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During the work, look for leakage paths, scoring, pitting, corrosion, broken springs, blocked leak-off passages, damaged shaft coatings and one-sided tooth rubbing. Carbon rings and labyrinth segments have different removal and installation sequences. Install matched parts in their marked positions, engage anti-rotation features, replace complete segment sets where specified, verify spring seating and measure OEM clearances. Before restart, restore gland suction and pressure control, confirm open leak-off lines and monitor leakage, rubbing, vibration, oil contamination and gland pressures during a controlled startup.
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What a steam-turbine seal does
A turbine seal controls the pressure boundary where the rotating shaft passes through a casing. It limits steam escaping from high-pressure sections and limits air entering low-pressure sections. The seal system is part of the turbine’s pressure-control arrangement, so a mechanically sound ring can still leak if gland suction, pressure-chamber steam or leak-off piping is wrong.
Carbon-ring packing
Carbon rings contact or nearly contact the shaft and are held in segments by garter springs. They are used where rubbing speed and differential pressure remain within the design envelope. API 611 permits carbon-ring packing in outer glands at rubbing speeds below 50 m/s, with a maximum average differential pressure of 2.4 bar per active sealing ring (API 611, American Petroleum Institute). These limits are design criteria, not a substitute for the turbine manufacturer’s data.
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Labyrinth packing
A labyrinth uses alternating teeth or strips and segmented rings. Each restriction converts pressure energy into velocity; turbulence in successive compartments dissipates that energy and reduces flow. Segments can move radially on springs to preserve a small running clearance. The arrangement is non-contacting in normal operation, but deposits, shaft movement, incorrect assembly or excessive vibration can produce a rub.
Labyrinth packing reduces leakage through a long series of throttling restrictions. Suction chambers remove leakage and air ingress, while pressure chambers supply controlled steam where required (EPRI seal-system description). A blocked line or incorrect valve position can therefore create symptoms that look like a damaged seal.
Before disassembly: establish the correct job and a safe boundary
Use the turbine’s own documents
Record the turbine model, serial number, casing arrangement and seal locations. Retrieve the OEM packing diagram and data sheet and identify:
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- Stamped marks, match marks, quadrant numbers and anti-rotation pins or notches.
- Spring type, spring recesses and whether rings are supplied only as complete sets.
- Specified cold radial or diametral clearances, measurement points and allowable repair limits.
- Shaft coating requirements, permitted polishing method and limits for coating damage.
- Cover, gland-housing and joint procedures, including the OEM bolt sequence and torque values.
There is no universal seal clearance, torque, leakage limit or isolation time for all steam turbines. A value from another model can turn a correct repair into a shaft rub or an uncontrolled leak.
Observe the operating condition when permitted
Before shutdown, document leakage, gland pressures, suction or vacuum, temperature, vibration and any bearing-oil contamination using the plant’s approved instruments and operating limits. Observe only from safe locations and under the site’s operating rules. Identify whether the apparent leak originates at the ring, casing split, shaft surface, gland piping or a flange. A wet or discoloured area at a joint is not proof that the seal rings are worn.
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Isolate, cool and verify zero energy
Follow the plant’s written shutdown and lockout/tagout procedure. The sequence normally includes isolation of steam and extraction connections, cooldown, depressurization, drain-down, electrical and mechanical isolation, and verification that hazardous pressure and temperature are absent. The exact devices, waiting periods and tests belong to the site procedure; do not infer them from another turbine manual. Disconnecting leak-off piping before it is depressurized can expose personnel to trapped steam and condensate.
Check the gland system before opening the case
Inspect leak-off, gland-suction and pressure-steam piping for blocked passages, corrosion, damaged valves, incorrect connections and abnormal pressure or vacuum. A blocked leak-off line can raise local pressure and increase leakage even when the packing is intact. Photograph and label connections, valve positions and instrument tubing so they can be restored correctly.
Inspection findings that determine repair
Carbon rings and spacers
During teardown, keep each segment identified if the OEM permits examination for possible reuse. Inspect faces and edges for nicks, pits, chips, cracks, scoring, dirt and loss of free movement. Check garter springs for distortion, corrosion, broken ends and loss of tension. Inspect spacer or partition rings for pitting, scoring, corrosion and warpage; a warped spacer can hold a ring out of position and create a local leak.
Labyrinth segments and springs
Examine every tooth or strip for bent metal, corrosion, deposits, scoring and evidence of a rub. Check locating springs for breakage, corrosion, correct length and free seating in their recesses. Look for crushed or displaced segment edges and for deposits that have reduced the intended clearance. The Dresser-Rand SST turbine manual states, “No field repairs of labyrinth are recommended”; its procedure requires unserviceable labyrinth segments to be replaced and the labyrinth to be replaced as a set (SST turbine manual).
Shaft and coating
Inspect the shaft under each seal for scoring, corrosion, deposits, heat discoloration and coating loss. A coating defect is not corrected by polishing it away. The SST manual requires shaft replacement when high-velocity oxy-fuel plating is bubbled, split or peeling. Do not return a shaft to service after disguising such a defect with abrasive work.
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Interpret rubbing patterns
One-sided rubbing on all labyrinth teeth is an important mechanical clue. The SST manual identifies this pattern as a possible indication of a worn or failed thrust bearing. Check thrust-bearing condition, axial position and rotor movement rather than simply installing new segments. A localized rub may instead point to a bent segment, misplaced spring, deposit or incorrect clearance.
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Carbon-ring replacement procedure
The following sequence is an OEM example; the applicable turbine manual controls the actual steps, lifting points and fasteners.
- Disconnect leak-off piping. Confirm the lines are isolated and drained, then label and remove the connections needed to lift the cover.
- Remove the turbine cover as specified. Use the approved lifting arrangement and protect machined joint faces. Do not force a cover against a trapped locating feature or connected pipe.
- Release the rings. Disconnect garter-spring ends, remove the springs and take out the carbon-ring segments. Preserve segment identity only where the OEM allows inspection for reuse; new rings should be installed as a complete matched set.
- Clean and inspect spacers. Remove deposits from sealing faces and inspect spacer or partition rings for pits, scores, corrosion and distortion. Replace any spacer outside the OEM repair limit or visibly warped.
- Inspect the shaft coating. Check the full sealing track. If plating is bubbled, split or peeling, stop and follow the OEM shaft-replacement disposition; do not polish over the defect.
- Install springs and the first complete set. Thread each garter spring into the spacer-ring slot. Install one complete ring set at a time, preserving the stamped-dot or other orientation marks. Engage every anti-rotation notch with its locating pin.
- Reconnect the spring ends. Verify that each joint is fully engaged and that the segments move as the design permits without binding.
- Refit the cover. Use the OEM gasket, joint preparation, bolt sequence and torque. Reconnect leak-off piping only after the cover and gland housing are correctly secured.
Do not mix segments from different sets or reverse a marked segment to make a gap appear uniform. Carbon rings are manufactured and matched for a defined position; orientation errors can create concentrated loading and rapid wear.
Labyrinth-seal replacement procedure
- Remove access components. Isolate and disconnect leak-off piping, then remove the turbine cover and gland-housing parts required by the packing diagram.
- Document the arrangement. Photograph and mark each quadrant, tooth direction, match mark and spring location before removing parts. Keep upper and lower halves identifiable.
- Remove and clean the segments. Take out the segments and locating springs without levering against the shaft. Clean deposits from segments, recesses and the shaft, then inspect for corrosion, scoring, bent teeth, cracks and broken springs.
- Replace unserviceable parts as a set. The SST manual requires labyrinth segments to be replaced as a set and recommends no field repair. Follow the OEM part number and set composition; do not substitute an individual tooth or weld-repair a damaged segment.
- Prepare the shaft. The SST manual instructs: “When installing new labyrinth rings, polish the shaft clean to remove any deposits which may have formed.” Polish only enough to remove deposits and only by the approved method; do not change shaft diameter, coating thickness or seal geometry.
- Install lower-half segments. Rotate each lower segment into position while compressing its locating spring into the designed recess. Confirm the segment sits fully home and can move as intended.
- Install upper-half segments. Fit the upper segments with their springs, keeping every match mark and quadrant in the position shown on the packing diagram. Check that no spring is trapped, cocked or omitted.
- Verify free movement and clearance. Rotate the rotor only when the approved procedure permits it. Confirm that segments do not bind and that measured cold clearances meet the OEM values.
- Close the gland housing and cover. Refit the cap and cover with the specified joint procedure, then reconnect and support leak-off piping without imposing external loads on the housing.
Some turbine designs allow outer seal rings to be removed for inspection and seal strips to be replaced during a short shutdown while the module remains in place. BHEL describes this access arrangement for certain designs, but it is not a general rule. The packing diagram and shutdown procedure must explicitly permit cover-only or in-place work before the rotor is left installed (BHEL turbine description).
Can seals be replaced without removing the rotor?
Sometimes, but only when the turbine was designed for that access. A cover-only job may be possible for segmented outer labyrinth or carbon packing if the segments can be withdrawn radially and the shaft remains supported. Other designs capture the seal axially, require rotor lifting, or make the required clearance measurements impossible with the rotor installed.
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| Question | What controls the answer |
|---|---|
| Can the cover be removed while the rotor stays in place? | The casing split, segment geometry, lifting clearance and OEM shutdown procedure. |
| Can individual labyrinth teeth be repaired in place? | The SST manual recommends no field repair; replace the specified complete segment set. |
| Can an outer ring be changed during a short shutdown? | Only on designs that provide radial segment access and approved in-place replacement, as described for some BHEL arrangements. |
| When is rotor removal likely? | When the seal is axially captured, shaft damage must be repaired, thrust position is in question, or required measurements cannot be made safely and accurately in place. |
Do not decide from the seal’s appearance alone. Confirm lifting and access dimensions, rotor support, shaft protection, measurement points and the required post-installation checks with the OEM procedure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Clearance and assembly controls
Measure cold clearances exactly as specified
Measure radial or diametral clearance at the locations and temperature stated on the packing diagram. API 611 requires temperature variation to be considered when establishing cold clearances. A cold measurement is not automatically a running clearance: shaft growth, casing growth, rotor eccentricity and thermal gradients change the relationship at operating temperature.
Record instrument, gauge size, temperature, measurement direction and result for every seal location. Do not average away a tight spot. If the OEM specifies a maximum and minimum, compare each reading with those limits and obtain engineering disposition for any out-of-limit value.
Use a final assembly checklist
- Every carbon ring or labyrinth segment belongs to the correct complete set.
- Stamped dots, arrows, match marks and quadrant positions agree with the packing diagram.
- Anti-rotation notches are engaged with their pins or keys.
- Garter springs and locating springs are present, correctly seated and free of damage.
- Segments move as designed and are not trapped by burrs, dirt or a pinched spring.
- Shaft coating and sealing surfaces are smooth, clean and within the OEM repair limit.
- Spacers and partitions are flat, correctly oriented and free of damaging pits or scores.
- Measured cold clearances are recorded and accepted by the responsible engineer.
- Cover, gland cap, joints and bolts follow the OEM procedure.
Restore the gland system before restart
Reconnect and test leak-off
Reconnect each leak-off line to its labeled port, remove temporary blinds as authorized and verify that passages are open. Check valves, drains, strainers and instruments for correct position and service. A disconnected or blocked line can defeat a correctly installed seal.
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Re-establish suction and pressure
Restore gland suction below atmospheric pressure where the design calls for it and restore controlled steam to pressure chambers. EPRI identifies both functions as integral to labyrinth sealing (EPRI seal-system description). Setpoints and valve lineups come from the turbine operating procedure, not from a generic pressure value.
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Use a controlled startup
During rolling, warming and loading, trend gland pressures, leakage, vibration, bearing-oil condition and any rubbing indication. Compare readings with plant-approved operating limits and the pre-shutdown baseline. Stop or hold the startup for abnormal leakage, rising vibration, evidence of contact, oil contamination or unstable gland pressure. The available standards and manuals do not publish one universal leakage acceptance number.
Common failure modes and practical responses
Leakage remains high after new rings are fitted
First verify gland suction, pressure-chamber steam and leak-off flow. Then check ring orientation, complete-set installation, anti-rotation engagement, spring seating and cold clearance. Inspect the shaft track and casing split for damage. Installing another set without correcting a blocked line or pressure-control error rarely solves the problem.
A labyrinth rubs immediately after assembly
Stop and protect the shaft. Check for a trapped locating spring, reversed quadrant, foreign material, insufficient cold clearance, shaft runout or axial movement. If all teeth show one-sided contact, investigate thrust-bearing condition and rotor position rather than grinding the teeth.
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Remove the damaged set, identify the installation force or interference that caused it and inspect spacers, pins and spring slots. Carbon pieces should not be patched or mixed with another set unless the OEM explicitly allows it. Replace the complete set when required.
Deposits return quickly
Look for steam-quality, corrosion-product or oil sources and verify drainage and leak-off capacity. Clean the shaft only within the approved method; recurring deposits indicate a system condition that seal replacement alone will not correct.
Records to retain
Keep the pre-shutdown operating observations, isolation verification, photographs, segment map, removed-part condition report, shaft-coating inspection, clearance readings, spring and anti-rotation checks, bolt records, leak-off test, gland-pressure lineup and controlled-startup trends. These records allow the next outage team to distinguish normal bedding from a developing mechanical or pressure-control fault.
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