Heat exchangers are essential to the operation of many systems in various industries, processing plants and electric power generation plants, for example. A unique type of heat exchanger (HX) was introduced several years ago: the Twisted Tube® HX. They offer the highest efficiency, but have always been a challenge for periodic inspections. Until now.

 Twisted Tube heat exchanger

The Challenge

A twisted tube is composed of two straight, circular sections near the tubesheets, separated by a length of helical, oval tube. This helix creates a small, central circular passage along the length of the tube where traditional bobbin probes can be pushed and pulled.

Twisted Tube Bundle

However, standard ECT probes deliver suboptimal performances — detecting, characterizing, and sizing flaws are problematic because of poor signal-to-noise ratios. Flaws on the wall furthest away from the tube’s center (crest) produce much smaller, distorted signals than same-size flaws in the narrow section, closest to the tube’s center (dip). This is because of the liftoff created by the tube’s geometry between its walls and the ECT probe. This amount of liftoff translates into a lower probability of detection (PoD) and less accurate sizing.

Twisted Tube Cross Section

Moreover, the common sizing method for determining flaw depth is inaccurate because of the reduced sensitivity and the large tube signal variations coming from the various flaw locations (crest or dip).

Internal rotary inspection systems (IRIS) also suffer from the non-linear geometry, as reflecting echoes are not perpendicular to the inspected wall. It can therefore not be considered as an alternative.

The tubes in such heat exchangers come in a variety of pitches, diameters (typically 19.1–25.4 mm [0.750–1 in]), and non-ferromagnetic and ferromagnetic materials.

The Solution

Eddyfi set forth to develop a probe that would fit into most Twisted Tube® HX and dynamically adjust to the changing geometry, minimizing liftoff and optimizing inspection results.

Twistec probe

The patent-pending Twistec™ probe is designed with a rotating head equipped with two “feelers” that press down the coils they contain against the twisted tube’s inner surface. The probe also incorporates a pair of oval coils that match the helix pitch of the twisted tube. The Twistec probe is therefore always close to surface under test, yielding significantly more precise inspection data. The feelers can compress, which enables the probe to fit through the circular section at the tubesheet.

At a pulling speed of approximately 0.3 m/s (12 in/s), the probe was able to inspect 1500 tubes at a rate of about 60 to 80 tubes an hour, yielding results that allow defects to be much more clearly identified. The Twistec probe’s signals are less noisy than those from a conventional bobbin probe, which yields more reliable results.

The Benefits

The Twistec’s revolutionary rotating probe head with its adaptive feelers and oval coils minimize liftoff and noise from the tube’s helix, while optimizing results.

The probe enables efficiently and reliably distinguishing between inner/outer-diameter defects, and determining whether they are in the crests or dips of tubes. These results are easier to analyze and much more accurate than those obtained with conventional bobbin probes in this particular application.

Twisted Tube® HX operators will have an easier time extending the useful life of this mission-critical piece of equipment with the high-quality results of the only solution capable of giving reliable results.

This is only one of the many demanding challenges that Eddyfi has risen up to until now. Don’t hesitate to ask one of our experts about your own project specifications.

Twistec results in Twisted Tubes

Twistec results in Twisted Tubes