Fossil fuel-fired power plants produce electricity under very harsh operating conditions with numerous pressurised components such as boiler tubing, headers and steam piping. These operate at very high temperatures ranging between 540 °C and 570 °C and stresses that can cause weld cracks eventually leading to pipe breakages.
The importance of early detection
Creep is a time-dependent, thermally assisted deformation of a component operating under stress. “It is now known that creep damage is occurring at a relatively early stage of the component projected lifetime. In many cases, weld cracks appear during the middle of their remaining service life,” notes Dr Marko Budimir, project coordinator of the EU-funded CreepUT project. Evidence shows that creep damage does not start on the surface and propagate inwards, but vice versa. It also develops much slower during the early stage. By the time it is detected at the surface, the pipe is almost ready to fail. Existing non-destructive testing methods such as replication metallography cannot adequately monitor below the surface. The CreepUT scientists have addressed these pressing problems by developing novel ultrasound technology. “Our smart ultrasonic scanner system provides a complementary volumetric coverage, enabling for the first time detection of sub-surface creep damage at an early stage. It can accurately measure creep deformation in the micrometre range – which is very difficult to detect with current technology – and extend component life beyond the original design limits in power plants,” explains Budimir.
How the system works
The CreepUT system is designed specifically for scanning critical welds susceptible to creep damage. It has an ergonomic mechanical manipulator that can carry and precisely position the ultrasonic testing inspection head over pipe testing surfaces. The inspection head, which consists of an ultrasonic probe and a wedge, is constructed around an ultrasonic transducer that can detect defects larger than 0.2 mm. The wavelength of the ultrasonic transducer can significantly affect the probability of detecting a crack. The discontinuity must be larger than the wavelength to stand a reasonable chance of being detected; otherwise, the sound wave will pass around the defect. “Most ultrasonic testing is performed at frequencies between 1 MHz and 15 MHz. Higher frequencies permit detection of smaller defects. However, after a certain point, sound energy tends to scatter and does not travel far in the material,” explains Budimir.
Challenges during operation
Testing the newly created CreepUT system in lignite-fired power plants will be challenging. The high concentration of small dust particles released during fuel burn can cause errors during the scanning process. The system must be carefully optimised to withstand harsh operating conditions inside a power plant, which include dust concentration, humidity and elevated temperatures. The sensitivity of the ultrasonic probe is also key to the success of the CreepUT system. The final surface roughness should be around 1 μm; if more, it can severely limit the probe ability to detect very small defects. Given that such fine roughness is expensive, the system is designed to scan over four points around the pipe, rotating by 90-degree increments. CreepUT technology provides enhanced safety assurance along with increased revenue via reduced power plant outages. Additional trials are required to convincingly demonstrate the merits of this new inspection process.
CreepUT, creep damage, power plant, ultrasound technology, non-destructive testing, sub-surface, weld crack, ultrasonic testing, ultrasonic transducer