Prime Movers: Steam and Gas Turbines
Prime-mover questions test stage action, governing, vacuum effects, water induction, overspeed protection and gas-turbine cycle behaviour. You need to distinguish impulse from reaction blading and understand why a condenser vacuum or compressor pressure ratio changes output and heat rate.
Steam turbines
- Impulse stages drop pressure mainly in stationary nozzles; reaction stages drop pressure in both stationary and moving blades. Velocity compounding uses multiple moving rows; pressure compounding uses repeated stages.
- Governing controls speed or load by throttling steam or adjusting nozzle admission. Overspeed trip systems are independent of normal control and are tested regularly.
- Condenser vacuum increases available enthalpy drop. Backpressure rises if air in-leakage, cooling-water loss or fouling occurs, reducing output and raising heat rate.
- Water induction is a severe hazard: it can warp rotors, crack blades and cause thermal shock. Drains, trap operation and steam-line warm-up are protective measures.
Gas turbines
- The Brayton cycle efficiency rises with compressor pressure ratio and turbine firing temperature. Compressor work consumes a large share of turbine output.
- Compressor surge is a flow instability that can reverse flow and damage blades; stall is local flow separation. Bleed valves, inlet guide vanes and anti-surge controls protect the machine.
- Blade cooling allows higher firing temperature than the blade metal could otherwise tolerate. Combined cycles recover exhaust heat in an HRSG to raise overall efficiency.
Worked example
A gas-turbine compressor takes air at 288 K and has a pressure ratio of 16 with k = 1.4. The isentropic outlet temperature is T2 = 288 x 16^(0.2857) = 288 x 2.208 = 636 K. With cp = 1.005 kJ/kg-K, compressor work is about 1.005 x (636 - 288) = 350 kJ/kg. If actual outlet is 660 K because of inefficiency, actual work is 374 kJ/kg and more turbine work is consumed.
Traps that cost marks
Confusing gauge vacuum with absolute pressure; assuming a higher vacuum always helps without considering condenser limits; treating surge and stall as identical; and ignoring cooling-air penalties when calculating gas-turbine output. In steam turbines, never assume the overspeed trip is a substitute for a functioning control valve.
Key points: impulse and reaction stages differ in where pressure drops; condenser vacuum affects available energy; water induction is a major turbine hazard; gas-turbine efficiency depends on pressure ratio and firing temperature; surge protection is essential.