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Fouling and corrosion of heat exchanger tubes and fin fan air-cooled condenser tubes contribute to lost heat transfer and production capacity in a variety of industrial settings, from petroleum refinery process units to chemical production facilities. When these tubes are mechanically cleaned and non-destructive tests are done to assess tube condition, fouling processes can be reversed and plant engineers can draw precise conclusions about future unit performance and plan for appropriate maintenance.
Tubes that are filled with deposits cannot perform as they are designed to, and the fouling deposition can result in tube damage like pitting and cracking, which over time can weaken and thin the wall of a tube. This can lead to tube leaks or forced unit outages due to tube failures. Preventing these events is vital to unit reliability and longevity. Even one tube leak can bring a unit off-line, where the cost for lost production, labor to repair the leak, and unit startup can result in extreme expense.
Figure 2, Conco
(Figure 2, right, shows a heat exchanger tube before and after mechanical cleaning.) Once the tubes are free of debris, it is possible to precisely evaluate tube integrity with the use of the non-destructive testing methods Eddy Current Testing (ECT), Remote Field Testing (RFT) and Near Field Testing (NFT). The data collected with these methods can provide vital information on the projected life expectancy of the unit.
Eddy Current Testing (ECT) utilizes electromagnetic induction to evaluate tube walls for imperfections and thickness. When the tube wall is uniform, a steady reading will result, but when pitting, corrosion, cracks or wall thinning are present, there is a deviation in the magnetic field. When an ECT inspection determines that a tube has poor integrity and is a risk to heat exchanger or condenser reliability, plugging is recommended.
Figure 1, below, shows an ECT probe and the magnetic field it creates.
Figure 1, Conco
Remote Field Testing (RFT) is strongly associated with ECT, and is often referred to as Remote Field Eddy Current Testing. RFT is primarily used to inspect ferromagnetic tubing since conventional Eddy Current techniques are ineffective on ferromagnetic materials because the permeability of such materials is often too high to get good penetration, unless operating at very low frequencies.
Near Field Testing (NFT) is a more recent non-destructive testing technology, and is a fast and cost-effective solution for the inspection of fin-fan carbon-steel tubing. The NFT technique relies on a simple Eddy Current probe design that provides very straightforward signal analysis. Like other non-destructive techniques, NFT targets the detection of internal corrosion, cracking and pitting in carbon steel tubing. While ECT penetration is limited to the inner surface of a heat exchanger or condenser tube, NFT probes are not affected by the unique fin geometry on the outside of air-cooled condenser tubes.
Figure 3, right, illustrates a Near Field probe in action.
Figure 3, Conco
Cleaning and non-destructive testing of heat exchangers and condensers is a proven strategy to maintain unit reliability for the long term. It is essential for plants to choose a reputable and experienced service provider for tube cleaning and non-destructive testing, and outcomes are markedly better when both of these necessary elements are conducted by one experienced provider.
Contact Conco Services Corporation for more information on mechanical tube cleaning and non-destructive testing. www.conco.net
