Coupling Check Requirements And Differences

ASTM E273-15 does not provide clear specifications for 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 raises a very important issue. How do we know that the ultrasound beam reached the inspection area, and whether 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 to perform a coupling check and explain why some are better than others.

The main method used by IIS Ltd and a few other NDE system manufacturers is called Pitch-Catch. It is the most robust way to ensure that the ultrasound beam reaches the inspection area and returns 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 the coupling signal is too 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 alarms mentioned above and serves as an additional layer of protection. It saves customers thousands of dollars should a coupling pump fail, or lines get clogged.

- Oil or grease on the pipe, as well as excessive rust or dust, can attenuate or block the coupling signal, triggering an alarm.

- An aging or damaged transducer will have an attenuated or missing coupling signal, triggering an alarm.

- Misaligned transducers can significantly attenuate the coupling signal, triggering a coupling alarm.

- 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 the 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, the beam can be reflected back to the probe, and the resulting signal is 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 or grease 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.

 - An aging or damaged transducer can still provide enough signal strength 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 of the internal reflector coupling check method.

- 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.

With the above, you have a better understanding of your system, which helps you troubleshoot and maintain it.

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 to show an API auditor that you truly tested the entire weld, volumetrically and along the entire pipe length. 

Daniel Lucaci

CEO IIS Ltd.