The design of many Heat Recovery Steam Generators (HRSGs) does not allow access to the inside surface of the tubes for examination without cutting open a header and using complex robotics. These finned tubes are also spaced tightly together. TesTex, Inc. uses the Low Frequency Electromagnetic Technique (LFET) to examine HRSG finned tubes from the outside diameter. The Remote Field Electromagnetic Technique is used when access is provided to the inside diameter of the tube. TesTex is currently working with EPRI on two projects to develop additional inspection tools for HRSGs.
The Low Frequency Electromagnetic Technique (LFET) is able to detect and quantify wall thinning on HRSG finned tubes from the outside surface through the fins. The LFET scanner is moved along the tube measuring the strength of the return electromagnetic signal that is emitted into the tube by the scanner’s exciter coil. The carbon steel fins allow the signal to penetrate into the base material of the tube. EPRI sponsored TesTex to perform an examination of HRSG finned tubes with pitting and general wall losses. The results showed that LFET can detect localized wall losses such as pitting as well as general wall losses like flow accelerated corrosion. The scanners are currently operated by hand although it may be possible to design a crawler that can scan the entire length of a tube without the use of scaffolding.
The TesTex developed Remote Field Electromagnetic Technique allows for the inspection of HRSG tubes from the I.D. when utilizing access holes drilled into the header. The flexible probe is able to go through a modest buildup inside the tube enabling inspection of the unit.
For the testing of HRSG finned tubes our studies show that an RFET inspection can be used to detect localized and general wall losses. Quantification of defects can be made using appropriate calibration standards.
EPRI has contracted TesTex to develop an “Internal Inspection Tool” that can be inserted inside a header to allow a Remote Field Electromagnetic Technique probe to be pushed up the length of the HRSG tubes to record thickness readings. Access will be provided removing the endplate from the header.
A Remote Field Electromagnetic Technique (RFET) probe equipped with a camera on the probe tip is carried by the crawler along the inside of the header. The crawler aligns the position of the probe for insertion into the desired test tube through the use of cameras. Once the probe is properly positioned, the crawler has a probe feed module that pushes in the probe’s poly to force the probe up the tube. The RFET and video data can be viewed in real time as the probe is inserted and as it is retrieved from the tube. This data is also stored for analysis.
TesTex uses our Balanced Field Electromagnetic Technique (BFET) to inspect the tube header welds of HRSG units. The BFET probes and cameras are placed on the welds using a C-clamp housing that attaches to the tube. Once attached the assembly moves circumferentially around the weld examining for cracking, lack of fusion, porosity, and other defects. TesTex is currently working on updating the Claw so it can be attached to a mechanical arm to examine the welds in the interior rows. Part of this project is being funded by EPRI.
Click here to view the Combined Cycle Journal Article on NDT Heat Recovery Steam Generators.
TesTex’s HRSG Inspections are available just contact us!