In the April 2012 issue of Energy-Tech magazine, we wrote about the importance of developing a long-term life-cycle management program for your Feedwater Heaters (FWH). That article stressed that the utility must be committed to periodic condition assessment and trending over time. We identified a number of complimentary inspection and testing techniques to accomplish that objective. As heaters degrade and eventually fail, the understanding of why damage is occurring is just as important as the short term resolution of arresting the current leakage event. It is for these reasons that failure cause analysis (FCA) is the primary objective in FWH life-cycle management and, as such, must remain an integral consideration throughout the corrective maintenance process.
This article will discuss details in maintenance repair procedures from two distinctive approaches once failures are experienced. One perspective is based on forced outage situations with very limited time available. Procedural guidelines are established about the approach of satisfying immediate resolution of
Forced outage approach
Based on an indication of leakage during operation, the heater should be isolated and removed from service as soon as practical to prevent collateral damage from leak impingement. The decision to authorize, or when to actually initiate a forced outage to address a leaking FWH, is strictly a Plant Management judgment. Certain FWHs pose different financial impacts on unit operating loss, depending on the respective locations within the system. Operations might be able to quantify the severity of the failure based on a tube-side pressure test. Maintenance should pre-stage tooling and equipment to address anticipated activities. Some of these activities include channel access, lift/rigging device operation, replacement gaskets, etc. Managers should review past job files (if applicable) regarding the specific heater to be addressed to look for trends. Once access is gained, and the partition plate covers removed, station mechanics should conduct channel side inspection, keeping in mind that working conditions might be significantly hampered by the constraints of the situation. The following are the recommended subsequent steps based on a forced maintenance outage.
- Shell Side Test: With the heater isolated, conduct a shell side air test (approximately 30-40 psig), to locate leaks in the tube field. Leaks must be differentiated as either tube failures vs. tube weld joint or roll joint failures, where leakage is apparent through the ligament surrounding the tube. Repair procedures are predicated on this determination.
- Limited Individual Tube Hydrotesting (ITHT): Forced outages often preclude the necessary time or required conditions for NDE/ECT activities. The specific situation might limit maintenance personnel to do nothing more than shell side testing to locate leaks and move directly to plugging the failures (temporarily) and plan to re-address them at the next scheduled opportunity. The risk in doing so must be understood and accepted by management, since leak impingement may have caused collateral damage to adjacent tubes. Also, failures may be catastrophic and/or tubes may be totally severed. Without the time available to employ the more fragile instruments like NDE probes or fiberoptic videoprobes, maintenance should – at a minimum – attempt to perform ITHT of 2-3 rings of the failure’s adjacent tubes to identify any weakened tubes and minimize the possibility that they will not subsequently fail upon return to service. This approach is preferred to “insurance” plugging of surrounding tubes. ITHT procedures also were previously discussed in our April 2011 Energy-Tech article.
- Tube Leak Location Determination (TLLD): If remaining forced outage time frame and conditions allow, maintenance should try to ascertain the location (in the tube span) for all failures identified by shell side test and ITHT procedures. If this cannot be done at this time, it should be addressed during the next planned outage.
- Videoprobe Inspection: Under the restrictions of a forced outage situation, videoprobe inspection of the failed area(s) might not be possible. Typically, videoprobe inspections are tabled for planned outage situations, but if the quantity of failures and the possibility of catastrophic damage is a concern, then further inspection utilizing fiberoptics should be considered. Of major concern is when the severity of failures indicates a tube might be severed and damaged so extensively that return to operation without addressing some sort of stabilization of loose and broken sections could cause additional collateral damage, even though tube ends might be plugged.
- Tube Plugging Under Forced Outage Conditions: Failed tube plugging under forced outage conditions must consider the primary objective for FWH maintenance – Failure Cause Analysis. Assuming that the limited outage time frame and the adverse conditions within the heater have prevented maintenance personnel from making all the necessary tests and inspections to permit analysis and condition assessment, it is recommended that plugging be categorized as temporary under these circumstances. Management must therefore be committed to revisiting the specific heater in question at the next available planned unit outage. To this end, the selection of a type of plugging device that allows reliable quick installation under the potential of adverse conditions, while offering the ease in removability to readily allow future inspections/tests, is recommended for forced outage situations.