Safety is a relative concept. Virtually every industrial process involves some hazard to persons and property. Solvents used in conventional paints present a fire hazard. The possibility of dust fires and explosions exists in powder coating. Which process is safer isn’t the point. The point is to design and operate any system to minimize the inherent hazards.
Safe, efficient design and operation of an electrostatic powder coating system is based on three fundamental rules:
- Maintain a powder-air concentration well below the Minimum Explosion Concentration (M.E.C.) of the powder being sprayed.
- Eliminate all potential sources of ignition.
- Incorporate and maintain appropriate safety features in the system.
Successful powder coating relies on the recovery of over-sprayed powder. Properly designed and operated, powder recovery systems are safe. However, as they involve handling large volumes of air and powder, they do present a potential hazard should an ignition accidentally occur.
The first powder coating systems employed conventional dust collection equipment to remove the powder from the booth exhaust as shown in Figure 1. These systems are effective, but call for large cyclones and bag houses and long runs of large diameter ducting. Subsequent development has been aimed at reducing the overall size of the powder recovery system and improving its efficiency and safety.
Replacing the bag house with cartridge filters reduces the overall size of the system by a factor of about 2.5 and eliminates the problems associated with filter bags. In recent designs the cartridge filters are located right in the base or on the side of the spray booth, eliminating much ducting. However, the cartridges still must handle the entire volume of booth exhaust air as shown in Figure 2. During filtration the powder-air concentration must pass through the hazardous range, so a potential hazard still exists with large volumes of powder present. The cartridge module, however, with its wide open front, is not considered a closed vessel and does not require explosion venting.
The cyclone has also been redesigned. The new systems employ several smaller cyclones (three to nine) that are more efficient and easier to clean. They are connected directly to the booth to eliminate the large cumbersome ducts. The cyclones are mounted vertically or horizontally for accessibility and overall area utilization as shown in Figure 3. In all new cyclone systems the large ducts are eliminated.
A third recovery system, extremely popular in the U.S. in the late 1970’s and still popular today internationally, employs a moving belt of filter fabric to remove the powder from the booth air stream right in the booth. When it strikes the belt the powder-air concentration passes through the hazardous range instantaneously. The powder is vacuumed from the belt with a low-volume air stream and separated from the air in a compact Cy-Cart unit. Having only about one-sixth the volume of an equivalent cartridge filter and one-sixteenth of an equivalent bag house, the Cy-Cart unit presents a small hazard should an ignition occur. The Cy-Cart must have a relief vent in case of an explosion.
The following basic principles of good design and operation apply equally to all systems.
Safe Powder-Air Concentration
Various United States authorities recommend that air flow through a powder coating booth and recovery system should be sufficient to insure that the maximum powder concentration will never exceed 50 per cent of the M.E.C. If the M.E.C. of the powder has not been established and certified, it is usually assumed to be 0.030 ounces per cubic foot. In such cases, the maximum concentration should not exceed 15 oz. of powder per 1,000 CFM of air.
Elimination of Ignition Sources
Do not permit smoking in the vicinity of any component of a powder coating system. All entry-ways into the powder room should be marked with “No Smoking” signs.
Under normal operating conditions, the energy output of a properly designed and maintained electrostatic powder spray gun is much lower than necessary to produce ignition in any powder-air mixture. However, guns should have a built-in safety device which positively limits their output current and simultaneously shuts down the system when that limit is reached. It is also good practice to make sure that the tip of the gun does not approach the workpiece or the sides of the grounded metal spray booth too closely.
All equipment, spray booth, conveyor, hangers, workpieces, ducting and powder recovery units, must be effectively and permanently grounded to prevent the build-up of a static charge. Codes dictate that the resistance to ground of conveyors and hangers be one megohm or less. The best way to ground a system is by using grounding rods.
Lights should be located outside the coating booth, if possible. Any lights or electrical equipment located inside the booth must be explosion-proof.
Because of the nature of their operation, it is impossible to maintain the concentration below the M.E.C. in certain system components. Powder recovery units, such as filter houses and cyclones, should be pressure relief vented.
Pressure relief vents should provide one square foot of area for each 30 cubic feet of enclosed volume. Preferably these vents should be ducted outside. Vent doors should be hinged or chained to prevent them from becoming missiles. If pressure relief vents are in accessible locations they should be caged to protect personnel in case they operate as a result of rapid pressure buildup.
All powder recovery filters should be equipped with differential pressure instruments. At the very least these instruments should give a visual indication of the pressure across the filter. Greater safety is provided by an instrument which also sounds an alarm when the pressure drop is excessive. A device that shuts the entire system down when filter pressure is abnormally high provides the most protection.
Exhaust fans should be of spark-proof and dust-ignition-proof construction. Totally enclosed, fan-cooled motors meet this requirement. Exhaust fans should be located downstream of powder recovery units, where powder concentrations above the M.E.C. are unlikely to occur.
An approved flame detection system is required in an automatic coating system. It shuts down the system and sounds an alarm. Inclusion of automatic shut-off valves in ducting will cut off air flow and reduce the possibility of propagation. Automatic explosion suppression or fire extinguishing systems will further reduce fire hazards.
All systems should have interlocking controls so that the malfunction of any component will shut down the entire system. Electrostatic spray equipment, conveyor and exhaust fan should be interlocked so that if one is inoperative, the others will shut down. It is also desirable to interlock the oven controls so that parts cannot be sprayed unless oven temperature is within the desired temperature range. If the conveyor shuts down for an extended period of time, the oven temperature should drop so that parts in the oven do not over bake. Incorporating control interlocks in the basic electrostatic equipment simplifies system installation.
Handling Powder in the System
Air flow through the coating booth should be sufficient to prevent the drift of powder into the room. In the United States, some agencies specify a minimum average air velocity of 60 FPM through the openings of an electrostatic coating booth. An average is specified because air flow through an opening is not uniform.
Although an average velocity of 60 FPM may be permitted, it is our experience that it should be higher. A normal design criteria is 125 FPM. Also, the shape of the opening affects air flow into the booth, and a booth with tall narrow openings may require higher average velocities.
Providing higher velocities than absolutely required is good practice because it gives added assurance that powder will not drift from the booth. Other than slightly higher initial cost, it imposes no economic penalty, because, unlike a liquid paint booth, exhaust air can be returned to the plant. There is no air make-up.
Air velocities in conventional recovery system ducting should be high enough to convey the powder without depositing it on duct surfaces. Experience indicates that conveying velocities from 3,500 to 4,000 FPM meet this requirement.
Spray booths and filter housing should have no horizontal surfaces on which powder can collect. Interior surfaces should be free of flanges, ledges and other projections. A coarse screen located in the exhaust openings of the spray booth will keep large foreign objects out of a conventional recovery system. Round ducting with large radius bends is preferred to rectangular ducting
Good Operating Practice
Proper design and construction are the first steps toward successful powder coating. Successful operation depends on good operating and maintenance practices.
Each powder coating system is unique in one way or another. Every new system should be monitored closely during its first few weeks of operation to determine its particular operating characteristics. The observations are then used to establish standard operating and maintenance procedures. This approach will enable the supervising operator to catch and correct any abnormal condition before a serious problem occurs.
Good housekeeping is a must. Although powder coating systems are much cleaner than wet paint systems, they cannot be neglected entirely. Powder should not be allowed to accumulate within the system or the surrounding area. Any object, such as a hanger or workpiece which may fall into the booth, should be removed immediately. In addition to providing a place for powder to collect, it may block the exhaust opening and reduce air flow through the booth.
Spilled powder should be cleaned up immediately, and powder drifting from the system should be cleaned up daily. As a rule of thumb the spray booth should be rough cleaned every day, and thoroughly cleaned once a week. Use a squeegee with the recovery system operating. Rags or brushes may introduce lint or bristles into the system, contaminating coatings or clogging spray guns. After squeegeeing, clean crevices with a non-sparking vacuum cleaner equipped with a grounded hose.
Powder collecting on booth walls will retain its charge. This presents no real hazard but could cause the operator to experience a static electricity shock after cleaning. To prevent this, the operator or his tools should be grounded while cleaning the booth.
All powder handling equipment and all nearby electrically conductive equipment must be grounded. Grounds should be checked regularly. Ground connections should be inspected visually every day and checked positively every month.
Check filters and traps in the compressed air supply daily and drain or replace as necessary.
Post “No Smoking” signs and enforce this rule.
Powder recovery equipment should be checked regularly for proper operation. Check connections for leaks which reduce operating efficiency. Check that pulsating air-type filters are operating properly. Don’t pulse at too short a time interval. This prevents a precoat of powder from being established on the filter surface, permitting powder to pass through the filter.
Check the readings of differential pressure instruments used to monitor the filters at the beginning of every shift. Table I indicates the normal differential pressure across various filters and the recommended settings for sounding an alarm and for system shut- down. If absolute filters are not equipped with differential pressure instruments, they should be changed at regular intervals.
Check the various system safety devices regularly, including the automatic gun shut-off and the system safety interlock. Check that pressure relief vents are free to operate.
Periodically, inspect all electric motors to make sure that they are not overheating. In most cases, fan motor bearings are sealed. If not, take steps to prevent them from becoming contaminated with powder and lubricate according to the manufacturer’s instructions.
Don’t use recovery system components as powder storage reservoirs. If recovered powder is returned manually, empty the drums regularly. If the system has an automatic powder return, run the recovery and return system for half an hour or so at the end of the day to return the bulk of the recovered powder to the supply hopper. Check the screen in the rotary sieve of an automatic powder return system regularly to insure that it is not torn or broken and is working properly.
If the operator wears gloves, then the gloves should be conductive, or the fingers and palm of the operator must be exposed to make contact with the gun trigger and the grounded handle.
Finally, do not store combustible materials near any portion of a powder handling system.
Assuming good initial design and proper installation, safe operation of a powder coating system is based on recognition of the possible hazards and a common sense approach to system operation and maintenance. The following “Do’s” and “Don’ts” are offered as a guide to operators and supervisors.
- Check operation of safety devices and interlocks regularly.
- Check for leaks and abnormal pressure drop across filters.
- Check sieve in the automatic powder return system. Where applicable, make sure that air bearings are supplied with clean, dry air.
- Check ground connections regularly, including ground between work-piece and hangers.
- Lubricate electrical motors regularly and check for overheating.
- Rough-clean booth and area daily with squeegee and non-sparking vacuum cleaner. Clean thoroughly every week.
- Clean and drain compressed air filters regularly. Replace as necessary.
- Strip hangers regularly to make sure that parts are properly grounded.
- Remove foreign objects which may fall into booth immediately.
- Spray powder with the booth air exhaust and powder recovery system off.
- Let spray guns approach grounded objects too closely.
- Permit foreign objects to remain in the booth.
- Permit smoking in area.
- Let powder accumulate anywhere.
- Permit fan motor bearings to become contaminated with powder.
- Use recovery system hoppers as powder storage reservoirs.
- Clean spray booth with rags or brushes.
- Block pressure relief vents.