Ultrasonic phased array technology

Steam turbine housing inspection with ultrasonic phased array technology

Ultrasonic testing is one field of non-destructive testing (NDT) in which rapid progress continues to be made. Ultrasonic techniques are suitable to examine large volumes of material and are less hazardous than volumetric, X-ray examination.

Project
Small crack formations, with a depth of <5 mm, in the most critical areas of steam-turbine inner housings are considered normal. However, during a major overhaul at a Dutch power plant in 2008, an original equipment manufacturer service group discovered a circumferential crack with a depth of almost 50 mm and a length of 450 mm in one of the customer’s turbine housings. It was decided initially to repair the casting with a weld. The contractors guaranteed the integrity of the repair weld for two years. When the guarantee period expired, instead of immediately ordering a new housing, the owner decided to ask KEMA to check the quality of the repair weld; however, ultrasonic testing was difficult to use, because weld site access was obstructed by a shrunk-ring construction.

KEMA examined the weld by inserting a manipulator into the inlet nozzle; however, the inlet nozzle had a circular cross-section, while the cavity near the repair weld had an oval cross-section. There were no accurately dimensioned drawings to show an accurate housing type – only the basic geometry was known during the design phase. Three weeks before the inspection, workers ascertained that the housing was one-third smaller than previously assumed, surmised from the sketchy data available. As a result, the manipulator needed to be modified with short notice.

Furthermore, casting materials are usually difficult to inspect using ultrasonic methods or standard ultrasonic sizing methods, such as TOFD. To deliver the project, both the manipulator and ultrasonic method had to be designed within three to four months. Several mock-ups and test pieces, with artificial defects, were fabricated to solve the problems. Test pieces were manufactured using the same casting material, whereas others were manufactured using mild steel 52. The ultrasonic phasedarray technique was used for the detection process as well as to size the manipulator. The manipulator was adaptable to inlet nozzle length and housing curvature changes. The method was tested on test blocks, with the same geometry as the steam-turbine housing, and demonstrated to the OEM and customer, with the final inspection carried out over a short time period during a brief turbine overhaul.

The inspection revealed no signs of the weld cracking, so the customer did not have to purchase a new housing. The original method that was developed and tested for this project can also be used on other HP housings.

Objectives
 This project’s goal was to determine the quality of a repair weld in an HP turbine housing and to demonstrate that there were no signs of cracking within the weld.

Benefits
> The customer saved the expense of purchasing a new HP turbine housing

Project coordinator
> KEMA, the Netherlands

Project details
> Duration: March – June 2010