Extracting the dust, fumes and sparks generated by plasma cutting heavy plate steel poses some specific problems, and it was this that prompted Bob Kilby, head of health and safety/quality and environment at the ArcelorMittal’s plant in Birkenhead, to seek the expertise of Western Air Ducts

As a specialist steel and mining company ArcelorMittal puts health and safety at the top of its list of priorities in every one of its operations – including, of course, at the steel plate stockholding and processing plant in Birkenhead, where 35 staff handle and cut plates up to 12m long by 3m wide for a wide range of applications, ranging from shipbuilding to the creating the new gates to the vegetable plots at Kew Gardens.

Cutting steel of these dimensions is no minor undertaking – and the plant has invested in machine tools to tackle the job, including an ESAB plasma cutting machine on a 28m x 7m bed. 

The plasma technology necessary to achieve this high-speed, accurate cutting heats the steel plate to temperatures of 2,000°C, causing the steel to vaporise. The resulting fumes and dust are extracted through slots in the machine bed, into an under-floor duct from where they are carried outside the building, where the air is filtered and cleaned before being emitted to the outside air via a chimney. The correct and efficient functioning of the extraction system is therefore a vital part of the plasma cutting operation for the plant.

The high temperature of the process generates sparks. In order to avoid these sparks reaching the air filters  (even though these are themselves fire-retardant) Western Air Ducts installed its unique Flashback Box, between the extract duct and the filters, to ensure that all sparks are extinguished before they reach the filters, therefore avoiding any possibility of a fire hazard.

The series of 15 filters serving the plasma machine are replaced on an annual basis, but during the intervening 12 months, as part of the planned maintenance procedures, they are refreshed by a ‘reverse pulse’ mechanism, which literally blows the dust off the filters and into a holding tray (from which it is extracted) leaving the filters clean once again. This process is planned in such a way that the filters are cleaned in sets, so that machine production is not disrupted. 

One of the effects of this planned cleaning process is the airflow through the extraction system varies week by week and even day by day, as air is extracted more rapidly through the cleaned filters. Western Air Ducts therefore proposed the incorporation of an in-line device to measure the flow rate, together with a system of dampers which can be opened and closed to exactly regulate the air flow.

As Kilby explains: “Being able not only to know the exact air flow rate at all times, but also to regulate it easily, enables us to fine-tune the system so it operates at optimal efficiency. Our plasma cutting machine represents a major investment for this plant and our board was keen to have it operating at maximum efficiency. The  extraction system enables us to achieve this, while maintaining our commitment to health, safety and the environment.”