62
Tube Products International January 2011
TWI Ltd
– UK
pass, most of the energy in the laser beam
and the assist gas are used to cut material
originally contacted by the beam. Only laser
energy and gas which have passed through
the upper section of the cut are available to
address the lower section and this is cut less
effectively. For the second pass, a kerf has
been previously opened in the top section and
now the majority of the laser energy passes
through this and acts more effectively on the
lower section of the tube.
As an example of performance, Figure 2
shows the dependence of the maximum
cutting speed at which the tube is severed as
a function of laser power. The results shown
are for a tube of 155mm diameter and 1.5mm
wall thickness, for two pass cutting, but similar
trends were observed for other diameters and
walls. Note that the cutting speed is fairly
linear with applied power, at least up to 5kW.
The optimum assist gas pressure appeared to
be about 8bar.
Figure 3 shows cut sections from 60mm diameter tube, with
wall thicknesses from 1.5 to 11mm, again for two pass cutting.
Process parameters can be found on the figure caption. In
this series of cuts, the only parameter to be varied was the
process speed, which was ten times faster on the 1.5mm wall,
compared to the 11mm wall. However, it should be noted that
all the different wall thicknesses could also be cut at a speed
of 100mm/min, thereby affecting cutting of all these tubes
without any change at all in the process parameters. The
largest tube to be cut in this work had a diameter of 170mm,
with a 7mm wall thickness (Figure 4). This is not believed to
be the limit for the laser in use. To go above this diameter,
a shorter nozzle assembly would be needed. Using a three
pass technique, this tube was severed in a time of 7min, using
4.8kW of laser power. Another possibility demonstrated was
the cutting of concentric tubes, for example, a 25mm diameter
tube located inside a 60mm diameter tube. In this case, a two
pass technique was effective in severing both tubes at once
(Figure 5).
In order to simulate a possible arrangement of different
diameter tubes in different locations and with different packing
density, the assembly of tubes shown on the left of Figure 6
was constructed. Over 50 cuts were employed to demolish
this assembly of tubes ranging from 25 to 155mm in diameter,
including tube severance, fixture severance and hole cutting
(in the larger diameter tubes). The assembly was reduced to
the state shown on the right of Figure 6 in one continuous
cutting sequence lasting just over 15 minutes.
Conclusions
A very effective and efficient system for cutting of stainless
steel pipes and other fixtures/fittings has been developed.
The cutting head is both lightweight and has a significant
stand-off tolerance and so is relatively simple to remotely
deploy and operate. Although this work was commissioned
by the NDA, primarily with a view to nuclear decommissioning,
the techniques developed could be useful in a wide range
of industries for cutting up pipework and vessels during
demolition.
Acknowledgements
The authors are grateful to the Nuclear Decommissioning
Authority for funding the work reported in this article and for
giving permission for its publication.
Figure 6: Cutting demonstrator, before and after cutting
Figure 5: Cutting of
concentric tubes, in
this case a 25mm
tube inside a 60mm
diameter tube
