Turbine Steam Chest Life Assessment

The optimal approach to condition assessment, regardless of the component involved, is to use a programmatic approach, and steam chest condition assessment is no exception. Steam chests typically vary significantly from one to the next in shape and complexity; consequently, stress distributions vary and damage occurs first and is most advanced at the high stress regions, accordingly. One of the most significant cost drivers in an overall program is an ongoing implementation of NDE that has little technical justification, i.e., implementing NDE as the means of identifying the high stress locations via flaw detection. Keep in mind that flaws can manifest themselves at both macro and micro levels. Therefore, inspection typically includes surface inspection using liquid dye penetrant and/or magnetic particle inspection for macro damage and metallographic replication for micro damage, plus ultrasonic inspection for volumetric inspection of subsurface flaws and flaws at otherwise inaccessible surfaces. In a programmatic approach, the first step is to accurately understand the stresses of the steam chest to determine the appropriate areas requiring examination and monitoring. Then, only after identifying the critical areas on the steam chest, attention turns to defining the optimal techniques and procedures to examine the areas identified. By implementing a focused inspection that concentrates on the critical areas, as opposed to a shotgun approach, the scope, cost, and the frequency of the inspection is significantly reduced. The programmatic approach identifies these critical areas up front and helps to determine the best method for their inspection. The best method is most often dictated by access constraints and limitations at the region of interest. In recent years, significant strides have been made in the use of advanced UT techniques such as linear phased array (LPA) and annular phased array (APA) ultrasonic inspection for sizing cracks in some of the least accessible areas. In many cases, once identified, the damage can subsequently be monitored periodically with only the local removal of insulation. The disassembly of the valve is not required on an on-going basis, nor is full insulation removal in most cases. Finally, once damage has been identified and characterized, be it early form cavitation through to defined cracks, the model used initially to identify the inspection locations is then used to assess the damage in terms of growth rates and failure potential. This information is utilized for a complete Fitness for Service Assessment of the unit. This would include definition of re-inspection intervals, monitoring requirements, and possibly to assess repair/replace options and schedules. These assessments meet the requirements of current Standards in Fitness for Service Assessment. The robust life assessment program presented here includes: 1. upfront analysis of the steam chest to identify problem areas including modeling of the valve, 2. focused baseline inspection of identified potential problem areas, 3. Fitness for Service Analysis utilizing focused baseline inspection results, 4. continued monitoring of critical areas of the valve. This programmatic approach results in a focused, optimized integrity assessment program at minimized cost.Copyright © 2010 by ASME