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In Figure 6, impingement damage of the DSH zone bottom plate from the adjacent tube failure can be seen. This illustrates the importance of removing leaking heaters from service as soon as practical after a leak is detected. The secondary failures caused by the leak can become more damaging than the initial mechanism.
Whenever the unit is coming down for an outage, suspected leaking heaters should be subjected to a tube-side leak test. This can be done simply by leaving the tube-side pressurized, and once the steam and drains to the shell side are secured, opening the shell side drains to check for leakage.
Of course, this is simply a go/no-go check for leaks; it will not tell you which tube is leaking.
In order to identify the leaking tube location, the heater should be subjected to a shell-side pressure test. This can be done a number of ways; the most common involves filling the shell-side with water or pressurizing the shell with air and checking the face of the tubesheet for leaking tubes. The advantage of the
The approach to the repairs (plugging) are predicated on this discovery.
Individual tube hydrotesting
Once a tube failure is confirmed, some utilities will insurance plug the 6 tubes surrounding the failure in order to prevent another forced outage in case one of the adjacent tubes had been weakened due to leak impingement. This tends to dramatically increase the percentage of tubes plugged in the heater (often a criteria for heater replacement). This also can result in localized hot spots in the tubesheet where there is no longer feedwater flow, which might cause additional damage or failure of weld repairs. An individual tube hydrotest of the surrounding tubes can prevent the need for insurance plugging by testing each tube to a much higher pressure than the FW operating pressure. Any weakened tubes on the verge of failure will not pass this go/no-go integrity test and will be plugged. Tubes that pass the test can be returned to operation with a high level of confidence.
Non-destructive examination (NDE)
One of the most common forms of NDE is Eddy Current Testing (ECT), which provides an assessment of the FWH tubing. ECT can identify the location of most defects, determine if the defect is ID or OD originated, and even estimate the size of the defect. ECT has the advantages of having a relatively fast inspection rate and can be used to evaluate heater condition through time by comparing current results with past inspections in order to trend defect growth rates. However, it must be understood that Eddy Current Testing is a subjective test. The results can be affected by such factors as operator experience and proficiency, fill factor (percentage of tube ID cross section filled by the probe) and calibration of the probe relative to a tube standard. Tubes must also be clean and free from deposits as a prerequisite. While ECT testing is very popular in the nuclear industry based on almost exclusive use of austenitic stainless steel tubing, plants with carbon steel and other ferritic tube material in their FWHs require specialized methods of ECT to compensate for the inherent magnetic permeability. ECT is known to have error factors, and in certain instances can miss thru-wall failures. Therefore, we advise caution when using ECT results as the only basis for tube plugging criteria.