Steam Turbine: Rows of Blades Turn Superheated Steam Into 3,600 rpm

How a Steam Turbine Works, Stage by Stage

A power-station steam turbine is a rotor carrying rows of moving blades inside a casing that carries alternating rows of fixed blades. Steam enters through stop valves, which shut it off in an emergency, and control valves, which set the flow and therefore the power. In a large fossil unit it then passes through several sections on one shaft:

• High-pressure (HP): steam enters at ~16–25 MPa at 540–600 °C and expands to a few MPa.
• Intermediate-pressure (IP): the steam is first reheated in the boiler to about its original temperature, which raises efficiency and keeps the final steam drier.
• Low-pressure (LP): usually double-flow, entering at the middle and flowing outward both ways to cancel axial thrust, then exhausting into a condenser at ~5 kPa (~33 °C).

Every stage is one fixed row plus one moving row. Step 1: the fixed row's passages narrow, so steam loses pressure and gains speed, leaving as angled jets with a large tangential (whirl) component. Step 2: the curved moving blades turn those jets back the other way; changing the steam's whirl momentum takes a force, and the equal and opposite force drives the blades round. Step 3: the slowed steam enters the next fixed row, is accelerated by the next pressure drop, and the sequence repeats a few dozen times. Exhausting into a near-vacuum instead of the atmosphere lets the same steam do roughly 30% more work.

The Governing Relations, With the Numbers Worked Through

Nozzle speed. An energy balance on a fixed row gives V = √(2Δh), with Δh in J/kg. A 100 kJ/kg drop gives V = √(2 × 100,000) ≈ 447 m/s, about 450 m/s.

Blade work. The Euler turbine equation gives specific work w = U·ΔV_w: blade speed times the change in the steam's whirl velocity across the moving row.

Worked example, impulse stage. Let the 447 m/s jet leave at α = 20° to the plane of the wheel. Its whirl component is 447 × cos 20° ≈ 420 m/s, and the best blade speed is U ≈ 0.47V ≈ 210 m/s. Relative to the blade the steam arrives with whirl 420 − 210 = 210 m/s; an ideal symmetric blade reverses that to −210 m/s, so the steam leaves with absolute whirl 210 − 210 = 0. So ΔV_w = 420 m/s and w = 210 × 420 ≈ 88,000 J/kg: 88% of the drop, the ideal ceiling cos²α. The other ~12 kJ/kg leaves as ~150 m/s of axial velocity. At 3,600 rpm (ω = 377 rad/s) that blade speed sits at a mean radius of 210 ÷ 377 ≈ 0.56 m.

Reaction stages. The degree of reaction is the fraction of a stage's enthalpy drop that occurs in the moving row. For a 50% reaction stage the best ratio is U/V ≈ cos α, about 0.9, and the ideal work is w = U²: at the same 210 m/s only ~44 kJ/kg, half the impulse stage's 2U². A reaction turbine therefore needs roughly twice as many stages.

Why so many stages? A reheat unit's total isentropic drop is roughly 1,500–2,000 kJ/kg. Released in one nozzle, 1,600 kJ/kg would make a jet of √(2 × 1,600,000) ≈ 1,790 m/s, demanding an impulse blade speed near 840 m/s at mid-blade, impractical for a large rotor. A few dozen smaller steps keep jets and blade speeds moderate.

Impulse vs Reaction: de Laval, Parsons, Curtis and Rateau

Gustaf de Laval built practical single-stage impulse turbines in the 1880s, using a convergent–divergent nozzle to accelerate steam past the speed of sound. His small wheels ran at up to ~30,000 rpm behind reduction gears, on slender flexible shafts that deliberately ran above their first critical speed.

Charles Parsons went the other way. Parsons' reaction turbine, patented 1884, spread the drop over many stages, with about half of each stage's expansion inside the moving blades; his first unit drove a ~7.5 kW dynamo at ~18,000 rpm. In 1897 his Turbinia raced uninvited through the Spithead naval review, faster than any ship present; her top speed was 34.5 knots. The pressure difference across every reaction row creates large axial thrust, carried by balance (dummy) pistons, opposed flows and a thrust bearing.

Charles Curtis (1896) velocity-compounded the impulse stage: one set of nozzles feeding two moving rows with a fixed redirecting row between, extracting a large drop at a best U/V of ~0.23. Auguste Rateau (c. 1900) pressure-compounded it as a series of simple impulse stages. Modern turbines blend these: often an impulse control stage (sometimes a two-row Curtis wheel) whose nozzle groups open in sequence as load rises, then low-reaction or ~50% reaction stages. On long, twisted LP blades, reaction rises from low values at the hub to well above 50% at the tip.

Why the Blades Grow From ~5 cm to Over 1 m

Steam swells enormously as it expands. At 25 MPa and 600 °C its specific volume is ~0.014 m³/kg; at a ~5 kPa exhaust with ~10% moisture it is ~25 m³/kg, about 1,800 times larger, so the steam specific volume grows ~2,000-fold from inlet to exhaust. Mass flow falls by only about a third (feedwater-heater extractions bleed it off), so by continuity, ṁ = ρ·A·V_axial, the flow area must grow almost as fast. Designers combine longer blades at larger radius, higher axial velocity near the exhaust, and 2–6 parallel LP exhaust flows. The result: blades grow from ~5 cm at the inlet to last-stage blades over 1 m long.

Centrifugal root stress scales with material density × ω² × exhaust annulus area, so the longest full-speed blades are titanium alloy (~4,400 kg/m³ against ~7,800 kg/m³ for steel), heavily tapered and twisted, with tips moving faster than sound in the exhaust steam. That steam carries ~8–12% water by the last stages, and each 1% of moisture costs roughly 1% of stage efficiency (the Baumann rule). Water films on the stator vanes tear off as coarse droplets that the blades strike at hundreds of metres per second, pitting the leading edge near the tip, so blades carry erosion shields such as brazed Stellite strips, or hardened leading edges.

Nuclear sets push further. A water-cooled reactor supplies ~7 MPa saturated steam at ~285 °C, so each kilogram does less work and roughly twice the steam flow is needed per megawatt. Most large nuclear sets run at half speed, 1,800 or 1,500 rpm with four-pole generators: halving ω quarters the centrifugal stress, letting last-stage blades reach ~1.75 m.

Locked to the Grid: 3,600 rpm, Governors and the Overspeed Trip

A grid-connected turbine's synchronous generator must turn in step with grid frequency, n = 120f/p for p poles, so a two-pole machine runs 3,600 rpm on a 60 Hz grid, 3,000 rpm on 50 Hz. More power means more torque at the same speed, admitted by opening control valves. Governors share load changes with a speed droop of ~4–5%: a unit swings from no load to full load for a 4–5% change in frequency.

Worked numbers. At 3,600 rpm, ω = 2π × 3,600 ÷ 60 ≈ 377 rad/s. A blade tip 2 m from the axis moves ~750 m/s at 3,600 rpm (~29,000 g): 377 × 2 = 754 m/s, and ω²r ≈ 284,000 m/s². Because stress scales with ω², a 10% overspeed raises it by 21%.

The dangerous moment is a load rejection: the generator breaker opens and electrical torque vanishes. With an inertia constant of a few seconds, a set still receiving full steam would gain roughly 10–15% speed per second. Control valves close, and reheat units add intercept valves at the IP inlet because steam stored in the reheater can keep driving the rotor. If speed still reaches the overspeed trip at ~110% of rated speed (~3,960 rpm on a 3,600 rpm set), every stop valve shuts within a fraction of a second, triggered by a spring-loaded eccentric bolt that flies out from the shaft end or by electronic 2-out-of-3 voted speed probes.

Specification and testing. Utility turbines are commonly specified to IEC 60045-1 and industrial drive turbines to API 611 and API 612, with API 670 covering vibration and overspeed protection. ASME PTC 6 tests measure heat rate and section efficiency: in the superheated HP and IP sections pressure and temperature fix enthalpy directly, but in the wet LP section they do not, so LP efficiency comes from an overall energy balance. Vibration is judged against ISO 20816-2, and trips are proved by real overspeed runs or oil-injection tests of the bolt.

How Steam Turbines Fail: Hinkley Point A and Other Lessons

Disc burst. In 1969 a low-pressure disc burst during a routine overspeed test at Hinkley Point A in England, wrecking the turbine-generator. A stress-corrosion crack at a keyway in a shrunk-on disc of low-toughness steel had grown to critical size, and the disc failed by brittle fracture. The lesson made fracture mechanics central to rotor design and drove tougher, cleaner disc steels and in-service ultrasonic inspection.

• Stress-corrosion cracking clusters where steam first condenses in the LP turbine, near the Wilson line, where impurities such as sodium hydroxide and chlorides concentrate.
• Water induction: water flowing back into a hot turbine from attemperators, extraction lines or drains can distort and crack casings and rotors; ASME TDP-1 sets design practice against it.
• Creep and thermal fatigue in HP and IP rotors at 540–600 °C, which limits how fast a unit may start.
• Blade high-cycle fatigue: Campbell diagrams keep blade natural frequencies clear of running-speed harmonics; at low load and poor vacuum, last-stage blades can flutter.
• Motoring: if steam is lost while the generator stays on line, windage overheats the LP blades, so a reverse-power relay trips the breaker.

The 1986 Chernobyl accident also began during a turbine test: whether a coasting-down turbine-generator could run the coolant pumps for the roughly one minute the backup diesels needed to take over.

Steam Turbine vs Gas Turbine vs the Rankine Cycle

Gas turbine. A gas turbine makes its own working fluid: a compressor on the same shaft squeezes air, fuel burns in it, and gas at ~1,300–1,600 °C drives internally cooled blades. That is the Brayton cycle, and the compressor absorbs roughly half the turbine's work. A steam turbine burns nothing and carries no compressor: its steam comes from a separate boiler or reactor, the feed pump takes only a few percent of its output, its blades are uncooled, and it exhausts into a vacuum. In a combined-cycle plant, gas-turbine exhaust raises steam for a steam turbine, lifting efficiency above ~60%.

Rankine cycle. The Rankine cycle is the loop (pump, boiler, turbine, condenser); the steam turbine is the one machine in it that makes work. Better loops, with higher steam conditions, reheat and feedwater heating, took fossil plants from ~35% efficiency toward ~45–47% in ultra-supercritical units.

Common misconceptions.

• “Reaction turbines push by reaction, impulse turbines by impact.” Both work by changing the steam's momentum; degree of reaction only says where the pressure drop happens.
• “The white clouds are turbine steam.” The turbine's steam stays in a closed loop; cooling-tower plumes are evaporated cooling water.

The steam turbine compared with the machines and ideas it is most often confused with

Machine or concept	What it actually is	Working fluid and conditions	Speed and scale
Steam turbine (multistage axial)	A machine: alternating fixed and moving blade rows extract the enthalpy of steam made in a separate boiler or reactor; no compressor on the shaft	Steam at ~16–25 MPa and 540–600 °C (fossil) or ~7 MPa saturated (nuclear), expanding to a condenser at ~5 kPa (~33 °C); uncooled blades	3,600 rpm (60 Hz) or 3,000 rpm (50 Hz); most large nuclear sets 1,800/1,500 rpm; single units above 1,000 MW
Gas turbine	A machine that makes its own hot gas: compressor, combustor and turbine on one shaft (Brayton cycle)	Air compressed ~15–25:1 in heavy-duty units, fuel burned to ~1,300–1,600 °C at the turbine inlet; internally cooled blades; exhausts near atmospheric pressure at ~550–650 °C	Large units drive generators at 3,000/3,600 rpm; the compressor absorbs roughly half the turbine's work
Rankine cycle	Not a machine: the thermodynamic loop of feed pump, boiler, turbine and condenser	Water and steam in a closed loop; feed-pump work is only ~1–3% of turbine work	Plant efficiency ~33–47% depending on steam conditions
Hydro turbine (Francis, Kaplan, Pelton)	A machine driven by liquid water falling through a head	Incompressible water: nothing expands, so usually one runner instead of rows of ever-longer blades	Tens to hundreds of rpm with many-pole generators; up to ~1,000 MW per unit
Tesla turbine	A bladeless machine: fluid spirals between closely spaced discs and drags them round by viscous friction	Steam, air or liquid; practical only in small sizes	High speed and small outputs; never displaced bladed turbines for power generation