This study, which was conducted in 2015, was an attempt to investigate how noise could be controlled in Sarcheshmeh Copper complex. The main objective was to design and manufacture a silencer for the cooling tower pump of the thermal power station.
Technical and acoustic properties of noise sources were initially identified. Then, a plan was presented to design, manufacture, and install a silencer for the cooling tower pump of the thermal power station.
3.1. Investigating Features and Properties of Noise Sources
3.1.1. Technical Properties of the Noise Sources
One of the main issues in designing silencers is identifying the sources of noise. Therefore, after an initial investigation, the following sources were identified as the main noise generating factors in the outlet of cooling tower pump:
(1) The valve that vents out the stem; (2) pressure difference before and after the valve, which generates noise within the valve; (3) high velocity of the steam that passes through the pipe outlet.
3.1.2. Acoustic Properties of the Noise Sources
In order to study the features of the noise generated by the outlet of the cooling tower pump, the sound pressure level was measured at different octave frequency bands based on the dBA weighting chart ranging from 31.5 to 8000 Hz. The measurement was conducted by the use of a sound level meter (B & K 2260). Sound power level (SWL) at a particular distance (R) from the source was calculated through the following formula (
SWL = SPL + 20 Log R - 11
SWL: sound power level (dB)
SPL: sound pressure level (dB)
R: distance from the noise source (m)
After calculating SWL for each frequency band, the total noise of the source was calculated through the following equation (
After conducting all necessary calculations, SWL for the outlet of the cooling tower pump was found to be 135 dBA. The level of noise varies at different distances from the source.
In the present study, noise measurement was conducted by the use of a sound level meter (B & K 2260). Measurement was carried out in the light of ISO 1996 standard. In order to enhance measurement reliability, the sound level meter was calibrated by a calibrator before each new measurement.
3.2. Designing, Manufacturing, and Installing Silencers
3.2.1. Limitations in Designing Noise-Controlling System
Since steam is emitted from a small cross section into the air with high velocity, attempts to prevent or constrain the emission of steam cause reverse pressure or severe blow to the system due to steam return. This may lead to a lot of damage. Thus, the major issue is creating no blockage in the free flow of steam. Diverting the path of the existing steam or creating winding paths may causes a blow to the system and eventually leads to damages. Consequently, we need to use direct paths in designing the system as much as possible. Another limitation is that the steam emitted from the outlet has a temperature over 162 °C. Hence, the materials that are used in the noise controlling system should be able to tolerate this high temperature and humidity.
Identifying the acoustic features of the noise source is the first step in designing instruments for controlling noise pollution. Knowing the acoustic features and physical characteristics (e.g. location, operational procedure, and limitations) of the noise source can help us design a noise control system with maximum efficiency. It should be noted that, in controlling and reducing noise pollution, there is no limitation for noise reduction, and various procedures can be used to reduce the amount of noise significantly. However, greater reduction in sound pressure level increases the costs of implementing noise control procedures. Therefore, the financial aspect is another factor that must be taken into account in designing noise-controlling systems. The proposed standards have presented a range of acceptable noise for occupational exposure. Thus, it is not advisable to spend more money to reduce noise to a level below the proposed standards. Some other important factors in designing the silencer are its location, its safety (i.e. making sure that the silencer will not fall over during the operation), and ease of installation.
In order to reduce and control the generated noise in the outlet of the vent of the cooling tower pump, the following three steps must be taken: (1) reducing air velocity; (2) absorbing the generated noise as the steam passes through the vent; and (3) preventing the noise regeneration. To reduce air velocity in the designed silencer, we should decrease the cross sectional area of the outlet pipe. Based on the following equation, the larger the difference in the cross sectional areas of the air passage, the lower the air velocity (
2 = V1S1/S2
2: final velocity of air front (m/s)
1: initial velocity of air front (m/s)
2: final cross section area (m2)
1: initial cross section area (m2)
As air passes from S1 to S2, it produces a lot of noise. In order to absorb this generated noise, suitable absorbents should be used throughout the air passage. In order to avoid noise regeneration in the designed silencer, the surface of the silencer must be resistant to air passage. Furthermore, the air passage should be designed to absorb the maximum amount of noise and the absorbents exert the minimum amount of mechanical resistance.
The generated noise in the vent outlet of the cooling tower pump is due to the high velocity of the air that passes through, a phenomenon known as jet noise. In this phenomenon, high-velocity air molecules collide with the ones in the surrounding environment (which is almost static), hence producing a noise of 135 dBA at the source and 127 dBA a meter from the vent outlet. Measurements indicate that sound pressure level at ground level is 125 dB. In order to have a noise of 85 dBA, we should reduce the noise level by 40 dBA. Given that the volume of the vented air is fixed, air velocity can decrease by raising the cross section area. To reduce air velocity, we need to use a silencer that is both absorptive and dispersive. Therefore, in this project, the designed silencer should not only absorb the noise generated by air passage, but also reduce its velocity so that the ventilation would happen in low speed. Some other parameters that are important in designing a silencer are: (1) flow of exhaust air; (2) cross sectional area; and (3) frequency band of the sound pressure level (