Prime Movers and Engines: Cycles, Ratings, and Auxiliary Systems
What this domain tests
Prime mover questions cover steam turbines, gas turbines, and reciprocating engines: their cycles, efficiency, ratings, starting and protection systems, and the auxiliary systems that keep them running. Expect items on turbine types, governors, overspeed trips, and engine subsystems.
Core rules and concepts
- Steam turbines: impulse (pressure drops across nozzles) and reaction (pressure drops across moving blades). Condensing turbines exhaust to vacuum; back-pressure turbines exhaust to process steam. Governors control speed or load; an overspeed trip is an independent protective device.
- Gas turbines: Brayton cycle; compressor, combustor, and turbine. Efficiency rises with firing temperature and pressure ratio. Starting requires cranking, ignition, and acceleration to self-sustaining speed.
- Reciprocating engines: four-stroke cycle (intake, compression, power, exhaust). Diesel uses compression ignition; spark-ignition uses a spark plug. Turbocharging raises power density.
- Auxiliaries: lube oil, cooling water, fuel supply, and air intake filtration. Loss of lube oil is a trip condition.
- Ratings are stated at reference conditions; output falls with altitude and ambient temperature.
Worked example
A gas turbine rated 10 MW at 15 C ambient is installed at 1000 m altitude where ambient is 35 C. Estimate the effect of the temperature rise alone if output falls about 0.7% per degree C above 15 C.
Reasoning: temperature rise = 20 C. Derate = 20 x 0.7% = 14%. Output falls to about 8.6 MW before any altitude correction. Altitude further reduces mass flow, so the combined derate is larger. This is why site rating differs from nameplate rating and why intake cooling is used in hot climates.
Traps that cost marks
- Treating the overspeed trip as part of the governor; it is an independent protective device.
- Confusing impulse and reaction blading pressure behaviour.
- Using nameplate rating without correcting for ambient temperature and altitude.
- Forgetting that loss of lube oil pressure is a trip, not an alarm only.