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|3. Passive isolation systems
It is usually not enough to vent or suppress an explosion. It is also necessary to isolate the explosion to make sure the flames and pressure wave will not reach, damage, or even trigger another explosion in an adjacent process section, typically connected by pipes to the place of the primary explosion.
The design and positioning in the process of proper isolation valves must be carefully studied during the dust explosion risk analysis.
It is possible to install active isolation valves, but also passive systems depending on the position in the process.
In process lines, like pneumatic conveying systems, it is necessary to install active isolation system to stop the propagation of an explosion if it happens. Indeed, in such process lines, the pipe section must be fully opened during normal production, but immediately closed in case of an explosion. This is done thanks to quick acting valves.
Quick acting valves are usually gate valves powered by an actuator able to close the valve very quickly, in around 25 ms.
Those valves must be controlled by a dedicated panel which is linked to pressure sensor or in some cases spark detectors. When the sensors detect that an explosion is initiated, the valve is closed.
Such system must be reliable, and the valve must be positioned at a defined distance of the source of explosion in order to have time to close. As a consequence, only a few companies worldwide can supply such valves, process owner should therefore always seek consultation prior to installing such system. It is also advise to commission the system with the help of the manufacturer.
Passive system have no actuator, they are by design tight to explosion, or will be closed due to the explosion pressure increase.
Star valves can block the explosion provided they have designed for this purpose. They must be explosion resistant at a pressure higher than Pmax explected, have a small clearance in between the rotor and the stator, and have at least 8 vanes so that there is no direct passage from 1 side to the other of the valve.
In case of detection of explosion, airlock rotary valves must be stopped, so that they don't bring product in the system, or in the contrary extract possible embers to the next process vessel.
Ventex valves are special valves usually positioned in clean air pipes - without product flow - that are designed to close tightly in case of explosion. They can be equipped with proximity switches to detect their closing.
Check valves may me considered in some low duty situations, for example at the aspiration of air to a dilute phase conveying system.
Explosion isolation needs must be defined thanks to a dust explosion risk analysis. Conclusions of the risks analysis must be implemented by the factory.