Explosion protection for aspiration systems

Explosion hazard in dust extraction systems

Ventilation is a mandatory part of the production process in industries where combustible dust is present: grain dust, coal dust, metal dust, wood dust. Dust extraction is needed to improve fire and explosion protection of facilities.

Grain dust, on the other hand, is a class 3 dust in terms of toxicity and explosion hazards. Its Low Explosion Limit (LEL) depends on particle size and humidity and varies from 40 to 20 g/m3. Particularly dangerous are fine dusts for which the LEL can be even lower.

In the storage and processing facilities of the former Soviet republics cyclones developed more than 50 years ago are often used for ventilation. They practically do not capture dust with dispersion less than 10 μm, which is the most explosive.

More modern bag filters show a higher efficiency, but may not provide the desired residual dust content either. The reason can be design errors, incorrect installation or the fact that the actual operating conditions were not taken into account.

The filters and ducts themselves can become contaminated with combustible dust if they are used incorrectly or if they are installed incorrectly. In contact with the air and the ignition source it causes a primary explosion. The shockwave and flame rapidly travel through the ducts to the unburned dust/air mixture, causing a series of secondary explosions. Accidents in an aspiration system can damage adjacent equipment and rooms where it is installed.

Sources of ignition initiation

EN 1127-1-2014 classifies 13 active ignition sources which, when in contact with a combustible dust/air mixture, may lead to an accident.

What can initiate an explosion in aspiration systems:

• Mechanical sparks from friction;
• Sparks from an electric motor;
• Static electricity;
• Electromagnetic waves;
• Sparks from faulty electrical wiring;
• Fire particles from the external environment entering the air ducts;
• Self-ignition of dust accumulations.

An aspiration system that does not comply with the operating conditions of the site only adds to the explosion hazard problems of the plant. It becomes an additional safety factor.

But a fully functioning, efficient aspiration system also does not prevent an explosion, because combustible dust is always present. Therefore, when planning a ventilation system, Ex-examination should already be considered at 10 % of the NEQ. In the case of grain dust, for example, explosion protection systems are required for dust concentrations from 2 g/m3.

Safety standards for the owner of a hazardous industrial facility

Due to the presence of combustible dusts, storage and processing facilities for raw plant material are classified as hazardous areas. The minimum requirements for their explosion protection are specified in Directive 99/92/EC.

the necessary actions to be taken when operating a hazardous facility:

• Assess the risks taking into account the likelihood of explosive atmospheres;
• Classify and mark hazardous areas;
• Install explosion protection devices on the equipment.

The general requirements of Directive 99/92/EC are specified in national legislation.

ATEX standards provide for explosion-protected versions of aspiration systems:

1. The filter housing is reinforced for explosion pressure.
2. The overpressure is relieved by means of explosion-proof deflectors and flame arresters.
3. Check valves are installed in the intake air duct to cut off the explosion and prevent the propagation of explosion products back to the aspirated points.

The explosion protection systems are selected on the basis of the operating conditions and the characteristics of the materials to be aspirated.

Technical solutions

We first carry out a risk analysis to determine the hazardous area class and the equipment's protection level in accordance with IEC 60079-10-2-2011.

We carry out investigations, including by means of an accredited laboratory. Based on the results, our experts determine the explosion protection parameters of the plant:

• Low Explosion Level / LEL
• Maximum rate of explosion pressure rise / (dp/dt)max
• explosion class / Kst
• Minimum explosion temperature / MIT
• Minimum ignition temperature / GT
• Minimum explosion energy / MIE
• Oxygen concentration limit / LOC

This data makes it possible to select specific explosion protection equipment to suit the operating conditions.

Together with RSBP, we offer four standardised technical solutions for the protection of aspiration systems: explosion suppression and cut-off; release of excess explosion pressure (explosion dischargers); flameless explosion release; explosion cut-off. We adapt them individually to the customer's site.

The HRD system quickly detects a primary explosion in a cyclone, bag filter or ducts and extinguishes it at the nucleation stage. Explosion suppressants are used for this purpose. The HRD explosion suppression system reduces the explosion pressure to safe values, at which point the vessel is not destroyed. The HRD barrier effectively cuts off an explosion in ducts. It works in the same way as the HRD system, but sprays an extinguishing agent in a flat plume. This covers a large surface area of the equipment to be protected.

• sensors detect an increase in pressure in the equipment to be protected within 1 ms;
• a signal via the control centre to the activators provides an almost instantaneous command to activate HRD cylinders with extinguishing agent;
• the sensitivity of the system allows for suppressing explosions below 0.2 bar, i.e. at the nucleation stage;
• spray nozzles on the HRD barrier cylinders fan the substance out, thus cutting off the explosion across the entire duct diameter;
• the system is assembled in only two steps using prefabricated drawings - installation of the elements and adjustment of the electrical connections;
• no modifications to the equipment design are required;
• system maintenance consists of visual cleanliness checks and pressure checks on the cylinders only;
• has a long service interval of 6 months to 10 years.

Used HRD cylinders are easy to replace. The inerted explosion suppression system, unlike standard explosion detectors, is safe for use where people are working. It also meets the strict hygiene requirements of the food and pharmaceutical industries. For example, water can be used as an extinguishing medium.

HRD complex can be used as a standalone solution in aspiration plants or in combination with VMP defoggers, FLEX flame arresters.

Explosion dischargers for explosion pressure relief

We offer VMP bursting discs certified to TP TC 012/2011 and EN 14797.

Burst dischargers are installed on cyclone casings, bag filters and dust collection hoppers. Special openings are made in the walls of the equipment to release the shock wave and flame during an explosion. Under normal operating conditions they are sealed by bursting discs of a certain size and shape. In the event of an accident they open and release the products of the explosion to the outside. As a result, the pressure in the tank drops to a safe level and the structure is not destroyed.

• quickly reduces the pressure to safe values;
• Low-cost installation and operation;
• Applicable to equipment already in operation;
• Up to 30 m clearance area, that is why explosion relief via VMP panels can only be used in areas free of people;
• Explosion relief devices are installed outdoors;
• The static opening pressure can be regulated;
• Panels are available in various shapes and sizes;
• The system is effective even when the explosion hazard class of the site is high.

VMP membranes are simple to install and easy to maintain during operation. VMP membranes are simple to install and maintain. After a failure, they are easy to replace. The membranes are also resistant to mechanical damage and adverse weather conditions.

fireless explosion release

This method is used for additional protection for aspiration systems in enclosed spaces. FLEX flame arresters certified in accordance with TP TC 012/2011 and EN 16009 are used in flameproof protection. They are used in conjunction with flame arresters and explosion suppression devices.

Flame arresters consist of an explosion panel and an outer casing with a scattering surface. They prevent the spread of open flames and rapidly reduce the pressure and temperature of the explosion.

The shockwave and combustion products pass through a system of special grids. The dissipation effect reduces the temperature of the explosion from 2600 °C to safe levels and prevents open flames from reaching people in the workplace. At the same time the overpressure is reduced to values to which the elevator design is resistant. Importantly, the process is automatic - no operator is needed to operate the flame arrestor correctly.

Special features:

• can be installed indoors;
• Hazardous area of smoke is no more than 1.5 m, which allows for safe areas near the air intakes for people to work;
• Flame arresters are certified for explosive dusts with very low minimum ignition energies (MIE) and minimum ignition temperatures (MIT).

The explosion extinguishing system is easy to install and maintain in operation. It does not require expensive construction work or modifications to existing equipment. Most importantly, the system reliably protects cyclones and filters from the damaging effects of an explosion and localises the consequences of an accident.

Check valves for explosion cut-off

We offer B-FLAP I devices that are certified to EN 16447, EN 15089, TP 012/2011 and TP 010/2011. These are passive explosion-proof mechanical dampers that are triggered by a shock wave, closing the air duct and preventing an explosion from spreading to the aspirated equipment.

Under normal process conditions, the b flap check valve is open and locked by the RPD mechanism. During an explosion, due to an air shock wave, the flap closes and locks in the closed position. This is how the mechanical check valve cuts off the explosion by localising it in the ductwork.

Special features:

• Suitable for ducts from DN 100 to DN 800;
• Independent of power supply;
• Maintenance work consists only of cleaning the valve;
• Can be operated at an ambient temperature of -40 °C to 80 °C;
• The performance is independent of the concentration of combustible dust in the pipework, provided that the maintenance intervals are observed.

The b-flap check valve is practically universal. It is suitable for both vertical and horizontal ducting, for overpressure and underpressure systems. The valve can be installed in both the inlet and outlet of suction systems.

Work stages

• Step 1. Evaluate the explosion hazard of the process and equipment.
• Step 2. Discuss the technical specifications with the customer.
• Step 3. Selection of an explosion protection solution in accordance with the applicable industrial safety legislation, the customer's wishes and the manufacturer's recommendations.
• Step 4. Preparation of the project and its approval by the supervisory authorities.
• Step 5. We deliver protection systems anywhere in the country.
• Step 6. We assemble equipment or provide quality control of third party installation works.
• Step 7. We carry out commissioning of explosion protection systems.
• Step 8. We provide service and warranty support.

ATEX.CENTER takes care of the effective explosion protection in your plant.

Guarantees

ATEX.CENTER is an official distributor of the leading European distributor of explosion protection RSBP spol. s r.o. Our specialists periodically confirm their qualifications in his commission of expertise.

In cooperation with the RSBP, we have implemented more than 20 large-scale explosion protection projects. Thanks to intelligent solutions we were able to prevent more than 1 260 000 EUR of damage and save the lives of over 600 employees in 5 companies.

We give all our customers the opportunity to visit an RSBP test facility in the Czech Republic to see explosion protection in action.

The RSBP explosion protection symbol on your equipment is a guarantee of safety for your company