Noise pollution
analysis in Tehran cement plant
Ahmadi Orkomi
A, PhD1*, Tavakoli B, PhD 2, Noorpoor A, PhD 3
1- PhD student at faculty of environment, University of Tehran,
Tehran, Iran. 2- Assistant Prof., faculty of natural resources, University of Guilan,
Rasht, Iran. 3- Associate Prof., faculty of environment, University of Tehran,
Tehran, Iran.
Abstract
Received:
August 2013, Accepted: February 2014
Background: Cement industry has many process units. Basically all of these
units can be considered as a source of noise. Since noise pollution is
defined based on its offensive hearing effects, the importance of the noise
sources depends directly on the number of workers in the unit. Materials and
Methods: An experimental study has been done at Tehran cement factory to
recognize and analyze the major noise pollution source machineries. In this
survey, first, the mean A-weighted sound pressure level and the number of
workers in each unit have been measured and the obtained data has been analyzed
in Microsoft Excel. Afterwards, based on the mentioned ISO, some experiments
have been done to calculate the A-weighted acoustic power level of the
crusher which emits the highest level of noise. Results: The noisiest units have been recognized and some applicable
suggestions have been offered to reduce the sound exposure level. Also by
calculating the sound power levels and considering the design parameters
which affect the sound power generated by the crusher, an empirical equation
has been presented to calculate the acoustic power level as a function of
horsepower, number of hammers and the weight of each hammer. Conclusion: The acoustic measurement results show that the crusher, cement
mill and row mill units are the three major noise emission sources. In
another experiment, based on ANSI S1.36 and ISO 3746 guidelines, the sound
power level of the crusher unit has been measured indirectly. Afterwards, by
using a multiple regression algorithm and minimizing the mean square error,
an empirical equation has been proposed to correlate the acoustic power level
as a function of the horsepower and the mass of all hammers of the crusher. |
Keywords: Sound, Measurement; Cement.
Introduction
Noise is one of the physical
environmental factors affecting human health. Because of the adverse effects on
people's, life in the large and industrial cities, noise pollution is becoming
a remarkable concern. Noise in work environment is the major cause of concern
for safety and health of the factory workers. Regulations limiting the noise
exposure of industrial workers have been instituted in different countries (1,
2). The aim of these noise regulations is to ensure that workers' hearing is
protected from excessive noise at their place of work, which could cause them to lose their
hearingor to suffer from tinnitus. Noise in work environment is the major cause
of concern for safety and health of the factory workers. Industrial laws in
many countries provide protection for workers against noise pollution as
described by*
the National Environmental Standards Committee of the Tanzanian Bureau of
Standards (TBS) (3).
Exposure
duration of 40 hr per week of equivalent noise level of 85 dBA is considered to
be safe and noise level above this limit is bound to cause induced temporary or permanent
hearing loss (4-6). Cement manufacturing is one of the major areas that play a
significant role in economy of countries. Sharma, et.al (7) compared
experimental sound pressure level (SPL) with the calculated SPL by proposed
formula and related the exposed SPL to the horsepower and the life time of the
source. Also some researchers have been done on measuring the noise level in
cement plants, analyzing the hearing damage to labors and providing some noise
control suggestions to reduce the damaging effects of noise exposure
(8-10).With the aim of noise generation parameters recognition, a vast research
has been done (11, 12). Makarewicz (13) and Jones and Hothersall (14) studied
the effective parameters on road vehicle noise generation. The cement
production machineries emit offensive noise because they have a high acoustic
power. In the design point of view, to investigate the sensitivity of the
emitted noise to the physical parameters, the influence of the parameters on
the sound power level should be conducted. But based on the writers’ knowledge,
the effect of design parameters on cement machineries noise emission has not
been published well. In this work an experimental survey has been done in
Tehran cement plant to demonstrate the most noise emission sources. To overcome
the aforementioned drawbacks in recognition the affecting parameters on the
acoustic power level, an empirical equation has been presented to correlate the
sound power level of the crusher to its design variables. The paper is
organized as follows. In the material and methods section, the algorithm of
sound pressure level measurement and the formulation to calculate sound power
level have been discussed. In the results section, the measurement results have
been reported and the noisiest units have been detected. Also, correlation
between sound power level and some key effecting parameters has been discussed.
Some conclusion remarks have been highlighted in the last section.
Figure1:
Schematic diagram of process units in a cement factory
Materials and
Methods
This study was conducted at Tehran
cement factory. Tehran cement factory is one of the highest ranking cement
producers in Iran and its product is type 2 Portland cement. Based on the researcher’s
knowledge, studies about the noise pollution from machinery and the noise
exposure rate in the workers zones have not been well established for this
industrial plant. A schematic diagram of the main process units in a cement
factory has been shown in Figure 1 and the overall map of the cement plant has
been demonstrated in Figure 2. All measurement procedures were according to the ISO 3746 (15)
and the noise levels were measured by a digital mini sound pressure level meter
(MODEL TES 1353H) in the workers main commuting spots. The measurements have
been done during the work hours on July 20, 2013, between 11.00 am and 4.00 pm,
in which all of the major machines were running and the number of workers was at
maximum level.
Figure 2: Perspective view of Tehran cement
plant with tagged SPL measuring areas
Firstly, the number of units and the
measuring areas in each unit have been chosen based on the noted ISO algorithm.
The measuring units in the plant are main stone crusher, row material mill, pre
heater, kiln, kiln to cement mill conveyer line, cement mill and packing unit.
These units also have been tagged in Fig. 2. To obtain a better and more
reliable data, for the large areas, the number of measuring point was
increased. Some of units in this plant have more than three floors. For example
in the row of material mill there is a separate floor for the mill, gearboxes
and centrifugal separators. So in such a unit to obtain a remarkable and
tangible result, the measurement has been done on all the floors separately. In
all the measurement points the sound level meter was set toward the mentioned
unit and 60 centimeters away from operator.
The pressure level of a source can
be measured directly and easily, but until now, there is not a devise or
algorithm to measure the sound power level of an acoustic source directly. The
importance of the sound power level has become evident from design and noise
control point of view. The pressure level of a distinct source depends on the
features of the environment in which the sound transmits and it is different
for a noise source in two different media. Then the SPL cannot be a sound feature
for design purposes. In contrast, the sound power level is only a function of
the design and operational parameters of the sound source. So if the value of
sound power level is calculated, by correlating the power level with the
physical parameters (like power, frequency, size, constructing materials and
etc.), a machine with a lower value of emitted sound power level can be
designed by altering the affecting parameters.
In this work, an empirical
relationship correlates the sound power level of crusher unit to some
independent effective physical parameters. In approach to the aforementioned
purpose, the value of sound power level was calculated in some different values
of independent variables. The sound power level was calculated from SPL data.
According to Barron (16), the method of calculation of the sound power level
from SPL data depends on the space in which the source is located. If the
desired environment is an ideal reverberant or an anechoic room, some national
and international standards like ANSI S1.31, ANSI S1.32, ANSI S1.33 and ANSI
S1.35 (17-20) and ISO 3741, ISO 3742, ISO 3743 and ISO 3745 (21-24) for
measuring the SPL are used. In this work the environment was not an ideal
reverberant or anechoic room so for this real room, the survey method
established by ANSI S1.36 (25) and ISO 3746 (15), has been applied.
In this work one microphone was used
for measurement. According to the survey method, there are nine key measurement
points around the source as depicted in figure 3.
Figure 3: Key measuring points around a
noise source in the survey method
According
to the survey algorithm described in Barron (16), the values of constants in figure
3 can be selected based on the dimension of the source. The acoustic intensity
for a direct and reverberant acoustic energy can be formulated as below,
|
[1] |
where
|
[2] |
R is the room
constant and may be calculated from Eq. [3].
|
[3] |
in
which
|
[4] |
|
|
|
|
|
[5] |
|
For
air at standard condition, the last term of Eq. [5] is approximately equal to 0.1
decibel (dB). So by measuring the sound pressure level the sound power level of
the noise source can be obtained during each operating condition by applying
the Eq.
[5].
By these data, the empirical relation between sound power level and the
effecting designing and operating parameters can be derived. The major effective parameters which has been
considered are the engine power, mass of hammers (mass of one hammer times the
number of hammers) and angular velocity (rpm). Four crusher units have been
studied in this survey. Three of them were the main stone crushers located in
unpainted block rooms. The surface absorption coefficient of such a room is
reported as 0.35 in Barron (16). The other crusher was a mini crusher installed
between kiln and cement mill units to operate on the clinker particles. It
should be noted that the sub crusher is located in an open field in which the
reverberant acoustic field is much lower than the direct field. So, this
environment can be dealt as an anechoic room and the sound power level
calculation procedure is as below,
|
[6] |
Where j is counter on the number of
measuring points. The total area of the hemisphere commonly has been divided to
Ns equal surfaces. Here the value of Ns is selected as 6.
Results
Regarding to the number of
measurement in a unit, the logarithmic average of the data has been obtained
and reported as the mean sound pressure level of the unit. The result of all
executed measurement has been demonstrated in table 1. It should be noted that
the units’ numbers in table 1 and the tagged numbers in figure 2 are identical.
Table 1: SPL values and the number of labours in noisy units of the
cement plant
SPL-min |
SPL-max |
SPL-LAeq |
Labor
NO. |
Sample
NO. |
Unit |
|
98 |
105 |
102.4 |
2 |
14 |
Crusher
(Ground) |
1 |
97 |
101 |
98.9 |
4 |
Crusher
(Underground-1) |
||
86 |
88 |
87 |
2 |
Crusher
(Underground-2) |
||
76.3 |
80 |
78.1 |
1 |
4 |
Conveyor
(Crusher to Raw Mill) |
2 |
93 |
103.3 |
99.15 |
3 |
16 |
Raw
Mill-First Floor (Gearbox Unit) |
3 |
88 |
94.1 |
92 |
6 |
Raw
Mill-Second Floor |
||
82.5 |
83 |
82.8 |
3 |
Raw
Mill-Third Floor (Separators) |
||
86.8 |
88.9 |
87.1 |
6 |
Raw
Mill- Fourth Floor (Gearbox Elevators) |
||
98 |
101 |
98.8 |
1 |
7 |
Raw
Mill Blower |
4 |
81 |
87 |
86.2 |
8 |
6 |
Pre
heater |
5 |
78 |
80.1 |
80 |
1 |
6 |
Kiln |
6 |
80 |
81.5 |
81 |
1 |
3 |
Conveyor
(Kiln to Cement Mill) |
7 |
99.3 |
103.8 |
100.83 |
2 |
12 |
Cement
Mill – First Floor (Gearbox Unit) |
8 |
98 |
100 |
98.9 |
4 |
Cement
Mill – Second Floor |
||
85 |
89 |
88 |
6 |
Cement
Mill – Third Floor |
||
83.3 |
85 |
84.1 |
6 |
Cement
Mill – Fourth Floor(Separators) |
||
79.2 |
85 |
81 |
16 |
8 |
Packing
Unit |
9 |
By using the data in table 1, the
most intensive noise emission sources in this factory are summarized in table
2. According to tables 1 and 2, it can be concluded that the noisiest source is
the crusher unit with the mean SPL 102.4 dB at its basement. Among the nine
units, the packing unit, kiln to cement mill and crusher to row mill conveyers,
kiln and pre heater have the equivalent sound pressure level (LAeq)
lower than 85 dB. So they are acoustically the safe zones for the labors. But
in the high noise exposure units (e.g. crusher unit), there is major risk of
hearing loss for the operators. Although the workers in these units use hearing
protection gears, it is not an efficient control strategy. Because despite
using the gears, the workers in these zones, have a sound lever induced hearing
damages.
To reduce the level of hearing
damages, it is recommended to update the installed control programs or employ
part time operators or alter their activity zones between safe and unsafe
acoustical zones or apply new control strategies in the noisy zones. The noise
emitted from the noted units in Tehran cement factory can be harmful for the
people in the surrounding residential zones.
As
can be seen in Figure 2, by planting trees around the factory, the noise level
has been reduced to a tolerable level at the residential area. As noted in the
preceding paragraphs, in this survey, the crusher unit emitted the maximum
level of noise in the plant. As can be seen in previous published works (10, 26),
this unit is one of the top ranked noise pollution sources. So regardless of
the type of stone in different factories, this unit naturally is a main source
of noise. In this regard, it cannot be a rough idea to consider the designing
parameters as the main cause of noise emission. So in the following section it
has been tried to correlate the emitted noise of the crusher unit to its
physical parameters.
Table 2: Most noisy units in Tehran cement factory
SPL-LAeq |
Unit Name |
102.4 |
Crusher
(Ground) |
100.83 |
Cement
Mill – First Floor (Gearbox Unit) |
99.15 |
Raw
Mill-First Floor (Gearbox Unit) |
98.9 |
Crusher
(Underground-1) |
98.9 |
Cement
Mill – Second Floor |
98.8 |
Raw
Mill Blower |
Based
on the equations [1-6], the crusher sound power level has been calculated. Some
features of the crushers and the result of power level calculation have been
summarized in table 3.To demonstrate the mathematical correlation among the
sound power level and the effective parameters, the Eq. [7] has been considered
as below,
|
[7] |
in
which hp and Mh are the horsepower of crusher and the
mass of hammers respectively.
Table 3: Measured sound power level and
some parameters of the crushers
Parameter Unit |
Dimensions |
Hammer
No. |
Mass
of Hammer (kg) |
Mh
(kg) |
Sample
No. |
Hp |
LW
(dB) |
Room
dimensions |
Stone crusher
1 |
2*1*2 |
70 |
75 |
5250 |
9 |
1502 |
111.8 |
8*8*3 |
Stone crusher
2 |
3*2*3 |
84 |
120 |
1080 |
9 |
1745 |
117.4 |
10*10*4 |
Stone crusher
3 |
3*2*2 |
70 |
90 |
6300 |
9 |
805 |
106.4 |
12*6*3 |
Sub crusher 1 |
2*1*1 |
60 |
52 |
3120 |
6 |
422 |
102.3 |
- |
By
using the data in table 3 and the proposed Eq. [7], a multiple regression
algorithm by minimizing the mean square error has been used to calculate the
unknown coefficients in Eq. [7]. The unknown coefficients A, B
and C in Eq.[7] have been obtained 15, 10 and 17 respectively. By the
obtained coefficients, the predicted value of LW has been
calculated by Eq. [7] and compared with the measured values in Figure 4.
As
it can be seen, there is a reliable consistency between the predicted values by
the proposed equation and the measured data. It should be noted that by
measuring more data in different factories with diverse types of crushers, the
effect of other physical parameters like angular velocity, capacity of the
machine and the aspect ratio of stone can be highlighted. This plan is one of
our next goals in this research area.
120 115 110 105 100 95
Figure 4.a: Measured and predicted values of LW versus horsepower.
Figure 4.b: Measured and predicted values of LW
versus total hammers’ mass
Discussion
In the present
study the noise pollution problem in the unit No.7 of Tehran cement company has
been addressed. The acoustic measurement results show that the crusher (SPL-LAeq
102.4 dB), cement mill (100.83) and row mill (99.15) units are the three major
noise emission sources and the packing unit, kiln to cement mill and crusher to
row mill conveyers, kiln and pre heater are acoustically the safe zones for the
laborers. After that, the crusher unit as the noisiest unit in the cement plant
has been studied separately to identify the physical and operational parameters
affecting the noise level of the equipment. By using a multiple regression
algorithm and minimizing the mean square error, an empirical equation has been
proposed to correlate the acoustic power level as a function of the horsepower
and the mass of all hammers of the crusher. Although, the number of crushers
which have been studied in this study is relatively low. Our next aim is to
study crushers in Guilan cement factory and modify the functionality between
noise level and physical properties.
Conclusion
In
this work a survey analysis has been done in Tehran cement factory to find the
noisiest sources which emit the noise in workers main commuting spots. It is
recommended to update the installed control programs in the factory to protect the
workers from the harmful levels of noise pollution. The SPL depends on the
features of the environment in which the sound transmitted, while the sound
power level is only a function of the design and operational parameters of the
sound source. So by calculating the sound power level and finding its
functionality with the physical parameters (like power, frequency, size,
constructing materials and etc.), a machine with a lower value of emitted sound
power level can be designed by altering the affecting parameters. Because of
the mentioned reason, in another experiment, based on the ANSI S1.36 and ISO
3746 guidelines, the sound power level of the crusher unit has been measured
indirectly. Finally an empirical equation which correlates the sound power
level with the physical parameters has been obtained.
Acknowledgements
The
authors would like to thank the staffs and managers of Tehran cement company
for their cooperation and useful guidance.
Conflict of Interest: Non declared
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* Corresponding author: Ali
Ahmadi Orkomi, PhD student at graduate faculty of environment, University of
Tehran, Tehran, Iran.
Email Address: Orkomi@ut.ac.ir