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Whereas cutting and grinding create sparks by particle removal at high speed that creates frictional heat, welding raises the temperature of a localized area to a level where the material melts. The use of high-temperature gases and electrical arcs in welding means that as no mechanical energy has imparted the distance the sparks travel is reduced. However, in both cases, the hot material in spark form is close to the melting point of the material being worked on.
Welding, grinding or cutting – which process has the greatest fire hazard potential?
The main difference for hazard potential that would need to be captured in a work permit is that in cutting and grinding operations the sparks can travel over 35 feet. In the case of a raised location above ground level at a worksite this distance can be increased further.
OHS online recommends the following to control hot work hazards using standard equipment:
“While designated hot work areas should have a 35-foot clear radius of combustibles typically maintained, there are very few areas in the average facility that are always clear of combustible material 35 feet in all directions from a given point.”
Spark creation while grinding
In grinding operations, the friction of the disc on ferrous metals i.e., those containing iron creates heat causing high temperature particles to be projected away from the tools being used.
Grinding discs are typically made from either Silicon carbide or Aluminium oxide of varying particle sizes, bonded together with a cementing matrix. As a rule, silicon carbide discs/wheels are used with lower strength materials such as Aluminium. Therefore, its Aluminium Oxide discs are typically used to cutting ferrous metals creating a “hot work” situation where spark generation occurs due to friction. It requires materials of different designs and materials based on tungsten ceramics to reduce the frictional heat generated. The greater lubricity of these ceramics reduces the temperature due to any frictional event like grinding or cutting.
The hot particles removed from the metal can reach temperatures of up to 1100 deg. C in a normal cutting or grinding operation. As these particles are usually occurring in the presence of oxygen in the atmosphere the presence of any combustible material near the operation is a potential fire hazard.
To avoid combustion conditions that create a fire hazard the alternatives are:
- To remove all oxygen from the atmosphere
- Surround the worksite in a habitat that is non-combustible, so all sparks are contained in it.
- Prevent sparks forming due to heat reduction.
The situation most easily controlled from the 3 scenarios above is the prevention of high temperature sparks so that the ignition source is removed. This especially the case where mobile repairs are required rather than in a factory process setting.
Factories with production line grinding or cutting –
Where a process line exists for standard multi repetition operations, high-temperature particles are handled by spark arresting systems. In factories where continual cutting or welding processes occur, ducts attached to the process machinery act as dust collectors and coolers for the sparks generated. The drawback to the ducting approach is that build-ups up of material that does not make it through the ducts to the filtration system are hidden and hence cannot be monitored accurately. One solution to this issue that would lower the temperature of particles produced by the process would be to reduce the frictional energy imparted to the metal. Selection of ceramic-type materials for cutting discs would ensure lower temperature cutting and therefore less likelihood of high-temperature events occurring at duct elbows for instance.
Worksite mobile applications
At a repair or inspection worksite where every individual situation could be unique, handheld grinding and cutting equipment is used. At these sites no ducting system and particle collection is available. Therefore, each location normally requires a risk assessment as part of a hot work permit process due to the expected spark creation.
One key element of the assessment is a fire watch being organized so that personnel can ensure that sparks being generated do not cause a fire. Both the risk assessment and fire watch personnel would not be required if a hot work permit were not needed due to certified “cold work tools” being used.
These non-sparking pneumatic tools need to be DNV certified as a third-party verification of their ability to lower the temperature during operations to not much above body temperature, such as our TFT Safety Tools line. This is achieved by careful material selection that lowers the frictional temperature created and disc matrix design to efficiently remove particles from the target whilst renewing the cutting surface of the disc. The cutting matrix removes larger particles in some patented designs which means the bigger mass of the particles also reduces the likelihood of the particle reaching higher temperatures. An added benefit of using an improved design is that rotational speeds can be lower, which again reduces frictional heat generation.