Steam Turbines, Prime Movers, and Power Generation
Domain focus
This domain tests Rankine cycle operation, turbine stage design, condensers, governors, overspeed protection, and generator output. Class 1 questions link steam conditions to efficiency and safe operation.
Core rules
- Impulse stages convert pressure to velocity in nozzles; reaction stages convert pressure in both stationary and moving blades.
- Condenser vacuum lowers exhaust pressure and increases available enthalpy drop; air in-leakage raises back pressure and reduces output.
- Heat rate is fuel energy input per kWh output; lower heat rate means better efficiency.
- Overspeed trip, turning gear, lube oil, and gland seal systems protect the turbine during startup, shutdown, and coastdown.
- Generator output depends on steam flow, enthalpy drop, and generator efficiency; excitation controls voltage and reactive power.
Worked example
A turbine receives 500,000 lb/h of steam with an enthalpy drop of 400 Btu/lb. Ideal power = (500,000 × 400)/3,412 = 58,617 kW. If actual output is 55,000 kW, the overall turbine-generator efficiency is 55,000/58,617 = 93.8%. The reasoning: 3,412 Btu/kWh converts thermal energy rate to electrical power; the difference is losses, leakage, and mechanical/electrical inefficiency.
Traps
Reading condenser vacuum as absolute pressure; ignoring air in-leakage; tripping overspeed protection to shorten a test; and assuming superheat has no effect on stage efficiency. Also distinguish throttle pressure from first-stage pressure.