Introduction

Recently a Process Plant requested an investigation of noise emissions causing adverse local community reactions at distances of over two kilometres from the industrial site.

Under specific atmospheric conditions the plant was intrusively audible to these distant residents. The noise was a rising and falling drone accompanied by feelable structural vibration within the residences. It occurred late at night when light winds blew from the north to north east.

The operations require over 27,000 tonnes of charcoal per annum for the furnaces. This charcoal is produced on site, in a pair of Charcoal Retorts, using local timbers.

Investigation

The intruding sounds and vibrations were a transient phenomena and very dependent on weather conditions. The investigations needed to locate the sources of the drones and vibrations and recommend solutions.

The Charcoal Retort was a structure 45 metres tall divided into ten levels. The two Charcoal Retorts extended through the structure from Ground Level right up to Level 9. On Level 10, there were winches for the wood skips. The Incinerator extended from Level 1 through to Level 6. The two Cooling Gas Scrubbers extended from Ground Level up to Level 4. Process equipment was located at Ground Level, and on Level 1, Level 2, Level 5, Level 6, and Level 9.

Wood is fed into the top of the Retorts. Volatiles are baked out of the wood as it descends through the Retort. These volatiles are drawn into the Incinerator and burnt. A portion of the incinerator waste gases are fed back up through the top half of the Charcoal Retorts to bake the wood to charcoal. Excess incinerator gases are vented to atmosphere via a 45metre tall exhaust stack. In the bottom half of the Retorts, Retort gases are cooled in the Cooling Gas Scrubbers and cycled through the processed charcoal to drop the temperature below the self-ignition temperature as it exits the base of the Charcoal Retorts.

In addressing the noise problem it was decided to investigate the problem at the source using field measurement and computer modelling. On each level one-third octave sound pressure levels were measured. The measured noise emissions included, process fans, ductwork noise emissions, and process vessel wall noise emissions. Based upon detailed plans of the processes, the sound power levels were calculated. These sound power sources were given a location in a three dimensional space and fed into a computer model for the Charcoal Retort. From this model component contributions were calculated at the residences over two kilometres away. The computer modelling was repeated for both normal and adverse weather conditions.

Conclusions

The results of the computer model clearly indicated that the process equipment (which were extremely loud at close range), were inaudible at the residences. The main sound power sources of audible sound proved to be located at the top of the Exhaust Stack, followed closely by the outer shell of the Exhaust Stack, and the outer shell of the Cooling Gas Scrubbers. The low frequency sound which was producing induced vibration at the residences was traced to an "organ pipe" effect in the Exhaust Stack, induced by the waste gas streams interacting with the transitions at the base of the Exhaust Stack and the Exhaust Stack itself.

The problem was traced to an instability in combustion conditions within the Incinerator, with an acoustic leakage occurring through the Cooling Gas Scrubbers, via the central process transition zone of the Charcoal Retorts.

It was suggested that the low frequency sound, inducing structural vibrations at the residences, be alleviated by the converting the lower third of the existing Exhaust Stack into a rock filled quarter wave length attenuator.

The client had been considering an alternate solution consisting of a large silencer on top of the Exhaust Stack. The computer model was able to demonstrate that the noise reduction of a silencer at this location would not have produced a noticeable change in the perceived noise emissions.

The combination of real-time third octave sound pressure level measurements with computer modelling proved a powerful combination in clarifying and explaining what the client had found to be a baffling and intractable problem. It changed a frightening unknown into a local novelty for nearby residents.

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