Principles of fi re and explosion
7.5 Explosion
7.5.5 Principles of explosion management
The principles of explosion management can be broken down into two discrete areas: those of control and miti-gation, each will be dealt with separately. The Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) require the application of a hierarchy of control measures to manage the risk of accidental explosion.
Control
Critical to the management of explosive atmospheres is the avoidance or reduction of potentially explosive mater-ials within an atmosphere.
The substitution of fl ammable substances by inert materials or limiting the concentrations of the fl amma-ble substances to avoid their explosive range must be con sidered at the top of any explosion management hierarchy.
Such controls may be the replacement of a fi ne dusty material by a less dusty granular material or reducing the fl ammable gas to the absolute minimum.
Limiting the concentration to avoid the explosive range with mechanical systems linked to ventilation which may be actuated via gas or fl ow detectors (including alarms) should be considered. In the case of combustible liquids the objective should be to reduce the concentration of any mist formed, below the lower explosion limit, which in turn will ensure it is suffi ciently below its fl ashpoint to prevent explosion.
An alternative mechanism may be the use of adding inert gases, e.g. nitrogen and carbon dioxide, utilising water vapour or inerting using a powdery substance such as calcium carbonate. With appropriate dispersal these materials can prevent the formation of an explosive atmosphere, which we term inerting.
Design and construction – equipment, protective systems and system components
When considering the types of equipment, protective systems and components that will contain fl ammable substances, reasonably practicable steps should be made to keep the substances enclosed at all times and the materials of construction should be non-combustible.
Where necessary leak detection systems should be fi tted and particular attention should be given to the following areas:
➤ Joints
➤ Piping
➤ Areas that may be subject to impact
➤ Areas that may be subject to hazardous interactions with other substances.
The detection systems should provide advanced warning of any leakage from the equipment, systems or compon-ents, so that appropriate steps can be taken to prevent the consequential build-up of any fl ammable atmosphere.
Dilution by ventilation
While quite effective with gas and vapour in relation to dusts, ventilation is of limited effectiveness and provides suffi cient protection only when the dust is extracted from the place of origin and deposits of combustible dust can be prevented.
DSEAR regulation 6(4)
(a) the reduction of the quantity of danger-ous substances to a minimum;
(b) the avoidance or minimising of the release of a dangerous substance;
(c) the control of a release of a dangerous substance at source;
(d) the prevention of the formation of an explosive atmosphere, including the appli-cation of appropriate ventilation;
(e) ensuring that any release of a danger-ous substance that may give rise to risk is suitably collected, safely contained, removed to a safe place, or otherwise rendered safe, as appropriate;
(f) the avoidance of –
(i) ignition sources including electro- static discharges; and
(ii) adverse conditions which could cause dangerous substances to give rise to harmful physical effects; and (g) the segregation of incompatible
danger-ous substances.
In order to prevent the formation of an explosive atmosphere from the dispersion of dust in air or by equipment, it is appropriate to design conveying and removal systems to an approved standard. Such equipment will remove the levels of dust within the enclosure which will help to avoid primary explosions;
however, secondary explosions could still easily occur if the lying dust is suffi ciently agitated leading to entrain-ment in the conveying and removal systems.
The avoidance of ignition sources that may precipi-tate an explosion for either gas or dust in the atmosphere provides a relatively good control measure; however, in relation to dusts, accumulations and moisture content must be managed effectively as together they have the potential for self-heating and therefore self-ignition, thus the management of temperature and moisture in relation to dust must be considered.
Mechanical inputs can also produce either glowing sparks or hot spots and while the sparks are not suffi ciently energetic to provide ignition repeated contact may run the risk of igniting a dust cloud.
The removal of any foreign objects from process streams and the use of non-sparking or spark-proof equipment (intrinsically safe) must be considered in either gaseous or dusty atmospheres.
Ensuring that any electrical equipment is subject to regular maintenance (planned preventive maintenance) must also be seen as a key area to prevent ignition sources initiating an explosion.
Electrostatic sparks from static electricity must also be minimised and the following should be considered:
➤ The use of conducting materials for equipment, plant, etc. to avoid charge build-up
➤ The earthing of any equipment that may become charged
➤ The earthing of workers
➤ Earth non-conducting materials via an earth rod through the storage vessel.
With regard to electrostatic discharges if there is any doubt earthing should take place.
Mitigation
In relation to dust the best way to contain a primary explosion is to ensure that the process equipment is strong enough to withstand it. Dust explosion pres-sures are usually within range of 5–12 bars. Designing the plant as though it were a pressure vessel is likely to cause it to be very expensive and beyond what is reasonably practicable. It is therefore quite often that designers will resort to explosion venting.
Explosion venting is one of the most effective ways to relieve pressure; however, it is often diffi cult to size
the vent correctly to ensure that suffi cient pressure relief is available as it must allow suffi cient outfl ow of the burnt fuel and air to relieve the pressure being generated by the heat of the explosion.
There are a wide variety of differing designs of venting dependent upon processes undertaken. These can be simple panels that are ejected, vent covers (attached to process vessels with clips and rubber seals), and hinged doors that can withstand explosions or where necessary redirect the explosion.
The vent area will depend upon the volume of the enclosure, the enclosure’s strength, the strength of the vent cover and burning rate of the dust cloud.
There are a number of hazards caused by venting which will need to be taken into consideration, these are:
➤ Emission of blast waves from the vent opening
➤ Ejection of fl ames from vent opening
➤ Fireballs can be ejected
➤ Emission of solid objects (parts of the vessel, vent covers, etc.)
➤ Reaction forces on the equipment, induced by the venting process
➤ Internal venting may also lead to secondary explosions.
Consideration must be given to the location of any explosion relief panel or venting, this must be con sidered at the design stage.
If venting an explosion cannot be achieved, explosion suppression may be considered. Any suppression unit must be permanently pressurised, fi tted with a large diameter discharge orifi ce and any valve required to operate the discharge mechanism should be of high speed which is quite often achieved via a small detonation charge.
Dust explosion
Figure 7.31 A vented dust explosion
Principles of fi re and explosion
The trigger of such a suppression system is likely to be via either a pressure sensor or fl ame sensor, each will have systems to prevent false alarms and actuation.
In addition to a suppression system an alterna-tive may be control by the addition of a liquid; however, there are environmental implications where water and dust residues have the potential to contaminate the environment.
Housekeeping in relation to the removal of dust accumulations must be considered although as previously discussed this should be undertaken with careful con -sideration to eliminate the possibility of secondary dust explosions. Installing good dust extraction systems and making arrangements for spilt dust to be removed immediately using vacuum systems or explosion-proof vacuum cleaners will assist in the management of dust control; however, those operated by individuals rely heavily upon human factors and the level of competence, training and instruction.
Ultimately, as is the case with all hierarchies of control the provision of suitable personal protective equipment may be considered.
Plant layout and classifi cation of zones
Any building or plant where there is the potential for gas-eous or dust explosions must be designed upon simi-lar lines to those in which explosives are manufactured,
EX
Figure 7.33 Warning sign for places where explosive atmospheres may exist
Figure 7.32 Explosion suppression system in operation
Supressor
Pressure rise detector
Pressure wave Nozzle
Ignition
Pressure wave
Flame
Zones for gases/vapours
Zone 0 A place in which an explosive atmosphere consisting of a mixture with air of fl ammable substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.
Note: In general these conditions, when they occur, arise inside containers, pipes and vessels, etc.
Zone 1 A place in which an explosive atmosphere consisting of a mixture with air of fl ammable substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally.
Note: This zone can include, among others, the immediate vicinity:
➤ Of zone 0
➤ Of feed openings
➤ Around fi lling and emptying openings
➤ Around fragile equipment, protective systems, and components made of glass, ceramics and the like
➤ Around inadequately sealed glands, for example on pumps and valves with stuffi ng-boxes.
Zone 2 A place in which an explosive atmosphere consisting of a mixture with air of fl ammable substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Note: This zone can include, among others, places surrounding zones 0 or 1.
Table 7.4 Showing the zones for gases and vapours
stored, etc. These should be located away from other buildings and actual parts of the plant should be as remote from one another as is possible. Ideally build-ings should be of single storey in nature but kept as low as possible and the explosion prone part of any pro -cess should be as high as possible, ideally on the roof to minimise the possibility of building collapse.
Where any hazardous part of the plant is located within a building the area should be reinforced and protected from the rest of the area by a blast wall. As discussed previously the area should be vented to avoid damage (structural) from any overpressure.
Escape routes and other emergency response planning must take into account the explosive nature, as should any electrical equipment.
Under the Dangerous Substances and Explosive Atmospheres Regulations 2002 there is a requirement to identify hazardous contents (containers and plant) to ensure that the selection of the correct equipment and systems can take into account the level of and likelihood of there being an explosive atmosphere.