Steam turbine cycle performance is formulated via high-pressure Rankine expansion across impulse and reaction stages per ASME PTC 6 and IEC 60045-1. Steam expansion is evaluated on the Mollier enthalpy-entropy ($h-s$) diagram.
If exhaust moisture exceeds 12%, last-stage titanium or stellite-shielded blades suffer rapid droplet erosion, leading to unbalance, severe bearing vibration, and forced plant outages.
When steam expands too far into the wet dome without reheat, fine mist coalesces on stationary diaphragm vanes into large water droplets. These droplets detach and impact the supersonic spinning tips of last-stage blades at relative velocities exceeding 1,200 ft/s. Moisture exceeding 12% creates severe leading-edge scalloping and erosion notches, triggering fatigue blade throwing that destroys the entire LP turbine rotor.
2. Over-Throttling at Part Load Destroying Rankine Efficiency
In throttle-governed turbines operating at 40% to 60% load, the main governor valve constricts steam flow, dropping inlet pressure from 900 psig down to 400 psig via isenthalpic throttling. Because throttling is purely isenthalpic, no shaft work is produced during the pressure drop. Available isentropic enthalpy drops by over 30%, causing part-load heat rates to spike and burning massive excess boiler fuel.
3. Wet Steam Water Induction Shock Warping Turbine Rotors
Inadequate attemperator spray control or failed boiler drum level trips can send liquid water slugs into the main steam piping. When water droplets enter the 900°F HP turbine casing, differential thermal shock contracts the bottom of the casing faster than the top. The casing bows upward into a banana shape, causing irreversible high-speed rubs against rotor labyrinth seals and catastrophic rotor seizure.
If condenser cooling water temperature rises or air removal ejectors fail, condenser backpressure rises from 1.5 inHgA to 6.0 inHgA. Because the pressure ratio across the LP stages collapses, the last-stage blades do zero work and begin churning dense steam like a fan. Windage friction generates intense localized heating, pushing exhaust hood temperatures past 200°F and distorting low-pressure bearing pedestals.
5. Loss of Gland Steam Sealing Pressure Inducing Air In-Leakage
Turbine rotor shaft ends penetrating sub-atmospheric casing zones must be sealed by positive-pressure gland steam (typically 2.5 to 5 psig). If the gland steam regulator fails, cold atmospheric air is sucked across shaft seals directly into the condenser. Oxygen dissolves into the condensate, triggering catastrophic pitting in deaerator tanks and feed-water preheaters while blanketing condenser tubes with non-condensable gas.
Frequently Asked Questions
What is the maximum allowable exhaust moisture fraction in a condensing steam turbine?+
Per ASME PTC 6 and steam turbine OEM standards (such as GE, Siemens, and Mitsubishi), exhaust steam moisture at the condenser flange should strictly not exceed 10% to 12% (steam quality x >= 88% to 90%). As steam expands into the two-phase wet region, microscopic water droplets condense and are accelerated by high-velocity steam jets. At relative peripheral blade tip velocities exceeding 1,200 to 1,500 ft/s (350 to 450 m/s), these water droplets act like high-speed abrasive projectiles, pitting and gouging the leading edges of last-stage low-pressure (LP) blades.
What is the Baumann rule for steam turbine wet expansion?+
The empirical Baumann rule states that for each 1% of average moisture present during steam expansion in the wet stages of a turbine, stage aerodynamic efficiency decreases by approximately 1%: eta_wet = eta_dry * (1 - y_avg). Water droplets cannot accelerate as quickly as vapor molecules, creating aerodynamic drag, blade surface droplet braking, and boundary layer disruption that drains mechanical shaft power.
What is Willans Line and how does it describe part-load steam turbine efficiency?+
Willans Line describes the linear relationship between total steam consumption (W, lb/hr) and gross electrical power output (P, kW) for a throttle-controlled steam turbine: W = W_0 + m * P. The parameter W_0 represents the no-load steam flow required simply to overcome mechanical bearing friction, windage losses, and keep the rotor spinning at synchronous speed (typically 10% to 18% of rated steam flow). Consequently, at low electrical loads, the specific steam consumption (lb of steam per kWh) increases dramatically.
What is the difference between an extraction-backpressure and a condensing steam turbine?+
A condensing steam turbine expands steam all the way down to sub-atmospheric vacuum pressures (typically 1.0 to 2.5 inHgA / 0.035 to 0.085 bar) in a water-cooled or air-cooled surface condenser, maximizing the enthalpy drop and electrical power generation. A backpressure (topping) turbine exhausts steam at positive gauge pressure (e.g. 50 to 150 psig) directly into an industrial process or district heating header, achieving higher overall combined thermal efficiency because the latent heat of condensation is utilized rather than rejected to cooling towers.
How does throttle governing cause thermodynamic irreversibility compared to nozzle governing?+
Throttle governing regulates turbine load by constricting a main throttle valve, which drops steam pressure via isenthalpic (constant enthalpy) expansion before entering the first stage. Because isenthalpic throttling destroys available pressure energy without producing work, entropy increases and available isentropic head drops. In contrast, nozzle governing uses multiple control valves that sequentially feed separate nozzle arcs, maintaining near-full throttle pressure across active nozzles and minimizing part-load throttling losses.