Background
A cement plant in Vietnam's Red River Delta was operating a limestone conveyor system adjacent to a residential area that had developed around the plant over the years. The plant had received formal noise complaints from the local People's Committee and was required to demonstrate noise reduction measures.
The conveyor in question was an 800mm wide EP250 belt running at 3.0 m/s, carrying crushed limestone from the quarry crusher to the raw mill. It ran 20 hours per day. The measured noise level at the nearest residential property boundary was 68 dB(A) β the permitted limit for industrial zones adjacent to residential areas was 60 dB(A).
An 8 dB(A) reduction was required for compliance.
Noise Source Analysis
A site survey identified the main noise contributors from the conveyor:
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Steel tube return idler vibration β the largest source. Steel tube rollers on the return run vibrated at their natural frequency as the belt passed over them, radiating noise like a drum. Measured contribution: approximately 18 dB(A) above background from this source alone.
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Limestone on steel transfer chute β stone impacting the steel chute walls at the loading point.
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Belt impact at loading zone β limestone dropping onto the belt at the crusher discharge.
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Drive gearbox β minor contributor, already isolated on anti-vibration mounts.
Solution
Return idler replacement with rubber-coated steel idlers: The 47 return idlers on the conveyor were replaced with rubber-disc type self-cleaning return idlers. The rubber disc rings dampen the vibration that steel tube rollers amplify. This was the single largest noise reduction measure.
Rubber lining of transfer chute: The limestone transfer chute at the loading point was lined with 40mm NR rubber panels. Limestone impacting rubber rather than steel reduced impact noise at the loading point significantly.
Impact bars in loading zone: UHMWPE impact bars replaced the standard trough idlers in the 4-meter loading zone, providing continuous support and reducing belt impact noise.
Noise Measurement Results
Post-modification measurements at the same boundary measurement point:
| Source | Before | After |
|---|---|---|
| Return idlers | Major | Significantly reduced |
| Transfer chute | Significant | Reduced |
| Loading zone | Moderate | Reduced |
| Total at boundary | 68 dB(A) | 59 dB(A) |
The 9 dB(A) reduction brought the plant within the 60 dB(A) limit, satisfying the regulatory requirement.
Cost-Effectiveness
The noise reduction measures had a secondary benefit: the rubber-disc return idlers eliminated return idler buildup (a persistent maintenance problem on the limestone conveyor) and the rubber chute liners extended liner replacement intervals from 6 weeks to approximately 7 months.
The noise compliance investment therefore also reduced maintenance costs β an outcome that was presented to the plant management as evidence that the compliance investment had operational value beyond regulatory compliance.
Key Points
Return idlers are typically the largest noise source on a conveyor. Replacing steel tube rollers with rubber-disc type rollers is usually the single most effective noise reduction measure and also eliminates carryback buildup.
Rubber lining of chutes reduces noise as well as wear. Rubber-lined chutes are a standard noise reduction measure in applications near sensitive receivers. The noise benefit is real and measurable.
Noise reduction and maintenance improvement often go together. The measures that reduce noise (rubber components, damped idlers) are often the same measures that reduce wear and maintenance frequency. The business case for noise mitigation is often stronger than compliance alone.
Elephant Rubber supplied the rubber-disc return idlers, rubber chute liners, and UHMWPE impact bars for this project.