A firm producing fibre mouldings for the automotive industry made use of a number of fibre pulp header tanks, pressurised to ensure a constant flow to the pulp presses. As the level of pulp in a tank was replenished, air pressure rose until a pre-set limit was reached, at which point the pressurised air was released to the atmosphere through a 50 mm port. This discharge contained moisture and, occasionally, some pulp from the header tank.
The tanks were located in working areas of the factory, exposing employees every 10 minutes to A-weighted noise levels of about 101 dB for about 3 to 4 seconds.
The solution was to install an absorptive jet discharge silencer, approximately 2 m long, onto the pressure vessel discharge. This had the following features:
£400 per unit. (1995)
Details of the measured noise reductions are shown in the table:
Sound pressure level at 1 m from the pressure vessel
| A-weighted | Octave band centre frequency (Hz) | ||||||
|---|---|---|---|---|---|---|---|
| 250 | 500 | 1000 | 2000 | 4000 | 8000 | ||
| Before treatment | 101 | 88 | 92 | 94 | 96 | 94 | 93 |
| After treatment | 91.2 | 87 | 88 | 84 | 81 | 85 | 84 |
| Attenuation | 9.8 | 1 | 4 | 10 | 15 | 9 | 9 |
Photographs courtesy of Marley Automotive Components Limited. Equipment designed by Ian Sharland Limited.