Secondary explosion protection

Secondary explosion protection means to prevent the reaction of still resulting mixtures by avoiding ignition sources. 

The rule described above forms a basis for the secondary type of explosion protection, which results in the avoidance of ignition sources, e.g. by using either a flameproof enclosure of a commutator motor or the pressurized enclosure of a gas analyser, or by limiting the electrical values in intrinsically safe circuits. 

This secondary type of explosion protection will be the main object throughout all the following chapters. It covers burnable substances like gases, vapours, mists as well as dusts, and usually refers to atmospheric air as the second component forming a hazardous atmosphere. Atmospheric conditions are defined as total pressures from 8 104Pa (0.8 bar) to 1.1 105Pa... 

According to [2] the following scenario is often encountered in industry a primary explosion occurs in a plant area. As a consequence of insufficient explosion protection the dust is finely dispersed in the room leading to a secondary explosion. The latter often involves a larger mass of dust and hence a larger energy release than the primary one. The probability of occurrence of a dust detonation, however, is considered to be small. 

Hq can essentially be influenced by measures of the primary explosion protection and Hz by measures of the secondary explosin protection. Measures of the tertiary explosion protection affect principally the probability of unwanted damage effects (S). 

The Directive 94/9/EC [1] aims mainly at constructors and manufacturers of work equipment intended for use in potentially explosive atmospheres. The secondary measures of explosion protection are discussed as a matter of priority in order to prevent effective ignition sources at the work equipment. 

General Design Safety. In addition to taking specific explosion prevention and protection steps, the conditions that can result in a secondary explosion and the frequency of ignition must be minimized. The following are important ... 

Minimize Significantly reduce the quantity of hazardous material or energy in the system, or eliminate the hazard entirely if possible. It is necessary to use small quantities of hazardous substances or energy in (i) storage, (ii) intermediate storage, (iii) piping and (iv) process equipment, as discussed in the previous sections. The benefits are to reduce the consequence of incident (explosion, fire, toxic material release), and improve the effectiveness and feasibility of other protective systems (e.g. secondary containment, reactor dump or quench systems). Process intensification (see below) is also a way to reach this objective. 

The upper hemisphere of the steel shell is surrounded by a shielding made of reinforced concrete with a wall thickness of about 2 m. This shielding protects the nuclear part of the plant against any external impact (e. g. gas explosion, military aircraft crash) it also significantly reduces the likelihood that radionuclides will escape to the environment. The interspace between the steel shell and the secondary containment is held at sub-atmospheric pressure, so that any radionuclides penetrating the steel shell via leaks in the event of a loss-of-coolant accident would be transported by the annulus air extraction system to the standby filters and retained here, thus preventing release to the environment. 

---