ULTRASONIC TRANSDUCERS FOR PIPE WELD TESTING
or NOW YOU SEE IT, NOW YOU DON'T
Back in the 1970's, a group of engineers at Republic Steel looked into ultrasonic testing of longseam tube welds. In particular, they were concerned with the huge variations in signal level as a conventional transducer is shifted towards or away from the weld line.
So severe are these echo variations that a defect can be lost completely if a probe is misplaced by as little as 2mm. Therefore if conventional transducers are used to test ERW welds, the probe to weld spacing must not only be exact,
it must remain exact throughout the test.
Graph 1 shows what happens when a conventional probe/wedge assembly is used to test an ERW weld in a typical Oilfield Tubular product. The test defect is a 10% o.d. notch, and it becomes quite “invisible” if the probe happens to be a millimetre or two away from its ideal position. Note, that for this and all other tests, we set the equipment gain to give 100% full screen signal at the highest peak, then assumed a 50% alarm trigger level (shown in red) is being used to decide detectability. This is quite similar to real operating conditions.
The Republic Steel group
came up with the concept
of the “involute” transducer,
which was configured to
produce a very large beam
at equal angles of incidence
to the tube surface, and thus
generate a more uniform
ultrasonic beam through the
weld profile. In 1980 U.S.
Patent 4,195,530 was
awarded to B.J. Ross et al.
and assigned to Republic
Steel. Republic licensed
the technology to other
users, but deep pockets
were needed to enjoy the
benefits of this innovation.
Republic Steel is no more, but
involute transducers have lived
on, and they are still obtainable
from a new owner of the patent.
InspecTech, a few years later, pursued a different line of enquiry, and we came up with our current technology, which is
unrelated to involutes.
Natalia (Rosa) D'Elia
Natalia (Rosa) has just passed her 15 year mark as an employee of Inspectech Analygas Group, and she deserves a medal for her dedicated long service to the Group.
Rosa looks after our electronic assembly functions, a big job considering the number and complexity of components that go into a modern circuit card. Today much of her work requires the use of optical instruments when dealing with surface mounted components.
It is thanks in part to Rosa's dedication to the highest quality standards that our electronic equipment has such an excellent record of longevity. Many of our systems in the field have 10 or more years of operation with never an electronic failure.
Meet The People
We wish all our readers a safe and happy holiday season and a defect-
Recently, a customer who uses both InspecTech probes and involute probes came to us with some testing questions, and we had the opportunity to make a direct comparison of probe technologies. The results were interesting enough that we would like to share them with our readers. Two important parameters were examined.
(1) The distance-
(2) The ability to detect small defects.
The results are shown in graphs 2, 3 and 4.
Comparing Graph 1 with Graphs 2 & 3, it is immediately obvious that the more advanced probes offer a huge benefit over the conventional unit. The signal level is more uniform with probe position, meaning that defects are much less likely to be missed because of weld wander. Graphs 2 & 3 both demonstrate a wide range of probe to weld positions that will not result in missed defects.
Graph 4 tells a different story. Here we see a
comparison of responses to different sizes and
types of defects. Most people standardise their
U/T on 10% notches that are 1” (25mm) long,
although some codes allow for twice that length.
What happens when the test defect gets shorter?
We have plotted the response of both probes to
exactly the same 10% test defect, but also when
it is reduced to 0.5” and 0.25” long, and the results
tell an interesting tale. What happens further,
when the defect becomes smaller and rounded
(penetrators anyone)? Here you can see why
InspecTech probes routinely pick up penetrators
less than 1mm long, whereas other systems cannot.
The conclusion is that while expensive involute
probes offer superior distance-
characteristics, standard InspecTech probes are
just as good, but work much better on small defects.
It is not difficult to do the tests that have been described here, and the test pieces are easy to make. Everybody concerned with weld line testing on ERW product should run similar tests. If you don't like the results you get, call us.