Constructional explosion protection in grinding plants
This is to be highly appreciated that nowadays in almost majority of the cement plants in India, employee safety is given topmost priority. Well recognised safety consultants and skilled agencies are appointed to educate the plant workmen and also the management about the global safety rules and standards. Not even the plant personnel, even the visitors are not allowed to walk in the plant without wearing PPEs and without attending safety related training. Lots of emphasis is given on the proper size and arrangement of working platforms with safety handrails, toe guards and walking clearances. In a nutshell, almost all the possible safety precautions are taken. However, one particular safety concern, which is mostly neglected even by the major cement manufacturing plants, is the safety against constructional explosion in coal grinding systems.
State-of-the-art constructional explosion protection e.g. in coal grinding systems as well as in alternative fuel processing and storage is nowadays regulated by international rules and directives. However, as the coal grinding system is only a part of a very large equipment scope, less attention is paid to this issue during the overall decision-making process. Non-proper design in terms of constructional explosion protection systems leads to serious consequences such as fatal injuries or injured workers during a big explosion. In recent years, several severe explosions in coal grinding systems, as well as in the processed industry of alternative fuels, have emphasised the necessity of properly-working constructional explosion protection. Therefore a.m. rules and directives should be implemented in the technical specification.
European ATEX directives divided in:
Directive 1999/92 EG relevant for operators covering the aspects of risk assessment, zoning, technical and organizational measures etc. pursuant the main target to the protection of health of workers acting in potentially hazardous areas.
Directive 2014/34/EC relevant for manufacturer of explosion protection devices covering the requirements of autonomous protection systems, components, safety and control equipment. The significance of this Directive is emphasized by the common requirements for equipment and protective systems. The part which is related to constructional explosion protection sounds as follows: "Should an explosion occur possibly endangering persons directly or indirectly (or for that matter domestic animals or property), such explosion is to be halted immediately and / or the range of explosion flames and explosion pressures to be limited to a sufficient level of safety".
EN European standards: These standards are of great significance e.g. for coal grinding systems in terms of explosion resistant equipment (EN 14460), dust explosion venting protective systems (EN 14491), explosion isolation systems (EN 15089).
A further decisive factor to have properly working constructional explosion protection in place, is generated as a result of changed coal properties in terms of high volatile fuels to be ground. Fuels of a VM content between 35 per cent and 45 per cent aren't a rarity. In some areas in the world the use of pet coke is already banned.
Other areas will probably follow. The fact that high VM coals have the tendency of creating a smoldering fire (even in the coal yard), reflects a potential risk of an explosion in the coal grinding system. At this point, it has to be emphasised that even the milling of pet coke creates an explosive atmosphere and may lead to an explosion.
Constructional explosion protection
The issue of constructional explosion protection in coal grinding plants is very multifaceted and needs attention with regard to the following themes:
Explosion venting in combination with explosion pressure shock resistance to the relevant degree. This is mainly applicable for: Protection of bag houses û taking the related Rule (EN 14491) into account. Apart from the coal characteristic values, K St / p max the enclosure's pressure shock resistance is one of the decisive factors to determine the necessary area in order to sufficiently protect the bag house as part of the coal grinding system. From experience, this value of pressure shock resistance is between 100 - 400 mbar g. Additionally, the obstruction of the venting path by filter bags should be avoided right from the beginning of the design stage.
Protection of pulverised fuel silos - taking the related rule (EN 14491) into account. As a result of the cylindrical shape of such enclosures, it is quite feasible to reach a pressure shock resistance of 2 bar g (max. value as stated in the relevant rule). This maximum pressure shock resistance leads to the lowest possible venting area, depending on the efficiency factor of the venting equipment to be installed.
"Explosion venting respectively explosion de-coupling in combination with explosion pressure shock resistance to the relevant degree.
In the past, explosion risks caused by dust explosion transmission through pipes and ducts of interconnected plant in units, have caused serious damages. This scenario is generated by a secondary dust explosion, the so called "Flame-jet-ignition", which results in very high explosion pressures in the connected enclosure. To limit the extend of dust explosions and to prevent violent secondary explosions due to explosion transmission via pipes and ducts, explosion protection using suitable explosion isolation techniques, are mandatory. In coal grinding plants, the installation of explosion diverters, designed in accordance to the relevant rule have proven their efficiency.
Techniques of intelligent concepts are available in order to effectively mitigate flame front propagation in dust-laden ducts. There have been made further research in form of various successful field tests with the French notified body, Ineris. Apart from field tests, the concept is now subject to computational modelling.
The main aim is the mitigation of pressure pushing the flame front into the riser duct, in combination with the diversion of the largest possible portion of flame body. In coal grinding plants, explosion isolation measures in form of explosion diverters are mainly required. E.g. in riser ducts between mill outlet and bag house inlet as well as in any kind of ducts used as de-dusting pipes. Other mechanically-operated explosion isolation devices (e.g. rotary airlocks, double flap systems of a certain pressure shock resistant degree) are necessary as well.
Explosion pressure shock resistance for full explosion pressure: An explosion pressure shock resistant design for full explosion pressure, is always necessary where venting as well as decoupling measures aren't feasible respectively uneconomical. Typical applications are intermediate hoppers e.g. pump pre-hoppers for pneumatic conveying systems. In coal grinding plants the full explosion pressure of 9 bar g as a design parameter is deemed as sufficient.
Self-re-closing, re-usable explosion venting devices: Self-re-closing explosion venting devices have been invented aiming at mitigating a fire inside the equipment. Mitigation respectively extinguishing of fire inside the equipment is only working effectively, provided that ingress of oxygen is prevented and the injected inerting gas e.g. CO2, N2 is kept in the system. Once open, the oxygen in the ambient air will increase the damages caused by fire while injected inerting gas is lost to the atmosphere without being effective.
The technical design of coal grinding systems has to be based on the latest available standards. Any non-consideration of a.m. standards may lead to fatal incidents. Scrutinising a proposed coal mill protection concept in the earliest stage of the acquisition procedure will prevent incorrect system protection. Investments in follow-up costs in terms of system's modifications are avoided.
Storage and handling of alternative fuels as well as of biomass products play more and more an important role. The substitution rate of coal by alternative fuels/biomass is steadily increasing, provided the infrastructure of the country concerned is given. Especially the processing and handling of wood pellets, wood shavings, saw dust, etc. is growing and require special attention in terms of constructional explosion protection. Explosive specific values of wood pellets are even higher than lignite, which underlines the hazard of a violent explosion.
It requires a special mention that employee safety is not limited to PPEs and the safety slogans but much more beyond and requires to be attended more specifically in case of the constructional explosion.
ABOUT THE AUTHORS:
Manoj Thakur, Director with Techcon Consulting and Engineering, and Berthold Bussieweke is Head of Sales with Thorwesten Vent GmbH, Thorwesten Vent GmbH. Thakur can be contacted on: Manoj.email@example.com or +91 9892189804, while Bussieweke can be contacted on: firstname.lastname@example.org or +49 172 2544100.