Seal clearance is a small distance with a large influence. In industrial gas and steam turbines, micrometer sized spaces determine the amount of leakage occurring in high-pressure vs. low-pressure areas. By managing the seal clearance correctly an organization can become more efficient, avoid costs associated with premature removal to service components, and reduce total operating costs. Read more on this site.
What Is Seal Clearance?
Seal clearance refers to the axial or radial distance between rotating parts and stationary parts at seals, which is most often seen at blade tips, shaft glands, and interstage locations. To start to arrive at an effective seal clearance, designers must understand that rotors grow thermally, and casings expand during operation.
Therefore, clearance must be set cold to have the correct running gap hot. If the seal clearance is too tight, rubs can occur resulting in rub damage; if clearance is too much clearance there is opportunity for leakage and lost work.
The most common seal families are labyrinth seals (tooth and landing or control flow), and brush seals (packing of metal bristles that conforms to the surface of the rotating rotor). Adjacent to the seal, you might find an abradable or honeycomb liner to create a contained “kiss” rub during transients (aftershocks). Managing clearance correctly is part of tip clearance control, rotor sealing strategy, and steam path optimization.
Why Clearance Matters for Efficiency
Clearance directly affects internal leakage, which bypasses the blades and produces no work. As leakage rises, stage pressure ratio and blade loading fall, raising specific fuel consumption. Small, disciplined reductions in clearance often translate into measurable heat rate improvements, especially on units that cycle or operate far from original design points.
After establishing the thermomechanical limits, teams pursue clearance reduction because it delivers:
- Lower leakage and higher stage efficiency, improving power output at a given firing temperature.
- Reduced fuel flow for the same megawatts, supporting fuel cost savings and emissions goals.
- More stable performance across load changes, with tighter control of startup transients and part-load operation.
How Brush Seals and Labyrinth Seals Work
Labyrinth seals use teeth to create throttling pockets. Each tooth drops pressure in steps, which makes it more difficult for fluid to leak across the seal. Labyrinth seals have some durability, they can tolerate a large diameter and are fairly easy to maintain, but labyrinth seals depend on a finite clearance to avoid hard rubs.
Brush seals replace the rigid teeth of the labyrinth seals, into dense wire bristles that gently contact or closely follow the rotor. This compliance affords an inherent tighter effective clearance and better leakage reduction. Modern brush seal designs now isolate any tooth wear from sealing action by pairing brushes with abradable backing or honeycomb, so minor rubs incidentally to performance are sacrificial and not catastrophic.
Many fleets have developed hybrid solutions, labyrinths in places with high rub severity and brush seals in places with the highest leakage sensitivity to trade-off durability and performance.
Field Services That Optimize Clearance
Materializing clearance reduction is not a parts swap, it is a discipline of field services. Outage teams put together measurements, modeling and precise rework to reach the target hot running gap, while avoiding rubs. The following services are often bundled during an outage, major or minor, to lock in leakage reduction and reliability:
- Metrology and borescope: Compile existing clearance, tooth wear and casing ovality baselines and locate rub marks, clearance assessments.
- Alignment checks of rotor and casing: Know where the shaft is positioned, offsets to bearings and thermal growth vectors prior to multiple cold clearance being set.
- On-site machining and shimming: Restore roundness, correct reductions in step heights, square off seal carriers, even fit abradable or honeycomb liners.
- Seal upgrade kits and coatings: Brush seals or advanced labyrinth teeth replacement and abradable coatings that allow controlled, low-energy rub-in.
- Performance verification: Conduct test runs and evidence leakage reduction and control of tip clearance against predicted target.
The above actions integrate with controls tuning and balance of plant checks to establish durable leakage reduction in combination with rub prevention at startup.
Case Studies of Performance Improvements
On a 200-MW combined-cycle gas turbine, the integrated benefits of replacing a set of worn labyrinth seals with a hybrid brush/labyrinth few geometries, and restoring casing roundness condition, provided for an estimated reduction of 20% to 30% of the gland and interstage leakage, modest improvement in net output leaving firing temperature unchanged, and a significant improvement in fuel per MWh. Operators confirmed they also had less variance at startup and fewer alarms due to differential expansion.
Check out https://www.sciencedirect.com/topics/engineering/combined-cycle-gas-turbine-power-plant to learn more.
On a 150-MW reheat steam turbine, reworking the seal carrier and provided the honeycomb liner renewal during a planned turnaround outage bottled up efficiency lost from over the last several years. As seen in the post-work performance-test, the plant experienced measurable leakage reduction, a more consistent pressure ratio across some critical stages, and improved reliability when cycling the turbine daily.
The magnitude of improvement for every machine is different; however, the discipline applied to clearance management remains amongst the most cost-effective processes available from today to reduce leakage and maintain performance and reliability over the long term for rotor sealing programs.
