Acoustic pulse reflectometry (APR) is a tube inspection method that has been gradually gaining acceptance as a tool for heat exchanger inspection. Different types of heat exchangers operating in different operating environments have different failure mechanisms, making some of them more suited than others for inspection by APR. Finned tube heat exchangers are a typical example of heat exchangers particularly conducive to APR inspection.
The reason APR is particularly useful on finned tube heat exchangers is that it is purely an internal pipe inspection method. This is a limitation where tubes are susceptible to external damage by corrosion or abrasion from support plates. In finned tube exchangers, however, the tubes often rest on top of each other, supported by the fins. There are no support plates, and in cases where the external fluid is forced air, there are no hostile chemicals on the exterior of the tubes.
Dual-cycle geothermal power plants often employ this type of heat exchangers. In this type of geothermal plant, hot brine heats an organic liquid having a low boiling temperature, turning it into vapor. This higher-pressure gas expands through the turbine, producing power. The lower-pressure gas is then condensed using finned tube heat exchangers (condensers in this case), which are cooled by air forced across the highly finned heat exchanger tubes by large fans. Thoroughly inspecting these heat exchanger pipes has been problematic, until now.
How APR Works
The theory behind APR is to inject a wideband acoustic pulse into a pipe. This pulse acts as a form of “virtual probe.” As long as the pulse encounters no discontinuities, it continues to propagate down the tube. Whenever a discontinuity is encountered—such as a blockage, expansion (due to wall loss, for example), or hole—a reflection is created. The reflected waves propagate back down the tube, where they are recorded for analysis (Figure 5).
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| 5. Different discontinuities have different signatures. In tubes belonging to heat exchangers, any sensed discontinuity represents a fault. Source: AcousticEye |
The ultimate purpose of tube inspection should be to examine tubes as rapidly as possible and then analyze the measurements both rapidly and using objective criteria. Keeping this in mind, APR is very well-suited to this task on both counts. The pulse acting as a probe travels through the tube at the speed of sound, resulting in inspection rates much faster than those possible with other techniques. Measurement of a single tube takes only several seconds, and there is no physical probe to push through the tubes or become stuck. The resultant measurements can then be analyzed by appropriate signal processing software, which is faster and more objective than human analysis.
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