Dual Mode Pulse Reflectometry – A New Approach to Heat Exchanger Tube Inspection

Background

The day the first heat exchanger came rolling off the assembly line, the need for efficient inspection and maintenance techniques followed closely on its heels. Tube and shell heat exchangers are required to operate continuously in tough conditions for years, coping with thermal cycling, corrosive fluids on the tube and shell side, vibration and fouling of many different types, all collaborating towards degrading the performance of the unit and causing its eventual failure. Research has shown that regular inspection and maintenance are usually more effective than run-to-failure strategies, and therefore heat exchanger inspection technologies have been continuously evolving.

Several physical principles have been employed by traditional inspection methods currently in widespread use:

1. Electromagnetics: inducing electric currents or magnetic fields in the tube walls, and measuring the degree to which they are disrupted by various defects.
2. Ultrasonics: sending ultrasonic waves perpendicular to the tube wall and measuring reflections caused by defects.
3. Optics: visual inspection of the internal tube surface through a long flexible optical fiber.

Methods employing the above principles utilize probes that can only inspect the section of tube in their immediate vicinity. Therefore the basic property common to all of them is the necessity of traversing the entire tube with a probe. This is not an issue to be taken lightly, as it has several consequences. The first is manpower; the minimal team for conducting such an inspection is usually two technicians, one performing the task of pushing and pulling probes, while the other is operating the equipment displaying or storing the measurements. The second issue is obviously wear and tear on the probes, especially when abrasive deposits can be found on the tubes' Inner Diameter (ID). In extreme cases probes might become stuck, to the point that they must be abandoned inside one of the tubes. The third issue is inspection time. Clearly, a limit on the inspection speed is the time required to push and pull a probe through a single tube. Considering the fact that heat exchangers range from tens to tens of thousands of tubes, a single additional second in inspection time per tube can add hours to the overall job. Given tight turnover periods found in most industries, the simple fact is that most inspection jobs resort to sampling (i.e. inspection of only a fraction of the overall tubes in a given heat exchanger). Evidently, a technique that can address all of these issues would be of great interest to heat exchanger operators, enabling them to reduce both the cost and time of inspections.