Problems of acoustic safety in power engineering

Environmental safety issues are becoming increasingly important in the life of society. Among environmental safety issues in power engineering, acoustic safety occupies a special place. The problem of acoustic safety is associated with the fact that the regular operation of power equipment leads to an increased noise level, and power facilities are located in close proximity to residential areas. In this work, acoustic calculations were performed to determine the sanitary protection zone for gas turbines units (GTU) and combined cycle gas turbine units (CCGT) of various capacities. A formula was obtained for calculating the width of the sanitary protection zone depending on the capacity of gas turbine units and combined cycle plants and their number. It is shown that the sanitary protection zone (SPZ) of a power unit of high capacity is smaller than the sanitary protection zone of several power units of the same capacity. It is found that the noise levels from individual groups of equipment can determine the noise level at the entire border of the sanitary protection zone or in its individual sections. At the same time, noise suppression measures should be taken for all sources that generate noise levels in excess of standards. It is necessary to start noise suppression measures from those sources that generate excess noise in a larger section of the sanitary protection zone.


Introduction
Environmental safety issues are now becoming increasingly important in the life of society. Among environmental safety issues in power engineering, acoustic safety occupies a special place. The problem of acoustic environmental safety is associated with the fact that the regular operation of power equipment produces increased noise, and power facilities are located in close proximity to residential areas.
New requirements SanPiN 1.2.3685-21 "Hygienic standards and requirements for ensuring the safety and (or) harmlessness of environmental factors for humans" [1], introduced in March 2021, limit permissible noise values at the border of sanitary protection zones to 55 dBA during daytime and 45 dBA at nighttime. For power facilities, the boundaries of sanitary protection zones can be located at 300, 500, and 1000 m [2].
A feature of a power facility is a large number of sources of constant noise, their continuous operation, close proximity to residential areas, especially in large cities. Sources of continuous noise vary greatly depending on the type of equipment used. Strong sources of noise for the surrounding area are the air intakes of the blower fans and compressors of gas turbine units, the nozzles of gas ducts of smoke exhausters and pipes of gas turbine and combined cycle power plants, transformers, gas distribution points, cooling towers, coal-grinding equipment, the bodies of draft machines, and  [3]. Axial draft machines are much more noisy than centrifugal ones. A classification of noise sources in turbine power plants (TPPs) is given in [3][4]. The most significant source of noise is the release of steam into the atmosphere. A large number of papers are devoted to the issues of noise from equipment and the creation of sanitary protection zones of power facilities [5][6][7][8][9][10][11][12]. Noise levels at the border of sanitary protection zones depend on the capacity and type of equipment used [11][12].A comparison of noise levels from steam power equipment with CCGT equipment is made in [12].
This paper presents calculations for both individual TPPs and individual groups of equipment. The width of sanitary protection zones of thermal power plants with advanced equipment for both gas turbine and CCGT units us discussed with regard to their capacity. In addition, the impact of equipment such as fan cooling towers and transformers is considered.

Results of calculations of SPZ aroundGTU and CCGT
The calculations were carried out for GTU and CCGT units of various capacities: GTU-78 MW and CCGT units with a capacity of 114 MW and 450 MW.
Acoustic calculations were performed in accordance with ISO 9613-2: 1996 [13]. In this case, the individual sound power levels of each piece of equipment, noise propagation conditions, and climatic and terrain features are taken into account. Such complex calculations are performed using specialized software products. The calculation is carried out according to the formula [13]: where LW is the octave-band sound power level in decibels produced by the point sound source relative to a reference sound power of 1 picowatt (1 pW); DC is the directivity correction in decibels that describes the extent to which the equivalent continuous sound pressure level from the point sound source deviates in a specified direction from the level of an omnidirectional point sound source producing sound power level LW; А is the octave-band attenuation in decibels that occurs during propagation from the point sound source to the receiver. The octave-band attenuation calculating by the formula A = Adiv + Aatm + Agr + Abar + Amisc, (2) where Adiv is the attenuation due to the geometrical divergence (the energy divergence upon emission into free space); Aatm is the attenuation due to atmospheric absorption; Agr is the attenuation due to the ground effect; Abar is the attenuation due to a barrier; Amisc is the attenuation due to miscellaneous other effects. The use of a specialized program for acoustic calculations makes it possible to simulate the location of TPP buildings with their overall dimensions with the main sources of noise. In the calculations, the shielding effect from buildings and structures of TPPs is taken into account. Acoustic calculations using a specialized program allow one to determine the noise level both from the main sources of noise and from individual sources or groups of sources at a distance from the TPP taking into account the relative position of sources in the area of the station and their height above the ground. The latter is especially important since artificial and natural barriers do not have a shielding effect on the radiation from a high-altitude source.
To determine the noise levels at the border of the sanitary protection zones, mathematical modeling of TPP with gas turbine equipment and CCGT was performed. At the same time, dry fan cooling towers are used at some stations, and natural draft counterflow cooling towers are installed at other TPPs. The calculation results in Figure 1 can be approximated by the formula for calculating the size of the sanitary protection zone R, m, from the electric power of the station N, MW: where N is the electric power of the station, MW; A and C are empirical coefficients, which are presented in Table 1. The confidence level R 2 for the obtained values according to formula (1) is about 0.99. The acoustic calculations performed make it possible to determine the main sources of noise responsible for the exceedance of the sanitary limits in the area surrounding the sanitary protection zone. For example, for supercritical pressure units, this is the noise from exit nozzles of axial smoke exhausters [3][4]; for TPPs with gas turbine units, this in some cases may be the noise from dry fan cooling towers [14].   [14] that fan cooling towers are always noisier than natural draft counterflow cooling towers for cooling the same amount of water from condensers. Therefore, the use of high-capacity dry fan cooling towers at TPPs instead of natural draft counterflow cooling towers increases the noise level in the surrounding area.  Figure 2 shows the maximum sound pressure levels at the design point at the border of the sanitary protection zone during operation of natural draft counterflow cooling and dry fan cooling towers required for the operation of a 78 MW GTU. The sound pressure levels during operation of dry fan cooling towers will be higher than sound pressure levels during operation of natural draft counterflow cooling towers for low-and medium-octave band frequencies. For example, the difference in sound pressure levels between a natural draft counterflow cooling tower and a dry fan cooling tower will be about 40 dB for octave band frequencies of 31.5, 63, and 125 Hz. During the operation of fan cooling towers, the sanitary limits will be exceeded at the control point at the boundary of the sanitary protection zone for octave band frequencies from 31.5 to 1000 Hz.  In any case, if the sanitary standards are exceeded, it is necessary to take measures to suppress noise. Possible measures to reduce the power of equipment are given in [15][16][17].
When developing measure to reduce the power and suppress the noise of power equipment, it is necessary to consider the regional climate factor [18][19][20].

Conclusion
 The noise from energy facilities depends on both the type and power of the energy units of which it is composed. Increasing the capacity of an energy facility leads to an increase in the sanitary protection zone. At the same time, the noise level at the SPZ border is often determined by a separate group of sources.  Formula (1) was obtained for calculating the width of a sanitary protection zone depending on the capacity of GTU and CCGT units and their number. It is shown that the sanitary protection zone of a power unit of high capacity is smaller than the sanitary protection zone of several units of the same capacity.  It is shown that the noise levels from individual groups of equipment can determine the noise at the entire border of the sanitary protection zone or at its individual sections. At the same time, noise suppression measures should be carried out for all sources that generate noise in excess of standards. It is necessary to start noise suppression measures from those sources that generate excess noise in a larger section of the sanitary protection zone.