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NV100 Acoustic Mill Monitor
NVMS Mill Monitors have found their place not only listening to Ball and SAG mills but also to crushers and any piece of plant machinery which exhibits certain characteristic noise during operation. Examples of this include the condition of the mill's contents and the characteristic tones produced as the steel balls hit the mill lining, or the noise produced by an ore crusher when unwanted objects are introduced.
The NVMS industrial noise monitors are essentially electronic versions of the human ear, but without the inconsistency between operators. The electronics are housed in a sealed IP66 rated stainless steel enclosure and the microphone is protected by an outdoor weather proof assembly. Power is drawn from a standard 110/240V supply and the measured sound level in decibels is converted to a 4-20mA signal which can be readily accessed by DCS systems such as Scan 3000 and LCN.
At the heart of the system is a Brüel & Kjær Sound Level Meter. This instrument is available in several versions and these complement perfectly the typical applications plant operators have in mind. A basic version of the sound level meter will provide a simple overall noise level whereas enhanced versions offer such features as octave and 1/3rd octave band filtering as well as data logging and frequency analysis. In many instances octave band or 1/3rd octave band filters are indispensable for plant monitoring, since being able to narrow down the instrument's 'hearing' sensitivity to a particular range of frequencies can allow characteristic tones to be monitored which are otherwise obscured by the general back-ground noise surrounding the equipment.
The entire solution is developed and manufactured by NVMS which means that customised solutions based upon noise monitoring can be readily available. This may include such options as a miniature computer to monitor, log, analyse and act upon results, alarm triggering, activating tape recorders... the flexibility is limited only by imagination
Just Listen...
Operators may wish to place microphones at strategic positions around the mill or specified equipment. In this case the cost of the installation can be reduced by including more than one Sound Level Meter (SLM) in the enclosure. Three versions of the NV100 are available; Models A, B and C. These correspond to enclosures containing 1,2 or 3 SLMs. This way the NV100 can be strategically located and the microphones positioned as required. Up to 50 meters of microphone extension cable can be used to position the microphones at nominated locations.
Hostile Environments...
As the microphone is typically located in a hostile environments, with mud, spray and other potential contaminants a constant hazard; NVMS have developed the NV100MT Hostile Microphone Housing. This device protects the delicate microphone and pre-amplifier from damage and corrosion whilst providing an acoustic window (Upilex) which can be readily replaced.
NV100MT Housing
In order to determine the acoustic effect of the NV100MT a series of tests were conducted. From these results a set of correction curves were established which can be viewed here:
The protective housing affects absolute levels to varying degrees across the audible spectrum.
If absolute levels are required they can be calculated from the correction values available. In most instances however this is not required as most applications require a subjective comparison between changing levels, not the actual true sound pressure level.
Onsite Preliminary Testing
NVMS offer a service which will provide you with detailed information on your particular mill(s), allowing you to confirm that acoustic mill monitoring is a viable option for your particular asset. The process is inexpensive and extremely worthwhile, Our site visit will establish the following:
- Which frequency band(s) are most appropriate for monitoring.
- What relationship (if any) exists when comparing this nominated frequency against other process parameters; by means of a time history graph.
- Where should the microphone and monitor be located with consideration of accessibility, cable runs and other factors
All measurements are taken using a Brüel & Kjær Type 2250 Logging Sound Level Meter. Post processing of recorded sound files for 3D FFT analysis is carried out using Brüel & Kjær Pulse
The process undertaken involves a gradual reduction in feed rate which should result in an increase in over-all noise levels resulting from the reduced dampening of ball / liner impact due to interaction with grinding material. Measurements taken includ both over-all (A-Weighted) sound pressure levels as well as analysis of 1/3rd octave bands.
Analysis of sound energy produced during the trial is represented below using a 3D contour plot off FFT (Narrowband analysis) versus Time, this particular load cycle took 80 minutes to complete.
When full (Start) the mill typically produces noise which is not broadband in nature, there are definite tonal characteristics at frequencies centered around 400, 800, 1k, 1.2k and 1.6kHz. As the mill becomes emptier the over-all loudness increases and the contributing frequencies do not shift however their contribution becomes more dominant especially in the higher frequencies (as would be expected due to reduced dampening of ball / rock to liner impact).
Sounds OK ?
Below are links to three 1 minute MP3 sound recordings which have been acoustically filtered in order to ascertain which specific frequency band would be most appropriate for continuous monitoring of the mill. The three files and their characteristics are as follows:
This file contains the un-filtered original sound recording covering the majority of the audible spectrum for 20Hz to 10kHz.
MillEmpty-1000Hz to 2200Hz.mp3
This file has upper and lower frequency limits which subjectively include the ‘rattle’ of rock and ball impact and filters out all of the lower frequency rumble as well as the extraneous higher frequency content.
MillEmpty-OneThirdOB-1600Hz-CF.mp3
This file is based on a 1/3rd Octave Band centre frequency of 1.6kHz; with upper and lower frequency limits of 1.779kHz and 1.412kHz respectively. Of the pre-defined filters available in the NV100 Mill Monitoring system this frequency span is the most appropriate as it encompasses the audible ‘rattle’ and pinging of impulsive ball / liner impact for this clients particular installation.
We recommend that quality headphones are used when listening to these recordings. The information above represents a small portion of the data we would provide you as a result of our onsite preliminary testing. We will also collect process data from you (including Mill Weight, TPH Feed-Rates etc) and include charting to compare these with measured (logged) noise levels.
NV100F Industrial Noise Ear - Full Noise Spectrum
Octave and One-third Octave Analysis...
The system above monitors a single octave or 1/3rd octave band of frequencies (band pass filtering) which is ideal in any situation where a price of equipment produces a defined tone under optimised operating conditions. A system we proposed recently provides a spectral output instead of a single 4-20mA signal. This is detailed as follows:
This design is ideal if you wish to monitor many frequencies quasi-simultaneously, using only ONE sound level meter. The principal difference in the system is that the output would no longer be a single 4-20mA constant current signal, it would be an ASCII STRING sent via RS-232. This 'text string' would be a comma-separated list of numbers which would be collected, trended, charted and interpreted by your plants computing infrastructure. The list represented the current SPL (Sound Pressure Level) in each frequency band. An example of a transmitted string would be:
70.2,72.4,77.0,79.9,89.3,90.3,92.9,97.7,99.2,101.0,99.4,99.1,99.7,100.3,98.6,96.6,94.8,92.8,89.9,90.3
Although your practical requirements may be for less frequencies the system can be readily configured to produce a string containing any number of frequencies. It is possible that having such a detailed signature of the noise characteristics of the equipment may provide operators with a powerful tool to optimise the running conditions.
The communication protocol provided (RS-232) is fairly arbitrary because at minimal cost you can convert this to virtually any telemetry method possible; TCP/IP for example. You would need to work with the plant DCS or Scan personnel in order to present this data in a meaningful manner (such as via Citect or as a Scan 3000 graph)