Noise Regulations
Which One Applies to Your Project and What Does it Mean?
Noise regulations can generally be separated into two categories, environmental noise regulations and Occupational Safety and Health Act (OSHA) regulations. The OSHA regulations generally apply to the sound levels within the facility itself and these regulations are intended to protect employees from noise induced hearing loss. It is important to be aware of the noise limits presented in these regulations. In Alberta the regulation is 85 dBA for 8 hours whereas in British Columbia it is 90 dBA for 8 hours. As the noise level goes up by either 3, 4 or 5 dBA depending upon the jurisdiction, the amount of allowable exposure time to the noise is halved.
Alberta uses 5 dBA and thus at 90 dBA, 4 hours of exposure time is allowed. In Alberta machinery is often specified to meet 85 dBA at 1 m because of the OSHA regulations. This in no way means that the regulation will not be exceeded as often there is more than one piece of noise generating equipment in close proximity to another or the machinery may be located in a reverberant building which raises the sound level even though the machinery itself meets the 85 dBA noise requirement. Typically the OSHA regulations are under provincial legislation under the Labour Act. There is also a federal occupational noise regulation and this applies to federal government employees, crown corporations, postal services, the military, and industries under federal control such as the railways.
Environmental noise regulations typically have to do with the noise emanating from a facility to the surrounding area. Environmental noise regulations can be instituted at a federal, provincial, county or municipal level and can overlap. Many of the environmental regulations are used as planning tools. For example, the Airport Vicinity Protection Act creates zones around an airport that disallow the construction of schools or residential development in the noisiest zones nearest to the airport. As one moves away from the airport housing can be built but additional noise control measures must be designed into the building shell. Many cities have similar legislation for development that is constructed close to major roadways forcing developers to either erect noise barriers along the road or design additional noise attenuation features into the buildings.
The Alberta ERCB Noise Control Directive ID 94-4 applies to energy related facilities. It sets out maximum allowable sound levels for both the daytime and nighttime periods for a resident located close to a facility. The allowable level is based on the density of development and proximity to major roads or flyways. The ERCB Noise Control Directive does allow a minimal noise impact and even when the guideline level is met, the noise from a facility can be clearly audible at the nearest resident. It is our experience that some people do not find this acceptable even when the allowable guideline level is met. The ERCB Noise Control Directive works on a complaint basis for existing facilities, as they must meet the required noise level but only if a complaint is received from an adjoining resident. In the case of new facilities or additions to existing facilities, a noise impact statement must be prepared showing that the new plant or addition will not exceed the guideline requirements at the nearest or most impacted residence. This allows the guideline to be used as a planning tool when designing a new facility that could be located at a number of different locations. The noise impact at each location can be assessed and the location with the least noise impact to adjoining residences could be evaluated along with the other criteria required to locate the plant.
It is possible to become involved in a jurisdiction that either has no noise regulations or where the noise regulation does not set out allowable noise levels. This makes the situation difficult because if a complaint is received after the facility is built, it can lead to expensive noise control upgrades if the complainant can convince the authorities that the noise is too loud. We suggest in these cases that the owner of the plant work together with the authorities and local residents to agree on an allowable noise level before the plant is constructed. The criteria used is often based on the noise regulation used by another jurisdiction for a similar situation. The plant can then be designed to that noise level and hopefully avoid problems after the plant is operating. If complaints are then forthcoming regarding noise from the facility, measurements can be conducted to confirm that the plant meets the required noise level. When constructing a plant in a quiet area it has been our experience that companies that establish good PR with the residents usually have far less complaints about noise
Alternate Methods of Improving Cooler Performance
In our article Improving Aerial Cooler Fan and Acoustic Performance from Newsletter #1, we discussed ways of improving aerial cooler performance using inexpensive tip seals and bell-mouth entries. For the operator with larger improvements in mind (and deeper pockets) we offer Variable Frequency Drives (VFD) for your consideration. These have been around for decades, but have found favor in gas processing only recently. Basically the VFD takes a fixed 60 Hz Ac electrical input and converts it to a variable frequency output with the result that the driven machinery is no longer constrained to a fixed, submultiple speed of 3600 rpm.
The immediate advantages are flexibility of control, reduced noise and part-load power conservation for any electric-driven equipment which previously was constrained to operate at fixed speed. In the case of reciprocating compressors, where power is a linear function of speed, the benefits are moderate and usually not worth the capital expense. However in the case of aerial cooler fans and some applications of centrifugal pumps and compressors, the three advantages may well justify the cost.
In Alberta aerial coolers are (theoretically) designed to work at ambients of 85 to 90°F. However average year-round ambient temperatures are 35 to 45°F. Likewise nighttime temperatures are typically 17°F lower than max day. This provides the engineer with good opportunities for VFD application if the intent is either nighttime noise reduction or overall power conservation. The current ERCB Noise Control Directive ID 94-4, requires that plant noise at most rural residences must be less than 40 dBA at night but may rise to 50 dBA during the day. In cases where plant noise is dominated by cooler fans, the VFD offers an excellent way to reduce nighttime noise without affecting daytime hot weather performance. An aerial cooler fan placed on VFD/temperature control will typically slow down to 75% speed at night - unless the unit is already undersized. As fan noise is a function of speed to the 5th exponent, this results in a noise reduction of 6 dB. As axial fan power is a function of speed to the 3rd exponent, the electric power saving is dramatic also; yielding a 55% saving at 75% speed. Lastly, on the positive side, VFD also gives much better and more linear control than conventional louvers.
As a word of caution however, VFDs do have some potential negatives and pitfalls for the unwary. For fan service be sure to specify variable speed/variable torque; as the torque will vary with the square of speed. Also the VFD should have programmable features to block out any running speeds coincident with fan blade flexural natural frequencies. Finally the engineer should assess whether there is a need for harmonic filters as VFD output signals are generally not tidy sine waves! Some operators with multi-fan coolers turn off sequential fans as ambient temperature drops and cannot see the point of VFD control. Because of the extremely non-linear speed/power and speed/noise relationships, there is still a distinct advantage in VFD control of all fans together rather than sequential shutoff. For example, all fans at half speed rather than half the fans at full speed and half shut off, has an electric power saving differential benefit in favor of VFD of 87.5% (vs. 50%) and a noise reduction benefit of 15 dB (vs. 3 dB)..
