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Comparison between the water chamber and contact shoe
technique |
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Water Chamber Technique
Most manufacturers of ultrasonic rotating systems offer heads using the
water chamber technique. The transducers are located within a rotating water
chamber through which the tube is made to pass as centrally as possible.
With small diameter tubes close tolerance guides can be used either side of
the rotating chamber to ensure the tube is maintained as concentric as
possible with the chamber during testing. Depending on the O.D. tolerance of
the tubes to be inspected there may have to be larger gaps between the
guides and the tube which nearly always leads to of centre conditions particularly at the tube ends; variation in the concentricity between the tube and rotating chamber gives rise to ultrasonic incident angle variation
with resulting sensitivity variations and the repeatability statistics of
the overall system deteriorates.
The main advantages of the water chamber is (i) higher rotational speeds
can be achieved and (ii) the ability to get more transducers in a shorter
space in terms of length; this point is important when trying to achieve
minimum untested end lengths and also achieving optimised testing conditions
at the tube ends.
Typically water chamber rotating heads are very good for small diameter
precision tubes with surfaces free from scale. If these type of heads are
used with hot finished (black) tubes then problems can occur with the scale
dust collecting in the chamber during production testing which ultimately
attenuates the ultrasound thereby affecting sensitivity.
Although manufacturers claim to be able to flush the scale away this is only
partially successful and generally to achieve a good test the tubes need to
be clean and free from scale. |
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Contact Shoe Technique
In this technique the transducers are housed in a water column block
which actually rides on the tube surface by means of a contact wear shoe.
The main advantage of this method is the ability to test hot finished
material even with rough surfaces and the ability of the probe assemblies to
test with typical tube off-centre conditions of ± 10mm without affecting the
test sensitivity.
Excellent repeatability figures can be achieved with rotary heads using this
technique and this inevitably leads to less spurious marking and prove up
requirements. Typical seamless tubes can suffer from some out of
straightness particularly at the pipe ends and the contact shoe design with
its tube following capacity is by far the better system for testing this
type of product. The overall length of the contact shoes containing the
transducers must be kept as short as possible to ensure testing can be
carried out as close as possible to the tube ends. The only disadvantages of
this technique are the slightly lower rotational speeds that can be used and
the minimum tube O.D. size that can be tested. Unicorn are probably the only
supplier which offer both types of rotating head; the water chamber
technique can be used for tube diameters up to 120mm maximum O.D. and above
this the contact shoe technique is recommended; the minimum diameter which
can be tested by the contact shoe range of rotary heads is about 40mm O.D. |
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Author |
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C. James |
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C.Phys., M.Inst. P., M.Inst.NDT |
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Company Ultrasonics
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