Condition Assessment of Dissimilar Metal Welds
DMW UT Scan with OD (Upper Left), Mid-Wall and ID (Lower Right) Damage
The Problem: Dissimilar metal welds in boiler tubing eventually fail due to intrinsic differences in thermal coefficient of expansion.
The Solution: Perform a volumetric examination of the DMWs to determine the extent of damage and repair/replace those exceeding the permissible limit of damage.
Traditionally, the volumetric examination was conducted with radiography or removal of tube samples. Structural Integrity (SI) has developed a system consisting of focused UT probes and a hand scanner that can reliably detect internal creep/fatigue damage in DMWs.
The above photo shows a scan of a heavily damaged tube that was subsequently removed from the boiler and the presence of damage confirmed by metallography.
SI can provide this testing service, which includes the following deliverables:
- The past performance of the unit is reviewed and estimates of tube damage are calculated using DMW LIFE, a program developed for ESEERCO. Plant operating data and results of previous assessments are reviewed to identify critical inspection areas.
- During inspection, SI can provide daily updates of tube inspection results in order that samples can be removed for metallographic confirmation of the UT findings. A site final report is generated with damage levels of all DMWs investigated.
- SI provides copies of the UT signals (see figure above) on computer diskettes for use as a baseline for future inspections and independent assessment.
By providing a map of relative damage levels throughout the boiler, a plan of periodic inspection and replacement can be developed for DMWs, which can maximize the boiler availability while minimizing the number of DMWs that need to be repaired/replaced during an outage.
"The UT technique for finding damage in DMWs is a good screening tool, and when combined with metallurgical confirmation, provides a very useful method for determining the condition of DMWs throughout the boiler"
- Alex Bonnington, Potomac Electric Power Company
For more information on SI's Tube Life Prediction capability, please contact SI.
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