Explosion protection of dust collection systems
The purpose of this article is to familiarize you with the strategy and regulations for explosion protection of dust collection systems.
In recent years, special attention has been focused on reducing the risks of explosive dust/gas explosions and preventing accidents at industrial enterprises. In this regard, the following documents establishing requirements for explosion protection systems have entered into force:
- Directive 1999/92/EC Minimum requirements for the safety and health of workers potentially exposed to risk in explosive atmospheres;
- Directive 2014/34/EU “On the approval of the Technical Regulation of equipment and protective systems intended for work in a potentially explosive environment.”
Does your company effectively manage the risks associated with working with a potentially explosive product, are you familiar with the requirements of current legislation?
Explosive zone
Releasing the explosion pressure
Where to start?
The employer must comply with all requirements specified in Directive 1999/92/EC. These are the minimum requirements for occupational safety and health protection of personnel working at enterprises with a potentially explosive product. The employer must comply with the requirements of this Directive and apply the specified safety measures. The business must have "Explosion Protection Documentation", which will describe the strategy and explosion protection measures are necessary.
Problems of the danger of dust explosion in dust collection systems
The dust collection system is designed to remove dust from technological processes, separate solid particles from the air using a filter and discharge dust from the filter into a storage bin or transportation system.
If the production processes a potentially explosive product, then in the dust collector we get the highest concentration of explosive dust and a high risk of explosion.
The principle of operation of the dust collector is that dust is separated from the air with the help of filter elements that accumulate a large amount of dust on their surface, in addition, there is always a cloud of explosive dust inside the filter due to the constant pulling of dust into the filter. When cleaning the filter elements, a pulse of compressed air is blowing to the filter element, which discharges dust from the filter elements into the dust hopper, from which the dust is discharged by a rotary valve. At the time of cleaning the filter elements, the highest concentration of explosive dust is formed inside the filter unit in a suspended state.
Dust collector operation
Bag cleaning
Why does a dust explosion occur?
As in other technological processes, an explosion in a dust collector occurs with the simultaneous presence of the following factors:
- Oxygen;
- Confinement (enclosed equipment space);
- Fuel (combustible dust);
- Ignition source;
- Fuel mixing (dust-air mixture in concentration LEL <…< UEL)
Oxygen + fuel + ignition source = combustion
Oxygen + fuel + ignition source + fuel mixing = flash
Oxygen + fuel + ignition source + fuel mixing + confinement = explosion
LEL – is the minimum dust concentration (g/m3) at which an explosion can occur.
UEL – is the maximum dust concentration (g/m3) at which an explosion cannot occur.
< LEL
> UEL
LEL ≤ ... ≤ UEL
Ignition sources
- Hot surfaces;
- Flames and hot gases (including hot particles);
- Sparks of mechanical origin;
- Electrical equipment;
- Stray currents;
- Static electricity;
- Lightning;
- Electromagnetic waves from 10^4 Hz to 3*10^12 Hz;
- Electromagnetic waves from 3*10^12 Hz to 3*10^15 Hz;
- Ionizing radiation;
- Ultrasound;
- Adiabatic compression and shock waves;
- Exothermic reactions, including self-ignition of dust.
Based on official statistics, dust collection systems are the most dangerous equipment in the enterprise in terms of the probability of explosion, so when designing a dust collection system to work with a potentially explosive product, do not forget about the need for explosion protection of dust collectors and explosion isolation systems.
Statistics of explosions
Processes
Industry
An explosion in the dust collection system leads to catastrophic consequences
Methodology for determining requirements for explosion protection of dust collection systems
* See Table 1
◊ See Table 2
Table 1
|
EX area in the place of installation of the dust collector |
Protection of external components of the dust collector |
|---|---|
|
Safe |
Not required |
|
Zone 22 |
Ex CAT 3D |
|
Zone 21 |
Ex CAT 2D |
|
Zone 2 |
Ex CAT 3G |
|
Zone 1 |
Ex CAT 2G |
Note: additional requirements for the protection of the external components of the filter, to be determined according to the results of tests of dust explosion parameters.
The explosion hazard of the equipment installation zone must be determined according to Hazardous area classification (EN 60079-10-2).
Table 2
| Type | Installation in Ex Zone | Protect according to EN requirements | Can safely handle a potentially explosive product | Protect according to |
|---|---|---|---|---|
|
Type-1 |
No |
Not required |
No |
Not required |
|
Type-2 |
No |
Not required |
Yes |
|
|
Type-3 |
Yes |
Ex CAT 3GD / 2GD |
No |
Not required |
|
Type-4 |
Yes |
Ex CAT 3GD / 2GD |
Yes |
|
Short description of systems:
Type 1 – the system is not intended for work with potentially explosive dust and is not intended for installation in an explosive zone;
Type 2 – the system is designed to work with potentially explosive dust, but cannot be installed in an explosive zone;
Type 3 – the system is not intended for work with potentially explosive dust and can be installed in an explosive zone;
Type 4 – the system is designed to work with potentially explosive dust and can be installed in an explosive zone.
- Antistatic or grounded filter elements;
- Grounding of air ducts;
- Filter grounding;
- Inspection hatches, service doors must withstand the residual pressure of the Pred explosion not lower than the resistance of the filter housing;
- If the filter elements do not withstand the residual pressure of the Pred, the full volume of the filter is taken into account when calculating the blast release area, not just the dirty side. Also, when returning air to the room, it is necessary to install an explosion isolation system or a flame filter on the duct for returning air to the room.
Options for explosion protection of dust collection systems
| № | Description of components | Type |
|---|---|---|
|
Explosion protection of the dust collector |
||
|
1 |
VMP venting panel
Explosion venting |
Mechanical
Passive |
|
2 |
Flex Flameless venting device
Explosion flameless venting |
Mechanical
Passive |
|
3 |
HRD system
Explosion suppression |
Chemical
Active |
|
Explosion isolation |
||
|
4 |
B-Flap Explosion Isolation flap valve
Explosion isolation |
Mechanical
Passive |
|
5 |
HRD barrier
Chemical explosion isolation |
Chemical
Active |
|
6 |
Flame Proof rotary valve
Mechanical explosion isolation |
Mechanical
Passive |
According to the mechanism of action, explosion protection systems are divided into 2 main types:
Passive – a protective system that is activated due to the pressure caused by an explosion, or is designed in such a way that the flame does not pass through it. For example, the rupture of the venting panel due to a pressure jump, the closing of the check valve flap with the help of a pressure wave. Or structural, such as gaps between the rotor and valve stators that an explosion cannot pass through, or a flame arrester mesh that does not allow flames to pass through.
Active – is a protective system that, with the help of a sensor, monitors the beginning of an explosion in the equipment (pressure surge or flash), sends a signal to the controller, which analyzes the sensor indicators, and in the event of an explosion, sends a signal to activate the explosion protection (closing dampers, cylinder activation).
Also, explosion protection systems are divided according to the principle of action:
Mechanical – protects the equipment in a mechanical way: membrane rupture, shutter closing, isolation due to MESG (Minimum Experimental Safe Gap).
Chemical – protects equipment by spraying a reagent.
Description of explosion protection methods in dust collection systems
Any of the described explosion protection methods consists in reducing the pressure inside the technological equipment to a pressure below the equipment’s resistance (Pred), so that in the event of an explosion, the equipment remains undamaged and the explosion occurs predictably and safely. Of course, an explosion can be completely safe only in combination with explosion isolation systems, the correct selection of the protection method and the size of the protective device.
Any of the following explosion protection methods can be combined with each other in any way convenient for you.
1. Venting panel VMP - explosion pressure relief
Venting panel is an explosion pressure relief valve that is installed on the dust collector, usually below the filter elements. The venting panel is the weak point of the filter housing and in the event of an explosion it ruptures and releases excess pressure from the filter, thus the filter housing remains undamaged.
The rupture membrane is usually installed on the dust collector, which is installed outside, where it is possible to release the explosion into a safe area. When designing, do not forget to take into account the length (Lf) and width (Wf) of the flame opposite the venting panel.
Note that the recommended place for installing the venting panel is to install it under the filter elements. Installing a vent panel opposite the filter elements will prevent the blast pressure from freely venting, resulting in ineffective pressure venting and damage to the filter housing.
Schedule of pressure increase in the equipment in the presence and absence of an explosion relief venting panel
Explosion venting – the pressure curve inside the equipment, in the presence of a venting panel for the explosion pressure relief;
Without explosion protection – pressure curve inside the equipment, without explosion protection;
Pstat – venting panel rupture pressure (usually equal to 0.1 bar);
Pred – residual pressure in the equipment, after explosion venting;
Pes – resistance of equipment to pressure;
Pmax – maximum explosion pressure (depends on the type of potentially explosive product).
Examples of explosion protection of dust collection systems by venting panel
2. Flex flameless venting device - flameless release of explosion pressure
Flex flameless venting device is equipped with a flame filter that filters the flame and resets the explosion from the dust filter is completely safe for equipment and personnel, the explosion can be release even indoors.
Since the component of the Flex is the venting panel, which is installed on the body of the dust collector, the principle of explosion protection of the equipment with the flameless venting device is completely similar to the protection by the venting panel, as described above.
Example of Flex flameless venting operation
Examples of explosion protection of dust collection systems with Flex
3. HRD system - explosion suppression
Principle of operation explosion suppression system: the controller constantly evaluates the readings from the pressure/optical sensor, in case of detection pressure jump or flash sensor, the controller registers this and sends a signal to activate the HRD container, which sprays a powder inside the equipment through a nozzle that suppresses the explosion.
The HRD system is characterized by extremely fast introduction of fire extinguishing agent inside the equipment. Thanks to high-speed sensors, the HRD system suppresses an explosion in 60 milliseconds.
When the dust concentration is in the LEL…UEL range, an explosion occurs in the presence of an ignition source.
The principle of operation of the HRD explosion suppression system is to dilute the explosive concentration of dust by rapidly spraying the powder inside the process equipment, due to which the explosion is stopped at its early stage, when the explosion pressure is still in safe values for the equipment.
Visualization of explosion suppression by the HRD system
Time: 0 ms
Pressure: 0 bar
5 - 35 ms
0,03 - 0,1 bar
40 ms
0,08 - 0,15 bar
60 ms
0,15 - 0,2 bar
Components of the HRD system
CONEX
The controller evaluates and records information from the sensors, sends a signal to activate the containers. Works as a user interface for staff.
DETEX / LUMEX
Explosion detection sensor in 5-35 ms. This information is sent to the controller in 5 ms.
HRD container
The HRD container is equipped with a quick-opening valve that sprays the powder into the protected equipment in 5 ms.
Telescopic nozzle
The nozzle is necessary to evenly spray the powder inside the equipment to suppress the explosion.
Examples of explosion protection of the dust collection system using HRD explosion suppression system
Description of explosion isolation methods in the dust collection system
In the event of an explosion in the dust collector, the explosion pressure and flame will not only be released through the venting panel, but will also spread through all possible openings in the filter, such as the inlet pipe and the dust discharge hole. To protect the equipment connected to the dust collector, it is necessary to use explosion isolation, which localizes the explosion in the dust filter.
4. B-Flap check valve - explosion isolation
B-Flap Explosion Isolation Valve is a mechanical device that is activated due to the explosion pressure wave . The pressure wave, which travels faster than the flame, closes the damper inside the valve, thus preventing the pressure and flame from spreading to the process equipment.
Installation options
B-Flap Explosion Isolation Check Valve Installation Examples
5. HRD barrier
The principle of operation of the HRD explosion isolation system: the controller constantly evaluates the readings of the optical sensor installed on the pipeline at inputs to the filter, in the case of detection of a flash by the sensor, the controller registers it and sends a signal to activate the HRD container, which sprays powder inside the pipeline through the nozzle, which isolates the spread of the flame through the pipeline.
The components of the HRD barrier are completely analogous to the HRD system:
- ComEx controller;
- LumEx sensor;
- HRD container;
- Telescopic nozzle.
Examples of HRD barrier installation
6. Flame Proof rotary valve
Rotor valve certified according to EN 15089 works as an explosion isolation device in the gravity discharge of dust from the filter, which prevents the explosion from filter to bin or conveyors into which collected dust is unloaded. The rotary valve isolates the explosion due to the fact that the body is made of cast steel with high precision grooving of the body in which the rotor rotates, and the rotor is made of sheet steel. Due to this, a minimum gap between the housing and the rotor (MESG) is ensured, which in the event of an explosion does not allow pressure and flame to pass through it.
Examples of rotary valve installation
for your technological process
