3B2 POWER ENGINEER COMPREHENSIVE EXAM QUESTIONS WITH COMPLETE VERIFIED ANSWERS AND DETAILED EXPLANATIONS

3B2 POWER ENGINEER COMPREHENSIVE EXAM QUESTIONS WITH COMPLETE VERIFIED ANSWERS AND DETAILED EXPLANATIONS QUESTION 1 When the exit pressure of a steam turbine nozzle is less than ________ times the inlet pressure, then a ________ nozzle will be used. A) 0.757, convergent B) 0.577 (critical pressure ratio for steam), convergent-divergent C) 0.757, convergent-divergent D) 0.757, convergent E) 0.577, convergent or convergent-divergent Correct Answer: B Explanation: For steam, the critical pressure ratio (choked flow) is approximately 0.577 (P2/P1 = 0.577). If the exit pressure is less than 0.577 times the inlet pressure, the nozzle must be convergent-divergent (De Laval nozzle) to expand the steam supersonically and avoid shock losses. If exit pressure is above the critical ratio, a simple convergent nozzle suffices. QUESTION 2 Most aero-derivative turbines' lube oil systems require: A) Two backup lube oil pumps B) Separate inlet and outlet filter systems C) Separate lube oil tanks and coolers D) Separate lube oil systems (for gas generator and power turbine) E) Separate backup pumps Correct Answer: D Explanation: Aero-derivative gas turbines (derived from aircraft jet engines) often have separate lube oil systems for the gas generator (engine) and the power turbine (free turbine). They also often have separate oil tanks and coolers for each system. QUESTION 3 It is general practice to let the thickness of the walls and flanges from the inlet to the exhaust end of a steam turbine casing: A) Decrease (thinner at the exhaust end) B) Fluctuate C) Be the same D) Vary E) Increase (thicker at the exhaust end) Correct Answer: A Explanation: The temperature and pressure of steam decrease from the inlet (high pressure, high temperature) to the exhaust (low pressure, lower temperature). Therefore, the casing walls and flanges can be made thinner towards the exhaust end, saving material and weight. QUESTION 4 In a two-stage refrigeration system, the 1st stage compressor is also known as a ________ compressor. A) Booster B) Standby C) Low volume D) Extraction E) Refrigerant Correct Answer: A Explanation: In a two-stage refrigeration system (for very low temperatures below -40°C to -80°C), the first stage compressor is called the booster or low-stage compressor. It compresses refrigerant from evaporator pressure to an intermediate pressure. The second stage (high-stage) compressor compresses from intermediate pressure to condenser pressure. QUESTION 5 In a rotary vane compressor: A) Vanes are kept close to the central hub by centrifugal force B) There is a rotor in the exact center of the casing C) Refrigerant is trapped between the roller and the decreasing volume of the eccentric casing D) The rotor oscillates E) The vanes move in and out of the rotor slots Correct Answer: E Explanation: In a rotary vane compressor, the rotor is eccentrically positioned in the cylinder. Vanes slide in and out of rotor slots due to centrifugal force (and sometimes spring pressure). As the rotor turns, the vanes maintain contact with the cylinder wall, creating chambers that decrease in volume, compressing the refrigerant. The vanes move in and out of the rotor slots. QUESTION 6 In a gas turbine, there are fewer turbine stages than compressor stages because: A) Energy can be extracted more efficiently in the turbine B) Turbine blades are bigger so require fewer C) Higher exhaust temperatures can be tolerated D) The hot gases occupy more volume E) Hot exhaust gases are required for the HRSG's Correct Answer: A Explanation: Energy can be extracted more efficiently in the turbine (expansion) than added in the compressor (compression). Each turbine stage can extract more energy than each compressor stage adds. Therefore, fewer turbine stages are needed to drive the compressor and produce useful output power. QUESTION 7 Pressure limiting devices of refrigeration systems shall be connected: A) Between the compressor and the stop valve on the suction side B) To a blowdown tank C) Between the inlet of the condenser and the outlet of the receiver D) Between the compressor and the stop valve on the discharge side E) To the receiver outlet Correct Answer: D Explanation: Pressure limiting devices (high pressure cut-outs or pressure relief valves) must be connected between the compressor discharge and the stop valve on the discharge side to protect the compressor and system from overpressure. They should be installed as close to the compressor as practical. QUESTION 8 The warm-up time required for a turbine is dependent on: A) How quickly the turbine is required B) The manufacturer's instructions C) The size of the turbine D) The type of turbine E) Whether the steam is dry or wet Correct Answer: C Explanation: Larger turbines have more thermal mass (more metal) and take longer to warm up evenly to prevent thermal stress, differential expansion, and rotor "hogging" (bending). Warm-up time is primarily determined by turbine size (rotor diameter, casing thickness). Manufacturer's instructions provide specific ramp rates, but size is the fundamental factor. QUESTION 9 ________ compressors require intake and discharge valves. A) Rotary B) Centrifugal C) Sliding vane D) Rotary lobe E) Reciprocating Correct Answer: E Explanation: Reciprocating compressors use automatic spring-loaded or pressureactuated intake and discharge valves that open and close based on pressure differences across the cylinder. When the piston moves down (intake stroke), the intake valve opens; when the piston moves up (compression stroke), the intake valve closes and the discharge valve opens when cylinder pressure exceeds discharge line pressure. QUESTION 10 Besides reducing NOx, water injection systems: A) Stabilize the combustion process, thereby increasing the life of hot section components B) Help keep the compressor blading clean C) Reduce the gas turbine's thermal efficiency D) Decrease the power output Correct Answer: C Explanation: Water injection reduces NOx formation by lowering flame temperature, but it also reduces thermal efficiency because energy is used to vaporize the water (heat of vaporization) rather than producing useful work. This decreases overall efficiency and may slightly reduce power output unless compensated by increased fuel flow. QUESTION 11 The pressure at the outlet from the moving blades is the same as the pressure at the inlet to the moving blades in a/an ________ turbine. A) Topping B) Reaction C) Back pressure D) Impulse E) Extraction Correct Answer: D Explanation: In an impulse turbine, all pressure drop (enthalpy drop) occurs in the stationary nozzles. The steam pressure remains constant as it passes through the moving blades; only velocity decreases (converted to mechanical energy). In a reaction turbine, pressure drops across both stationary and moving blades. QUESTION 12 A condenser shell may be fitted with an expansion joint if the: