ' 1000 to 2000 ft from continuous heavy traffic ' .2 3 3 . ,. 4 4 5 300 to 1000 ft from continuous medium.density traffic ., 300 to 1000 ft from continuous heavy traffic " Within '300 ft of continuous' medium density Within 300. ft of continuous heavy traffic ., Nighttime: business or enwmwmnial area Daytime; business or commercial area Nighttime; industrial or manufacturing area Daytime; industrial or manufacturing area large equipment, it is recommended that large cooling towers be selected from sound pressure, level ratings tabulated for various locations rather than from sound power, ratings. Pn> cedures and work sheets for this are av&Hable.from Reference 69. These also allow for the shielding effect of. large walls. . 1965 Guide And Data Boole A comparison of the sound ratings:of available equipment with the acceptable levels will indicate how much, if any, silencing treatment will be required under the prevailing con ditions of distance and directivity. Considerable design infor mation is available in References 69 and 70. Silenoers for sp cific units, are often available from the 'manufacturer. Acceptable octave band sound pressure levels inside adja cent buildings can be' estimated from Table 4. This intrp- duces the additional variable of the noise. reduction (NR) value of the building waff. Table 24 shows .that the most im- portant factor is the. extent to which' windowB and. vents'are open. When the neighbor's building' is hot air condition^ Row Bof .Table 24 should be used.. "' STEP 6--ISOLATION OF MACHINE VIBRATIONS The relatively, .high speeds of modem machinery and .the relatively lightweight construction of modem buildings nudtp it necessary to, take,.the following precautions to prevent excessive' vibration. .Faflure to observe any one of these-w3] almost' certainly cause- objectionable-'noise and vibration, often in remote parts of-- the building, and may even mjw fatigue failures. 1. Specify that machines shall be balanced, both statically and dynamically, within the limits of best commercial practice, limits of balance which can.be reasonably expected for various types of machines11 are given in Table 25.. 2. Evaluate the inherent quietness o! various machine types! Centrifugal pumps and compressors generally run smoother then reciprocating ones, and compressors with four or more cylinders run smoother than one or two cylinder machines. Whenever dust or lint is a problem, fans should be of a design which minimises the accumulation of foreign material that would create imbalance. . ' 3. Specify that machines shall- not-have any critical speeds within 30 percent of any contemplated operating speed. Before changing the. operating speed of a major machine, or of^any machine.that is running rough already, check,for structural resonances and contact the manufacturer to make sure that the Table 22 .... Decibel Difference Between Power Level of Outdoor Equipment None and Corresponding-Sound Pressure Level at Any Distance, r* Pufmea, r, fa Feet (Lw -L,) for Q - 2 (Lw - L.) for Q - 4 (Lw - L,) for Q - 8 10 18 15 12 15 - 21 18 15 20 24 21 18 30 ' 27 24 21 40 30 - 27- 24 60 80 >20 200. ' '500 ' 1000* 33 '. 36 .39 30 '. 33, 36 . 27 30 . 33 43 40 37 . . 51 , 57r-62' 48. - 54-59 45 51-56, - * Table does Dot apply when r fa fa** (has twice the nurinma dSmraaoa of tbe equipment. Vafaee may be op to ft db low for diauace* between S end ft turn* tyhnum equipment dimcanou. v .. * At above 1000 ft, eir ebeorptieu end e&iK*pherie eonditiona become important. . .< Table 23 .... Approximate Power Levels* for Cooling Tower Noise, m db re-10"1* Watt*7 CoeGng Tower fan* Octet* Band Center, cps" Tjrp* Horsepower 63 106 425 '850 1700 3400 - 6900 125 250 500 ` . 1000. 2000 4000 8000 Induced Draft Propeller ' Forced Draft Centrifugal 10 hp* . 20hp* 50 hp* 100 bp* 20 hp 50 bp . 100 hp 97-102 100-105 104-109 107-112 90- 95 94- 99 97-102 96-101 99-104 103-108 106-111 89- 94 93- 98 ' 96-101 94-99 97-102 101-106 ,,104~109 87- 92 91- 96 94- 99 91-96 87-92 " ' 94- 99 ' 90- 95 98-103 101-106 94- 99 97-102 85- 90 83- 88 89- 94 87- 92 92- 97 90- 95' 83-89 86-9290-96 93-99 83-88 87-92 90-65 80--87. / 83-90 . 87-94 90-97, 79-84 '83-88 ' . 80-91 ; '73-83 76-88 80-90 83-93 - 76-81 80-85 83-88 -* Them powet leveb db'ttot"provide mlcarnation eb Ufa rmdistioo pattern (directivity) of the mood. A cafetgr factor of up to 1ft db ahoatd be applied. iniirltllf in the father frequency beads when tbe eir discharge or intake openings (ace e critical kfaeticn. Final mfactiona should always be baaed co manufaetunsr'e ratiaf* far the particular orientation of the eoolina towov " * For propeller fans addfftdb to fisted wwf fa bead in which tbe blade frequency ocean. Blade frequency (RPM/BOj X number ai hfadea,- Sourid.Conl/v-J 2T3 Table 24 .... Approximate Noise Reduction Provided by Typical Wall Constructions) in db Description a_No wail; outside conditions B--Any typical wall construction with windows nor<>_Any typieaTwall construction, windows closed but frTnrirb' vent* oorroally open D--Any typical wall construction, with no crackB or E--Apprwdmately 20 Ib/aq ft solid wall no windows, 806,1F ft waJL no windows, cracks or openings 53 63 0 10 15 20 26 32 Octet* Bond Center, 91 106 212 425 850' 1700 3400 6900 125 250 500 1000 2000 . 4000 . 8000 000000 0 10 12 14 16 18 . 18 18 15 17 19 21 23 . 23 23 20 23 26 29 32 32 32 27 31 35 39 43; 45- -- 47 33 - 37 \ 1 Ml 45 - . 49 51 . :s3.>.=t sew speed will not be too close to any critical speed. Critical speeds are usually due to the natural frequency of shafts, framework, piping or fan blades. Critical speeds of multiple shafts anoe estimated easily from nomograms in Refer ence 71. Bear is mnd that the critical shaft speed may be lowered by the flexibility of the bearing supports. 4. Design the supporting structure so that it docs uot have any natural frequencies within 30 percent of the operating speed of any one of the machines to be mounted on this structure. There is a dfr*TM* advantage in designing supporting structures simple ^wntgh so that their natural frequencies can be estimated from n^m^gntme Buch as those in Reference 72. Foundations for heavy highspeed machines such as centrifugal compressors should t pK--lrgH particularly carefully. Step-by-step design-information is available in Reference 73. 5. M*ka sure that the machinery room is constructed maa- gjvdy with effective "**1* around all pipe and duct openings into adjacent rooms. This is necessary in order to reduce the transmis sion of airborne sound to other parts of the building. <x Analyse the vibrational forces and motion for each machine installation in sufficient detail to insure that no excessive forces are transmitted to the building structure and no excessive strain impfMeH on connecting pipes,- ducts, etc. It must be understood that no'matter how rigidly a machine seems to be supported, there is a certain amount of flexibility in either tbe supporting structure or the framework of the machine itself. If the combina tion of this flexibility, the mwa of the machine, and its speed approaches a certain critical value, violent vibration will result even if the machine is most carefully balanced. This condition is called resonance and is bound to cause trouble and often prema ture failure. It is absolutely necessary and always. posable to avoid resonance, either by malring the flexibility of the entire supporting structure muck fairer th*n the. flexibility which , pro duces resonance, or by using machine mounts which have a much higher flexibility. For the reciprocating machines) the analysis for resonant condi tion has to include low-order multiples of the machine speed be cause there are secondary forces and couples which cannot be completely balanced out. For all electrical equipment, the analysis must include the frequency of magnetic distortion which occurs at twice the line frequency (120 cps).- > - ';. It must also be borne in mind that.toe.centrifugal force due to imbalance produces not only vertical, t but also horizontal vibration at right angles to the shaft. Unbalanced couples, or the customary location of .vibration mounts wcll bdow the.center of gravity of equipment, cause vibration in, axial, rolling,'rocking and twisting modes. Thus, there are always 6 possible resonant frequencies which have to be avoided. Calculation of Vertical Natural Frequency . Vibration in a vertical direction is generally most serious because of tbe low rigidity of common floor structures in the vertical direction. Tbe vertical forces acting on a machine are shown schematically in Fig. 22. The symbol of a spring is used between the machine and the foundation to show that every supporting structure has a certain amount of flexibility.- When the Tnftihing is rigidly mounted on a heavy concrete pad rest ing on soil,,this flexibility may be relatively, small, but'not gn-iftU enough to be negligible. Data on soil compressibility can be found in Reference 73. Table 25 .... Typical Machinery Balancing Specifications. Source 1. Mil-Std 167 (Ships) 2. Tool Engineers Hand book!* (Chapter on Balanring Machines. ^ W. L. Senger, p. Sources 1 and 2 3. NEMA Standard MG 1-4.23 Rev. 11-15-56 ApptkabS&y Utataag Uaits end Parameter* Rotors for Naval Machinery 700 rpm; OA-iri. I.l---------- 100 lb 950 rpm . os-in. 0.62--------- 1001b 1140 rpm . os-in. 0.35--------- . 100 lb 1750 rpm os-in. 0.23 --------- 1001b 3450 rpm os-in. 0)12--------- ` 100 lb Rotors for any type machine with suffi ciently rigid'structure 500rpm 1000 rpm> 0.0005 to 0.001 in. 0.0005-0.001 Displacementoffreely 0.0002 to supported bearings 0.0005 in. - 5000 rpm 0.0005-0.001 0.0002-0.0005 0.0001-0.0002 a Rotor weight in percent of mufluhe'' 25%upi wei^it.'. Rotors for any type . Dynamic (2-plane) balancing required if RPM > 1000 rrmobin* 'or length/diameter ratio of rotor > H Completely asembled electric motors elasti cally supported for test Frame Diameter Series. 'ISO, 200, 210 and 220 250, 280 and 320 360, 400, 440 and 500 Max. Peak-Peak Displacement -0.001 ml . ` - \ 1 0.0015 in. 0.002 in. ... MlWtd 167. althoafh xpiee*od Is different tern*, axo'roa*lily equivalent to those from Tebfa 84-7 of Tool Pmiiw-ri' Handbook, far rotors " wcw Percent of w^Ki*. wefaht. ,, , r M allow* unbalance* Ur in cum of tboee Gated fa'the other two aocrcea, ny for S-pdfa motom. Better balance* can usually be obtained