Energy-Tech Magazine

ECT vendors will typically provide a color coded tubesheet map that shows the location of tubes with defects and the amount of wall loss determined by the test. While this shows where in the tube field damage is occurring, it is just as important to identify the location of the defect down the length of the tube (i.e. is the defect within the Drain Cooling Zone, Condensing Zone or Desuperheating zone, and is the defect at a baffle location or at the midspan). This information is critical to the primary objective, failure cause analysis.

A baseline ECT is recommended for future survey comparisons. Even new FWHs with no apparent issues should be opened, inspected and evaluated after the first 5 years of operation.

Tube leak location detection
Regardless of the limits of a particular outage, it is important to determine where in the span the tube failed. This information is critical in determining the root cause. For example, failures experienced in close proximity to steam or drains inlet locations might indicate undersized or dislodged impact

Advertisement

plates. Tube failures at baffle/support plates or at the midspan of a tube can be signs of tube vibration. Failures in the U-bends or at the transitions between stressed and unstressed portions of the tube can reveal a material problem. A group of failures in the same location might indicate the presence of foreign material or an operational problem, such as a “wet wall” condition at the exit of the desuperheating zone.  

The location of a tube failure can be quickly determined by a number of methods. Probably the easiest method is by using a video probe to locate the failure and then measuring the length of probe inserted into the tube. Another method is to introduce compressed air into the failed tube from one end, and then insert a tight fitting rod or probe down the opposite end until the leak is covered, at which point the pressure in the tube will increase. The length of rod inserted will give the location of the leak. In vertical channel down heaters, the manometer principle can be employed using clear plastic tubing coupled to the failed tube leg.

When documenting and trending tube failures, it is important to look at the heater three dimensionally by superimposing the location of baffle cuts on to the tubesheet in order to determine which baffles the failed tube contained (see Figure 12). This can provide useful information when determining potential failure causes. Sometimes it might take a little expertise in determining the location of the baffles and cut lines, especially if heater internal drawings are not available.

Failed tube sampling
Following ECT or a tube failure, the best way to get a first-hand look at the nature of the defect is through tube sampling. Pulling a tube section from the heater has several advantages; 1) the defect can be viewed visually or under a microscope, 2) it can be used to confirm the credibility and reliability of the ECT results and 3) the tube can be sent to a metallurgical lab in order to identify suspected failure mechanisms. When sending a tube sample to a third-party laboratory, the utility should provide certain background data to the lab such as material compositions, typical pressures and temperatures experienced, and typical water chemistry. A good lab should be able to provide useful information, such as photo micrographs, chemical and metallurgical analysis, and provide likely failure and/or corrosion mechanisms.

When it comes to prolonging the life of feedwater heaters, the primary objective for the system engineer is to establish a programmatic approach for heater maintenance and to commit to conduct failure cause analysis. A complete life cycle management program is one where the results of all of the complementary tests and inspections are evaluated. The findings of which should all point to the same potential contributing factors, and thus the most likely root cause(s).