In the physical sense an explosion is a simple combustion reaction. The great danger, however, is the high reaction rate. The temperature rises in fractions of a second, and the explosion overpressure increases very quickly, potentially causing catastrophic damage to equipment components and endangering people.
Unlike a fire, it is not possible to ‘run away’ from an explosion due to the high dynamics. This circumstance makes the field of explosion protection a very important.
Explosion protection measures can be broken down into different groups. Various measures are aimed at precluding the possibility of an explosion in the first place. This can be done by preventing the formation of an explosive atmosphere, or also by reliably preventing effective sources of ignition.
Additionally, there are facilities or processes where the occurrence and convergence of conditions for an explosion cannot be reliably prevented.
To ensure safety in these cases as well, the equipment components must be designed so that no injuries occur even if an explosion takes place.
Since the protection lies in the construction of the equipment, this area is referred to as ‘constructive explosion protection’.
Venting as a protective measure
The objective of venting is to lower the overpressure that occurs in the event of an explosion to a previously defined level, known as the reduced maximum explosion overpressure.
During explosion venting, vent areas are opened at a defined static activation pressure to dissipate the overpressure to the surrounding atmosphere. Starting from the time of ignition, the pressure rises until the static activation pressure of the vent device has been reached.
At this value, the vent area is opened, and the pressure curves diverge. The rate of pressure rise decreases and the reached maximum is significantly lower because the overpressure can be exhausted to the surrounding area.
The size of the vent areas is calculated based on the reduced maximum allowable pressure and other parameters in accordance with the two European standards EN 14491 Dust Explosion Venting Protective Systems and EN 14994 Gas Explosion Venting Protective Systems.
Various approaches are possible for venting. In addition to bursting panels, which are implemented as rupturing membranes made of metal, valves can also be used for this purpose.
HOERBIGER relief valves of the EV series provide a reusable and virtually maintenance-free solution, while also reducing flame propagation lengths compared to bursting panels.
Explosion venting using rupture disks or blow-out flaps result in considerable external flame development.
At times, process equipment must be situated inside a production hall or in other areas, so that the propagation of flames by several meters in the event of an explosion poses serious danger.
HOERBIGER offers flameless explosion venting for these instances. The relief valves of the EVN2.0 type are equipped with a flame arrester and allow flameless venting even in enclosed spaces.
In the event of an explosion, the valve plate on the inside is lifted when the static activation pressure is exceeded and the flow is deflected by 90º in the circumferential direction, similarly to the EV series valves.
The flame arrester is arranged on the outer circumference of the valve and splits the hot gas stream into small sub-streams, which are guided through small individual channels that have a defined cross-section and specified length. With this technique, the heat is dissipated to the flame arrester material, and the temperature of the gas is reduced below the flash point. Only smoke and combustion products (ash) exit to the outside of the flame arrester.
Each flameless venting device is tested in accordance with the European standard EN 16009 Flameless Explosion Venting Devices.
The key criteria of the design of a flameless venting device are listed in the Type Examination Certificate (ATEX certificate) that is subsequently issued, venting efficiency being one of the most important parameters.
All venting devices, including bursting disks, incorporate a certain flow restriction to the outflow of explosive gases. Venting efficiency is expressed as the efficiency factor in percent. This factor indicates the percentage of the physically installed vent area that provides effective relief. This value serves as the basis for increasing the nominal width of the venting device to provide the vent area as required according to the calculation. The objective is to reduce the effects that occur during an explosion to a predetermined, safe level.