In this third and final installment of our inspection speed series, the case of rotating probes vs. eddy current array probes.
Rotating Pancake Coil Probes
Rotating pancake coils (RPC) probes have been around for decades. They were developed in the 90s to detect stress-corrosion cracking (SCC) and are still common today, complementing eddy current testing (ECT) probe inspections when more precision is necessary. Rotating pancake coils probes use a motor making them turn in a helical pattern inside the tube. As this probe’s name suggests, its surface-riding pancake coils scan inside tubes looking for defects that can elude eddy current testing probes, such as small circumferential cracks. RPC probes are precise, as they’ve demonstrated time and again. They are, however, somewhat fragile and have one major drawback: their low inspection speed.
It’s indeed common to see the inspection speed under 25 mm/s (1 in/s) with rotating probes, 12 mm/s (0.5 in/s) being fairly typical. Inspections involving rotating pancake coils probes are therefore very time consuming, relegating the them to tubesheet area inspection and acting as backup to conventional eddy current testing probes for short sections of tubing only.
Eddy Current Array Tubing Probes
On the other hand, you have eddy current array technology. This technology doesn’t rely on a small motor or rotation speeds, so it offers a better inspection speed and can perform single-pass inspections.
Eddy current array tubing probes rely on an array of coils that’s multiplexed, which yields high-definition data very much like rotating pancake coils probes, without the inspection speed limitation. With eddy current array probes, the inspection speed depends on the number of channels, which relates to the type of probe you’re using, the size of the tube you’re inspecting, and the lowest frequency of your configuration. The lower the frequency, the longer it takes to perform acquisitions during each multiplexed timeslot. Thin walls and low conductivity, however, make for fast inspections because, in this case, the lowest frequency remains quite high.
For example, for the following tubes, the typical inspection speed would be:
- Stainless steel with an outer diameter (OD) of 19.1 mm (3/4 in): 350–750 mm/s (14–30 in/s)
- Brass with an OD of 19.1 mm (3/4 in): 250–400 mm/s (10–16 in/s)
- Stainless steel with an OD of 25.4 mm (1 in): 350–520 mm/s (14–21 in/s)
- Brass with an OD of 25.4 mm (1 in): 200–300 mm/s (8–12 in/s)
In most cases, this is at least 10 times faster than RPC probes.
Of course, this speed comparison is completely worthless if the quality of the data cannot be compared, too.
ECA Probe Data, How Does it Hold Up?
Look at the following data of scans performed on a condenser tube:
ECA Probe Inspection Results 1
You can see that a deep circumferential crack is detected. The results from the ECA probe clearly shows that the crack is circumferential because the the axial C-scan (bottom center) does not display a response signal.
ECA Probe Inspection Results 2
As you can see, the ECA probe shows not only the axial crack, but also a number of smaller circumferential cracks, that elude the RPC probe.
There you have it. It had to be demonstrated, but as you can see, ECA probes can yield the same levels of coverage and precision as those of RPC probes, at unmatched inspection speeds. Actual field use of ECA probes revealed that they can be successfully used in full tube length examinations. Eddy current array tubing probes have detected defects at baffles that went previously undetected simply because RPC probes had only been used at the tubesheet… on account that it was too time consuming.