TURBINES & TURBINE COMPONENT INSPECTION
There are several components that are inspected in a turbine. These include bore, disk keyway, disk blade attachment area, blades, nozzles, casing, bolts, etc...
The mechanism of crack growth in a rotor bore is due to the combined action of creep and fatigue. Creep is more prevalent in rotors that operate at temperatures around 1000°F. Fatigue is more prevalent in rotors that operate at lower temperatures.
Three methods are commonly used for bore inspection:
• Magnetic Particle Testing, Eddy Current and Ultrasonics.
• MT & ET are limited to surface cracking.
• Ultrasonic Inspection can perform a complete volumetric exam.
- Solid Rotor
The main advantage of a boreless rotor is its lower level of stress compared to the bored rotor. The lower level of stress makes the boreless rotor tolerant to larger flaws. Because of this reason, inspection procedures for boreless rotors are less stringent. The inspection of a solid rotor is performed by using a combination of L-wave UT and Shearwave UT.
• The L-wave UT can detect flaws directly below the transducer.
• Rotor sections directly below the disks cannot be inspected. These locations are inspected using refracted Shearwave.
- Disk Keyway Cracking
The primary cause of disk keyway cracking is stress corrosion. Inspection of keyway cracking is performed using a range of ultrasonic refracted angles and most recently phased array inspection has been used as a viable inspection.
- Blades - Stator Vanes
ACTMS™ On-line Stator Vane Crack Detection System
The failure mechanism of turbine blades is dependent on their temperature, environment and stress state. Corrosion fatigue is the major failure mechanism of blades in the next-to-last stage of the low-pressure turbine. Creep blade failures are limited to HP turbines. Cracking of blades occurs at the following three locations:
• Blade attachments
The inspection methods chosen for each of these locations depends on whether the inspection is performed with the blade removed from the disk or not.
Eddy current and magnetic particle are the two methods used to inspect the blade. Eddy Current inspection is a good method since it can be performed without removal of the turbine from the casing.
Once the turbine has been removed from the casing, the blade attachments become directly accessible for inspection. Either eddy current or magnetic particle testing may be used at this stage.
Wet Mag is the most commonly used method for inspecting blade length. The only method available to inspect blade tenons is ultrasonics.
Creep-rupture and brittle fracture are two primary reasons for bolt failures. The low toughness that leads to brittle fracture is due to the inherent high strength of bolts. The failures are usually initiation controlled. The failure time is very short after the crack initiation.
Ultrasonic testing is the method that can inspect bolts without removing them from the casing.