This paper describes an investigation by SRL into the cause of tonal noise alleged to be emanating from a UK aluminum conversion plant in the United Kingdom. The case was politically very sensitive, almost resulting in a court hearing. Consequently the name of the factory and other references have been omitted from this paper.

SRL's investigation arose as a result of an appeal from the factory management against the extent of the remedial work that the Local Authority required to abate the nuisance. The Local Authority had issued an Abatement Notice in respect of Noise Nuisance under the terms of the Environmental Protection Act, 1990, section 80. This paper starts at the issuing of the abatement notice and highlights the conflict between the need to impose suitable works to remedy a problem whilst avoiding measures that are unduly restrictive.

Context

Plant

The Factory under consideration is one which recycles scrap aluminum and recasts it into billets. The process

Figure 1 : The plant layout.

involves the use of furnaces, or ;Melters', followed by casting and a gradual cooling process. The plant layout is shown in Figure 1. The factory building which is constructed largely of profiled steel cladding houses most of the plant including the furnaces. At the rear of the factory, there is some external plant comprising a dust and fume extraction system and several cooler units.

This paper deals largely with the fume extraction and filtration system. This needs to be a fairly comprehensive system, because of the requirements of the Environmental Agency to trap particulate emissions, and comprises of three bag filtration units. Two of the bag plants, numbers 1 and 2, are served by centrifugal fans which discharge into a common stack.

The filter bags are mounted at high level and are periodically shaken by a mechanical excitation in the 'Scavenger Area'. This area is mounted above the plant. There are also two cyclones which collect the larger particulate emissions. One of the features of this assessment was that it was essential that the efficiency of the plant was unimpaired in the attempt to satisfy the demands of the Environmental Agency for the local residents.

Location

The area surrounding the factory can be characterized as classic British Standard BS4142 territory, namely mixed residential and industrial development. The location is shown on Figure 2, where the factory is

Figure 2 shows the factory location in relation to the nearest properties

highlighted in blue and several of the nearest properties are highlighted in red. Other factories in the area, can be clearly seen to the north of the residential estate.

The entrance for vehicles is at the front or east side of the factory and the rear or west side of the factory faces a residential estate some 200m distance.

Complaints

For several years some of the residents of the estate had been complaining about their perception of unpleasant low frequency noise which appeared to emanate from the factory. The noise was identified as being tonal in nature.

This noise had been investigated by the local authority and by other consultants previously leading to the definition of the noise as a nuisance. Review of this information at the start of the project had convinced us that there was a nuisance that needed to be addressed.

A previous study demonstrated that there were measurable tones at 12 Hz, 16 Hz and 38 Hz at the complainants properties. Figure 3 illustrates how close the back of the factory was to the nearest properties.

Abatement Notice

A notice was served by the local Environmental Directorate, based on the recommendations from their external consultant. This notice turned out to be very specific, in several respects.

Level

Compliance with the Noise Abatement Notice was defined as follows .....

" The requirements in this notice will be considered to be satisfied if the sound pressure level of 48 dB in the one-third Octave band centered on 40 Hertz is not exceeded in any of the 5 minute measuring period..."

This was based on the level described in the German Standard DIN 45680. The typical level existing at the complainants when the plant was operating was 56 to 57 Leq dB in the 40 Hz one third octave band.

Much discussion was had with the client and with the Environmental Inspectorate and their consultant about the merits of using a German Standard when it could be argued that BS 4142 used in conjunction with BS 7445 part II would be appropriate. However, this debate is beyond the scope of this paper.

Schedule

In addition to specifying a noise limit within a defined third-octave frequency band, the Notice contained a detailed schedule of work split into two phases. The following is a summary of the mitigation schedule that was specified, as it was written in the notice ;

Phase 1

• Acoustically treat the scavenger area on Bagging plant 1 and 2 by fully enclosing the area using acoustic panels ( 3mm mild steel with 50mm Class 'O' foam with lead lining )
• Fit/replace inlet and exhaust silencers on the main fans of Bagging plants 1 and 2 to attenuate low frequency noise ( 300mm wide splitters, 150mm airways )
• Acoustically clad the fan casing with 100mm thick insulation
• Stiffen and clad the existing/replaced outlet silencer casing
• Fit silencer to the homogeniser cooler designed to attenuate low frequency noise

Phase 2

• A secondary stack silencer shall be introduced
• A frequency tuned expansion box should be fitted inside the factory to control low frequency duct born noise from the Melters
• The cyclones should be sealed and stiffened to prevent drumming.
• The platform which sits close to the cyclones should be isolated using anti-vibration mounts

Timetable

Not only was a specific level set, together with specific mitigation measures, but a defined timescale was imposed by which all of the above measures had to be implemented. This timetable was defined as 6 weeks for the most urgent, Phase1 works and 12 weeks for the Phase 2 works.

Investigation

Method

SRL was not convinced about the need to do all of the work set out in the schedule nor about the priority given to each item. It was decided that the best way was to adopt a structured approach to analysing the noise and identifying the actual noise source.

In particular, noise measurements were made under a number of different operating conditions to isolate the source of the low frequency noise from the offending areas. The areas which were comprehensively surveyed included the three bag plants, No 5 stack, the roof area and the ductwork from Melters 3 and 4 to the bagging plant. We also measured noise from the homogeniser cooler fans on the roof and on the caster 2 cooling tower. DAT recordings of specific noise sources were made for subsequent post analysis using a laboratory-based narrow band analyser. Some vibration measurements on the cyclones and the scavenger area building were also taken with particular attention being given to vibration in the 40 Hz region.

An aerial platform was used to gain access to the top of the bagging plant stacks and the high level extraction ductwork.

Findings

A large number of readings were taken, with the following summary of results, as shown in Table 1 below :

In summary, the only sources that were found to be generating significant levels of noise at 38 Hz or anywhere within the 40 Hz third octave frequency band were sources associated with Bag Plants 1 and 2. An illustration of this is given by Figure 4, which shows print outs from the narrow band analysis of two DAT recordings. The first is a recording made on the site boundary and the second was recorded by the fan casing for Bag Plant 1. The peak at 38 Hz is shown clearly in both cases.

Predictions

Having performed the measurements set out above, SRL established the sound power level of the individual plant items in one-third octaves and predicted the sound pressure level in the 40 Hz one-third octave band which would have occurred at the claimants property as follows :

Thus, just taking the contribution from these key sources, it can be seen that the predicted sound pressure level at the complainants property in the 40 Hz one-third octave band is very similar to the level of 56-57 that was measured in practice. All of the above pointed to the fans and ductwork associated with Bag Plants 1 or 2 being responsible for the noise source.

One of the key limiting factors experienced throughout this project was the need to maintain effective atmospheric pollution control meant that the fans operating by Plants 1 and 2 could not be switched off so that we could have a look inside. It was known at the outset that the rotational speed of the centrifugal fan on No 1 Bag plant was 2280 rpm, which coincides with a frequency of 38 Hz and so appeared to be a strong candidate for the noise source. However, we had been assured that the fan had been regularly balanced and was in good order. The fan was belt driven so the prospect of misalignment being a cause of unacceptable vibration at the rotational frequency was unlikely.

With the fan discounted, we were left with the possible conclusion that the 38 Hz tone was due to a strange effect in the ductwork. For example, we found a closed off section of ductwork having a length of 4.5 metres, which happens to be approximately 1/2 the wavelength of a 38 Hz noise.

Mitigation

The investigation described above meant that we were able to approach the original Schedule of Works with a better idea of what would actually be effective. We therefore provided a revised scope of work which differed considerably from the schedule that had been included in the notice.

So, taking each of the items as listed in the Schedule of Works, we defined whether the mitigation was required, as follows :

Phase 1

Acoustically Treat Scavenger Area.

Our measurements had determined that the scavenger area on the bagging plants was not a cause of noise in the 40 Hz one-third octave band and so this item was eliminated.

Fit/replace inlet and exhaust silencers on the main fans of bagging plants

Fitting or replacing inlet or exhaust silencers on the main fans on bagging Plants 1 and 2 was physically not possible because of space restrictions and implications of additional pressure drop. The additional pressure drop could not have been tolerated by the Factory Inspectorate or the Environmental Agency.

Acoustically clad the fan casing

Acoustically cladding the fan casing was an option but for the fan casing to be adequately clad it would have required cladding of some 400mm thickness.

Fit silencer to the homogeniser cooler

We were able to avoid the need to fit a silencer to the homogeniser cooler as the factory management made a commitment not to use the unit.

Stiffen and clad the existing/replaced outlet silencer casing

We agreed that stiffening of the ductwork was a good first step to combating the problem.

Thus all of the items suggested in the first phase of the schedule of works were considered not to be necessary except perhaps for stiffening of the ductwork. This work was therefore implemented as is shown in Figure 5, where the added cross-bracing can be clearly seen.

Phase 2

Expansion box to control noise from melters.

Our investigation had already shown that the noise radiating from the ductwork was not low frequency transfer from the melters and so mitigation in this area would have no purpose.

Cyclones should be sealed and stiffened, with isolated platform

We had demonstrated that the cyclones are not a significant source of noise in the 40 Hz one-third octave band, negating the need for this mitigation work.

Introduce a secondary stack silencer.

We believed that this measure should be the first priority as it would have the effect of reducing noise emanating from the exhaust stack of bagging Plants 1 and 2, albeit the reduction being broad band in nature.

Thus all of the items suggested in the second phase of the schedule of works were not considered to be necessary except the introduction of a stack silencer. Consequently a silencer in the form of a 4m x 300mm diameter pod was inserted into the stack and a reduction of 5 dB at 38 Hz plus a broad band reduction in noise was obtained.

Residual Noise Levels

There was, however. still a much reduced but noticeable tonal component in the 40 Hz one-third octave band which was found to be radiating from the stiffened ductwork.

At this stage of the exercise it was possible to shutdown the bagging plant and we were able to inspect the rotor of the fan on bagging Plant 1. we were somewhat surprised when we found that the fan rotor had actually been repaired rather crudely and the blades of the rotor were very asymmetrical, a discovery which effectively justified our original conclusions about the source! Although a fan rotor may be in balance, if it is asymmetrical, one naturally would expect that a tone at the rotational frequency would be generated. In finding this, we advised the client to install a new fan with a symmetrical, well balanced rotor. This was done and the fan was chosen so that it had a different rotational speed to the original one but was still able to perform the same duty.

Outcome

The result of changing the fan to the new type, installing the stack silencer and stiffening the ductwork was such that the noise level in the 40 Hz one-third octave band was reduced to 46 dB, thus achieving the Environmental Directorate's target even though there had been some dispute about the standard to be used.

Figure 6 shows the one-third octave spectrum at the complainants property before and after the implementation of the few noise control measures described above. It is important to remember that this was achieved at a fraction of the cost that would have been involved had the client followed the schedule of works recommended by the Environmental Director Consultant. Amicable consultation with the local authority throughout the whole process did ensure that a satisfactory resolution was achieved.

It is also worth noting that the comprehensive measures proposed by the local authority will have been based on recommendations from their external consultant, who in turn, will have been working to a specific brief of doing all that is required to abate the nuisance. Maybe a problem with the existing confrontational system is that extreme positions tend to be adopted on both sides, without sufficient consideration of what is actually required.

The message is that we, as consultants, should never adopt the 'broad brush' approach to a noise issue. By careful investigation and analysis, problems can be resolved without incurring unnecessary expense.