As heat exchanger tubes age, their failure rate becomes exponential (Figure 1). Excessive forced outage time is the consequential result of tube failures becoming so excessive that, e.g. entire strings of feedwater heaters must be valved out of service for months until a new feedwater heater can be specified, purchased, delivered and installed.

 

Consequently, high priority should be given to performing condition assessments and monitoring.  The results provide early warning of impending or existing problems.  Therefore, appropriate corrective action(s) can be taken prior to the occurrence of an adversely signifi­cant impact on availability and/or plant performance.  Furthermore, the utility staff gains assurance that the heat exchangers will be main­tained in the non-exponential approach portion of the tube failure rate usage curve.

 

The distant advantage of starting such a program early in plant life is clear.  However, implementation and timely corrective action (repair, retube, rebundle or replace) at any stage of a plant's life will provide beneficial in meeting life extension goals.

 

HES assisted EPRI in formulating the three level approach method we employ.  We believe this method provides an essential means to obtaining long term, cost effective plant operation.  It encompasses discreet increments of effort and outage requirements and allows a condition assessment to be concluded as soon as the desired level of confidence in heat ex­changer assessment is achieved.

 

Use of this three level approach promotes coherent planning since the need for potentially expensive and time consuming inspections can be determined in advance.  That information permits integra­tion of heat exchanger life man­agement requirements - including inspection, repair and replacement - with cash flow planning input to minimize budget impact.  Moreo­ver, this can be accomplished within the normal unit outage time frame.

 

Level I

The Level I assessment utilizes:

(a) original heat exchanger, design data, (b) unit historical data, (c) minimal visual inspection (e.g. Figure 2), (d) industry experience to determine the most likely damage mechanisms and (f) the extent of probable damage experienced throughout the operating history of the unit.

 

Level I results, coupled with the current and expected future operat­ing modes of the unit, permits experienced engineers to estimate the length of time the component may be operated before failure is likely to occur (remaining life).  Con-sequently, these results permit a Level II examination to be sched­uled at a convenient time, before failure is expected.

 

Level II

The Level II assessment involves performance of detailed nonde­structive  examination (NDE) of the heat exchanger to refine the Level I remaining life estimate.  An example is shown in Figure 3.  The Level 2 effort is more expen­sive and time consuming than Level I.  But, it is often necessary because the available data is often insufficient to obtain an accurate condition assessment.

 

Level III

The Level III assessment is used less frequently.  It involves more invasive techniques, e.g. perfor­mance of corrosion failure analysis,  in-stallation of corrosion monitoring equipment, etc.   See Table.

 

Experience has shown that t`he three level approach is cost effec­tive.  It provides a means to achieve excellent integration of component life assessment into normal utility planning, operating and main­tenance practices.