The objective of the LTA program is to give the operators of gas turbine and opportunity to operate the gas turbine beyond the limited design lifetime of the gas turbine components. Turbine rotors of gas and steam turbines can be greatly affected by stress, fatigue, corrosion, embrittlement, or cracking. All these factors can limit operational lifetime. Our lifetime assessment can maximize the life cycle of your rotor.
Until recently, turbine rotors were regarded as everlasting machines so long as no apparent damage was found. In recent years, a number of OEMs have stated that turbine rotors have a limited lifespan.
The following processes can limit the lifetime of turbine rotors, and even lead to complete destruction of the rotor:
Petrotech maximizes the operational lifetime of your turbine rotors. We offer:
Nondestructive Evaluation (NDE) Methods:
NDE is a well established and proven tool to help determine the integrity of gas turbine and compressor components during their life cycle in operational environments. Conventional methods such as penetrant testing (PT) and magnetic particle testing (MT) are well suited to detect surface and slightly subsurface discontinuities. This methods are particularly sensitive to small surface service induced cracking in components. Penetrant testing, which requires the discontinuity to be open to the surface for detection and magnetic particle testing can only be used on magnetic materials. Visual inspection (VT), performed either with the human eye or with high- resolution cameras, is limited to dimensional measurements, usually the detection of large open discontinuities or component condition assessment. Longitudinal and shear wave ultrasonic testing (UT) is used for full volumetric integration of a component while eddy current testing (ET), an electromagnetic method, is sensitive to small surface or slightly subsurface indications in many materials.
Assessing rotor lifetime
Most industrial gas turbine manufacturers currently use Equivalent Operating Hours (EOH) as a basis for determining the remaining lifetime of gas turbine rotors.
The lifetime for a typical rotor is from 100,000 to 150,000 EOH. With a timely inspection of rotor parts, it is usually possible to achieve a single extension of about 50,000 to 100,000 EOH.
High temperature and high stress, either alone or in combination, can create lifetime-limiting effects. These effects can be calculated and checked, and the remaining lifetime can be assessed.
Lifetime-limiting processes progress slowly. Many processes are not directly related to EOH at all, unless a specific rate-of-attack is known and taken into account.
Possible causes of failure must be interpreted with care:
Low-alloy steel rotors have low metal temperatures (350°C to 400°C, or 600°F to 750°F). Therefore, creep is not considered to be a major problem except in local areas with increased temperature and/or stress levels, such as fir tree serrations. Stress corrosion is only to be expected when very high static stress is present under a corrosive environment.
Loss of yield strength/stiffness and creep, however, are the only processes that could limit the lifetime of a rotor based on its EOH.
Extending rotor lifetime
In order to extend rotor lifetime, you must carefully calculate both steady state (centrifugal) stress and dynamic (thermo-mechanical) stress levels. To calculate the effect of a defect on rotor lifetime, you will need both alloy toughness data and a fracture mechanical analysis so as to:
Alloy steels exhibit a transition from low ductility at low temperatures to much higher ductility at higher temperatures. This transition temperature, called the FATT (Fracture Appearance Transition Temperature), occurs after the appearance of the fracture of Charpy-V-notch test samples. The FATT in new materials is usually around 0°C to 120°C (32°F to 250°F). The alloy is brittle below the FATT; it is ductile above the FATT.
Temper embrittlement is a phenomenon in alloy steels created by the migration of specific trace elements (tin, antimony, phosphorus, arsenic) to the grain boundaries. Increased levels of these elements at the grain boundaries can shift the FATT to a higher temperature.
Neither the lower nor the upper platform ductility values are much affected by temper embrittlement; only the shift in the FATT extends the low ductility to a higher temperature.
Sensitivity to changes in the FATT is also determined by nickel and chromium, elements that are not the root cause of the shift. The presence of chromium and nickel increases this sensitivity significantly. By definition, nickel containing alloys do have good ductility. Their properties may change when exposed to high temperatures, however.
Consequences of temper embrittlement
When material properties (especially ductility and toughness) and operational stress levels are known, permissible defect sizes can be calculated.
There are two important criteria:
Ideally parts should not contain defects larger than the minimum defect size for growth.
Defect growth behavior can be determined when both the stress loading and the material behavior are known. In this case, a “safe-life” concept can be used to assess the significance of defects.
Gas turbine rotors are not designed with a “safe-life” concept. Thus defects are supposed to be below the minimum defect size for growth. In temper-embrittled material, the acceptance criteria for minimal and critical defect sizes are both reduced considerably.
Defects in turbine rotors
Fracture mechanics use simplified models for cracks and other defects. Defects can have a wide variety of shapes and dimensions. They can also occur in clusters.
Most, if not all, inspection technologies only report “equivalent defect size” of detected indications. This means the indications look like defects with a standard shape and dimension. The real defect could therefore be either larger or smaller, or more or less dangerous.
Therefore, it is highly recommended to use large safety factors for defect assessment after the initial lifetime assessment inspection. Once defects are detected, their potential growth or stability can be determined during future inspections. Follow-up inspections can use this information to reduce uncertainty about the significance of indications.
Ideally, the first inspection of a turbine rotor should take place at about 50% to 70% of the rotor’s specified lifetime. The inspection reports can be used as a reliable basis to assess the lifetime extension.