Steam Engines, Turbines, and Prime Movers
Domain 4: Steam Engines, Turbines, and Prime Movers
This domain tests how steam expands through a turbine or engine to produce shaft work: impulse and reaction blading, compounding, governors, overspeed protection, condensers, and auxiliaries.
Core rules
- In an impulse stage, pressure drops in the stationary nozzles and the rotor blades see velocity change at nearly constant pressure. In a reaction stage, pressure drops across both stationary and moving blades.
- Compounding reduces rotor speed by dividing the heat drop among stages: velocity compounding uses repeated rows, pressure compounding uses successive stages.
- A condenser lowers exhaust pressure and increases available enthalpy drop. Air in-leakage raises back pressure and reduces output.
- Overspeed trip, turning gear, gland seals, and water-induction protection are safety-critical. Never admit steam to a turbine with inadequate drainage or without turning gear procedure.
Worked example
A turbine receives 100,000 lb/hr steam with an enthalpy drop of 200 Btu/lb. Ideal power = 100,000 x 200 = 20,000,000 Btu/hr. Using 2,545 Btu/hr per horsepower, ideal output = 20,000,000 / 2,545 = 7,858 hp. At 80% efficiency, shaft output is about 6,286 hp. The reasoning: heat drop gives ideal work; efficiency corrects for blade, leakage, and mechanical losses.
Traps
Treating all turbine stages as impulse; ignoring condenser vacuum; starting without draining; and testing overspeed trip only at normal speed.