Comprehensive explosion protection of technological equipment in a coal mill

WOLFF GROUP

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Krzysztof Tetla
Explosion protection of technological equipment in a coal mill

Problem:

  • explosion hazard of a coal dust/air mixture upon contact with an ignition source (e.g. hot surface, mechanical and electrostatic sparks, open fire source) in apparatus involved in the storage, dust extraction, grinding and pneumatic transport of coal on the premises of a coal mill
  • an uncovered conveyor belt preventing the installation of decoupling cylinders

Solution:

  • improving safety by means of appropriate technical solutions to protect process equipment against the effects of a potential explosion
  • securing the coal dust container with the HRD explosion suppression system
  • reinforcing the transfer and covering partially the conveyor belt supplying coal dust from the coal hall to the coal dust tank to enable the installation of the HRD type explosion decoupling system on the charge to the container
  • protecting the coal mill with HRD type explosion suppression and decoupling systems
  • protecting the pipeline transporting coal dust vertically downwards from the mill to the filter by installing a certified load relief panel on the diverter
  • design and installation of adaptation frames for new, certified decompression panels on the technological filter
  • applying an explosion isolation/decoupling system using an HRD cylinder on the dust channel (transport of coal dust from the mill to the filter) and to the secure system a small buffer tank
  • preparing an as-built explosion risk assessment in the scope of the modernisation carried out

Check the details of the implemented equipment and services:

Formation of explosive atmospheres inside process equipment and installations

Due to the specificity of technological processes that take place in production equipment such as silos, air filters or built-up transports, the elimination of the formation of explosive atmospheres inside these equipment can be not only complicated and expensive, but sometimes simply unfeasible.

This is due to the fact that the processes of crushing, mixing, transferring, grinding, shredding the product during handling, etc. naturally generate dust and potential ignition sources.

Explosion hazard in a coal mill

The explosion risk assessment carried out at the cement plant indicated that appropriate measures were needed to protect the technological equipment and to contribute to reducing the level of risk. Explosive safety work was commissioned to GRUPA WOLFF specialists.

Modernisation of the coal mill in terms of explosion safety was necessitated the hazards posed by the coal dust/air mixture at the time of contact with e.g. hot surface, mechanical and electrostatic sparks, open fire source as a source of ignition. The risk was related mainly to the equipment involved in storage, grinding and pneumatic transport processes.

The task of our engineers was to select appropriate technical solutions to protect the equipment forming the entire installation, i.e. coal dust container, mill with dust duct, technological filter and coal conveyors system against the effects of a possible explosion.

Comprehensive explosion protection of technological equipment

As part of the order, our engineers developed the project, as well as delivered and assembled explosion-proof systems. Knowledge and experience in the field of explosion safety and knowledge of the processes carried out by the cement plant helped in the efficient course of work. Below, we demonstrate how the protection of the individual devices was ensured:

  • coal dust buffer tank with transport system to the mill

As part of the work carried out, the conveyor belt supplying coal dust from the coal hall to the coal dust tank was partially covered and the transfer of coal dust was reinforced. This enabled the installation of an explosion decoupling system by means of HRD cylinders on the hopper.

The coal dust tank itself had not previously been protected against the effects of explosion. If an explosion occurred, the tank would probably be torn apart, and the explosion could spread to the other apparatus of the production plant. Therefore, the tank was secured with the HRD explosion suppression and decoupling system on the side of the conveyor belt.

  • a combined roller and bowl type mill with a coal dust transport system to the filter

The mill is equipped with a CO2 inerting system and an inerting system using mineral dust the purpose of which is to reduce the risk of ignition of the product. However, in the event of ignition, these installations would not be able to protect the device from explosion effects.

In view of the above, the coal mill was secured by explosion suppression and decoupling systems – a total of 6 HRD cylinders. Five of them were mounted on a coal mill and one on a dust duct transporting dust from the mill to the filter (cutting off the explosion).

The pipeline transporting coal dust vertically downwards from the mill to the filter was additionally protected by a decompression panel on the diverter. In the event of an explosion, the construction will provide a reduction of the explosion pressure in the pipeline and also reduce the risk of explosion transfer from the filter to the mill and vice versa.

  • technological filter with a system transporting dust to silos

Until then, the fabric filter inside the mill had been protected by uncertified explosion relief dampers. As part of the modernisation, they were replaced with certified decompression panels which would ensure even more reliable protection of the device. In the event of an explosion inside the filter, other apparatus of the system are also at risk. For the apparatus protection, the explosion isolation/decoupling system was applied by means of HRD cylinders on the dust channel (transport of coal dust from the mill to the filter). One of the HRD cylinders was used for protecting a small buffer tank located between the filter and the screw pump.

As-built explosion risk assessment

The modernization of the coal mill was subjected to the as-built explosion risk assessment prepared by our ATEX specialists. It was aimed at assessing the technical condition, effectiveness and usability of the technical solutions applied.

Undoubtedly, the modernisation of the coal mill has significantly improved the explosion safety of the plant and therefore no further risk reduction measures are required. It should be noted here that in the event of changes within the area covered by the modernisation that are important from the point of view of security, it will be necessary to update ATEX documents.

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