The hidden dangers of dust explosion accidents in powder electrostatic spraying enterprises and their improvement measures

Zeng Invented 1,2, Xu Weiwei1,2, Huang Daimin1,2

1.
Guangzhou Special Mechanical and Electrical Equipment Inspection and Research Institute; 2.
National Quality Supervision and Inspection Center for Explosion-proof Equipment

Abstract: This paper introduces the explosive characteristic parameters of powder coatings commonly used by powder electrostatic spraying enterprises, and combines with daily work practices, summarizes the common hidden dangers of dust explosion accidents, and proposes improvement measures.

Keywords: powder coating; electrostatic spraying; dust; explosion; hidden danger; improvement

0 Preface

Powder electrostatic spraying is a coating method in which powder coating particles are charged or polarized and adsorbed on the surface of the workpiece under the action of corona discharge with a certain electric field strength and aerodynamic force.

It has low energy consumption and high raw material recycling rate.
Containing volatile organic compounds, it can realize semi-automatic or automatic operation, and is widely used in household appliances, automobiles and other fields.

The particle size range of powder coating particles is below 350μm, most of which are less than 75μm, and there is a possibility of combustion and explosion under certain conditions.
All powder electrostatic spraying companies shall be included in the scope of companies involved in dust explosions, and they shall accept supervision and carry out daily safety management in accordance with relevant requirements.
Based on the daily dust explosion-proof work experience, the author summarized the common hidden dangers of powder electrostatic spraying enterprises, and proposed corresponding improvement measures in accordance with the requirements of relevant standards and normative documents for the reference of enterprises.

1 Explosive characteristics of powder coatings

Powder coatings are mainly solid powders mixed with resins, curing agents, pigments, fillers, additives, etc.

Resin is the main film-forming substance, which determines its performance.
According to the type of resin, powder coatings can be divided into two types: thermosetting and thermoplastic.
Thermosetting powder coatings have good performance, large output and a wide range of applications.

The powder coating will explode when it meets 5 conditions (ie combustible dust, combustible material, ignition source with certain energy, relatively closed space, and suspended dust).
Its explosion parameters are divided into two categories: one is the sensitivity of explosion, which reflects the difficulty of explosion, such as the minimum ignition energy of dust cloud, the minimum ignition temperature of dust layer, the minimum ignition temperature of dust cloud, the lower limit of dust cloud explosion, etc.
; The other is the intensity of the explosion, which reflects the intensity of the explosion, such as the maximum explosion pressure, the maximum explosion pressure rise rate, and the explosion index.
Because the composition of powder coatings is different, there are many factors that affect the explosive characteristics, so the data obtained by many documents are quite different.

Table 1 lists the explosive characteristic parameters of resins and powder coatings provided by some documents.
Document [7] is a normative document issued by the State Administration of Work Safety.
The explosive characteristic parameters are the basis for supervision by the safety supervision department.

2 Determination of powder spraying area

Determining the equipment and areas where powder coatings exist in the process (ie powder spraying area) is a prerequisite for proposing reasonable and effective dust explosion prevention and control measures.

The electrostatic spraying equipment is composed of a powder supply device, a spraying control system, a spray gun, a powder spraying room, an air source, a powder recovery device, and a curing furnace.
The process flow is shown in Figure 1.

From the equipment composition and process, it can be determined that the powder spraying area should generally include: (1) powder spraying room, powder supply device, recovery device; (2) the opening of the powder spraying room within 3m horizontally and 1m vertically; 3) Places where powder coatings are stored or accumulated at the spraying site; (4) Inside the exhaust duct, inside the air circulation filter and its maintenance structure, and other areas where explosive suspended dust or accumulated dust may be generated.

In particular, the powder coating recovery device, including a fan, a purification device and a powder conveying pipeline connected to it, has the function of realizing the two-phase separation of the gas-solid mixed fluid.
A common recovery device is a recovery system (as shown in Figure 2) formed by "series connection" of a primary cyclone separator and a secondary filter element (or bag) separator.
The recovery device constitutes a relatively closed flow channel of air and powder mixed fluid (dust cloud), which should be monitored as a key part of dust explosion hazard.

3 The common hidden dangers of dust explosion accidents in powder electrostatic spraying enterprises

With the promulgation and implementation of new dust explosion-proof standards and normative documents, some production equipment of the original powder electrostatic spraying enterprises no longer meet the new dust explosion-proof requirements.

Relevant normative documents also clearly list non-conformance items as hidden dangers of dust explosion accidents, and identify 10 “hidden dangers of major production safety accidents”.

3.
1 The powder recovery device uses positive pressure to blow powder

There are usually two schemes for the equipment layout of the powder recovery device (see Figure 3): The first scheme is that the mixed fluid of air and powder first passes through the fan, and then enters the secondary recovery separator to collect the powder and discharge the purified air; the second scheme is The mixed fluid of air and powder first passes through a secondary recovery separator.
After the powder is collected, the purified air is discharged through a fan.

Companies often use the equipment layout as shown in Scheme 1, and the safety is poor.
The mixed fluid of air and powder (dust cloud) enters the recovery separator through the high-speed rotating parts of the fan.
Without reliable measures to prevent ignition sources, the high-speed rotating parts generate heat or metal impurities brought in through the powder spraying room and the fan rotate at high speed Sparks generated by the collision of components will form an ignition source, and dust explosions are prone to occur when they enter the inside of the separator.
In addition, the destructive power of a powder explosion under positive pressure is also greater than that of an explosion under negative pressure.

3.

2 The powder recovery device does not take measures to prevent and control dust explosions, and does not use explosion-proof devices that reduce the risk of explosions

The dry dust removal system in dust explosion hazardous locations should adopt preventive and control measures according to the characteristics of combustible dust explosion and select one or more explosion-proof devices that reduce the explosion risk.
However, the existing powder recovery devices of most powder electrostatic spraying companies do not meet this requirement, and explosion-proof devices must be added.

The current measures to prevent and control dust explosions mainly include explosion venting, inerting, explosion isolation and suppression.
Corresponding to there are 4 different types of explosion-proof devices: explosion venting devices can resist explosions when the explosion pressure has not reached the dust collector and air duct Exhaust the explosive products before the strength, so that the powder recovery device and the air pipe will not be damaged; the inerting device can fill the dust collector with inert gas or powder to make the combustible dust lose the explosiveness; the explosion-proof device can be installed on the air pipe, The flame and explosion wave are blocked within a certain range; the explosion suppression device can be installed on the air duct and/or the dust collector, and the air duct and/or dust collector are filled with physical equipment for extinguishing the flame at the initial stage of the explosion.
Or chemical fire extinguishing media to inhibit the development or spread of the explosion.

3.
3 The powder recovery device is not set or the air lock powder unloading device is not set correctly

The lower part of the powder recovery separator should be equipped with an air-locking powder unloading device (see Figure 4), and it should run continuously, and the powder unloading cycle should be set so that there is no dust accumulation in the separator.
The continuous operation of the air-locking and powder unloading device can not only prevent the lower part of the separator from leaking and cause the separation efficiency to drop, but also avoid the accumulation of excessive powder inside the separator and affect the dust collection effect.

When powder electrostatic spraying companies use the secondary recovery device as shown in Figure 2 to recover powder, the large particle size powder separated by the primary cyclone separator can be directly reused, and the fine powder separated by the secondary bag separator can be discarded.
, Or mix with new powder according to the specified ratio before use.
For the powder recycling of the primary cyclone separator, companies often manually remove the powder collecting bucket at the bottom of the cyclone or use a Venturi powder pump to directly return the powder to the powder supply device, while the powder processing of the secondary bag separator is basically The powder collecting bucket is regularly removed by hand to unload the powder.

In previous inspections, it was often found that some enterprises did not have air-locked powder unloading devices.
After the powder accumulated under the separator, the powder was cleaned manually or returned to the powder supply device by a Venturi powder pump; in addition, some enterprises were equipped with air-locked powder unloading devices.
The device is not set for continuous operation, and the powder is unloaded only by manually turning on the device on a regular basis.

3.
4 The structure design of the powder spraying room is unreasonable, and the exhaust air volume is insufficient

During powder electrostatic spraying operation, the unadsorbed powder coating is deposited on the bottom of the powder spraying room due to the effect of gravity.
When the powder spraying room is designed unreasonably or the exhaust air volume of the fan is insufficient, excessive powder is easy to accumulate at the bottom of the powder spraying room.
If the workpiece is accidentally dropped when passing through the powder spraying room with the conveying mechanism, it will arouse dust, cause collision sparks, and even cause an explosion.

Insufficient exhaust air volume can also easily lead to two phenomena, "powder escape" at the opening of the powder spray booth and "powder accumulation" in the recovery pipe.
Continuous escaping powder will worsen the environment of the workplace and make it not meet the requirements of occupational hygiene limits.
Dust accumulation is likely to form a dust cloud with a concentration higher than the lower explosive limit of the powder coating in the pipeline, and there is a risk of dust explosion.

During the inspection, it was often found that the structure design of the powder spraying room of some enterprises was unreasonable, the exhaust air volume of the fan was insufficient, and the cleaning system was not established for the powder spraying room, and dust accumulated on the bottom of the powder spraying room.

3.
5 The electrical equipment in the powder spraying area does not meet the explosion-proof and dust-proof requirements, and the entire equipment is not completely grounded

The electrical equipment, such as electrical appliances, lighting, wiring, switches, connectors, etc.
within the powder spraying area of ​​some spraying companies, cannot meet the explosion-proof safety requirements.
In some enterprises that use manual powder spraying operations, it is found that workers use non-explosion-proof lines and sockets installed on the outer wall of the powder spraying room to charge the mobile phones in order to facilitate the use of mobile phones during intermittent powder spraying operations.
Most companies do not pay attention to the overall grounding of spraying equipment, and only conduct anti-static grounding for high-voltage electrostatic generators, spray guns and other equipment, but do not pay attention to the anti-static grounding of other equipment such as powder spraying rooms and recycling devices.

4 Improvement measures

4.

1 Adopt negative pressure to convey powder, and reasonably calculate the air volume required by the recovery device

Powder electrostatic spraying companies should adopt the negative pressure method shown in Figure 3 for powder transportation and recovery.

In order to ensure sufficient exhaust air volume, the average concentration of suspended powder (that is, the concentration in the exhaust duct at the outlet of the powder spraying room) should be less than 50% of the lower explosion limit of the powder coating except for local areas such as the spray gun outlet in the powder spraying room.
.

4.
2 Add one or more explosion-proof devices to the recovery device

The powder recovery device should take measures to prevent and control dust explosion according to the explosive characteristics of the powder coating, and select one or more explosion-proof devices that reduce the explosion risk.
Based on the currently commonly used powder coating explosive characteristics parameters, combined with practical cases of powder coating recovery device transformation, enterprises can use a combination of explosion venting devices (such as explosion venting discs, see Figure 5a) and explosion-proof devices (such as explosion-proof valves, see Figure 5b) Modify the powder recovery device to meet the requirements.

4.
3 Improve the separator powder unloading device and set it to continuous powder unloading mode

For the first-stage cyclone separator, a continuous air-locking and powder unloading device and a powder collecting bucket can be installed at the bottom of the separator.

The venturi powder pump is installed on the powder collecting bucket for powder recovery, or a double-dense valve squeezing powder pump (See Figure 6) Continuous automatic powder unloading and powder conveying are carried out to ensure that there is no powder accumulation in the cyclone separator.

4.
4 Formulate a powder cleaning system to regulate and clean up dust on the job site in a timely manner

Powder electrostatic spraying companies should formulate a complete cleaning system to clarify the cleaning time, location, method, and responsibilities of cleaning personnel.

The cleaning of the inner wall and bottom of the powder spraying room should be emphasized, and the observation port of the recovery pipeline should be regularly checked for dust accumulation.

4.
5 Other suggestions

Powder electrostatic spraying companies should ensure that the electrical safety in the powder spraying area meets the requirements of overall explosion protection, and should be based on the classification of combustible dust (powder coatings), the ignition temperature of powder coatings, and the minimum ignition of dust clouds and dust layers formed by powder coatings Temperature selection, and reliable grounding.
Equipment conductors within the powder spraying area, including transmission devices, powder spraying rooms, recycling devices, etc.
, must be reliably grounded to prevent accumulation of charges that can generate electrostatic discharge.
Choosing a spray gun that meets the technical requirements described in the literature and ensuring that the distance between the spray gun and the workpiece is reasonable is also a measure to prevent electrostatic sparks from igniting the source.

Powder electrostatic spraying companies should formulate hot work safety management systems, operating procedures and emergency plans based on their own conditions.
Before improving the equipment for dust explosion protection, it shall be approved by the person in charge of safety of the enterprise and the implementation of the hot work permit shall be obtained.

5 Conclusion

The promulgation and implementation of dust explosion-related standards and normative documents have put forward new requirements and challenges for powder electrostatic spraying companies.
Powder electrostatic spraying enterprises should update, understand and digest the standards and normative documents in a timely manner, and investigate and rectify the hidden dangers of dust explosion accidents as required.