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Inspecting Heat Exchangers Across Different Industries

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This page outlines important considerations for eddy current inspection equipment required to effectively inspect various tubes used across industries. The focus is on identifying tube alloys, wall thicknesses, and the corresponding frequency ranges necessary for inspection.

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Rather than detailing every tube size and material within each industry, the focus here will be on determining the minimum and maximum frequencies required for inspection. This is done by identifying the lowest and highest frequencies needed for different tube types.  A range of test frequencies centered around an optimum detection frequency is the normal approach.

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For each tube’s alloy and wall thickness, the F90 frequency will be calculated.  The F90 frequency is the optimum detection frequency and gets its name because this frequency provides a separation angle of approximately 90 degrees between shallow I.D. and shallow O.D. flaws. The F90 can be calculated using an eddy current slide rule if the resistivity or conductivity of the material is known. Inspections typically occur at this frequency, as well as at half and twice the F90. Selecting equipment based on these calculated minimum and maximum frequencies will ensure reliable performance. For the best results, it may be advisable to use equipment capable of operating at half the minimum and twice the maximum frequency.  Most modern instruments will have a wide-frequency range available for use.

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Tubes without fins, often referred to as prime surface tubes, are typically inspected using a differential bobbin coil. The F90 and twice that value are often mixed to suppress signals from support plates, helping to detect small volume defects, especially near support plates. A secondary differential channel operating at half the F90 is useful for confirming whether detected anomalies are actual pit defects or other harmless features such as roll stops or magnetic inclusions. A fourth channel, in absolute mode, is recommended for detecting general thinning caused by erosion or corrosion. A four-channel eddy current instrument is ideal for these inspections.  Even though small, portable instruments normally used for surface testing can be used for limited tube testing, large scope exams require the use of instruments geared more for tube testing.  Those instruments will have more mixing capability, more display options, and typically have the means to record and store large quantities of raw data, as tube testing is normally performed after the data has been recorded.  Tubing data records are almost always used during subsequent inspections to review historical signals for change and growth.

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Fossil-Fuel and Nuclear Balance-of-Plant (BOP) Heat Exchangers

Historically, most main steam condensers in power plants were made from aluminum brass or copper-nickel alloys. These tubes, with diameters of 1 inch and a wall thickness of 0.049 inches, require inspection frequencies as low as 6 kHz, one of the lowest used in power plants.

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More recently, condenser tubes are increasingly made of titanium with wall thicknesses between 0.020 and 0.028 inches. For these thin-walled titanium tubes, inspection frequencies as high as 1.1 MHz (twice the F90 for 0.020-inch titanium) may be required.

Low-pressure feedwater heaters typically use aluminum brass or 90/10 copper-nickel tubes with similar wall thicknesses to condenser tubes, so lower frequencies are not necessary. High-pressure feedwater heaters, made from stainless steel or high-alloy steel, may require frequencies below those used for titanium condenser tubes due to their thicker walls (e.g., 0.049 inches or greater).

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Pressurized Nuclear Reactor (PWR) Steam Generator (SG) tubing inspections

Nuclear steam generator tubing is normally tubed with Incalloy 800 or Inconel 600/690 tubing. Most domestic first-generation nuclear power plant steam generators were tubed with mill-annealed Inconel 600, but those generators were plagued with corrosion cracking and almost all of those steam generators have been replaced.  Replacement steam generators are almost always tubed with thermally treated Inconel 690 (I690TT), due to its high resistance to corrosion.  Due to the low conductivity of Inconel, higher test frequencies are used.  ASME code governs PWR SG exams, and dictates that multi-frequency exams must be used, operating in both the differential and absolute modes. The “Prime” test frequency must be capable of producing a separation angle between 50-120 degrees between the signal from the 100% TWH and the 4X20% flat bottom holes.  For I600 having a wall thickness of 50 mils (.050), the F90 frequency is around 200 kHz, so a 2X F90 frequency is necessary to produce the increased phase separation required by the ASME code.  A good guideline for determining acceptable test frequency is to determine F90 by using a slide rule or by calculation, and then selecting a frequency 2X F90, half F90, and 1/10th prime frequency for locating support structures and confirming deposits, permeability, etc.  Keep in mind that there is no strict or exact test frequency.  Acceptable test frequencies are often determined by trial and error, and consist of a range of test frequencies enveloping the optimum (F90) detection frequency.

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Chemical, Petrochemical, and Fertilizer Industries

In chemical plants, heat exchangers use a variety of materials, including carbon steel, aluminum brass, high-alloy steels, and titanium. As many heat exchangers in chemical plants are constructed from carbon steel, remote field testing (RFT) is normally used instead of conventional eddy current testing.  Aluminum brass tubes in these plants often have thicker walls due to the corrosive chemicals used. For example, a tube with a 0.083-inch wall thickness requires an F90 of 4 kHz and a one-half F90 of 2 kHz.

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Air Conditioning Heat Exchangers

Heat exchanger tubes in air conditioning systems are typically made from copper alloys and feature integral fins, with the fins skipped at support plate areas. Occasionally, these tubes are made from 90/10 copper-nickel or titanium. Non-finned tubes, or “prime surface tubes,” are sometimes encountered as well.  The wall thickness in the skipped zone of a copper tube can reach up to 0.049 inches, requiring inspection frequencies as low as 1.5 kHz. Due to the sampling rate limitations of time-multiplexed instruments, these are not ideal for inspecting such tubes at low frequencies.  On the higher end, finned titanium tubes with a thickness of 0.032 inches may require frequencies up to 400 kHz. Most modern multi-frequency instruments are equipped to handle this.

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In air conditioning systems, finned tubes have support plates positioned further from the probe coils, and the lower frequencies used in copper tubes generate smaller support plate signals. As a result, these tubes can often be inspected without the need to mix frequencies in differential mode to eliminate support plate signals.

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For air conditioning heat exchanger inspections, a two-frequency, two-channel eddy current instrument is generally sufficient. However, inspection companies should be equipped with four-channel instruments for heat exchangers that use prime surface tubes.

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