The rotor (https://www.sciencedirect.com/topics/engineering/rotors) is the soul of a gas turbine. This long, precision machined item spins at thousands of rpm to convert the chemical energy in fuel to shaft power. It is important in the students’ frame of reference, because when they can visualize how rotors are constructed, stressed, and maintained, they can easily see, dependability is a function of both design and service practices.
This framework will describe how the rotor operates, what challenges it faces in operation, and how power plants maintain rotors for long service.
What Is a Gas Turbine Rotor?
A gas turbine rotor is the primary rotating assembly which consists of compressor and turbine disks, blades, and the main shaft. In a heavy-duty unit, the rotor will be one forged shaft, with several shrunk-on disks; in the other configuration, there is a bolted array of disks which are tensioned together with tie bolts.
The rotor imparts energy to the air as it moves through the compressor section; then hot gas expands through a series of turbine stages, again extracting work from a rotor of the same portion of the machine to drive both the compressor and the generator. Precision fits, tight clearance, and careful balancing are critical to avoid vibration and heat build-up in these systems.
Why Are Rotors Essential in Power Plants?
In a power plant, rotors are the connection between thermodynamics and the electrical grid. They transmit torque to the generator, store rotational kinetic energy to smooth out electrical grid fluctuations and provide inertia to stabilize the machine to emissions-compliance transients that occur on start-up and load changes. A rotor’s condition defines its upper limit for safe output and efficiency.
Before we analyze them, let’s first consider what a sound rotor must do every second it runs. It must be straight during heat-up, prevent cracking, and maintain the blades in the correct orientation at high RPM (read here for details). From this obligation flow four significant duties:
- Deliver high torque while maintaining precise alignment with the bearings and seals.
- Maintain precise blade tip clearances to maintain efficiency and minimize rubs.
- Direct cooling air through internal passagework to prevent hot section parts from overheating.
- Maintain balance to keep the machine from developing damaging amplitudes during coast-up and coast-down.
Common Stresses Rotors Face
Rotors live with combined mechanical and thermal stresses. High centrifugal forces pull on disks and blade roots; high-start and stop speeds introduce low-cycle fatigue. Varying temperatures on the shaft create temperature gradients causing differential expansion and thermal bow. Combined with time, creeps, which stretch the material in turbine stages at high temperature, and possibly corrosion or hot-salt attacks from extreme environments further complicate this.
Changing vibrations from mass changes, due to deposits or blade damage, are diagnosed by vibration analysis used by technicians to discover problems early. As risk increases from these stresses it is always advisable to do planned-maintenance, including rotor repair, during outages to keep minor defects from going to full forced-outage events.
Rotor Lifespan Explained
Rotor life is based on the strength of materials, and operational cycles, as well as how the machine is operated. A baseload unit that typically runs at steady state may have more life than a peaker that cycles with regularity. Burdened with numerous thermal shocks will reduce life like improper warm-up, ramping, passing dirty fuel, etc.
Engineers track life on a rotor using monitoring data and predictive forms of analytics. Many contributory factors dominate the life curve:
- Documented condition of stress critical features such as dovetails, bores, keyways, and wheel rims identified by nondestructive testing (NDT).
- Balance quality and runout control established by rotor balancing of the turbine and shaft alignment.
- Heat history is based on the number of starts, trips, and temperature excursions; fatigue and creep history factors in this.
- The quality of overhaul work is initiated during the outage for field service support, clean fits, proper bolt ties, correct torque on tie bolts, correct procedure for materials, etc.
Can Rotors Be Repaired Instead of Replaced?
Yes—many defects can be repaired rather than replaced in total. Shops have proven procedures that might include weld restoration of minor cracks, bored and journal machined, heat treat stress relief, blade groove refurbished and even straightened in the field. Disks may be re-stacked, tie bolts replaced, and re-balanced to OEM or engineered limits. With combined cycle fleets similar repair and refurb methods may be applied to steam machines; refurbishment of a steam-turbine returns an aged rotor to reliable service.
Following repair, the rotor will be checked for overspeed and balance, then reinstalled and aligned precisely. With good engineering controls and sound, qualified maintenance of industrial gas and steam turbines, repaired rotors will run safe and predictable operation protecting availability and avoiding costs and lead-time of a new unit.