Explosion protection pressurized enclosure

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

Explosion suppression is a proven technology and should be considered as a candidate for explosion protection. The NFPA has published a standard reference (NFPA 69, 1986) on explosion-suppression protection. Manufacturers should be consulted on design, installation, and maintenance. But, even with explosion suppression, it is common for the explosion pressure to reach 1 atm before it is suppressed. The added pressure surge from the injection of the suppressing agent must also be considered. Therefore, sufficient mechanical strength is always required for enclosures protected by explosion suppression. 

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. 

As an example of dust explosion protected apparatus standard IEC 61241-4, 1st edition 2001-3, types of protection pD describes a pressurization technique the interior of this apparatus is subjected to a continuous pressure from a supply of air (not containing any combustible substances) or other inert gases while electrical components within the enclosure are energized. 

Special protection comprises all protection techniques such as powder filling, static pressurization and encapsulation (by plastics materials). At that time, these techniques started their career in the field of explosion protection and advanced to autonomous types of protection described by IEC or European Standards individually. VDE 0170/0171/1969-01 does not contain any specific requirements for these techniques. It has been the decision of the certifying body that an s -apparatus will operate at the same level of safety compared with other well-defined types of protection, e.g. flameproof enclosure d ... 

Apart from inserting the complete gas analyser into a p-enclosure, the analyser head may be explosion protected for zone 1 application (e.g. according to flameproof enclosure - d - ) on its own, whereas the central control unit may be installed in a non-hazardous area, or alternatively, in a pressurized enclosure (Fig. 6.25) when zone 1 installation is required. 

In zone 2, the pressurization technique presents the greatest allowable bandwidth in selecting the electrical components to be installed inside an explosion protected enclosure in comparison with other types of explosion protection techniques as stated in IEC 60079-15, EN 50021 or VDE 0165. As a special feature, pressurization enables intervention and access of personnel... 

Pressurized enclosures - application in zone 2 and for dust explosion protection... 

The principle of explosion venting is that a gas explosion in an enclosure causes the vent(s) of sufficient area to open rapidly at a pressure well below the enclosure s strength, releasing hot gases to a safe location. In other words, the vessel fails in a predictable way such that people and plant are protected from the pressure effects of the explosion. 

Pressure detection shall be used for closed enclosure applications. Threshold detectors provide an electric signal when a preset overpressure is exceeded. Dynamic detectors provide an electric signal to the control and indicating equipment (CIE). Typically they have both rate-of-rise and pressure threshold triggering points that can be configured specifically to the application conditions. Although this type of detector minimizes spurious activation of the isolation system (due to pressure fluctuations other than explosion pressure rise), care shall be taken to set up such detectors to meet appropriate detection response criteria for the particular application and protected enclosure geometry. 

Relief venting to reduce dust explosion pressure requires the equipment to be protected to have a certain minimum strength. If the enclosure strength is too low, the enclosure will be... 

Even with explosion suppression, it is common for the explosion pressure to reach one atmosphere before it is suppressed. The added pressure surge from the injection of the suppressing agent must also be considered. Therefore, sufficient mechanical strength is always required for enclosures protected by explosion suppression. 

So, a purging process (performed with the protective gas maintaining the positive pressure differential during operation) shall flush the hazardous mixture inside the p enclosure before energizing the apparatus. More precisely during the purging process, and with air as protective gas, the potentially explosive atmosphere inside the enclosure shall be diluted to a safe value of gas concentration, i.e. 25 per cent of LEL, or, with inert gas as protective gas, the oxygen content of the atmosphere inside the enclosure shall be reduced to 2 per cent (v/v). 

Various protection techniques can be used to prevent electrical equipment from becoming an ignition source. Explosion-proof enclosures, pressurized and purged equipment, and intrinsically safe circuits can be used in Division 1 and Division 2 locations. Nonsparking equipment can be used in Division 2 locations. Details of the types of equipment permitted in each classified location are given in NFPA 70. 

Optical flame sensors can be used for detecting the initial explosion, provided interference of environmental conditions can be reliably prevented. Pressure transducers are often used because the pressure wave travels at the speed of sound and can be detected at various angles. Vibrations and other mechanical movements can interfere pressure sensors. Therefore, efforts have to be made to minimize the influence of these interferences. The suppressant must be effective in flame quenching and compatible with product and the material of the plant. The suppressors must be designed and arranged adequately, so that the suppressant is rapidly and uniformly injected to the gas mixture in the protected enclosure. 

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