Problem to Solution
An unwelcome by-product of generating power is noise. As in many industrial processes, noise is a natural occurrence associated with motion and its consequent disturbance of the surrounding air. That disturbance produces pressure waves in the air, which are received by the human ear and recognised as noise.
The process of generating power involves motion in many forms, for example in rotating and reciprocating equipment and the flow of gas, air, water and steam. With so much motion we might expect the levels of noise generated to be considerable and this is generally the case for large power plant. In fact, without proper noise control, the effects can be literally painful.
To avoid the pain and comply with requirements relating to noise at work, whilst giving due consideration to the environmental impact, requires effective methods of controlling noise. This has become a pre-requisite for new plant and an often necessary retro fit for older plant.
One of the main noise sources when generating power are diesel engines to which both air intake and exhaust gas silencers can be fitted to reduce noise.
A generic exhaust silencer system collects hot exhaust gases from the engine and discharges them to atmosphere as quietly and efficiently as possible.
An exhaust arrangement with minimised back pressure and satisfactory noise attenuation will usually be the result of a well designed system.
The exhaust termination point should not be in close proximity to the intake system for the engine or any ventilation systems of adjacent structures or buildings.
Exhaust silencers can be divided into three fundamental types, reactive, absorptive and combination of reactive/absorptive.
In addition to the three main silencer types, other functionalities such as spark arresting, emission control, heat recovery etc can also be incorporated into the silencer design.
Each type of silencer has specific performance attributes that can be used individually or in combination to achieve the required resultant noise level.
Absorptive silencers usually have relatively wide band noise reduction characteristics at middle and higher frequencies. These silencers are often used to attenuate the engine intake noise or supplement the performance of reactive silencers for the engine exhaust noise control. Absorptive silencers contain fibrous or porous sound absorbing materials and attenuate noise by converting the sound energy propagating in the passages into heat caused by friction in the voids between the oscillating gas particles and the fibrous or porous material.
A reactive type silencer generally consists of several smaller bore pipes (bypass tubes) that interconnect with a number of larger chambers. The noise reduction mechanism of a reactive silencer is that the area discontinuity provides an impedance mismatch for the sound wave travelling along the pipe. This impedance mismatch results in a reflection of part of the sound wave back towards the source or back and forth within the chambers. The reflective effect of the silencer chamber and piping, typically referred to as resonators, essentially prevents some sound wave elements from being transmitted past the silencer. The reactive silencers are more effective at lower frequencies than at higher frequencies and are most widely used to attenuate the exhaust noise of internal combustion engines.
The frequency at which maximum attenuation is obtained is when the chamber length is equal to ¼ of the wavelength.
Some silencers combine both reactive and absorptive sections to give noise attenuation over a broader noise spectrum.
If an engine is to be sited in an area of high risk due to hazardous or combustible materials, a certified spark arrestor type silencer should be used.
As well as internal combustion engines, noise is also generated due to the sudden release of gas or steam under pressure to atmospheric pressure due to the opening of a valve or similar.
Atmospheric vent silencers are designed to cut the noise generated by the expansion of gases from an elevated pressure to atmospheric pressure. The noise produced is a combination of ‘shock noise’ generated by the rapid reduction in pressure across the relief device and the noise created by the turbulent flow of gas downstream. Many vent silencers consist of an inlet diffuser, an expansion chamber and an absorptive section. The vented gas enters the inlet diffuser of the silencer and expands across a series of small holes in the diffuser body as it proceeds into the expansion chamber. From there it flows into the absorptive section before being passed into the atmosphere.
The diffuser forces the vented gas to pass through small radial holes normal to the initial gas flow. This breaks the single axial gas jet into a series of smaller radial jets, increasing the frequency of the generated noise so that it can be better attenuated by the absorptive section of the silencer.
In circumstances where the gas velocity in the inlet pipe is low, a simple open pipe and simple target plate can be used in place of the multi-holed diffuser.
Vent silencers are used to suppress the noise generated by safety relief valves, process vents, dumping gas to atmosphere during compressor or boiler set up sequences, de-pressurising lines for maintenance, ejector discharges and steam exhausts and the blow down of pipelines for cleaning purposes.
For more details on the industrial silencers and their applications—or any industrial plant maintenance and repair needs—reach out to the experts at IMS. We’re ready to assist you.
