Ambient plant noise levels are under the scrutiny of OSHA as well as plant Health and Safety departments. Knowing that normal speech is almost impossible at 85-95 dBA and OSHA permissible worker exposure is 90 dBA over an 8 hour period (see Table 1) we need to select equipment responsibly. The sound levels in this chart are sound pressure (Lp), generally understood as pressure on our ear drums while sound power (Lw) as shown in Table 3, is an energy value used for acoustic calculations.
Table 1: Permissible Noise Exposures Lp
|Duration per Day, in Hours||Sound Level in dB|
|0.25 or less||115|
We need to understand how the noise sources scattered throughout the plant contribute to the work environment overall noise level. To do that, we first need to understand logarithmic “decibel addition” (see Table 2). Decibel addition states that if two noise sources have a noise value of say 85 dBA, the resultant noise value is plus 3 dBA added to the lowest value or 88 dBA. Given multiple sources we can see how the ambient noise level continues to increase even if we randomly set an interior maximum level of 85 dBA for all new equipment.
Table 2: Rules for Combining Sound Levels by “Decibel Addition”
|When two decibel values differ by||Add the following amount to the higher value|
|0 or 1 dB||3 dB|
|2 or 3 dB||2 dB|
|4 to 9 dB||1 dB|
|10 dB or more||0 dB|
Attenuating additional noise sources makes good sense when installing new equipment. If a process fan cannot be selected at acceptable noise levels (dependent upon performance criteria), the traditional way to handle noise is to install an absorptive inline silencer on the fan inlet or more often the outlet. Silencers might be selected for 85 dBA or as low as 70 dBA depending upon fan location and if the exhaust airstream is to be recirculated back into the facility.
There are two common noise related mistakes made when designing air systems and selecting fans and air handlers; the first is ignoring breakout noise from unhoused areas of the system or components, specifically the shaft opening in the fan housing and the second, failing to account for breakout noise from an inlet or outlet flex connector. Breakout noise is defined as undesirable sound that is caused by the transmission of sound through the walls of a duct, building or other enclosure.
Not including a shaft seal at the fan housing can often be a major noise source. Shaft seals are options and available as high density Teflon disks up to gas tight labyrinth seals. Where the shaft enters the housing, noise easily travels and sound propagates. If the levels in the fan are exceeding 100 dBA (see example in Table 3) we have a critical noise source that is not attenuated.
Table 3: Fan Lw dBA Noise Levels
|Unsilenced Lw at Exp Joint (dB)||104||112||113||110||103||102||100||95||114*|
* the Lw of 114 hjas a corresponding Lp of 102 dBA @ 5 feet.
Table 4: flex connector Attenuation
|Description||Silenced Lp at 5 ft. (dB)||Overall dBA|
|Flex with Flow Liner||87||92||90||83||73||68||63||55||85|
|Flex with Acoustic Liner||89||92||86||74||60||47||35||30||80|
|Flex with Acoustic Liner and Flow Liner||81||83||77||65||51||38||26||21||72|
The second potential noise related issue is not including acoustic flex connectors or joints. If the fan or air handler is on RIS or spring isolators, the inlets and outlets of the equipment needs to be isolated with flex connectors. If a typically applied EPDM flex connector is installed, the connector has minimal noise attenuation characteristics.
Table 4 Flex Connector Attenuation points out important performance differences. The overall dBA values for an EPDM flex with and without liner and acoustic flex with and without liner are shown. The flow liner in both cases is used for high pressure, high velocities and/or dusty airstream conditions. As you can see, a standard flex only attenuates 8 dBA while 30 dBA attenuation is possible for an acoustic flex connector with flow liner. Considering these values along with the aforementioned “decibel addition”, the equipment designer/purchaser can make decisions which will meet both existing and long term plant noise levels.
In future articles, we will discuss other noise considerations and solutions available when new equipment is designed and installed.