ASTM
E273-15 does not have clear specifications when it comes to coupling checks. It
does however state that 100% volumetric inspection of the weld and entire pipe
length shall be made by the NDE system.
This
gives rise to a very important issue. How do we know that the ultrasound beam
made it to the inspection area and if it can return to the ultrasonic probes?
All
serious NDE system manufacturers therefore provide a coupling check during inspection.
Below we will cover the main methods used in achieving coupling check and why
some methods are better than others.
The main
method used by IIS Ltd and a few other NDE system manufactures is called Pitch-Catch.
It is the most robust way to assure the ultrasound beam made it to the
inspection area and back to the probes. This setup is illustrated in the
picture attached.
Pitch-Catch
Coupling Check Benefits
- Requires two
opposing probes used in pulse-echo for inspection and pitch-catch for coupling
check. One
probe sends a pulse and its pair across the weld listens for this pulse.
- Coupling
Gates/Alarms/Paint indicate when coupling signal is to low or not at all.
- In
addition, at IIS Ltd we also use a flow meter on the main coupling lines.
Should the flow decrease, the test head lifts to save the probes from excessive
heat damage. This also triggers the above alarms and is used as another layer
of protection. It saves customers thousands of dollars should a coupling pump
fail, or lines get clogged.
- Oil on
the pipe, as well as excessive rust or dust, attenuates or blocks the coupling
signal triggering an alarm.
- Aging or
damaged transducer will have an attenuated or missing coupling signal triggering
an alarm.
- Misaligned
transducers can have a significantly attenuated coupling signal, triggering a
coupling alarm as well.
- This
technique also provides valuable monitoring of the transducers.
Unfortunately,
not all NDE system manufacturers use a Pitch-Catch coupling check. Below you
will see why other methods aren’t as robust and effective at ensuring coupling
check is performed.
The most
common coupling check is performed using an internal reflector in the probe
holder. Usually this is very common when using focused probes. A percentage of
the beam energy gets reflected from the pipe surface (entry point) back inside
the probe holder. Using a reflector, this beam can be reflected back to the
probe, and the signal produced gets monitored.
Internal
Reflector Coupling Check Issues
- By using
this method, we essentially check that the ultrasound beam made it to the top
of the pipe and back to the transducer NOT to the weld.
- Oil on
the pipe can significantly attenuate or block the beam from reaching the weld. However,
in this case the coupling signal might not even be lost or reduced at all.
- Depending on
the design of the transducer holder and reflector, air bubbles may accumulate
on the reflector causing a false coupling alarm.
- Aging or
damaged transducer can still provide enough signal for a coupling check but not
enough for a full inspection.
- Misaligned
transducers will still produce a coupling signal as this signal comes from the
internal probe reflector and not another transducer across the weld.
Other
NDE systems also perform coupling checks using the following methods which are
even worse and should be avoided.
- Monitoring
the interface signal from the surface of the pipe. This has all the issues that
the internal reflector coupling check method has.
- Monitoring
the Main Bang (initial pulse) signal. This method should not be allowed at all.
Even with a damaged transducer the Main Bang signal is still strong.
Knowing
the above, you have a
better understanding of your system and it helps you troubleshooting and maintain the system.
At the
end of the day, if you want to test a weld using ultrasound, make sure that the
beam made it there and back successfully. And be sure this is recorded during
your inspection in order to show an API auditor that you truly tested the
entire weld volumetrically and through the entire pipe length.
Daniel
Lucaci
CEO IIS
Ltd.