AERZEN combines effective reactions with preventive measures / vibrational monitoring as early warning system
Author: Thorsten Sienk, freelance specialist editor, Bodenwerder, www.sienk.de
Combustible dust, spatial distribution, oxygen share, ignition source and the closed reservoir are components of a dust explosion and form the so-called explosion pentagon. The mutual relationships of the five fields can be used to prevent explosions effectively – mainly due to the fact, that processing of fine combustible dusts always bears the risk of a dust explosion. Particularly the pneumatic transport of cereal flour, cellulose or chemical precursors in powder form has tremendous effects on the explosion pentagon. As specialist in blower and compressor technology for conveying dust and gas, the AERZEN Ex-protection within different ATEX-zones is a part of day-to-day business.
Dust-air mixtures are explosive when the dust consists of combustible material. This includes besides timber, flour and cellulose cocoa, coffee, starch and inorganic substances such as the elements magnesium, aluminium and iron. And precisely these raw materials are typical when pneumatic conveying systems are used in companies for which blowers or compressors made by AERZEN ensure the continuous supply of the production by the silo.
Compared with traditional conveyor belts for bulk material pneumatic conveying uses a piping system for the material flow. Here, in the sense of the explosion pentagon this concerns a closed reservoir, without which an explosion is not possible, and without which a pneumatically driven material flow would not work.
The process air generated by blowers or comrpessors conveys fine dusts and powders through the piping system by means of a pressure and volume flow precisely designed in advance. Then, inside the piping, the conveying air is mixed with the product. The fine distribution increases the risk of a dust explosion as the combustible substance can ignite easily due to its large surface and the maximum availability of oxygen.
If a substance burns, this conceals an oxidation. In case of a fire, this proceeds slowly, in case of an explosion abruptly. Both of them have in common that oxygen is needed to generate a fire as well as an explosion. Therefore, explosives always carry their own oxidant. No matter whether timber or explosives: Both of them need activation energy fed in from the outside to be able to burn or explode.
What a match is for timber and a lit fuse for explosives is for pneumatic conveying mechanical friction, electrostatic charges or hot sparks. Frequently, they are sufficient as activation energy to detonate a dust-air mixture.
Influence of the blower technology on the explosion pentagon
Concerning pressure conveying of dust or combustible gases, the only way to intervene with the conception of the suitable blower or compressor technology is at the ignition source of the explosion pentagon. It must be excluded that an ignitable energy source is introduced into the conveying medium. This may be the case when due to a defect or due to lack of maintenance sparks originate in the blower or compressor stage, and these enter the pressure pipe uncontrolledly and in hot condition.
Concerning assemblies with ATEX approval (2014/34/EU) AERZEN most effectively and elegantly integrated a spark arrester in the discharge side silencer. The highlight of this solution is that the spark arrester works simultaneously as effective silencer. Compared with silencers with dampening material, concerning reactive silencers a slotted piping system eliminates the noise by means of a time-delayed interference of sound curves. This functional principle on the one hand works free from wear with regard to the otherwise eroding, loosening filter fabric and on the other hand ensures that sparks or hot metal flakes peter out and cool down across the long way through the filter.
Cause for sparks
With all demand for high production quality, durability and operational safety, potential risks in Ex-protection areas cannot be excluded even with a blower. If it comes to an input of sparks into the pressure pipe, as a rule, this results from a defect of the compressor stage. A too high bearing clearance may be the reason why inside the rotary pistons do not rotate any longer with the defined air gap but hit together and generate sparks. In application ranges coming under the ATEX regulations, the spark arrester within the discharge silencers represents an effective measure to safely prevent the entry of activation energy into the material conveying flow. Thus, the silencer is among the reactive ATEX measures, as it is used when a damage has already occurred.
Mainly in ATEX relevant application ranges AERZEN uses early warning systems to prevent serious and safety relevant damage from the start. Here, this concerns the active mechanisms of Condition Monitoring, with which potential damage can be detected reliably – mainly by means of vibrational monitoring. “We offer an expansion stage covering three levels,” explains Fabian Pasimeni, product manager for blowers and Delta Hybrid at AERZEN. The easiest way to monitor vibrations of a compressor stage are special measuring nipples installed near the bearing positions. “So, we can measure the vibrations selectively and detect developing bearing damage.” With PCH sensors as second expansion stage AERZEN takes a step towards continuous monitoring. The sensors installed at the sides of the compressor stage measure continuously and emit corresponding 4-20 mA signals which can be monitored and evaluated by the control system. If the AERZEN blower itself is installed in an ATEX zone, the sensors are supplied with a corresponding approval. “Meanwhile, our modular system of blower solutions is so nuanced that we are able to quote for different applications with varying ATEX requirements as standard,” states Pasimeni. “We are permanently working on conveying combustible process gases safely. Therefore, the effective Ex-protection along with the resulting ATEX-requirements is for AERZEN part of the day-to-day business.”
Based on this extensive know-how, and with sensors made by IFM, AERZEN developed the third stage of real-time monitoring. With up to four sensors per compressor stage “we can look precisely into the inside and find out the condition of the corresponding bearing,” explains the product manager. This includes condition monitoring of internal rings, Pitting (local material damage) and touching of the rotor against the internal housing wall as a consequence of thermal deformations. “By means of an exact analysis of the frequencies measured we can find out what kind of error occurs and, above all, its origin.” While the above measures aim to guarantee the Ex-protection, particularly for pressure pipes, AERZEN considers also for vacuum conveying the ATEX requirements as integral part of a blower solution from one single source.
Vacuum conveying of gases and powders
While concerning the Ex-protection of pneumatic conveying systems the traget is to prevent activation energy from entering into the conveying atmosphere, concerning vacuum conveying the penetration of the material into the blower must be excluded safely. For this, mainly filter inserts are used, creating a barrier between the material to be transported and the assembly, in this case providing the negative pressure. If the assemblies themselves are installed in an ATEX zone, AERZEN serves this application with motors of different rating and further components with ATEX approval. “So, for our customers, we facilitate the inspection process of the entire plant considerably,” explains Fabian Pasimeni. Furthermore, AERZEN developed their own zone separation filters, representing as so-called sentinel filter a further decoupling between the upstream process and the assembly. “If it comes to a filter fracture we can react accordingly quickly and shut down the assembly, before an explosive atmosphere can generate inside the compressor stage.”
If, however, as a result of a chain of unfortunate circumstances it comes to an explosion the effects on the environment can be reduced effectively by using special material for the construction of the compressor stage. “If the ATEX requirements and the explosion risk are high, for the housing of the compressor stage we no longer use normal grey cast iron, but special spheroidal graphite cast iron.” Thanks to the carbon contained in spherical shape, this kind of cast iron has mechanical properties that are similar to those of steel. If it comes to an explosion, the parts are not hurled through the hall as the nodular iron has a damping effect. “With all the striving for standardisation, particularly in ATEX-relevant applications the safety in the process still plays the most important part,” summarises Pasimeni.