4.4.3 Explosion mitigation
If explosive atmospheres may be present and ignition sources cannot be eliminated, then measures to mitigate the effects of the explosion, should an ignition occur, and prevent the explosion propagating to surrounding areas are required. There are a number of techniques available that can be employed to reduce the explosion pressure generated and/or contain the explosion within a given area.
4.4.3.1 Explosion venting
In this technique, weak areas (explosion vents) that fail early on in the explosion are deliberately incorporated in the item of equipment, venting the combustion products and so reducing the explosion pressure generated inside the equipment. There are a number of methods used to seal the vents, such as thin membranes, bursting discs, lightweight covers held in place by magnetic fasteners and spring loaded doors. The opening pressure of the covers and the size of the vents are chosen to give explosion pressures below that which would damage the equipment. It may, however, be acceptable to allow some damage to the equipment, e.g. bowing of panels, provided it does not result in damage to the adjacent area or injuries to nearby personnel. It should also be ensured that the explosion is vented to safe areas so it causes no damage or injuries. EN 14797:2006 [42], EN 14994:2007 [43] and NFPA 68 [44] provide guidance on the design of explosion relief systems and the methods of available for vent sizing. An innovative vent sizing technique is described in Appendix A6 and compared with the NFPA 68 requirements.
4.4.3.2 Explosion suppression
Explosion suppression is achieved by injecting a suppressant agent, either water or a liquid or powder suppressant, into a developing explosion to quench it before the maximum explosion pressure is attained. Suppressing hydrogen explosions is particularly challenging due to the high flame speeds of hydrogen explosions. Basic requirements for the design and application of explosion suppression systems are given in EN 14373:2005 [45].
4.4.3.3 Isolation systems
Explosion isolation is a technique that prevents an explosion pressure wave and a flame, complete isolation, or only a flame, partial isolation, from propagating via connecting pipes or ducts into other parts of the plant. The distinction between the two types is important as in some applications it may only be necessary to achieve flame isolation. The systems can be either be an active type, which requires a means of detecting the explosion and initiating an action to implement the isolation, or passive and requires no additional equipment to function. Examples of an active system are a quick acting valve, a complete isolation system, or an extinguishing barrier. The later system provides partial isolation by injecting a curtain of suppressant into the pipe or duct to quench the explosion. An example of a passive partial isolation system is a flame arrester. This device contains an arresting element, comprising a matrix of small apertures or convoluted gas pathways, with dimensions large enough to allow gas flow with minimal pressure drop, but small enough to quench and prevent the passage of flame through the element. A standard (prEN 15089 [28]) is under development that will specify the general requirements for explosion isolation systems, excluding flame arresters, and the methods for evaluating the effectiveness of different systems. EN 12874:2001 [46] specifies the performance requirements, test methods and limits for use of flame arresters.
4.4.3.4Containment systems
An alternative mitigation technique to those that aim to reduce the explosion pressure is to use equipment, for example process vessels, strong enough to contain the explosion. Equipment intended to withstand an internal explosion are classed as one of two types. Explosion-pressure-resistant equipment is designed to withstand the expected internal explosion pressure without becoming permanently deformed. Explosion-pressure-shock resistant equipment is designed to withstand the expected internal explosion pressure without rupturing, but allowing for some permanent deformation. EN 14460:2006 [47] specifies the requirements of the two classes of equipment.
4.4.3.5 Blast walls
Equipment and plant vulnerable to blast damage can be protected by blast walls. These are strong walls positioned between the item to be protected and the expected source of blast that will deflect the blast wave and thus reduce the intensity of explosion pressure experienced. They can also provide protection from missiles generated by the explosion. The possible beneficial and detrimental effects of blast walls on the dispersion of leaking gas need to be taken into account in the assessment of the explosion hazards. Depending on the circumstances, for example wind direction and site layout, blast walls may limit the spread of an explosive gas/air cloud. On the other hand, walls may extend the time an explosive cloud is present and thus the likelihood of an ignition, by inhibiting the dispersion of the gas by the wind. These effects are more likely to be important for gases other than hydrogen, as due to its low density there will be a significant upward dispersal due to buoyancy. An experimental and modeling programme on the effects of walls and barriers has been carried out within HYPER and details can be found in Appendices A5 and A6.