Prevention of explosion hazard

Prevention of fire and explosion hazard

Technical measures for explosion protection 

Oil Platform Explosion

What to do with a Hazardous Explosive Atmosphere?

If circumstances can lead to hazardous explosive atmospheres, measures must be taken to protect against the risk of explosion. Priority here is to prevent the formation of explosive atmospheres by, for example, replacing the flammable substance and preventing leakage.

If, for practical reasons, this is not possible or cannot be done to a sufficient extent, the area is zoned and ignition of the explosive atmosphere must be prevented.

In some cases, the probability of an explosion cannot be avoided entirely and the consequences of an explosion must be reduced as much as possible in order to guarantee the health and safety of workers.

Prevention of hazardous explosive atmospheres 

According to Article 3 "Explosion prevention and protection" of Directive 1999/92/EC, priority must be given to preventing the formation of hazardous explosive atmospheres.

Replacing the flammable substances

A hazardous explosive atmosphere can be prevented by replacing flammables. For example:

• Replacing flammable solvents and cleaning products with water-based solutions;
• Increasing the particle size of substances used;
• Moistening dusts or using pastes to prevent dusting.

Limiting the concentration

By keeping the concentration of gases and dusts outside the explosion limit range, no explosive atmosphere is formed. In closed containers and installations this is often relatively easy to achieve. Keeping the surface temperature of the liquid well below the flash point ensures the lower explosion limit is not exceeded.

In a dust environment it is virtually impossible to prevent explosive mixtures by limiting the concentration, due to whirling.

Avoid a hazardous explosive with oxygen

A hazardous explosive atmosphere can also be avoided by reducing the level of oxygen in an installation. For this, the highest oxygen concentration (oxygen concentration limit)at which no explosion can take place must be known. Inertization is usually done with nitrogen, carbon dioxide, noble gases, combustion gases or water vapor.

Incorrect usage or equipment malfunctions must be taken into account when inerting. Workers are at risk of suffocation of poisoning when inert gas leaks occur.

Prevention of leakage

Using closed system installations can prevent the formation of hazardous explosive atmospheres on the outside. Installations must be designed in such a way that under foreseeable operating conditions no significant leakage will occur. One way to ensure this is, among others, through regular maintenance.

Ventilation to prevent explosion hazards 

If leakage of flammable substances cannot be prevented, the formation of hazardous explosive atmospheres can often be prevented by good ventilation. Ventilation can also reduce the risk of hazardous explosive atmospheres or reduce the size of hazardous areas (zones). The maximum amount of possibly releasedgases and vapors and the location of the source must be taken into account.

The position of ventilation openings for gases heavier than air

Removal of dust to prevent explosive hazards 

In case of dust, generally only source extraction offers sufficient protection. Dangerous accumulation of dust must be prevented anyway by regular cleaning. This should certainly include poorly visible or difficult to access surfaces on which a considerable amount of dust can settle over time. If larger amounts of dust are released as a result of a malfunction such as a leak, additional measures should be taken to remove dust deposits immediately if possible.

Wet cleaning and the use of vacuum cleaners are preferred for removing dust accumulation. Methods that cause thedust to whirl or blowing away precipitated dust should be avoided.

Removing dust

Cleaning measures can be regulated within the framework of the company’s operating instructions for the use of flammable solids.

Prevention of ignition sources

If an explosive atmosphere cannot be prevented, a hazardous area classification must be carried out. By classifying a hazardous area into hazardous zones, the safety measures for each zone can be tailored to the risks involved. Zoning also determines which standards any equipment used must meet. Hazard zone classification is based on the properties of the liquids, gases and substances present in the room. 

First of all, it should be determined whether the quantity of flammable substances present and the release of these substances call for zoning.

Zoning the area depends on a number of factors:

• Location and number of danger sources;
• Hazard class of the sources (continuous, primary or secondary);
• Flow rate of the sources;
• Ventilation around the sources (natural, artificial);
• Explosion limits (LEL, UEL);
• Molecular weight relative to air;
• Temperature of the gas or vapor in relation to the flash point;
• Degree of containment by a vessel, tank or pipe. 

Coal dust explosions and fibres 

Zoning of areas where flammable dust and/or fibers are present is partly different from that of areas containing gases and vapors.

The reasons are:

• Gases and vapors consist of molecules that can diffuse and circulate freely. However, dust and fiber particlesare much larger and have far more weight, so normally they are present as a layer of dust. An external force (gust of wind or pouring) is necessary to (temporarily) disperse them into the air;
• Some types of dust are conductive (metal dust, coal dust) which can cause short circuits when they penetrate electrical equipment;
• Dust and fibers tend to sag and form layers, also on electrical or mechanical equipment and often these layers form an excellent thermal insulator, resulting in overheating of the equipment.

Types of ignition sources

There is only a risk of explosion if, in addition to an explosive mixture, an ignition source is present. In fact, there are only two ways to ignite an explosive mixture, either by a spark or by a hot surface. However, these may occur in very different ways.

In order to increase the legibility of this book, the thirteen possible ignition sources as mentioned in EN 1127-1 and the measures that can be taken to prevent ignition are listed in our article about sources of ignition.

Limitation of impact (constructive protection)

Sometimes explosive atmospheres cannot be prevented, nor is it possible to eliminate all potential sources of ignition. In that case, measures must be taken to limit the consequences of an explosion to a harmlessscope. Such measures are:

• Explosion-proof construction method
• Explosion pressure relief
• Explosion suppression
• Preventing flames and explosions from spreading 

These measures usually pertain to limiting the hazardous consequences of explosions occurring within the installation. In general, devices and protective systems used meet the requirements of Directive 2014/34/EU (ATEX 114). Structural measures such as walls to protect against pressure waves can also be taken.

Explosion-proof structure

Installations components, such as tanks, equipment and pipelines, are built to withstand an internal explosion without bursting. In general, a distinction is made between the following explosion-resistant versions:

• A version for maximum explosion pressure;
• A version for reduced explosion pressure in combination with explosion relief or suppression.

The design of an installation can be either explosion pressure resistant or explosion pressure wave resistant. 

Explosion pressure resistant containers and devices can withstand the expected explosion pressure without permanent deformation. The pressure value used in design calculations is based on the expected explosion pressure.

Explosion wave-resistant containers and devices are constructed to withstand a pressure wave with a force corresponding to the expected explosion pressure. Permanent deformations are allowed. After an explosion, the affected parts of the installation must be checked for deformations.

Explosion pressure relief

The term "explosion pressure relief" includes all that is necessary to open the installation in a safedirection when an explosion occurs. The pressure at which the installation opens is called the activation pressure.

The explosion pressure relief device is meant to ensure that the installation is not loaded above its explosion resistance. There will be a reduced explosion overpressure. For example, approved safety membrane or explosion valves can be used to relieve the pressure of an explosion.

Explosion relief is not permitted where persons are endangered or damage is caused to the environment.

Explosion suppression 

In case of an explosion, an explosion suppression device prevents the maximum explosion pressure from being reached by quickly blowing extinguishing agents into containers and installations. This means that protected devices only have to be designed and constructed for reduced explosion pressure.

In contrast to explosion pressure relief, the effects of an explosion on the interior of the installation are limited. Depending on the version, explosion overpressure can be limited to approximately 0.2 bar.

Preventing the spread of explosions

When an explosion occurs within an installation, it can spread to connected parts of installations and cause further explosions there. Acceleration effects by parts of the installations or by extension to pipelines may increase theharmful consequences. The resulting explosion pressure can be much higher than the maximum explosion pressure under normal conditions and can even lead to destruction of parts inside an explosion-proof or explosion-proof wave-resistant construction. It is therefore important to limit any explosions to individual installation parts. This is achieved by explosion-safe decoupling.

The following systems are available for explosion-safe decoupling of installation parts:

• Mechanical quick-release devices;
• Extinguishing flames using narrow gaps or by using extinguishing agents;
• Suppression of flames by a high counter-current;
• Immersion;
• Air locks.

Requirements for work equipment

In areas where hazardous explosive atmospheres may occur, equipment and protective systems must, in principle, be designed in accordance with the categories set out in Directive 2014/34/EU. Exceptions can only be allowed when based on an appropriate risk assessment and must be indicated in the explosion protection document.

If transportable work equipment is used in areas with a different hazard classification (different zoning), it must be designed for the least favorable conditions. If work equipment is used in zone 2 and zone 1, then it must comply with the requirements for use in zone 1. These measures must be described in the work permit and/or in the explosion protection document.

For safe operation, the following must be taken into account when selecting tools and equipment:

• Equipment group;
• Category;
• Explosion group;
• Temperature class.

The first option is related to the intended use and here a distinction is made between two groups of equipment:

Group I        equipment for use in underground mines;

Group II       equipment for all other (aboveground) locations.

Next, a relationship is established between the zone with its associated risk and the required category (safety certification) of the equipment or work tool. As indicated in § 4.2.1, a distinction is made between three zones for gas hazardous areas and three zones for dust hazardous areas.

Table 4.2 Zones, risk and ATEX categories

Each category is marked with the letter 'G' and/or 'D' where the 'G' stands for gas and the 'D' for dust. Here are few examples:

• A device marked 1D is an ATEX category 1 product and suitable for use in an explosive substance (dust) environment classified as zone 20;
• A device marked 2G falls into ATEX category 2 and is suitable for use in explosive gas atmospheres classified as zone 1.

All flammable gases are classified into one of the following groups. The designation is used both for the equipment and to indicate which gases may be present in the zone.

Table 4.3 Representative gas group

For equipment used in an explosive gas environment, the classification of the equipment must be either equal or higher than the classification of the gas. Equipment suitable for use in an environment containing gases of group IIC may therefore also be used for gases of group IIA and IIB. The reverse is not permitted!

Equipment for use in a hazardous dust environment is classified in a similar way, but based of particle size and conductivity. The dust groups IIIA, IIIB and IIIC are distinguished.

Table 4.4 Dust group, particle size and conductivity

Similarly, the classification of equipment used must be equal to or higher than the classification of the dust present. 

Lastly, the maximum surface temperature of the equipment must also be permitted for the environment.

The temperature class is used both to classify flammable gases or vapors and to select equipment to be used in areas where flammable gases may be present.

Table 4.5 Temperature class and maximum occuring temperatures

With equipment suitable for a dust environment, the temperature is indicated directly, bymarkings like T80°C or T210°C. This article explains both ignition and smoldering temperatures, the relationship between the maximum surface temperature and what is permitted for specific gases and dusts.

In practice, employees can easily move from, for example, a zone 2 to a zone 1 area. In that case it is very likely they will forget to leave a flashlight or mobile phone suitable for zone 2 behind in that zone and not take it into the zone 1 area. It is therefore strongly recommended to choose portable work equipment from category II 2 G which is suitable for use in zone 1.