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|Seminar Events Exhibitions|
|Safe Powder Coating (Part 1)|
Here is the first part of
the "Safe Powder Coating Guideline" from the The European Council of the
Paint, Printing Ink and Artists‘ Colours Industry (CEPE). CEPE
represents the interests of about 1100 paint and 75 printing ink and over 20
artists‘ colours companies at European level. It has 23 Member Associations
and 4 Affiliated Associations in 21 countries, 14 Company Members as well as
affiliated paint manufacturers in Poland, Romania, Russia and Turkey.
Participating companies to this Guide are :
Akzo Nobel Powder Coatings, BASF Coatings, Becker Powder Coatings, DuPont
Powder Coatings, IGP Pulvertechnik, INVER, Jotun Powder Coatings, Oxyplast
Belgium, PPG Industries, Rohm&Haas, Sigma Coatings, Tigerwerk, Teknos
Safe Powder Coating Guideline
7th Edition, 2005
Table of Contents
2. Dust explosion and fire hazards
3. Electrical hazards5
4. Health Hazards
5. Compressed Air
6. Construction of Plant and Equipment
6.1 Full guidance.
6.2 The spraying facility location
6.3 Spray Booths.
6.4 Stoving Ovens.
6.5 Application Equipment.
6.6 Ventilation and ducting
6.7 Ventilation and powder collection system
Industrial thermosetting powder coatings are typically produced by blending and extruding together resins, curing agents, pigments and additives. The resultant matrix is ground into fine discrete particles. Such powders are applied to a substrate or workpiece via a pressurised spray application system, complete with electrostatic charging of the powder coating to charge the particles and effect a high level of transfer on to the work piece.
Application can be via either fully automated or manual systems, with the workpiece transported through a spray zone containing a number of guns and into a stoving oven via an overhead conveyor.
Air pressures in the order of 0.8-2.0 kPa (10-30 psi) and electrical potentials in the order of 10-90 kV at a current between 1,5µA ( Tribo ) and 100 µA are typically employed.
Systems are designed to minimise the amount of overspray. Excess powder is removed by exhaust extraction and collected for re-use or disposal. From the information in this Guide relating to safe working procedures, it is clear that there is a lower level of hazard when using powder coatings compared to conventional solvent-based paints. Dust clouds in air require 50-100 times the energy necessary to ignite a solvent vapour/air mixture, and are therefore inherently more difficult to ignite. Also powder mixtures in air need to be above a certain concentration, known as the lower explosion limit (LEL) before ignition or explosion can occur.
However, certain hazards do exist when using powders depending on various factors. Precautions must be taken to avoid them, and these are referred to here as safe working procedures. If these are followed, any risk should be reduced to a minimum.
The main hazards involved in the electrostatic application of powder coatings are:
(i) Dust explosion and fire
(ii) Electrical shock
(iii) Exposure to hazardous substances
(iv) Compressed air
2. Dust explosion and fire hazards
2.1.1 Powder coatings, being fine organic materials, can give rise to dust explosions. A dust explosion may occur when both:
(i) the concentration of dust in the air is between the Lower Explosive Limit (LEL) and Upper Explosion Limit (UEL).
(ii) a source of ignition of the required energy for the dust cloud is present. Such sources of ignition can include:
(a) hot surfaces or flames
(b) electrical discharges or sparks
(c) electrostatic discharges
2.1.2 A fire may occur when a layer of deposited powder coating or a cloud comes into contact with an ignition source such as those listed in 2.11 (ii) above. A fire within the powder coating system may result in a dust explosion if either burning particles are allowed to enter confined sections of equipment, such as dust collectors, or if burning dust deposits are disturbed.
2.2 Prevention of powder dust explosion
2.2.1 An explosion can be prevented if both or either of the conditions shown in 2.1.1 are avoided. Powder coating systems should be designed to prevent both conditions occurring, but due to the difficulty of totally eliminating sources of ignition, more reliance should be placed on the prevention of explosive concentrations of powder. This can be achieved by ensuring that the powder in air concentration is kept below 50% of the LEL.
2.2.2 Determined LELs on the range of typical powder coatings lie between 20 g/m3 and 70 g/m3 dependent on the specific chemical and physical properties.
It is recommended that a twofold safety margin is applied to maximum dust concentrations. Spray booths and associated equipment such as dust collectors, should, therefore, be designed
and constructed to ensure that dust concentrations never exceed 10 g/m3. Dust concentrations should be reduced as far below this value as is reasonably practicable. Reference should be made to Pr EN 12981 and Pr EN 50177 for information on the design and operation of booths and spray equipment.
2.2.3 The application unit should be clearly marked with the capacity of the extraction unit and the maximum number and capacity of the spray guns. The configuration of the unit and coating powder usage should be regularly checked against stated values to ensure that airborne
concentrations do not exceed 10 g/m3
2.2.4 A regular maintenance and cleaning schedule should be introduced to prevent accumulation and build up of dusts. In the case of electrical equipment, build up of dusts can result in their ignition through overheating. The temperature of external surfaces, or surfaces liable to be exposed to dust, of electrical equipment should not exceed 270ºC. This value is 2/3 of the minimum ignition temperature.
2.2.5 The use of compressed air or dry brushing for cleaning up spills for cleaning down equipment should be avoided. Suitably designed dust-tight vacuum cleaners to Standard IP6X/IP54 of EN60529, or wet brushing are preferred methods.
2.2.6 Smoking should be strictly prohibited and all sources of ignition, such as matches and lighters, should be excluded.
2.2.7 The principles of avoiding the creation of dust clouds together with ignition sources are equally applicable to general handling and cleaning processes.
To be continued.........
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