The inspection team from SVI Dynamics recently performed a thorough inspection of the exhaust duct and stack for a Virginia power plant and made recommendations for important repairs.
What Type of Inspection Was Performed?
An inspection of the plant’s exhaust duct and stack was performed using thermal imaging to find outer casing hot spots. The exhaust duct liner was inspected, as well as the silencer duct roof liner, to find areas needing repair.
What Did the Exhaust Duct and Stack Inspection Find?
The SVI Dynamics team found multiple areas of concern during the exhaust duct and stack inspection. There were several hot spots in the outer casing plate discovered with thermal imaging. Hot spots that exposed temperatures higher than 300⁰F were documented. The main concerns for hot spots on an exhaust system are personnel protection and material fatigue. Typically areas that are reachable by personnel should maintain a 140F average surface temperature for safe operation. In areas that exceed that temperature, the liner may need to be repaired or upgraded, or personnel protection on the exterior of the duct or stack may need to be installed. The exhaust system has an internal liner, so the outer carbon steel casing stays within operating conditions for maximum temperature. If carbon steel is exposed to excessive heat on an ongoing basis, the material will fatigue and possibly crack, leading to structural integrity issues of the system.
A visual inspection of the exhaust duct liner allowed SVI to discover multiple liner studs broken off from the scallop plates, which will cause a rapid failure in the duct liner. Inspectors found multiple batten channel studs were broken off, which caused the channel to become displaced from its position, exposing the liner sheet edge to the flow. Additionally, broken welds were found in the test ports plug plate and CEM port liner plate.
Upon inspection of the silencer duct roof liner in the baffle trailing end area, it was found that the majority of the baffle upper guides and roof liner sheets were damaged during operation and during the removal of the existing trailing baffles. Some of the studs securing the baffle guides had pulled out of the liner. This was caused by the side dynamic loads applied on the baffle guides. In the baffle leading end area, the roof liner sheets and upper guides were pushed up, reducing the guide lateral support area for the baffles.
Thermal imaging for the east side of the stack above the breach found the stack plate was exposed to a temperature of 859⁰F, and a visual inspection of the stack plate from outside found there were no visible cracks in either side of the stack.
What Exhaust Duct and Stack Solutions Were Offered?
SVI proposed repairs of a total of 6 vertical field joints (3 joints on each wall side) and a total of 80 ft horizontal joints (40 ft on each wall side). It was recommended that the old liner insulation be removed and new insulation be installed (using four layers instead of three) to compress the insulation and close any possible gaps between the insulation and scallop plates. Modification of the liner sheets (a diagramed sketch was provided) to eliminate the possibility of the flow entering between the liner sheets was another recommendation, as were repairing cracked welds and installing applicable gaskets. Missing studs were to be replaced – welded to the existing scallop plate, then installing a new washer and nut. Test port rods and the CEM port liner plate should be rewelded.
SVI made recommendations regarding the silencer duct roof liner, including installing new liner baffle support brackets, liner studs, liner insulation and liner sheets, new baffle upper guides similar to the floor baffle guides with longer guide angles.
Lastly, the SVI inspection recommended metallurgy testing for three samples of the stack plate (locations documented) due to the stack plate A36 carbon steel being exposed to temperatures above 750⁰F for extended periods of time.
Does Your Plant’s Exhaust Duct and Stack Need an Inspection Like This?
SVI Dynamics designs and implements advanced solutions for industrial power plants’ exhaust ducts and stacks. If you need a comprehensive duct and stack inspection at your plant, contact us today to schedule our team to come to your site.