Summary

A furnace dust cleaning plant was refurbished. Prior to refurbishment, the nearest residential noise levels were tonal and 60 dB(A). After refurbishment, the noise levels rose to a very tonal 90 dB(A). Amidst the dust cleaning plant pipe work, sound intensity measurements were required to precisely locate the actual noise radiating surfaces. The client then independently sourced a commercial silencer for $3,000,000 which was subsequently shown to give only 3 dB(A) noise reductions. The poor noise reduction were due to pre-silencer noise breakout. Camets designed and supervised the construction and installation of a custom designed silencer, which reduced the nearest residential noise level from very tonal 90 dB(A) down to a very non-tonal 50 dB(A) (at 1/30th of the cost of the non-viable commercial silencer).

Introduction

The client is located near Sydney and is a manufacturer of centrifical spun cast iron pipes. The pipes are made from shredded scrap metal which is melted in a 25 tonne per hour cupola furnace. The liquid cast iron from the furnace is poured into a bank of centrifical spinning casting machines, producing a range of basic pipe sizes.

During the annual maintenance, major modifications to the furnace gas treatment plant resulted in substantial increases in the overall environmental factory noise emissions. The increase was 30dB above the recommended community noise criteria. The noise from the furnace gas treatment plant was highly intrusive for the local community and very tonal in character. Many tens of daily complaints from residents were received.

Immediate remedial action was sought to permit the factory to continue operating within the permitted legal environmental noise emissions permitted by the Environment Protection Authority. To solve the immediate problem, and to integrate the furnace gas treatment plant into the long term noise reduction strategy of the company, a noise reduction of 45 dB for these noise emissions were essential. The noise emissions were so bad, the plant operating hours were drastically reduced and the plant capacity was also reduced.

Investigations

The noise reductions were undertaken in two stages. Initially, a radical quick solution was required to permit continued operation in the short term. The "quick-fix" allowed sufficient lead time for stage two, the design, construction and installation of long term noise control equipment to achieve the permissible license noise emission requirements.

Investigations indicated that the noise was emanating primarily from the external surface of the gas discharge stack, and not from the top of the open ended 2.5 metre diameter gas discharge stack. Immediate noise control treatment consisted of constraining the stack walls, which were "ringing", to cut noise emissions from this source. Temporary stack silencers were inserted to control noise which was passing up to the open end of the stack. Ultimately, the noise was controlled prior to it entering the stack.

Remedial Action - Stage I

The design was constrained by the properties of the furnace gas entering the stack. The gas flow is 180°C and heavily laden with moist acidic furnace dust. The moisture and dust load resulted in a rapid build up on any exposed internal surface with solidly baked mud. The internal silencers were heavily caked with solid mud after one month of service.

Over a three day period, the plant noise emissions were measured and analysed; a set of acoustic absorption pods were designed; a local company fabricated the pods to those designs; and the pods were installed. Simultaneously, the external surface of the stack was shotcreted to control surface noise emissions.

Due to the dust and vibration conditions, these temporary measures were give a life expectancy of only three weeks before they would deteriorate. When these pods were finally removed the leading edges had extended by about 300 millimetres due to dust build up. It was found that one of the items removed during plant modifications had been an old absorptive sound attenuator which had become completely filled with baked mud, except for the narrow passages forced through the baked mud by the hot furnace gases.

Remedial Action - Stage II

During the following weeks a large multi chambered, tuned active silencer was designed, and constructed, to provide long term noise control. The multi chambered silencer was installed, and the temporary measures were removed.

This long term silencer was designed to provide, and achieve, a 45dB noise reduction. It was installed at ground level adjacent to the noise source, which enabled easy ground level access for weekly removal of accumulated dust, and also avoided costly structural support. An automated dust removal system was offered as an option, but not implemented by the client who planned to rebuild the entire dust collection plant within twelve months.

This silencer is still operating after eighteen months and has shown no deterioration in measured acoustic performance (47dB noise reduction) even in this hostile gas and dust environment. About 10 tonnes of mud is removed each week from one of the lower active tuned chambers where air particals are floated across the chamber as part of the silencer design.

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