Explosion-proof safety renovation of epoxy powder coating production line

Abstract: This article provides an analysis of the causes of dust explosions and protective measures, and provides an example of production line safety transformation: safety compliance transformation of epoxy powder coating production line

1.

The causes of dust explosions are analyzed as follows

Figure 1 (a) log burning; (b) bonfire with firewood; (c) dust burning paint online at coatingol.

There are five conditions for dust explosion, in addition to the three elements of combustion, oxygen, combustibles, and ignition sources, plus two: confined space and powder distribution

"Oxygen" exists in the air, and people also need oxygen to survive, and it is difficult to be removed during the production process

2.

(1) Kst - the time from ignition to the maximum explosion pressure

(2) Pmax - the maximum explosive force (determined by the dust material)

(3) Mie——minimum detonation energy, when MIE<100mJ (millijoules), dust explosion-proof control must be provided

(4) Mec - the concentration of dust reaching the explosion point

3.
Terms related to dust explosion

(1) SECOND EXPLOSION
.
An explosion in one container that causes a dust explosion in another location through a pipe or equipment is called a secondary explosion
.
The second blast would be massively destructive and more dangerous
.
Explosion-proof devices are usually used to prevent this from happening
.

(2) Reduced pressure (Pred)
.
In order to reduce the destructive force of the explosion, the pressure relief device is usually designed to reduce the explosion force in the container, and the Pred is generally designed to be two-thirds of the ultimate pressure resistance level of the container
.

(3) Explosion isolation
.
Blast isolation is preventing an explosion from being passed from one place to another
.
There are usually several forms of delivery: the pressure released by the explosion, the spark before the explosion, the pressure superimposed by the explosion, and the accompanying explosion
.

4.
The relationship between dust solubility/particle size and dust explosion

The danger of explosion in the dust space is judged as follows
.

(1) The 25W lamp cannot be seen clearly at a distance of 2 meters, and the dust concentration is about 40 grams per cubic meter
.

(2) The color of the surface of the equipment is not clear or the thickness of the ash is greater than 0.
8mm (about the thickness of a paper clip)
.

(3) Ultrafine powder is more dangerous and easy to ignite first
.
The same material with different particle size will show different risk levels (such as white sugar and soft white sugar)
.

5.
Specifications for safety and explosion-proofing of powder product production lines

The National Fire Protection Association (NFPA) has developed a series of standards to regulate the basic requirements for combustible dust production, processes, and any disposal equipment
.

NFPA 652 is the latest version of the National Fire Protection Association's standard for combustible powders
.
The following is an example of the highlighted parts of the new standard to help us identify problems and correct them
.

Key point 1, put forward the requirements of powder hazard analysis (DHA, dust hazard analysis)
.
The new standard introduces the requirements and methods of DHA (dusthazardanalysis) powder hazard analysis
.

The hazard analysis of powder production process and equipment must be recorded and archived
.
The dust hazard analysis consists of the following three parts
.

(1) Confirm the danger area where the hazard exists
.

(2) Confirm existing safety devices
.

(3) Confirm the safety guards that need to be added
.

DHA (Powder Hazard Analysis) documentation must be traceable
.
Modifications to existing processes, equipment, equipment owners and operators must complete a powder hazard analysis (DHA) document
.
If the process and equipment have not changed, the powder hazard analysis (DHA) document review and revision must also be carried out every 3 years
.

Focus 2, OSHA is introduced into NFPA standard OSHA (Occupational Safety and Health Administration) occupational safety and health is required to be included in the control documents of combustible dust
.

NFPA 654 (National Fire Protection Association Standard 654) applies to any production, process, and disposal (storage and transport) of combustible dust
.
Explosion-proof devices must be designed in accordance with NFPA 69 and NFPA 68
.

Point 3, isolators are required to be introduced into NFPA69 as described in NFPA 69, when an explosion occurs, the isolator provides a rapid barrier to the connection between the explosion-producing equipment and other equipment
.
NFPA69 provides the following methods (but not limited to the following examples)
.

(1) Chemical explosion protection
.

(2) Fast-acting mechanical block valve
.

(3) External control block valve
.

(4) Passive pinch valve
.

The function and application of the isolation device of the above example will be explained in the example of explosion-proof safety retrofit of epoxy powder coating production line
.

Since the dust particle size and other conditions (such as heat generation in the production process) are different in each area of ​​the production line, the probability of an explosion accident area in the dust product production line is also different
.

6.
Example of explosion-proof safety renovation of dust production line (renovation of epoxy anti-corrosion powder coating production line)

(1) Determine the combustible dust in the production process of epoxy anti-corrosion powder coatings (combustibles in the five elements of dust explosion).
The main raw materials of anti-corrosion powder coatings are epoxy resin, fillers and several additives.
Epoxy resin accounts for 63% and filler accounts for 30%.
%, other various additives 7%
.

Epoxy resin is combustible, but the raw material provided is granular and not powdery, and there is an explosion hazard (the hazard level is not high), but control measures are required
.

The filler is wollastonite, which is not combustible and has no danger of explosion
.
All kinds of additives are combustible and have the risk of explosion.
The amount is not large, but control measures are also required
.

The final product (epoxy anti-corrosion coating powder) is combustible and in powder form, with explosion risk and high hazard level.
Due to the large proportion of workshops, it is the main risk source of explosive combustibles, and it is necessary to focus on strengthening control
.

The explosion parameters of the powder can be known by consulting the relevant technical data or by conducting the powder sample test.
There are professional institutions abroad to provide this service
.

We know the explosion parameters MIE<100mJ millijoules by providing the produced samples to professional institutions for testing; Pmax (maximum explosion pressure) = 9.
7 kg/square centimeter
.

According to the American NFPA Fire Protection Association standard, the explosion protection level of the production line is set as Class 2, Division 2, Group G (Class II, Division 2, Group G,) dust production line All contain epoxy powder products Every link in the space loop, such as equipment, devices, and pipelines, must be able to withstand an explosion pressure of 11 kg/square centimeter (PSR11 for short), and we complete the production line explosion-proof safety upgrade and transformation based on this maximum explosion pressure Pmax value.
of
.

(2) Delineate explosion-proof areas of production lines (workshops)
.

Simple production process of anti-corrosion powder coating: ① Automatically add raw materials according to the formula ratio → ② Pre-mixer raw materials are mixed and pre-mixed → ③ Extruder raw materials are mixed and extruded → ④ Extruded into flakes and cooled → ⑤ Crushed by chip pulverizer → ⑥ Raw materials Air separation → ⑦ dust particle size screening → ⑧ finished product packaging passed DHA (dust hazard analysis);

Procedure ①Weighing and batching; ②Premixer; ⑤Crusher; ⑥Air classifier for raw material air separation; ⑦Product particle size screening machine and auxiliary production device: intermediate dust transmission pipeline and ultrafine powder process integrator are defined as dust Explosion hazard area
.
Explosion-proof safety devices must be installed in explosion-proof areas to carry out comprehensive upgrading and transformation of production line safety production
.

(3) Install safety protection devices in explosion-proof areas
.

Figure 2 is the flow chart of the upgraded process equipment
.

#1 The feeding screw of the batching station is changed to a sealed structure
.
The purpose is to prevent dust from leaking out and eliminate the danger of dust explosion in the workshop
.

#2 Retrofit the premixer with an explosion-proof electrical rating to meet Class II, Division 2, Group G (Class II, Division 2, Group up G) electrical explosion-proof standards, with thicker outer walls to improve energy performance Under the pressure of 11 kg/cm 2 dust explosion (the maximum explosion force of epoxy powder products is 9.
7 kg/cm 2 ), a temperature sensor is added at the bearing of the rotating shaft to monitor the internal temperature of the cavity
.
Immediately shut down once the temperature exceeds the standard setting
.

#3 Epoxy anti-corrosion coating finished pipeline system is replaced with PN16/20 standard seamless pipe, the pressure resistance exceeds 11 kg/cm 2 to meet the requirements of explosion-proof
.

#4 On-line air classifier and cyclone retrofit, with materials thickened to withstand the pressures produced by dust explosions exceeding 11 kg/cm²
.

The #5 rotary valve was upgraded to Westinghouse's explosion-proof rotary valve
.

#6 Install an isolation valve between the dust collector and the in-line airflow classifier
.
When a dust explosion occurs in the dust collector, the valve can block the connection between the dust collector and the on-line airflow classifier to prevent the second explosion of the airflow classifier
.
The dust collector stores all ultrafine powder, and this area has the highest level of dust explosion hazard
.
According to the statistics of dust explosion accident probability, 52% of dust explosion accidents occur in dust collectors
.

#7 Install a BS&B explosion suppression device in the dust collector, which can spray dry powder to extinguish the dust flames at the moment of the explosion, so as to kill the dust explosion in the bud
.

#8 Install an isolation valve between the dust collector and the process fan, the purpose is the same, to prevent the possible explosion of the dust collector from affecting the process fan and the equipment behind it
.

#9 Install an isolation valve between the pulverizer and the dry blower
.
When the dust of the pulverizer explodes, the explosion pressure will push the valve core to cut off the connection between the pulverizer and the drying fan, so as to prevent the dust explosion of the pulverizer from affecting the drying fan and subsequent equipment
.
The pulverizer is explosion-proof design and can withstand dust explosion pressure of more than 11 kg/cm²
.

In addition to the 9 safety and explosion-proof devices marked in the process flow chart, we also control the static electricity that may be generated in the production line and workshop.
Static electricity is a terrible ignition source for dust explosions
.

All equipment units on the production line should be jumpered and grounded together with wires to eliminate static electricity from all equipment, devices and pipes between equipment
.

There will be dust accumulation on the workshop floor, and it must also conduct electricity to eliminate static electricity
.
At first, we used the traditional conductive epoxy floor, just laid it and passed the conductivity test, there is no problem
.

But after a few months, it was found that the produced product (epoxy coating powder) stuck to the surface of the conductive epoxy floor, affecting its original conductivity
.
Later, the problem was solved by laying steel plates on the floor
.
Its cost is lower than that of conductive epoxy floor
.

(4) Maintenance of safety and explosion-proof devices for epoxy anti-corrosion coating production lines
.

The maintenance of the safety protection device after the safety transformation of the epoxy anti-corrosion coating production line is also very important
.
The key points are listed below
.

First, the explosion suppression device installed on the dust collector (there is also a pressure relief device installed on the production line) should be regularly tested to see if its function is effective
.
(It is recommended to start every 3 months, and if you are confident later, you can extend it to half a year or 1 year)
.

Second, all isolation valves should be regularly tested to see if their functions are effective.
Isolation valves are either mechanical or gas-electrically controlled
.
It is recommended to use a mechanical type.
When the conditions do not allow the use of a mechanical isolation valve, the gas-electric control valve should be considered.
The maintenance of the mechanical isolation valve is relatively simple
.
(It is also recommended to start every 3 months, and if you are confident later, you can extend it to half a year or 1 year)
.

Third, the dust product conveying pipeline should be regularly tested for the thickness of the pipe wall.
The detection point is generally selected at the elbow, and a permanent mark is made, and the same place is measured each time to observe its changes
.
Once the thickness of the pipe wall is found to be close to the limit, consider replacing it with a new seamless steel pipe
.
(It is recommended to measure once a year, and record and save the thickness of the pipe wall)
.

Fourth, static electricity is the ignition source of dust explosions, and it must be absolutely eliminated
.
Maintenance technicians must regularly check that the resistance between the equipment and the ground is less than 4 ohms, and the resistance between the workshop floor and the ground is less than 10 ohms.
It is recommended to check once a year
.

Fifth, the spare parts related to the explosion-proof safety device of the production line must be of the brand and model specified in the design of the production line, and the quality assurance certificate must be kept
.
If the spare parts brand is changed for various reasons, the model must go through the DHA (Dust Hazard Analysis) management procedure
.

7.
Conclusion

For any powder production, storage and other operations with ignition energy MIE<100mJ millijoules, there must be a DHA (dust hazard analysis) process and safety control measures
.

According to the statistics of dust explosion accidents, the integration room is the most dangerous area.
The ultra-fine powder in the integrator is very easy to be ignited and exploded.
This area must be equipped with a pressure relief device or a chemical explosion suppression device, and an isolation valve should be installed before and after.
, the connection with the front and rear equipment is cut off
.

The pulverizer is the second dangerous area, and an explosion-proof pulverizer must be selected, and an isolation valve must be installed so that there is a barrier between the pulverizer and the equipment (when the pulverizer explodes)
.
In addition to upgrading the hardware of the powder production line, a complete set of safety production rules and regulations must be established
.
Note the following points
.

(1) The workshop should be cleaned frequently, and the deposited dust should not exceed 0.
8mm
.

(2) Production line equipment and workshops should have anti-static measures
.
Static electricity is a terrible source of ignition for dust explosions
.

(3) Be sure to establish a complete set of production lines and a PM (preventive maintenance) plan for workshop safety protection devices
.

(4) The powder raw material/finished product packaging bag must have a ground wire and be grounded
.

(5) Smoking and related hot work are prohibited in the workshop.
If hot work is necessary, production must be stopped and the powder raw materials and finished products must be removed
.

(6) Dust production safety training is very important for all personnel engaged in powder product production (managers, engineers, maintenance technicians, production line operators), so that all employees engaged in powder production can master the knowledge of dust explosion and prevention, and prevent operation any mistakes
.

In a word, everyone is responsible for safe production, especially the production line with dust explosion hazards.
Starting from me, we can truly achieve zero accident safe production
.