News & Views, Volume 49 | Hydroelectric Penstock Inspection: Field NDE Services
By: Jason Van Velsor and Jeff Milligan
Our talented experts, using the latest technology and methods, deliver unmatched value, actionable information, and engineering knowledge for the management of your most critical assets.
Many of the penstocks used in the hydroelectric power industry have been in service for over 50 years. Often with older components, historical documents like, as-built drawings and proof of material composition no longer exist. This information is critical for inspection, repair and replacement decisions. SI has the expertise to assist hydro clients with everything from material verification, inspection, and fitness-for-service analysis to keep penstock assets in-service for many more years to come.
Structural Integrity (SI)
Longitudinal seam-welded hot-reheat steam piping operating in the creep regime is a continuing life-management challenge for many older fossil-fired power plants.
Structural Integrity recently had the opportunity to support a client’s emergent needs when their Standby Service Water (SSW) piping system experienced a pinhole leak just downstream of a valve. Concerned about other locations in the piping system with similar configurations, the site asked SI to assist with the expedited development of assessment and disposition plans for these other components. In response, SI was able to lean on our core competencies in failure analysis, advanced NDE inspection, and flaw evaluation to develop and deploy a comprehensive solution that met our client’s expedited timeline and helped them to mitigate the threat of future unplanned outages. The following sections outline how SI utilized our in-depth knowledge, cutting-edge technology, and world-class engineering to meet our client’s needs.
Grade 23 is a creep strength enhanced ferritic (CSEF) steel that was designed to offer similar creep strength to Grade 91 but with lower Cr content and, in the original concept, fabrication without pre- and post-weld heat treatment making the material attractive for the furnace wall tubes of ultra-supercritical coal plants where T12 has insufficient strength and T91 would be too complex to fabricate. Experience gained with T23 has shown that pre-heat is necessary and that post-weld heat treatment should also be performed when the material is employed in “high restraint” applications such as furnace wall tubes. Like other CSEF steels, T23 is very sensitive to heat treatment, and care must be taken to ensure that hard, brittle microstructures do not enter service – particularly in high restraint applications such as furnace wall tubes.
Acid dewpoint corrosion can occur in conventional and HRSG units in locations where temperatures fall below the sulfuric acid dewpoint temperature. This can occur when either the tube metal temperatures are below the acid dewpoint so that condensate forms on the metal surface, or when flue gas temperatures are below the acid dewpoint, so that the condensate will form on fly ash particles.
