Shaft Seals & Gland Steam System
Sealing a Rotating Shaft Without Touching It
Everywhere the rotor shaft passes through a stationary boundary — through the casing ends, through each diaphragm — steam pressure needs to be contained (or, in vacuum sections, air kept out) without the shaft physically contacting the stationary structure around it. This module covers how that's achieved through non-contact labyrinth seals and the dedicated gland steam system that supports them.
Labyrinth Seals — Controlled, Not Zero, Leakage
A labyrinth seal uses a series of closely spaced, non-contacting fins — on the shaft, the stationary housing, or both — that force any leaking steam through a tortuous path of small restrictions. Each restriction creates a pressure drop, and the cumulative effect across many fins progressively reduces leakage flow to a small, acceptable level. Because the fins never touch the shaft, labyrinth seals produce no friction wear from rubbing — leakage is controlled purely by maintaining small clearance gaps, not by contact sealing.
Labyrinth seals are designed around controlled leakage, not zero leakage. Some steam or air passage through the labyrinth is expected and normal — the design goal is reducing that flow to an acceptable level, not eliminating it entirely.
The Gland Steam System — Why LP Needs Steam In, Not Out
Where the rotor passes through the casing at each shaft end, dedicated gland (shaft end) seals require a small, controlled supply of sealing steam from the gland steam supply. This is where an easy-to-miss detail matters: HP shaft ends, above atmospheric pressure, need to prevent steam from leaking outward — but LP shaft ends, operating below atmospheric pressure because of condenser vacuum, actually need steam supplied inward, to prevent atmospheric air from leaking into the vacuum. Both cases are managed by the same general gland steam system, just working in opposite directions.
A seal pressure regulator maintains gland sealing steam at a controlled setpoint pressure across the turbine's full load range, since steam conditions from other plant sources vary with load and can't be used directly. At low loads or during startup, when the turbine's own leak-off steam may be insufficient for self-sealing, many units draw from an auxiliary steam source through this regulator, switching over to self-sealing (using the turbine's own extraction steam) once load increases.
Collecting the Leak-Off — Gland Steam Condenser and Exhauster
The steam/air mixture that does leak through the gland seals is collected and routed to a small dedicated gland steam condenser, which condenses the steam portion (recovering it to the condensate system) while venting the remaining non-condensable air. A small exhauster or vacuum blower maintains a slight vacuum on the condenser's outlet, continuously drawing the leak-off mixture through and venting the leftover air to atmosphere — this exhauster is what makes the entire system function as a controlled, continuously managed leak path rather than steam and air simply accumulating at each shaft penetration.
Gland seal system health is both an efficiency and a safety matter — steam venting visibly at a shaft penetration usually indicates a gland seal or gland steam system problem, not something to dismiss as cosmetic.
Internal Sealing — Diaphragms and Valve Stems
Beyond the shaft ends, labyrinth-style diaphragm packing is also used at each diaphragm's inner bore, minimizing steam that would otherwise leak around the diaphragm rather than through its intended nozzle passages — a direct efficiency loss conceptually similar to blade tip leakage from Module 2.3. Governor and stop valve stems, which move linearly rather than rotating, use a different sealing technology entirely: carbon ring seals, segmented carbon rings providing low-friction sealing around a reciprocating stem — a different technology from labyrinth seals precisely because the sealing challenge (linear motion vs. rotation) is fundamentally different.