In the April 2011 issue of Energy-Tech Magazine, we wrote about the importance of conducting Quality Assurance (QA) checks when replacing a feedwater heater. Assuming a properly specified and conservative design for the full range of load imposition, with the right materials of construction, and fabrication under a stringent quality program by an experienced manufacturer, most current feedwater heater (FWH) replacements are capable of a 40+ year life if they are properly operated and maintained. This article offers an overview of various inspection and testing methods that should be done periodically in order to obtain a picture of the overall health of this important piece of equipment.
With time, all feedwater heaters will degrade. None are immune to failure. The most important thing that can be done to preserve the life of the feedwater heater is to take a proactive maintenance approach. Periodic condition assessment and trending with time is the responsibility of the system/component engineers. The utility must be committed to establishing a programmatic approach
The best life cycle management programs are ones that use a variety of complimentary techniques in order to validate failure mechanisms and help to identify their root cause(s). The most successful maintenance programs do not rely on only one method for assessment. They include, but are not limited to:
- Visual inspections
- Leak testing
- Individual tube hydrotesting
- Non-destructive examination (NDE)
- Tube leak location detection
- Failed tube sampling
The results of all of the above techniques should point to the same root cause(s). However, do not eliminate the possibility of more than one contributing factor to failures.
The simplest inspection that can be done on a feedwater heater is a visual inspection of the accessible areas channel side while the heater is open. While station mechanics might enter the feedwater heater to conduct plugging or other tests, they often are not instructed to also inspect the conditions of other areas susceptible to damage inside the heater channel. This is important since there might be early visible signs of failure. Responsible engineers should take the time to conduct (and document) a channel-side inspection whenever their FWH is open for maintenance. Areas to inspect include:
- Tube inlets and tube-to-tubesheet joint welds for signs of erosion/wastage.
- Pass partition components and sealing surfaces for signs of feedwater bypassing and damage.
- Channel barrel area, especially tubesheet, to channel transition areas (i.e. corner weld joint or radius).
- Inlet and outlet nozzles for signs of wear/surface cracking.
- Manway area – check seating surfaces condition/cleanliness.
The best way to get a look at the inaccessible internals of the FWH is through the use of fiber optic video probes. Probes might be introduced down the tube ID to locate and get a first-hand look at tube failures and characterize the nature of the failure (i.e. is the failure a crack in the tube or a hole, or is the tube completely severed). Additionally, video probes can be introduced through nearby available shell-side connections to try to determine sources and the extent of failed/damaged areas.