Safety knowledge about electrostatic spraying

The definition of electrostatic spraying is to form a high-voltage corona discharge electric field between the spray gun and the workpiece. When the powder particles are ejected from the spray gun mouth through the discharge area, they will collect a large number of electrons and become negatively charged particles. Under the action of electrostatic attraction, they are adsorbed to the positively charged workpiece. When the powder adheres to a certain thickness, the effect of "like repels like" will occur, and the powder can no longer be adsorbed, so that the thickness of the powder layer of each part is uniform, and then the powder layer is leveled into a uniform film layer after heating and baking.

Characteristics of electrostatic spray powder: Explosion risks need to be vigilant

According to the "Catalogue of Key Combustible Dusts in Industry and Trade (2015)", electrostatic powder coatings (epoxy polyester powders) belong to the high explosion hazard level and are typical explosion-related dusts. Electrostatic spray rooms are also typical dust-related explosion places.

Electrostatic spraying components: multiple components work together

Electrostatic spraying components include powder spraying room, high-voltage electrostatic generator, electrostatic spray gun, powder feeder, powder recovery device, hanging conveyor, electric lifting machine, large cyclone device, central electric control cabinet, dust collector, etc. The core of electrostatic spraying is the electrostatic spray gun, as shown below. Usually, the spray gun can release a DC voltage of 100KV, but the current is only about 0.1mA, so it will not cause harm to the human body. The current mainstream spray gun is the corona spray gun.

Powder electrostatic spraying process: multiple processes are linked together

Powder electrostatic spraying process: loading → pre-degreasing → main degreasing → water washing and polishing → water washing → ceramic → water washing → drying → powder electrostatic spraying → curing → cooling → unloading

Safety precautions for electrostatic spraying: all-round prevention to ensure safety

1. The spraying line is solidified and dried. Personnel need to enter and clean it every week. Although there is a supply and exhaust system, it involves the possibility of natural gas leakage. The Emergency Management Bureau generally recommends that enterprises manage it as a limited space.

2. The hooks hanging on the conveyor line must be cleaned, otherwise there will be poor conductivity and ignition, especially when passing through the powder room, which may cause fire and explosion.

3. The grounding method of powder spraying equipment is as follows: 3-5 copper rods with a diameter of 15-25mm and a length of about 1500-2000mm are used. They are directly driven into the ground every 3000mm inside/outside the factory and all connected. If the soil is too dry, salt water should be used to enter effective grounding. The grounding resistance value of the measuring equipment should be less than 1 megohm, and the grounding wire should be 4-6mm2 copper core wire. All workpieces, hooks, spray guns, feed pipes, hangers, conveying systems, and powder supply centers need to be grounded. The grounding resistance of each set of dedicated electrostatic contacts is less than 100Ω.

4. Wind speed. All dust-generating points should be equipped with dust hoods according to the requirements, and the dust suction air volume and wind speed should be greater than 1m/s. The design wind speed of the vertical air duct of the main pipeline system should be greater than 19m/s, and the design wind speed of the horizontal air duct should be greater than 23m/s. The inner wall of the dust collector hopper should be smooth, and the angle between the hopper drop wall and the horizontal plane should be greater than 65°. 5. All electrical equipment in the electrostatic spraying room must be explosion-proof electrical equipment, and should be conductive dust explosion-proof electrical equipment. (The distinction between Da and Ga)

6. Dust explosion hazardous workplaces should adopt anti-static grounding according to regulations for all metal equipment, device housings, metal pipes, brackets, components, parts, etc. All metal pipe connections (such as flanges) should be bridged. The electrostatic grounding of the dust removal system must be set up independently.

7. Dust collectors, filters, and pipes that handle explosive dust and debris should be equipped with pressure relief devices, and explosion vents should be set up as required. Dry dust collectors and filters that purify explosive dust should be arranged on the negative pressure section of the system.

8. Compressed air blowing shall not be used for on-site dust cleaning in the powder spraying area. A cleaning method that does not generate dust should be used. It is advisable to use negative pressure dust suction to clean and wipe.

9. The connection between the dust removal pipeline, dust collector, inlet and outlet ducts, and air lock and ash removal device is welded. If flange connection is used, a conductive bridge is required, and its resistance is not greater than 0.03Ω. 10. Except for local areas such as the powder spraying gun outlet, the maximum concentration of suspended powder in the powder spraying room is not allowed to exceed 15g/m³. The permissible concentration of total dust in the workplace air is 8mg/m³, and the wind speed at the opening of the powder spraying room should be 0.3-0.6m/s.

Safety equipment related to the powder spraying room: multiple safeguards to prevent accidents

1. Dust concentration detector. Install a dust concentration detector at the outlet of the powder room. When the set concentration exceeds 10mg/m3, an alarm will be triggered, the fan will be delayed to stop, and the powder spraying equipment will stop.

2. Carbon dioxide fire extinguishing system. Once the fire detector in the powder room detects an open flame, the carbon dioxide fire extinguishing system will be activated immediately, and the carbon dioxide will be transported to the large cyclone filter device and the outdoor dust collector through the pipeline. At the same time, the fan stops and the spraying line equipment stops.

3. Temperature detection system. Install 2 temperature detectors on the top of the dust collector and 1 temperature detector on the inlet pipe connected to the dust collector. Once the temperature exceeds 70℃, the temperature monitoring device should send an audible and visual alarm signal. The fan stops, the spraying line equipment stops, and the explosion-proof valve of the pipeline falls and closes due to gravity.

4. The dust collector should be equipped with an inlet and outlet air pressure difference monitoring and alarm device. When the dust collector is installed or the filter bag is replaced, the monitoring values of the inlet and outlet air pressure of the dust collector should be recorded after 8 hours of use. When the inlet and outlet air pressure of the dust collector is greater than 20% of the recorded value during the subsequent operation, the monitoring device should issue an audible and visual alarm signal.

5. The dry dust removal system shall be equipped with an air-locking ash unloading device. The monitoring device for abnormal operation and fault shutdown of the ash unloading device shall issue an audible and visual alarm signal when abnormal operation and fault shutdown occur.