Zhang Hongsong
[China Sinoma International Engineering Co., Ltd. (Nanjing)]
01 Introduction
Noise interferes with and damages human hearing, sleep, and emotions and is extremely harmful. It is of great significance to prevent and control noise pollution, control and reduce noise generated by human activities, prevent noise from harming human health, protect and improve the living environment, ensure human health, promote sustainable economic and social development, promote the construction of ecological civilization, and promote the high-quality development of green and environmentally friendly industries.
02 Main noise sources in cement plants
The types of noise sources in cement plants are complex, the sound sources are widely distributed, and the sound pressure levels are high, resulting in the characteristics of large-scale, high-intensity, and long-term noise in cement plants.
2.1 fan noise
Fan noise exhibits mid- to low-frequency characteristics (see Figure 1), which mainly includes (1) rotational noise, (2) vortex noise, and (3) mechanical noise, which are generated by aerodynamic and mechanical motion.
Figure1 Centrifugal fan fan noise spectrum diagram
2.2 ball mill noise
The noise of the ball mill mainly comes from the mill cylinder, motor, reducer, large gear, etc. It is a large noise generated by the impact and vibration caused by the impact and friction between the steel balls, lining plates and materials of the mill, and has full-band characteristics. Noise covers the entire range of high, medium and low frequencies. In the process of noise control, the low-frequency components need to be focused and effective measures must be taken to thoroughly control them.
2.3 Air compressor noise
Air compressor noise has broadband characteristics (Figure 2), which is mainly composed of aerodynamic noise (air compressor intake and exhaust noise), mechanical noise generated by the impact and friction of mechanical moving parts, and noise generated by electric motors.
Figure 2 Noise spectrum diagram of an air compressor
2.4 Belt conveyor noise
Cement plants mainly use heavy-duty belt conveyors, consisting of a frame, conveyor belt, idlers, pulleys, tensioning device, drive, and other components. Power is transmitted by friction between the drive pulley and belt. The main noise sources during operation are therefore: (1) electromagnetic and mechanical noise from the drive motor and gearbox; (2) friction between the drive pulley and belt, and noise from transmission components such as bearings and idlers; (3) collisions between pieces of material, frame vibration, and similar sources.
2.5 Hoist noise
The noise of the elevator mainly comes from (1) electromagnetic noise generated when the motor is running; (2) mechanical noise generated by the transmission system such as bearings; (3) mechanical noise generated by the friction between the sprocket and the chain; (4) vibration noise generated by the platform when the elevator is running; (5) noise generated by the collision of the material and the casing when the lifting material is spilled.
2.6 Crusher noise
The noise sources of the crusher mainly include: (1) the noise caused by the impact of the ore on the crusher shell when the ore falls; (2) the noise caused by the extrusion, impact, and crushing of the ore during the operation of the crusher, causing vibration of the entire machine body such as the casing and supporting devices; (3) the noise caused by the impact of the ore on the chute when it falls from the discharge port of the crusher after crushing; (4) the noise generated by the motor. When the crusher is running to crush materials, the noise generated is relatively high, up to 100-105dB.
2.7 Roots blower noise
Roots blowers have the advantages of high pressure head, small flow rate affected by resistance, and stable air supply. However, the disadvantages of low efficiency and high noise are important links in energy conservation, consumption reduction, and noise reduction management in the cement industry. The use of new energy-saving magnetic levitation Roots blowers can not only improve efficiency and reduce energy consumption, but also reduce noise from 110dB of traditional Roots blowers to 75~80dB, with obvious effects.
03 Propagation of noise from cement plants
There are two main forms of noise transmission in cement plants: (1) outward propagation through the air medium; (2) direct vibration of solid components through sound sources, which propagates outward and radiates in the form of elastic waves in the foundations, floors, and walls of buildings, structures, and equipment to produce noise.
3.1 Sound leakage from building envelope
Major noise-generating equipment in cement plants, such as cement mills, roller presses, and other grinding equipment, is installed inside buildings. However, the walls and ceilings of these buildings are mostly concrete or steel-frame structures clad with color-coated steel sheets. Although they provide some sound insulation, they cannot completely prevent indoor noise from radiating and propagating outdoors. Vibrations generated during equipment operation travel through connected piping, supports, foundations, and other facilities, as well as steel frames, reinforcing mesh, and other building structures, radiating outward and affecting the surroundings. Indoor noise also escapes through air inlets, exhaust outlets, doors, windows, holes, and other openings.
Vertical mills, large fans, small dust collection fans, bucket elevators and other open-air equipment in cement plants directly produce noise to the surroundings through airborne sound transmission.
04 Cement plant noise control
Noise control in a cement plant should be based on its process characteristics and distribution of noise sources, combined with the overall layout of the plant, to establish areas, evaluate the noise impact of each area's noise sources on the surrounding environment, and determine the amount of noise reduction for each area's noise sources. The main steps are:
(1) Conduct a survey on the current status of the acoustic environment and arrange system test points for noise values at the factory boundaries on the east, west, south and north sides of the factory (see Figure 3).
(2) Based on the factory noise value, determine its noise control target requirements. In order to control the day and night noise emissions at the factory boundary within the national standard 53dB(A), based on the measured data, determine the difference between the current acoustic environment status and the target value:
(3) Through on-site sound field analysis, computer acoustic calculation and acoustic simulation, and noise control engineering experience correction, a noise control acoustic model was established (see figure 4), and compared with the measured data, the final feasible noise reduction plan was determined (see table 1).
Figure3 Test location map of noise measuring points at the boundary of a cement factory
Figure4 Simulation diagram of the current acoustic environment of a cement plant
Table1 Comparison table of simulation diagrams of actual measured exceeding standards at the factory boundary
It can be seen from the table 1 that the simulation curve is basically consistent with the actual measured data. The noise control acoustic model is very reliable. Based on this, the noise reduction solution is determined to be feasible.
(4) Effective noise control is achieved through comprehensive sound insulation, equivalent outdoor sound source attenuation, air absorption, barrier attenuation, sound attenuation and sound absorption and noise reduction, ventilation and heat dissipation and other system management measures.
05 Practical Application
5.1 governance measures
Take the noise reduction project of the raw meal mill system of a cement plant as an example. The raw meal vertical mill is about 180m from the factory boundary. Since the vertical mill is open all around, the measured noise value of the vertical mill near the field is 95dB(A). The noise has a great impact on the acoustic environment at the factory boundary. For noise reduction treatment, a cyclone tower and a feeding building are used, and sound insulation barriers are installed on both sides of the vertical mill to control the spread of noise to the surrounding environment. Specific measures include:
(1) Borrow the structure of the frame building to build a new foundation and make and install a sound insulation barrier steel structure;
(2) Use the building frame building around the mill to install sound insulation barriers, adopt an overall enclosure for the mill, and leave the top empty. The sound insulation barriers use sound-absorbing and insulating outdoor panels with Rw ≥ 30dB (A) and NRC ≥ 0.9;
(3) Install inspection soundproof doors at appropriate locations on the soundproof barrier wall, with soundproof door Rw≥30dB(A);
(4) The sound insulation barrier wall adopts edge processing, and all waistline layout, plate and edge part colors, and steel structure paint colors are coordinated with the overall factory plan (see picture 5).
Figure5 Vertical mill sound insulation barrier layout
5.2 governance effect
After adopting the above noise reduction measures and noise reduction amounts, the noise of this project is less than 53dB(A) day and night, which fully complies with and meets the control requirements stipulated in the applicable areas of various national noise standards.
During the noise reduction process, only sound insulation barriers are installed in the raw meal vertical mill system, and the maintenance of the grinding roller and motor can be hoisted from the top; daily inspections can be entered and exited through the inspection door, which does not affect the normal maintenance of the equipment. The top of the sound barrier is open, there is no enclosed space, and it has very little impact on the ventilation and heat dissipation of the equipment.
Source: "Cement Engineering" 2022
(Copyright belongs to the original author or institution)