4 - DUST CONTROL MEASURES: A GENERAL OVERVIEW
The general principles of dust control apply to the manufacture of asbestos-cement pipes and sheets, friction products (such as car brake linings), sealants, gaskets, coatings, textiles and to the installation and repair of these products. The methods of dust control described in this chapter are simple, effective and are readily available or can be made locally.
The reason it is necessary to control asbestos dust is because long term exposure to high concentrations can cause occupational diseases amongst workers. The inhalable dust of concern is not visible to the human eye. There are many locations in asbestos using factories where asbestos dust could be generated and released into the air. Controls are needed to prevent this from happening. This chapter is intended to provide guidance as to when controls are needed and what controls are available. Work practices and ventilation installations are described, along with methods for using them efficiently. Information concerning low level airborne concentrations which can result from an effective dust control program is also provided.
Good control measures should include:
€ Control at the source;
€ Engineering controls;
€ Work practices;
€ Housekeeping;
€ Personal protective equipment.
Control of dust should be achieved as near the source as possible. This increases the efficiency of the control process, minimizes costs and prevents the spread of dust into adjacent areas. The process of control must prevent emissions to the environment in order that a localized hazard in the factory is not converted into an environmental hazard. Therefore, if enclosure of the source is possible, this would be the first step to be taken.
Good engineering controls should include the following equipment:
€ Dust hoods;
€ Ductwork;
€ Dust collector and fan;
€ Make up air.
Proper engineering of each one of these components is essential. Among the different operations where industrial ventilation will be used are debagging, mixing, weighing, cutting, grinding, etc.
The elements of a complete local ventilation system are illustrated in Figure 1. A source of make up air or air to replace that exhausted through the hood must always be provided. The hood encloses the operation to the extent possible and provides a face velocity of air into the hood. As an example, for manual debagging, bags must be cut, emptied and disposed of inside a hood fitted with an exhaust connection. Cutting bags in half and handling empty bags outside the hood should be avoided. Figure 2 shows one type of bag opening station.
Enclosure is practical when the operator does not need to have contact with the operation. In asbestos-cement plants a number of machine operations, such as the interior machining of couplings, can be enclosed. Enclosure is more common for the processing of smaller pieces but it is also used on larger equipment like belt conveyors, carding machines as shown on Figures 3 and 4.
It is important that all parts of the enclosure should be at a lower pressure than the associated worker's area. This may be achieved with a small amount of suction air. This air, after passing through the entrance to the hood, is exhausted through a series of proper size ducts to an air cleaner which is usually an industrial cloth filter or "bag house". The ducting may be joined with any number of other hoods and cleaning systems and may have precleaning cyclones or settling chambers along the ducting. Good practice requires that the ducting have no blast gates or dampers, that the velocity be sufficiently high everywhere that the dust not fall out and plug the ducting and that the corners and bends of the ducting be designed so as to minimize wear and erosion. The bag house must be sized to handle the quantity of air flow being exhausted through the hoods. The clean air passes through the suction fan and is exhausted to the outside. If consideration is to be given to environmental concerns, a dust monitor can be placed at this point. However, this dust monitor is not considered an accurate measurement of asbestos dust but rather of the relative changes in total dust concentrations over time. (Permissible concentration of respirable dust in recirculated air is 0.1 mg/m3). In modern practice the bag filter is the universally approved method of removing asbestos dust from air. Cotton is the most efficient filter fabric for asbestos dust collection.
The environment can be well controlled at all work places provided good preventive maintenance on all dust control equipment is performed and that inspections are made on a regular basis.
Preventive Maintenance
Assuming that care has been taken to ensure that the selected equipment has been designed to allow easy maintenance, a proper preventive maintenance schedule should be introduced to keep the equipment in good order. Air moving and cleaning devices are hardworking and need good preventive maintenance. There are many examples of systems that have been satisfactorily installed, only to become inefficient soon afterwards due to lack of care.
To ensure a safe working environment, maintenance work of dust control equipments should be given priority over maintenance of the manufacturing machinery, equipment and process itself. Preventive maintenance should include the following:
Ductwork System
€ Proper adjustment or replacement of flexible connections between hoods and ductwork or between ductwork and fans;
€ Removal of any accumulation which could create blockages and dusty conditions at different operations;
€ Taking a static pressure reading at each dust hood and comparing it with the original reading. It indicates immediately if air flow-rate is what it should be at each hood. This can also be recorded on a form as shown in Figure 5.
Stop any water vapor leaking into the negative or suction side of the exhaust system before it causes accumulations of dust to solidify and to create blockages.
Dust Collector
If not initiated automatically, cleaning of filter bags must be made on a regular basis. This is necessary in order to maintain the resistance of the bags within the range for which they have been designed. In so doing, proper air flow within the dust collection system as a whole will be maintained.
The resistance of the bags must be taken and recorded on a regular basis. In the short term, it will indicate whether bag cleaning is necessary. On the long term, it will show whether bags are blinding or if other problems occur. Bag life will vary widely from one application to another. However, bags should be replaced before they blind too much or before they start breaking on a regular basis. This is why it is important to record the date and the location of the bags being replaced.
Any accumulation of dust in the hopper of the dust collector should be cleaned. If cyclone collectors and fabric-type dust collectors are equipped with rotary air-locks, the wiper seals of these air-locks must be replaced before they start leaking abnormally. If this is not done, there would be re-entrainment of dust inside the collector and blinding of the bags.
Special care should also be given to mechanical parts that require lubrication, such as bearings. Water should be flushed from compressed air supply dryers on a regular basis in order to prevent the introduction of water into the filter bags. Forms similar to those shown on Figures 6 AND 7 can be used to record the above information.
Fans
If the system is designed to permit the dust to pass through the fan prior to reaching the dust collector, the fan wheel or lining must be replaced before the wheel breaks or before wear holes begin to show on fan casings. Bearings must be lubricated according to manufacturer's specifications. If the fan is belt driven, belt tension must be checked and adjusted regularly. The belt must be replaced before breakage.
Inspections
A procedure should be laid down for the regular inspection and testing of the complete installation. Of course, this is over and above the daily inspection which should be [Figure 5] conducted by the operator and by the supervisor in order to detect leaks, breakages, holes in ductwork, etc. Several relatively simple techniques can be used.
Since the small particles produced by industrial processes cannot be seen by the naked eye, inspection can be difficult without the help of special lighting techniques to make the dust cloud visible (see Figure 8). The Tyndall Beam backlighting technique highlights dust released into the atmosphere which can then be recorded by normal photographs if desired.
Smoke Capsules or Puffer Tube is another technique. When used, they produce a white or yellow smoke that can be used to observe flow patterns around an exhaust aperture. They may also be used to detect leaks in sealed hoods, ducting or filter units.
As mentioned previously, hood static pressure readings can give a very good estimate of the amount of air flowing at each hood. However, if more precise results are required, many instruments can be used, i.e. pilot tube and manometer, rotating vane anemometer, velometer, hot wire anemometer, etc.
The monitoring of workplace ambient concentrations and measurements of personal exposure to fibres, which is also a part of environmental control, is explained in detail in the chapter named "Monitoring of Fibres".
Wet Methods and Processes
There are very few viable alternatives to exhaust ventilation, except the use of water in specific instances. Generally, this consists of the application of a fine spray directed at the source of dust, such as a cutting tool or saw, drilling, material deposited on the floor, etc. The spray must be gentle or asbestos is likely to be distributed together with very small water droplets. Also, care must be taken to collect and properly dispose of the wetted material and/or water containing the material. Since this is usually to be carried out by a worker rather than through the use of a piece of equipment, it is essential that the workers have the know-how and tools to perform these tasks. If not, training should be provided.
Wet processing is extremely effective in reducing the possibility of dust generation. Processes which may be wetted typically have much lower dust levels than processes that must be run entirely dry.
Dust in an asbestos-cement plant, from the asbestos-cement vat to the curing tanks, can be controlled by keeping the product, equipment and floor area wet at all times. If proper tools and wet spraying are used in conjunction with immediate clean-up of debris, the finishing section of an asbestos-cement plant can also be kept clean.
Of course, the wetting procedure requires some attention to electrical safety and other operational problems associated with water in the presence of lathes, drills, saws, etc. Many plants spray water over machining processes such as drilling and lathe cutting. This process usually produces significant reductions in fibre levels.
Due to the water droplets which frequently remain in the air during spraying, wetting in the presence of local exhaust ventilation systems is not recommended. The water spray will enter the exhaust ventilation system and produce a slurry with the asbestos, cement and other additives. When hardened, this slurry can contaminate the ventilation ducting and destroy the effectiveness of the bags in the baghouse (an industrial filter which cleans the dust from the air).
A possible problem with floor spraying is that over time, asbestos and water paste can collect in floor cracks, forming a hard, cement-like substance. If allowed to dry out, this material can be a minor source of dust when walked on. The solution is to perform complete sluicing operations during floor cleaning.
The water used for wetting purposes may be incorporated into the process if it is appropriate for asbestos-cement application. Otherwise, it may be directed to settling tanks where the solids can settle before the slurry is removed.
For the disposal of bag house waste or other operations where large quantities of asbestos chips and powder are present, wetting can be made significantly more effective by the use of wetting agents. The two most common agents are ordinary liquid detergents or ethylene glycol added to water in the proportion of approximately 1 to 1,000. These agents considerably increase the dust suppression capacity of the water and prevent emissions from the surface of disposal sites. However, wetting is not applicable everywhere.
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