Turbine Operation (TO) cheat sheet

Rankine cycle efficiency: η = (W_turbine − W_pump)/Q_in; reheating and feedwater heating raise cycle efficiency by raising mean heat-addition temperature.

Isentropic turbine work: W = h_in − h_out,s; actual work = (h_in − h_out,s) × η_isentropic, where η_isentropic is the stage or cylinder isentropic efficiency.

Carnot limit: η_max = 1 − T_cold/T_hot (absolute temperatures); no real cycle exceeds it, and condenser vacuum sets T_cold.

Stage reaction R = (static enthalpy rise in rotor)/(static enthalpy rise in stage); R ≈ 0.5 for a 50% reaction (Parsons-type) stage.

Velocity ratio and blade speed: optimum u/C_1 for maximum work is roughly 0.5 for a single impulse stage and about 0.25 per row for a Curtis stage.

Nozzle flow chokes when pressure ratio p_out/p_in falls to the critical value (about 0.546 for steam with k ≈ 1.3); beyond that, flow depends only on upstream conditions.

Degree of superheat at exhaust and last-stage annulus area limit the maximum steam flow per unit exhaust area; exhaust hood pressure relates to condenser pressure.

Heat rate: HR (kJ/kWh) = 3600/η_thermal; a 40% efficient unit has a heat rate of about 9000 kJ/kWh.

Rotor types: solid forged rotor for high-temperature HP sections; built-up or welded rotor for LP sections; shrunk-on discs are used on some LP rotors and are sensitive to stress corrosion cracking at keyways.

Blade attachment: fir-tree root for high centrifugal load with controlled fretting; T-root for smaller LP and shorter blades; lashing wires and shrouds damp vibration.

Seals: labyrinth (gland) seals control steam leakage at shaft ends; the gland steam system maintains slight positive pressure at the outer glands to prevent air ingress.

Bearings: journal bearings are pressure-fed tilting-pad or cylindrical types; thrust bearing (Kingsbury type) carries residual axial thrust not balanced by the balance piston/dummy piston.

Condenser: surface condenser maintains vacuum; air removal via steam-jet air ejectors or vacuum pumps; vacuum degrades with cooling-water temperature, fouling, or air in-leakage.

Feedwater heating: closed heaters with drain cascades and a deaerator (open heater) that removes dissolved oxygen and heats feedwater to saturation.

Bypass system: HP and LP bypass routes steam to the condenser during startup or load rejection, protecting the reheater and condenser from overpressure and overtemperature.

Turning gear (barring gear) rotates the rotor at low speed after shutdown to prevent bowing from uneven cooling; interlocked so it cannot engage above a set shaft speed.

Cold, warm, and hot starts are classified by first-stage metal temperature or by the time since last shutdown; hotter starts require faster loading but tighter differential-expansion limits.

Rolling off turning gear: admit steam to raise speed through critical speeds quickly; critical speeds must be passed rapidly and never held.

Differential expansion (rotor vs casing) is the key startup limit; excessive positive or negative differential expansion trips the unit or blocks loading.

Loading rate is governed by metal temperature matching; the turbine follows the boiler or is loaded by the speed/load controller according to a startup curve.

Normal shutdown: unload, trip or close main stop valves, verify no reverse rotation from generator motoring, and start turning gear once speed drops below the engagement threshold.

Cool-down rate limits are set to avoid thermal stress; casing temperature differences and steam-to-metal temperature mismatch are the controlling parameters.

Water induction prevention: drain lines, superheat margins, and main steam temperature above saturation are verified before and during loading.

Overspeed testing and valve stroke tests are performed at scheduled intervals; main stop and control valve freedom is verified before each start.

Speed/load control: droop characteristic - a 4-5% speed droop means a frequency change of 4-5% moves output from zero to full load in isochronous-off (droop) mode.

Electro-hydraulic control (EHC) converts electronic demand to hydraulic actuator position; loss of hydraulic pressure causes valves to fail closed (trip).

Overspeed protection: independent bolt/mechanical overspeed trip plus electronic overspeed trip; typical trip setpoint is 110-111% of rated speed.

Trip protection monitors include axial displacement, differential expansion, eccentricity, bearing vibration, bearing metal temperature, lube-oil pressure, and condenser vacuum.

Vibration protection: high vibration trips at a set level, with alarm at a lower level; shaft-relative vibration is measured by proximity probes, casing vibration by accelerometers.

Turbine trip relays de-energize to trip (fail-safe), closing main stop and control valves and tripping the generator breaker.

Temperature monitoring: thermocouples on steam inlet, metal, and bearing drains detect abnormal conditions; RTDs measure bearing metal temperature.

First-out trip indication identifies the initiating cause; alarm and trip setpoints are verified during periodic protection tests.

Overspeed: if speed exceeds trip setpoint and valves do not close, isolate steam at the boiler or main stop valve and trip the generator; do not re-close on a runaway machine.

Loss of lube oil pressure: standby pump auto-starts; if pressure falls below trip level, the unit trips to protect bearings; do not defeat the trip.

Water induction: indicated by sudden temperature drop, water hammer, or high drain levels; trip the unit and isolate the source, then inspect for distortion and cracking.

Load rejection: bypass valves open to protect the reheater and condenser; the unit may run back to house load or trip depending on control response.

Bearing failure signs: rising bearing metal temperature, vibration, oil discoloration, and metal particles in oil; trip and inspect before restart.

Condenser vacuum loss: exhaust hood temperature rises and LP blade overheating risk increases; reduce load and restore vacuum or trip.

Blade failure or rub: sudden vibration change, noise, and load loss; trip immediately and bar over only under controlled conditions.

Emergency response sequence: trip, verify valves closed and speed decreasing, confirm lube oil and turning gear, then investigate first-out and trend data before restart.

Major overhaul includes rotor removal, blade inspection, seal replacement, bearing inspection, and alignment checks; interval is set by operating hours and starts.

Non-destructive examination: dye penetrant, magnetic particle, ultrasonic, and eddy-current testing on blades, roots, and discs for cracks and stress corrosion.

Blade inspection focuses on last-stage LP blades for erosion, corrosion pitting, and fatigue cracks at roots and tenons.

Valve testing: main stop and control valve stroke tests verify travel and closing time; overspeed trip test verifies trip setpoint by actual overspeed or by test device.

Alignment: coupling alignment checked with laser or dial indicators; thermal growth offsets are applied to cold alignment targets.

Oil system maintenance: oil analysis for water, particles, and acidity; filter changes; reservoir cleaning; varnish and sludge monitoring.

Vibration signature analysis: changes in 1X, 2X, and subsynchronous components indicate unbalance, misalignment, rub, or oil whirl/whip.

Post-overhaul checks: turning gear operation, oil flush, valve calibration, protection trip checks, and controlled first start with vibration and temperature monitoring.

Lockout/tagout (LOTO) isolates steam, water, oil, and electrical energy before maintenance; verify zero energy state before work.

Confined space entry into condensers, drums, and vessels requires atmosphere testing, ventilation, attendant, and permit.

Hot work permits, fire watch, and combustible gas monitoring are required near oil systems and hydrogen-cooled generators.

Steam and hot-surface hazards: PPE, insulation integrity, and barricades around high-temperature piping and valves; steam leaks are invisible and must be approached cautiously.

Environmental limits: thermal discharge temperature limits, cooling-water intake velocity limits for aquatic life, and condenser cooling-water chemistry controls.

Air emissions: NOx limits for combustion turbines and auxiliary boilers; continuous emission monitoring where required by permit.

Noise exposure limits and hearing conservation programs apply in turbine halls; oil and chemical spill prevention and response plans are required.

Regulatory compliance: pressure vessel and piping inspection codes, operator licensing/certification, and periodic safety valve testing records must be maintained.

Lube oil viscosity grade is selected for bearing design; viscosity falls with temperature, so oil temperature control at the cooler outlet is critical.

Oil pressure: normal supply pressure set by the system design; low pressure starts the auxiliary pump, lower pressure trips the unit.

Oil temperature control: cooler outlet temperature maintained in a narrow band; high temperature reduces viscosity and load capacity, low temperature raises viscosity and flow resistance.

Steam chemistry limits: cation conductivity, sodium, silica, chloride, and dissolved oxygen are monitored; carryover causes deposits and corrosion.

Oxygen control: feedwater deaeration and oxygen scavengers reduce corrosion; pH is maintained in the alkaline range to protect carbon steel.

Condensate polishing removes dissolved impurities and particulate matter; resin performance monitored by conductivity and silica.

Cooling water: biocide treatment controls biological fouling; scale and corrosion inhibited by chemical dosing and blowdown control.

Hydrogen purity in hydrogen-cooled generators is maintained above a set percentage; moisture and oxygen in hydrogen are limited to prevent arcing and corrosion.

Synchronization conditions: voltage magnitude, frequency, phase angle, and phase sequence must match the grid before breaker closure.

Generator motoring occurs when steam flow stops but the breaker remains closed; reverse power protection trips the unit to prevent turbine blade overheating and damage.

Frequency droop: a 4-5% droop governor responds to grid frequency deviation by changing output, supporting primary frequency response.

Grid faults cause voltage dips and transient torque; the unit must ride through specified voltage/frequency disturbances or trip by protection design.

Load rejection and islanding: if the grid connection is lost, the unit either runs back to house load or trips; overspeed protection is the last line of defense.

Reactive power and voltage control: the generator AVR controls terminal voltage and reactive output; under- and over-excitation limits protect the rotor and stator.

Frequency limits: operation outside the allowed frequency band causes blade fatigue and generator overheating; protection trips or restricts operation.

Breaker and protection coordination: generator breaker failure, reverse power, loss-of-field, and out-of-step protection schemes must be tested periodically.