5.8 NOISE FROM GAS VENTS

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One of the more serious noise problems in industrial plants is the noise

produced by the discharge of air, steam, or process gas into the atmosphere.

Blow-off nozzles, steam vents, and pneumatic control discharge vents are

some examples of noisy venting situations. The noise generated by the jet

discharged through these devices is a result of turbulent mixing in a highshear

region near the exit plane of the vent. In this region, turbulent eddies

are small, and the noise radiated from the eddies is predominantly higherfrequency

noise. Sound is also radiated from the fluid stream further from

182 Chapter 5

Copyright © 2003 Marcel Dekker, Inc.

the jet exit plane as a result of larger turbulent eddies in this region of the jet.

Lower-frequency noise is radiated from this region of the fluid.

The overall sound power level for noise radiated from a vent may be

calculated from the following correlation (Burgess Industries, 1966):

LW ј 114:4 ю 20 log10

P1T1Mad

PoToMdo

_ _

(5-39)

The quantities in Eq. (5-39) are defined as follows:

P1 ј upstream absolute pressure of the gas

Po ј reference pressure ј 101:3 kPa ј 14:7 psia

T1 ј upstream absolute temperature of the gas

To ј reference temperature ј 300K ј 5408R

M ј gas molecular weight

Ma ј molecular weight of air ј 28:95 g/mol

d ј inside diameter of the gas vent

do ј reference diameter ј 1:000m ј 39:37 in

The octave band sound power level spectrum may be determined for a

gas vent from the following conversion.

LWрoctave bandЮ ј LW _ CF7 (5-40)

Values of the conversion factor CF7 are given in Table 5-10. The frequency

at which the maximum sound power level occurs for the gas jet is given by

the following relationship:

fo ј

0:20c

d

(5-41)

The quantity c is the sonic velocity of the flowing gas at temperature T1.

Noise Sources 183

TABLE 5-10 Conversion Factors CF7 (dB) to Convert

from the Overall Sound Power Level for a Gas Vent to

the Octave Band Sound Power Levels

Frequency CF7 Frequency CF7 Frequency CF7

fo=32 26 fo=2 7 8fo 17

fo=16 21 fo 5 16fo 25

fo=8a 15 2fo 7 32fo 31

fo=4 10 4fo 10 64fo 37

aThe table entry fo/8, for example, refers to the octave band that

includes the frequency fo=8, where fo is given by Eq. (5-41).

Copyright © 2003 Marcel Dekker, Inc.

The noise from gas vents is highly directional, so the directivity factor

is not unity in general (American Gas Association, 1969). The directivity

factor Q_ depends on the angle _ measured from the vent axis. Values of the

directivity factor and the directivity index, DI ј 10 log10 Q_, are given in

Table 5-11.

Example 5-5. A steam vent has an inner diameter of 154mm (6.065 in) and

vents steam (molecular weight, 18.016 g/mol; sonic velocity, 500 m/s) at

615K (3428C or 6478F) and 1480 kPa (215 psia). The vent is located outdoors.

Determine the overall sound pressure level and the A-weighted level

at a distance of 150m (492 ft) and at 908 from the vent axis.

The sound power level is found from Eq. (5-39):

LW ј 114:4 ю 20 log10 р1480Юр615Юр28:95Юр0:154Ю

р101:3Юр300Юр18:016Юр1:00Ю

_ _

LW ј 114:4 ю 17:4 ј 131:8dB

The directivity index for a location 908 off the vent axis is found from Table

5-11.

DI ј _5:5dB

The overall sound pressure level at a distance of 150m from the vent is

found from Eq. (5-4) for negligible atmospheric air attenuation:

Lp ј LW ю DI _ 20 log10рrЮ _ 10:9

Lp ј 131:8 ю р_5:5Ю _ 20 log10р150Ю _ 10:9

Lp ј 131:8 _ 5:5 _ 43:5 _ 10:9 ј 7:19 dB

184 Chapter 5

TABLE 5-11 Directivity Factor Q_ and

Directivity Index DI for a Gas Vent

_a Q_ DI, dB _a Q_ DI, dB

08 1.00 0.0 608 0.80 _1:0

108 1.80 2.6 758 0.447 _3:5

158 2.16 3.3 808 0.381 _4:2

208 2.52 4.0 908 0.282 _5:5

308 3.00 4.8 1058 0.200 _7:0

408 2.50 4.0 1208 0.158 _8:0

458 2.00 3.0 1508 0.118 _9:3

508 1.53 1.8 1808 0.100 _10

aThe angle _ is measured between the axis of the vent

and the receiver.

Copyright © 2003 Marcel Dekker, Inc.

The peak frequency in the vent noise spectrum is found from Eq.

(5-41):

fo ј

0:20c

d ј р0:20Юр500Ю

р0:154Ю ј 649 Hz

This frequency lies in the 500Hz octave band (354–707 Hz). The conversion

to octave band sound pressure level is found using Eq. (5-39). For example,

for the 500Hz octave band, we find the following value:

Lpрoctave bandЮ ј 71:9_5 ј 66:9dB

For the 250Hz octave band, which includes the frequency 1

2 fo ј 325 Hz, the

octave band sound pressure level is as follows:

Lpрoctave bandЮ ј 71:9_7 ј 64:9dB

The sound pressure levels for the other octave bands are given in Table 5-12.

The A-weighted sound pressure level may be determined from the

octave band sound pressure level values using Eq. (2-46). For the 500 Hz

octave band, CFA ј _3:2 dBA from Table 2-4. The calculation for the Aweighted

sound level is summarized in Table 5-12. The A-weighted sound

level is determined, as follows:

LA ј 10 log10f_10ЅLрoctave bandЮюCFA_=10g

LA ј 10 log10р103:07 ю104:58 ю105:60 ю_ _ _Ю ј 69:3 dBA

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