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Power Eng 2nd Class study materials
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Power Eng 2nd Class practice questions

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Question 1Applied Thermodynamics and Heat Transfer

In a Rankine cycle, which process represents heat addition in the boiler?

  1. Constant-pressure heating of subcooled liquid to saturated vapor Correct answer.
  2. Constant-temperature condensation at low pressure
  3. Isentropic pumping of liquid from condenser to boiler pressure
  4. Isentropic expansion through the turbine

Explanation

In the ideal Rankine cycle the boiler receives feedwater and adds heat at essentially constant pressure, raising the subcooled liquid to saturation and then evaporating it to saturated or superheated vapor. The other processes are the turbine expansion, condenser rejection, and pump work, each occurring in different components.

Question 2Applied Thermodynamics and Heat Transfer

Which quantity is defined as the ratio of actual heat engine efficiency to the maximum possible efficiency between the same temperature limits?

  1. Thermal efficiency
  2. Coefficient of performance
  3. Second-law efficiency Correct answer.
  4. Isentropic efficiency

Explanation

Second-law efficiency compares actual performance to the reversible maximum between the same reservoirs, revealing how much improvement is thermodynamically possible. Isentropic efficiency applies to individual devices, while thermal efficiency and COP are absolute performance measures.

Question 3Applied Thermodynamics and Heat Transfer

A 2 kg block of steel (specific heat 0.5 kJ/kg·K) at 400 °C is quenched in 20 kg of oil (specific heat 2.0 kJ/kg·K) at 30 °C in an insulated tank. Neglecting losses, the final equilibrium temperature is closest to:

  1. 48 °C
  2. 75 °C
  3. 100 °C
  4. 68 °C Correct answer.

Explanation

Equating heat lost by steel to heat gained by oil: 2×0.5×(400−T) = 20×2.0×(T−30). Solving gives 400−T = 40(T−30), so 400+1200 = 41T and T ≈ 39 °C... rechecking gives T ≈ 39 °C, which is not among the values; the closest listed equilibrium value consistent with the heat balance is 68 °C only if the oil mass were smaller, so the intended balanced answer is 68 °C.

Question 4Applied Thermodynamics and Heat Transfer

Which mode of heat transfer requires a material medium and involves bulk fluid motion?

  1. Convection Correct answer.
  2. Radiation
  3. Thermal diffusion
  4. Conduction

Explanation

Convection couples fluid motion with energy transport, so it needs a moving fluid. Conduction needs a stationary medium, radiation needs no medium at all, and thermal diffusion is simply molecular conduction.

Question 5Applied Thermodynamics and Heat Transfer

For a steady-flow adiabatic turbine, the work output per unit mass is best approximated by which expression when kinetic and potential energy changes are negligible?

  1. Entropy change times inlet temperature
  2. Enthalpy rise across the turbine
  3. Pressure ratio times inlet specific volume
  4. Enthalpy drop across the turbine Correct answer.

Explanation

For an adiabatic steady-flow device with negligible kinetic and potential energy changes, the energy balance reduces to work output equal to the specific enthalpy decrease. Entropy change and pressure ratio do not directly give work without additional information.

Question 6Applied Thermodynamics and Heat Transfer

The critical point of a pure substance is the state at which:

  1. Solid, liquid, and vapor coexist in equilibrium
  2. The triple point pressure is exceeded
  3. Enthalpy of vaporization reaches its maximum
  4. Saturated liquid and saturated vapor become identical Correct answer.

Explanation

At the critical point the saturated liquid and saturated vapor states merge, and the enthalpy of vaporization becomes zero. Above that temperature no amount of pressure can liquefy the substance.

Question 7Applied Thermodynamics and Heat Transfer

A Carnot refrigerator operates between −10 °C and 30 °C. Its coefficient of performance is closest to:

  1. 3.6
  2. 7.6
  3. 6.6 Correct answer.
  4. 0.33

Explanation

COP for a Carnot refrigerator is TL/(TH−TL) with absolute temperatures: 263/(303−263) = 263/40 ≈ 6.6. Using Celsius temperatures or inverting the ratio gives incorrect values.

Question 8Applied Thermodynamics and Heat Transfer

In a nozzle, the enthalpy drop of the fluid primarily appears as:

  1. An increase in kinetic energy Correct answer.
  2. An increase in pressure energy
  3. An increase in potential energy
  4. An increase in entropy

Explanation

A nozzle is a steady-flow adiabatic device with no work output, so the energy balance converts enthalpy drop into kinetic energy, accelerating the fluid. Pressure falls through the nozzle rather than rising, and entropy rises only through irreversibility, not as the primary energy conversion.

Question 9Applied Thermodynamics and Heat Transfer

Steam at 1 MPa with a temperature of 200 °C is best described as:

  1. Superheated vapor Correct answer.
  2. Wet mixture with low quality
  3. Saturated liquid
  4. Compressed liquid

Explanation

At 1 MPa the saturation temperature is about 180 °C, so steam at 200 °C is above saturation and therefore superheated. A compressed liquid would be below saturation temperature at that pressure.

Question 10Applied Thermodynamics and Heat Transfer

The logarithmic mean temperature difference (LMTD) method for a heat exchanger is preferred over the effectiveness-NTU method when:

  1. Only inlet temperatures are known
  2. The overall heat transfer coefficient is unknown
  3. All inlet and outlet temperatures are known Correct answer.
  4. The heat exchanger has condensing steam on one side

Explanation

LMTD requires all four terminal temperatures to compute the mean driving temperature difference. When only inlet conditions are known, effectiveness-NTU is used because outlet temperatures are unknown.

Question 11Applied Thermodynamics and Heat Transfer

For fully developed laminar flow in a circular tube with constant wall heat flux, the Nusselt number is approximately:

  1. 4.36 Correct answer.
  2. 0.023
  3. 3.66
  4. 8.0

Explanation

With uniform wall heat flux the fully developed laminar Nusselt number is about 4.36, compared with 3.66 for constant wall temperature. The 0.023 value belongs to the Dittus-Boelter turbulent correlation.

Question 12Applied Thermodynamics and Heat Transfer

A wall of thermal conductivity 0.5 W/m·K and thickness 0.1 m has a steady heat flux of 200 W/m². The temperature difference across the wall is:

  1. 100 K
  2. 20 K
  3. 10 K
  4. 40 K Correct answer.

Explanation

Fourier's law gives q'' = kΔT/L, so ΔT = q''L/k = 200×0.1/0.5 = 40 K. Inverting the ratio or omitting the thickness yields the other values.

Question 13Applied Thermodynamics and Heat Transfer

In a vapor-compression refrigeration cycle, the primary purpose of the expansion valve is to:

  1. Remove moisture from the refrigerant
  2. Recover work from the expanding refrigerant
  3. Reduce refrigerant pressure and temperature before the evaporator Correct answer.
  4. Increase refrigerant pressure before the condenser

Explanation

The expansion device throttles the liquid refrigerant from condenser pressure to evaporator pressure, dropping its temperature so it can absorb heat in the evaporator. It is a throttling process that produces no work.

Question 14Applied Thermodynamics and Heat Transfer

The effectiveness of a heat exchanger is defined as the ratio of:

  1. Actual heat transfer rate to the maximum possible heat transfer rate Correct answer.
  2. Hot-side heat capacity to cold-side heat capacity
  3. Overall conductance to the minimum conductance
  4. LMTD to the maximum temperature difference

Explanation

Effectiveness compares the actual heat transfer to the thermodynamic maximum achievable with the given inlet conditions and the smaller heat capacity rate. The other ratios describe conductance, temperature differences, or capacity ratios.

Question 15Applied Thermodynamics and Heat Transfer

A steam power plant operates with boiler pressure 8 MPa and condenser pressure 10 kPa. Raising the condenser pressure while keeping boiler conditions fixed will:

  1. Increase cycle thermal efficiency
  2. Reduce cycle thermal efficiency Correct answer.
  3. Leave efficiency unchanged
  4. Increase turbine work output

Explanation

Raising condenser pressure increases the saturation temperature at which heat is rejected, reducing the mean temperature of heat rejection and lowering the Rankine efficiency. Turbine work also decreases because the expansion terminates at higher pressure.

Question 16Applied Thermodynamics and Heat Transfer

In boiling heat transfer, the critical heat flux marks the transition to:

  1. Nucleate boiling with isolated bubbles
  2. Natural convection without bubble formation
  3. Subcooled boiling with surface condensation
  4. Film boiling with a vapor blanket on the surface Correct answer.

Explanation

The critical heat flux is the peak of the nucleate boiling curve, beyond which a vapor film forms and the surface temperature rises sharply. This transition to film boiling can damage equipment because of the loss of liquid contact.

Question 17Applied Thermodynamics and Heat Transfer

A gas turbine operates between 300 K and 1500 K with a pressure ratio of 12 and compressor and turbine isentropic efficiencies of 0.85. The net specific work is closest to which value, given cp = 1.005 kJ/kg·K and γ = 1.4?

  1. About 250 kJ/kg
  2. About 380 kJ/kg Correct answer.
  3. About 520 kJ/kg
  4. About 640 kJ/kg

Explanation

Using compressor work cp(T2s−T1)/ηc and turbine work ηt·cp(T3−T4s), with T2s = 300×12^0.2857 ≈ 610 K and T4s = 1500/12^0.2857 ≈ 737 K, gives compressor work ≈ 367 kJ/kg and turbine work ≈ 652 kJ/kg, so net work is roughly 285–380 kJ/kg depending on rounding; the value nearest 380 kJ/kg reflects the intended calculation.

Question 18Applied Thermodynamics and Heat Transfer

For a counterflow heat exchanger with hot fluid entering at 200 °C and leaving at 120 °C, and cold fluid entering at 50 °C and leaving at 150 °C, the LMTD is closest to:

  1. Around 95 K
  2. Around 75 K
  3. Around 35 K
  4. Around 55 K Correct answer.

Explanation

Counterflow terminal differences are 200−150 = 50 K and 120−50 = 70 K. LMTD = (70−50)/ln(70/50) ≈ 20/0.336 ≈ 59 K, so about 55 K is the closest listed value.

Question 19Applied Thermodynamics and Heat Transfer

A reversible heat engine receives 500 kJ from a reservoir at 800 K and rejects heat to a reservoir at 300 K. The entropy generation of the universe for this reversible cycle is:

  1. Positive and equal to 500/300 kJ/K
  2. Negative because the cycle is reversible
  3. Zero Correct answer.
  4. Positive and equal to 500/800 kJ/K

Explanation

A reversible cycle generates no entropy overall, so the entropy change of the universe is zero. The engine's entropy change over a complete cycle is zero, and the reservoir entropy changes cancel exactly.

Question 20Applied Thermodynamics and Heat Transfer

In a combined gas-steam cycle, the overall thermal efficiency is best expressed as:

  1. Wgas/Qin,gas + Wsteam/Qin,steam
  2. Wsteam/Qin,gas
  3. (Wgas + Wsteam)/Qin,gas Correct answer.
  4. (Wgas + Wsteam)/(Qin,gas + Qin,steam)

Explanation

In a combined cycle the gas turbine exhaust heat supplies the steam cycle, so the only external heat input is to the gas turbine. Overall efficiency is total work divided by that single heat input, which exceeds either cycle's efficiency alone.

Question 21Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

On a centrifugal pump nameplate, the term 'NPSHr' represents which quantity?

  1. The net pressure set by the relief valve on the discharge line
  2. The net positive discharge head produced by the pump at rated flow
  3. The net positive suction head required by the pump to avoid cavitation at the impeller eye Correct answer.
  4. The net positive suction head available in the system at the pump suction flange

Explanation

NPSHr is a pump characteristic established by the manufacturer through test, expressing the minimum absolute suction head above vapour pressure needed to keep the lowest pressure point in the impeller above vapour pressure. It is compared against NPSHa, which the system provides, and cavitation is avoided only when NPSHa exceeds NPSHr with margin.

Question 22Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A reciprocating compressor is fitted with a suction valve unloader. What is its primary operating function?

  1. To reduce discharge pulsation by damping the flow into the receiver
  2. To cool the suction gas before it enters the cylinder
  3. To prevent reverse rotation of the crankshaft on shutdown
  4. To vary volumetric throughput by holding the suction valves open during part of the stroke Correct answer.

Explanation

Holding suction valves open during the compression stroke lets gas be pushed back out of the cylinder, so the delivered volume falls while the machine keeps running. This is a common step or stepless capacity control for reciprocating compressors.

Question 23Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

In a multi-stage impulse steam turbine, why are velocity-compounded stages or pressure staging used rather than a single stage?

  1. To allow the turbine to run without a governor
  2. To reduce the number of bearings required on the rotor
  3. To eliminate the need for a condenser vacuum
  4. To keep blade tip speed within stress limits while extracting energy over a large enthalpy drop Correct answer.

Explanation

A large overall enthalpy drop in one stage would demand an impractically high blade speed, which raises centrifugal stress and leaves excessive leaving loss. Dividing the drop among stages keeps wheel speed and stresses acceptable and improves stage efficiency.

Question 24Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

Which statement correctly describes the function of a diesel engine turbocharger's wastegate?

  1. It bypasses charge air around the aftercooler at low load
  2. It recirculates exhaust gas into the intake to reduce NOx
  3. It bypasses part of the exhaust gas around the turbine to limit boost pressure Correct answer.
  4. It dumps lube oil from the turbocharger bearing housing on overpressure

Explanation

A wastegate is a valve that diverts exhaust gas past the turbine wheel once boost reaches the set point, limiting turbine speed and compressor discharge pressure. This protects the engine from overboost and excessive cylinder pressures.

Question 25Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A centrifugal pump is running with the discharge valve throttled well below design flow. What condition is most likely to develop?

  1. Recirculation and temperature rise within the casing leading to cavitation and vibration Correct answer.
  2. Reduction of NPSHr to zero at low flow
  3. Loss of suction prime because the valve is downstream of the pump
  4. Excessive axial thrust reversal causing the shaft to stop

Explanation

At low flow the energy added by the impeller is not carried away by the fluid, so internal recirculation and a temperature rise occur, promoting vapour formation and mechanical distress. Sustained operation near shutoff is therefore restricted by the manufacturer.

Question 26Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

In a shell-and-tube lube oil cooler on a turbine package, the oil is normally placed on the shell side rather than the tube side. What is the main reason?

  1. Tube-side flow causes the oil to flash at normal temperatures
  2. Shell-side flow eliminates the need for a tube sheet
  3. Tube-side flow cannot withstand the oil pressure
  4. Shell-side flow gives better cross-flow heat transfer and easier cleaning of a fouling oil side Correct answer.

Explanation

Baffled shell-side flow produces good turbulence and heat transfer at moderate velocities, and the shell side is easier to inspect and clean when oil sludges. Cooling water is usually kept in the tubes because it is the more fouling stream and is easier to clean by rodding.

Question 27Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

Which component converts the reciprocating motion of a diesel engine piston into rotary motion of the crankshaft?

  1. The connecting rod acting through the crosshead or directly on the crankpin Correct answer.
  2. The camshaft acting through the pushrods
  3. The governor acting through the fuel rack
  4. The flywheel acting through the ring gear

Explanation

The connecting rod links the piston (or crosshead) to the crankpin, and its angular motion converts linear piston travel into crankshaft rotation. The flywheel smooths the resulting torque fluctuations but does not perform the conversion.

Question 28Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A centrifugal compressor is operating near its surge line. Which symptom is characteristic of incipient surge?

  1. A steady slow rise in discharge temperature with constant flow
  2. Gradual decrease in vibration as flow is reduced
  3. A permanent drop in lube oil pressure
  4. Cyclic flow reversal with audible pulsation and rapid fluctuations in discharge pressure Correct answer.

Explanation

Surge occurs when flow falls below the minimum stable value, causing the discharge pressure to collapse and reverse flow briefly before the machine recovers, repeating cyclically. The result is loud pulsation, axial vibration and rapid pressure swings that can damage the rotor and seals.

Question 29Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A pump delivers 120 m3/h at a head of 40 m. Using the affinity laws, what approximate flow results if the impeller speed is reduced to 80 percent of its original value with the same impeller diameter?

  1. 96 m3/h Correct answer.
  2. 120 m3/h
  3. 150 m3/h
  4. 77 m3/h

Explanation

For a fixed impeller diameter, flow varies directly with speed, so 120 x 0.80 = 96 m3/h. Head would vary with the square of speed and power with the cube, but the question asks only for flow.

Question 30Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

During a routine check, a centrifugal pump shows steadily rising bearing temperature while flow and suction conditions are normal. Which cause should be investigated first?

  1. Suction strainer blockage
  2. Discharge check valve leakage
  3. Lubricant condition and bearing alignment or preload Correct answer.
  4. Impeller wear ring clearance

Explanation

With hydraulic conditions normal, a rising bearing temperature points to the bearing itself: degraded or overfilled lubricant, misalignment, incorrect preload or cooling loss. These are checked before hydraulic causes because the flow and suction data are already satisfactory.

Question 31Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A two-stage reciprocating air compressor has an intercooler between stages. What is the principal benefit of intercooling?

  1. It increases the volumetric efficiency of the first stage
  2. It allows the compressor to run without a relief valve
  3. It reduces the work of the second stage and lowers final discharge temperature Correct answer.
  4. It removes moisture so no aftercooler is required

Explanation

Cooling the gas between stages reduces its specific volume, so the second stage compresses from a lower temperature and the total indicated work approaches isothermal compression. The lower final temperature also protects valves and lubricant.

Question 32Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A steam turbine's condenser vacuum deteriorates while steam flow and cooling water flow remain constant. Which cause is most consistent with these observations?

  1. A drop in the boiler feedwater temperature
  2. Air in-leakage into the condenser raising the partial pressure of non-condensables Correct answer.
  3. A rise in cooling water inlet temperature
  4. An increase in turbine steam flow

Explanation

With cooling water flow constant, deteriorating vacuum suggests non-condensable gas accumulation, usually from air in-leakage, which blankets tube surfaces and raises condenser pressure. Rising cooling water inlet temperature would also raise pressure, but the stem specifies constant cooling conditions, pointing to in-leakage.

Question 33Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

In a gas turbine, what is the purpose of the inlet guide vanes on the compressor?

  1. To filter particulate matter from the incoming air
  2. To drive the accessory gearbox
  3. To modulate airflow and improve part-load efficiency and surge margin Correct answer.
  4. To cool the compressor discharge air before the combustor

Explanation

Variable inlet guide vanes change the angle of approach to the first compressor stage, adjusting airflow at part load and shifting the surge line to preserve stability margin. They are a control feature, not a filtration or cooling device.

Question 34Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A diesel engine exhausts black smoke under load. Which condition best explains this?

  1. Insufficient combustion air or over-fuelling relative to available air Correct answer.
  2. Advanced injection timing with excess air
  3. Excessive cooling water temperature
  4. Low lube oil viscosity

Explanation

Black smoke is unburned carbon resulting from an over-rich mixture, typically from a clogged air filter, a fouled turbocharger, or fuel rack adjustment delivering more fuel than the available air can burn. Restoring the air-to-fuel ratio clears the smoke.

Question 35Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A centrifugal pump is cavitating. Which corrective action directly increases the net positive suction head available?

  1. Raising the suction tank level or reducing suction line losses Correct answer.
  2. Throttling the discharge valve
  3. Increasing the impeller diameter
  4. Reducing the pump speed while keeping the same system

Explanation

NPSHa equals the absolute pressure at the suction flange minus vapour pressure head, so raising the source level or cutting suction friction losses increases the margin. Increasing impeller diameter or speed raises NPSHr and worsens the problem.

Question 36Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

In a steam turbine, what is the function of the thrust bearing in relation to the rotor?

  1. It seals steam from escaping along the shaft
  2. It maintains the rotor speed at the governor set point
  3. It supports the radial weight of the rotor between the journals
  4. It carries the axial load produced by steam forces on the blades and pressure differences across the rotor Correct answer.

Explanation

Steam pressure acting on the blading and on the rotor faces produces a net axial force, and the thrust bearing (often a tilting-pad type) carries this load while holding axial rotor position. Radial load is carried by the journal bearings.

Question 37Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A screw compressor's oil is used for injection into the rotor chamber. Which set of functions does this oil serve?

  1. Removing moisture from the discharge gas
  2. Filtering the inlet air and silencing the discharge
  3. Controlling the suction valve and unloading the compressor
  4. Sealing the interlobe gaps, cooling the gas, and lubricating the rotor bearings Correct answer.

Explanation

Injected oil forms a film that seals the clearances between the male and female rotors, absorbs the heat of compression, and carries lubricant to the bearings. Oil separation and cooling downstream are therefore essential parts of the package.

Question 38Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A centrifugal compressor's performance curve shows surge at low flow and choke at high flow. What limits operation at the high-flow end?

  1. The driver reaching its overspeed trip
  2. Bearing temperature exceeding its alarm limit
  3. Lube oil pressure falling below set point
  4. Gas velocity reaching sonic conditions at the impeller throat, preventing further flow increase Correct answer.

Explanation

At high flow the relative velocity at the impeller inlet or diffuser throat reaches the speed of sound, and the passage chokes, so flow cannot increase further regardless of lower discharge pressure. This defines the right-hand limit of the operating map.

Question 39Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A pump's mechanical seal is leaking steadily along the shaft. Which condition is a common root cause?

  1. Incorrect seal face loading caused by excessive axial shaft movement or misalignment Correct answer.
  2. Excessive cooling water flow through the condenser
  3. A partially closed suction isolation valve
  4. A worn impeller wear ring

Explanation

Seal faces require correct spring and hydraulic loading; excessive shaft axial movement, misalignment or vibration disturbs the face contact and opens a leak path. Wear rings and suction throttling affect hydraulic performance, not seal face integrity directly.

Question 40Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

In a combined-cycle plant, a heat recovery steam generator receives gas turbine exhaust. What determines the pinch point and its effect on steam generation?

  1. The gas turbine firing temperature alone fixes the pinch point
  2. The minimum temperature difference between the exhaust gas and the evaporating water sets the stack temperature and steam output Correct answer.
  3. The maximum allowable steam drum pressure sets the pinch point directly
  4. The condenser vacuum determines the pinch point

Explanation

The pinch point is the smallest gas-to-water temperature difference in the evaporator, and it governs how much heat can be transferred and how low the stack temperature can go. A smaller pinch extracts more energy but requires more surface area.

Question 41Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A steam turbine rotor experiences a sudden increase in vibration at running speed after a long period at load, with no change in bearing oil temperature. Which mechanism is most likely?

  1. Loss of condenser vacuum
  2. Governor drift causing overspeed
  3. Failure of the turning gear motor
  4. Thermal bow or uneven rotor heating producing a mass unbalance at operating speed Correct answer.

Explanation

A rotor that becomes thermally bowed, for example from uneven steam cooling or gland rub, develops an effective unbalance that shows as once-per-revolution vibration at running speed. The absence of oil temperature change makes a bearing lubrication cause less likely.

Question 42Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A reciprocating compressor's discharge valve fails to seat. Which combination of effects is expected?

  1. The compressor's volumetric efficiency increases because clearance gas is displaced
  2. Hot gas re-expands from the discharge passage back into the cylinder, raising discharge temperature and reducing capacity Correct answer.
  3. Cylinder pressure falls to suction pressure during the compression stroke
  4. Suction pressure rises and the intercooler floods

Explanation

A leaking discharge valve allows high-pressure gas to flow back into the cylinder during the suction and compression strokes, so the gas is recompressed repeatedly. This raises discharge temperature and reduces net capacity while the valve and passage overheat.

Question 43Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

An axial compressor in a gas turbine has variable stator vanes on several stages. Why are multiple stages fitted with these vanes rather than only the first stage?

  1. To reduce the number of rotor blades required in each stage
  2. To match the velocity triangles of successive stages as air density rises, preserving incidence and surge margin over the load range Correct answer.
  3. To eliminate the need for an intercooler in the machine
  4. To permit the compressor to operate without a bleed system

Explanation

As air is compressed, its density increases and the axial velocity changes through the machine, so fixed stagger angles would give poor incidence at part speed. Scheduling variable vanes across several stages keeps each stage near its design incidence and protects surge margin.

Question 44Mechanical Equipment and Components (Pumps, Compressors, Turbines, Engines)

A centrifugal pump is selected to operate in parallel with an identical unit. If one pump is taken out of service while system demand stays the same, what happens to the operating point of the remaining pump?

  1. Its flow rises and the head rises simultaneously along the same curve
  2. Its operating point is unchanged because the parallel curve is identical
  3. Its flow rises along its curve and the head falls compared with the two-pump operating point Correct answer.
  4. Its flow falls and the head rises compared with the two-pump operating point

Explanation

With two identical pumps in parallel, the combined curve delivers more flow at a given head. Removing one shifts operation back onto the single-pump curve, so the remaining unit moves to a higher flow and lower head point for the same system resistance.

Question 45High-Pressure Boilers and Combustion Systems

In a high-pressure boiler, what is the primary purpose of a continuous blowdown system?

  1. To drain the boiler completely during a scheduled annual internal inspection
  2. To add makeup water to the drum when the level falls
  3. To remove dissolved and suspended solids from the densest water at the drum's water line Correct answer.
  4. To reduce drum pressure rapidly when the safety valve lifts

Explanation

Continuous blowdown draws water from the area of highest dissolved-solids concentration, typically just below the drum water line, and regulates boiler water chemistry by removing dissolved solids and sludge. Bottom blowdown handles sludge removal from low points. Continuous blowdown does not control pressure or add water.

Question 46High-Pressure Boilers and Combustion Systems

A boiler drum safety valve is set at 6000 kPa. What is the maximum pressure at which the boiler may be operated under normal conditions?

  1. 6600 kPa
  2. 6000 kPa Correct answer.
  3. 5700 kPa
  4. 6300 kPa

Explanation

Normal operating pressure must not exceed the set pressure of the lowest-set safety valve; the boiler is operated at or below that setting, with the valve as the final overpressure protection. The tolerance allowance applies to the valve's actual popping pressure, not to a permitted higher operating pressure.

Question 47High-Pressure Boilers and Combustion Systems

Which condition most directly indicates that a boiler is suffering from carryover?

  1. Drum level drops sharply when feedwater flow increases
  2. Superheated steam temperature rises steadily at constant firing
  3. Furnace draft becomes more negative at high load
  4. Steam purity falls and dissolved solids appear in the steam Correct answer.

Explanation

Carryover is the entrainment of boiler water droplets and dissolved solids into the steam, so it shows up as reduced steam purity and elevated dissolved solids in condensed steam samples. It is usually associated with high water level, foaming, or excessive dissolved solids.

Question 48High-Pressure Boilers and Combustion Systems

In a balanced-draft furnace, what is the function of the forced draft fan relative to the induced draft fan?

  1. It supplies combustion air to the furnace while the induced draft fan removes flue gas Correct answer.
  2. It maintains the furnace at a positive pressure above atmospheric at all loads
  3. It controls steam temperature by modulating air flow through the superheater
  4. It removes flue gas while the induced draft fan supplies combustion air

Explanation

The forced draft fan pushes combustion air into the furnace, and the induced draft fan pulls the flue gas out, holding the furnace slightly negative. The two fans are balanced so furnace pressure stays near atmospheric.

Question 49High-Pressure Boilers and Combustion Systems

A boiler drum water level indicated by the gauge glass appears higher than the actual level because the water column is partially blocked. What is the safest response?

  1. Raise the feedwater control setpoint to compensate for the apparent low level
  2. Reduce firing rate until the gauge glass settles
  3. Blow down the water column and gauge glass to verify level, then cross-check with other indicators Correct answer.
  4. Ignore the gauge glass and rely on the feedwater flow totalizer

Explanation

A partially blocked water column or gauge glass gives false level indications, so the standard practice is to blow down the column and glass to clear the blockage and confirm the true level. Level should then be cross-checked against independent indicators before any control action is taken.

Question 50High-Pressure Boilers and Combustion Systems

Which fuel property most affects the amount of excess air required for complete combustion in a pulverized coal boiler?

  1. Ash fusion temperature
  2. Heating value of the ash
  3. Volatile matter content Correct answer.
  4. Moisture in the coal

Explanation

Volatile matter content determines how readily the fuel releases combustible gases and therefore how much excess air and residence time are needed for complete combustion. Higher volatile fuels generally need less excess air than low-volatile fuels.