Alleviate heat exchanger flaws through testing, design and materials

Sheila Kennedy, contributing editor, says use these methods to confirm the integrity of tubes and tube sheets.

By Sheila Kennedy, contributing editor

Heat exchanger flaws and fouling increase fuel usage and maintenance costs and put operations and production at risk. New choices in heat exchanger testing, design and materials are alleviating these concerns.

The depth of corrosion within a tube is an indicator of the heat exchanger’s remaining life.

– Sheila Kennedy, contributing editor

Acoustic testing: Acoustic pulse reflectometry (APR), used in seismic studies, duct investigation and musical instrument research, is the basis for AcousticEye’s non-invasive testing method for heat exchanger systems. APR measures one-dimensional acoustic-wave propagation in tubes and then records and analyzes the reflections produced by changes in the cross-sectional area of the tubular system to detect flaws.

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AcousticEye’s Dolphin 2000 uses the technology to detect leaks, wall loss, full or partial blockages, corrosion and fouling. The Dolphin system emits a series of acoustic waves down the tube and measures the resulting reflections, and its patented software identifies each defect’s type, location and severity. The heat exchanger’s owner might assign thresholds for each type of flaw so that only measurements exceeding the threshold are categorized as flaws. Before accepting or rejecting flagged items, the technician might drill down for further measurement information, including the signal graph.

Because the analysis is automated, tubes can be tested in less than 9 sec without the aid of experts. The Dolphin’s speed and suitability for any tube configuration, size or material makes 100% inspection coverage possible. By comparison, traditional sampling methods, such as standard eddy current or ultrasound testing, are more time-consuming and require skilled professionals to interpret test results.

Helium testing: Another non-destructive approach that detects leaks in heat exchanger tubes and tube-to-tube sheet connections uses helium. The helium leak testing process from SGS, once limited to LNG carriers, has been extended to heat exchangers for the first time by SGS Korea.

The gas is introduced into the heat exchanger’s internal space and overpressurized, causing the gas to pass through welding flaws, cracks and pin holes before entering a sniper attached to a helium mass spectrometer. After an electronic beam ionizes the gas in an ion chamber, the helium-ion collector sends an amplified signal to the indicator. The leak is proportional to signal strength.

Ultrasonic testing: The depth of corrosion within a tube is an indicator of the heat exchanger’s remaining life. The heat exchanger life assessment system (HELAS) from SGS Group measures the ultrasonic immersion length, which is then converted into the corrosion depth inside cooling water or air-fin type tubes. The remaining life is calculated by estimating the maximum corrosion using two extreme-value analysis methods: minimum variance linear unbiased estimate (MVLUE) and maximum likelihood (MLH).

HELAS is faster and more efficient than a more common nondestructive ultrasonic testing method, the internal rotating inspection system (IRIS). IRIS measures wall thinning and pitting in small-bore pipes caused by corrosion and erosion by inserting a probe in a tube flooded with water and removing it as the data is displayed and recorded. Corrosion avoidance: An all-aluminum evaporator eliminates the problem of formicary corrosion seen in traditional copper tube evaporator coils. Delphi’s new MCHX evaporator for indoor settings performs efficiently in both condensing and evaporating modes, while consuming 40% less mass than a competitive product. It has a brazed aluminum construction and microchannel condenser technology that requires less refrigerant. Delphi also announced a new condenser with an optimized alloy that is more corrosion resistant for outdoor applications. The MCHX condenser, originally launched in 2003, was updated to improve robustness and simplify installation. A Delphi MCHX heat pump coil is currently in development.

Anti-corrosive materials: A team of scientists at MIT’s Pappalardo Micro and Nano Engineering Laboratories recently discovered a way to transform polyethylene into a material that conducts heat as well as most metals, while remaining an electrical insulator. The material’s high thermal conductivity could someday be applied to heat dissipating applications such as heat exchangers. In large quantities, the fibers could be potentially cheaper than metals for heat exchanger fins. So far the researchers produced individual fibers in a laboratory setting, and they hope to produce whole sheets of material with the same properties and integrate them into real-world applications.

E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at sheila@addcomm.com.

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