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Laser safety is controllable

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onsdag 31. oktober 2012

Managing laser safety is a matter of strict adherence to established industrial standards.

By Kent Krøyer
It sounds dangerous: an ultra high-power laser beam that evaporates metal in fractions of a second. However, in practice the risks are manageable and can be reduced to acceptable minima.

The risks connected to laser welding derive from very high levels of energy. This is both concentrated energy at a small focal point at the workpiece and scattered light from this focal point. Because the human eye is very sensitive, and since the infrared laser light is invisible, strict measures must be in place to avoid scattered laser light and in particular risk of stray reflections from the workpiece must be dealt with.

Eye damage from intense light. Graphics: Lasersafe,

Public standards require three levels: of safety measures the main barrier is engineering controls at the equipment, secondly comes administrative controls such as access restrictions for trained and untrained personnel, and finally protective equipment is the last resort.

“Personal safety is our highest priority and therefore we have established proper safety standards concerning the use of our 32 kW laser system, which we will strictly adhere to. One of the parameters for selecting our laser system was that it is of the highest safety standard,” says Christian Højerslev, CEO of Lindoe Welding Technology (LWT).

This new laser welding fabrication lab, dedicated to the heavy industries, is established at the premises of Lindø Industrial Park, a former shipyard which is fenced and has a guarded gate.

There LWT is situated in a former production facility, which is now refurbished and prepared for this special application – the operation of a very powerful laser system, consisting of two 16 kW Trumpf disk lasers. The laser system will be housed in a facility allowing large-scale workpieces to be handled. All technical installations are reengineered for the laser system with its associated control and robotic equipment.

“All entrances will be locked at all times and only trained people will have numbered keys. Access to the working area will only be allowed through the LWT office areas, and only after appointment. In addition, the gate and doors to the working area will be relay monitored so that the laser will shut down immediately should anyone try to enter when laser welding is in progress. This safety precaution is installed to protect unauthorized persons from entering the facility during processing. However, specially trained staff may operate the laser using special goggles, ” says Christian Højerslev.

Design is the first means of risk prevention
The safety of personnel working with industrial lasers is covered by the public standards ISO 60825-1 and ISO 11553-1. The first deals with general safety aspects of lasers and safe threshold limits while the second explains how the laser manufacturer is required to design the machines in ways that makes them safe to operate.

Every conceivable incident which could lead to human injuries must be analyzed with respect to probability, associated maximum exposure time and damage assessment. The combination of these results is given in an overall safety evaluation, in which each part can be dealt with systematically.

A central concept is the maximum permissible exposure (MPE) to the body. The eyes, being the most sensitive organs, set the limit. At the wavelength range of welding lasers (700 – 1400 mm) this limit is 50 W/m2 at the cornea of the eye, according to ISO 60825-1.

The light from the laser is divergent, and thus the irradiated area increases with increasing distance. With the MPE and the beam parameters of the laser in question it is possible to calculate the maximum distance at which an eye injury is possible. This is the nominal ocular hazard distance (NOHD). The NOHD constitutes a perimeter around the laser equipment in which safety measures must be established.

The engineering of built-in safety features in the laser equipment is the first and main barrier against accidents. The manufacturer must foresee and identify any conceivable incident, estimate a probability for a hazard analysis and design adequate measures to prevent such incidents from occurring.

The second level of safety is guarding the personnel by administrative procedures, e.g. by controlling access to the working area for skilled and unskilled personnel, such as visiting guests.

Finally, as the third level, the use of relevant safety equipment is mandatory. This stipulates among other things the use of blocking walls, filtering windows or that special goggles are to be worn.

Source: “Laser safety in non-confined welding areas”, by M. Honoré and J.K. Kristensen from Force Technology, Denmark, and F. Boekhoff from Meyer Werft, Germany.

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