Power Eng 4th Class 4A flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Factor of evaporation

Ratio of heat absorbed per kilogram of steam to the latent heat of steam at 100 °C, about 2257 kJ/kg. Used to express actual evaporation as equivalent evaporation from and at 100 °C.

Rankine cycle efficiency

Net work output divided by heat added in the boiler, using steam-table enthalpies. Superheating, reheating, and lowering condenser pressure raise efficiency by increasing the average temperature of heat addition.

Sensible versus latent heat

Sensible heat changes temperature without a phase change, Q = m·c·ΔT. Latent heat changes phase at constant temperature, such as boiling water to steam at saturation conditions.

Boiler horsepower definition

Evaporation of 15.7 kg of water per hour from 100 °C feedwater into dry saturated steam at 100 °C, equal to 33 475 kJ/h or about 9.81 kW.

Three-element drum level control

Feedwater control using drum level, steam flow, and feedwater flow signals. It compensates for swell and shrink during load changes, keeping level stable when a single-level signal would mislead.

Low-water fuel cutoff

Safety device that shuts off the burner when water level falls below a safe minimum. It must require manual reset and be independent of the operating level controller.

Furnace prepurge requirement

Before ignition, the furnace must be purged with airflow of at least 25 percent of full-load airflow for not less than five minutes, removing combustible gases to prevent explosion.

Excess air and flue oxygen

Air supplied above stoichiometric requirement. Measured as percent O₂ in flue gas, about 3 percent O₂ corresponds roughly to 15 percent excess air for natural gas firing.

Three T's of combustion

Time, temperature, and turbulence. Sufficient residence time, high enough temperature, and good mixing of fuel and air are required for complete combustion and low carbon monoxide.

NPSH available

Net positive suction head at the pump suction, calculated from suction pressure minus vapour pressure plus static head minus friction losses. It must exceed the pump's required NPSH to avoid cavitation.

Pump affinity laws

For a centrifugal pump, flow varies directly with speed, head with speed squared, and power with speed cubed. Impeller diameter changes follow similar relationships for flow and head.

Best efficiency point

The flow rate where a pump operates at highest efficiency. Running far from it causes recirculation, vibration, cavitation, and shaft deflection, shortening seal and bearing life.

Safety valve versus relief valve

A safety valve opens with pop action for steam or gas service. A relief valve opens gradually in proportion to overpressure, typically for liquid service. Both protect against overpressure.

Steam trap function

Automatic valve that discharges condensate and non-condensable gases while retaining steam. Types include thermostatic, thermodynamic, and float-and-thermostatic. A failed trap causes water hammer or steam loss.

Pipe schedule number

Approximate wall thickness designation for pipe, calculated as 1000 times design pressure divided by allowable stress. Higher schedule numbers indicate thicker walls and higher pressure ratings.

Gate versus globe valve

Gate valves provide straight-through flow and are used for isolation, not throttling. Globe valves force flow through a seat, giving better throttling control but higher pressure drop.

Ion exchange softening

Process where calcium and magnesium ions in feedwater exchange for sodium ions on resin, reducing hardness and preventing scale. Resin is regenerated with brine solution.

Oxygen scavenger sulfite

Chemical such as sodium sulfite that reacts with dissolved oxygen in boiler water, preventing pitting corrosion. Residual is maintained at roughly 20 to 40 mg/L in the boiler water.

Caustic embrittlement

Stress corrosion cracking of stressed steel at riveted seams or tube ends caused by concentrated caustic. Coordinated phosphate treatment and avoiding caustic concentration prevent it.

Deaerator purpose

Removes dissolved oxygen and carbon dioxide from feedwater by heating and mechanical scrubbing. It reduces corrosion in the boiler and condensate system and also preheats feedwater.

Higher heating value

Heat released by complete combustion when water vapour in products is condensed, so latent heat is included. Lower heating value excludes that latent heat and is lower.

Selective catalytic reduction

NOx control method injecting ammonia or urea into flue gas over a catalyst, converting nitrogen oxides to nitrogen and water. Efficiency typically 70 to 90 percent.

Electrostatic precipitator

Particulate control device charging ash particles and collecting them on plates by electrostatic attraction. Efficiency depends on particle resistivity, field strength, and rapping.

Three-phase power formula

Real power in kW equals the square root of three times line voltage times line current times power factor, divided by 1000. Apparent power in kVA omits power factor.

Motor synchronous speed

Speed of the rotating magnetic field, calculated as 120 times frequency divided by number of poles. A four-pole motor on 60 Hz has a synchronous speed of 1800 rpm.

Locked-rotor current

Starting current drawn by a motor at standstill, typically five to eight times full-load current. Reduced-voltage starters limit inrush to protect the supply and driven equipment.

4-20 mA transmitter signal

Analog current standard where 4 mA represents zero percent of span and 20 mA represents 100 percent. Live zero allows detection of a broken wire or failed transmitter.

PID integral action

Control action that integrates accumulated error over time, eliminating steady-state offset. Too much integral gain causes cycling and overshoot; too little leaves persistent deviation.

ASME Section I scope

Section of the ASME Boiler and Pressure Vessel Code covering construction of power boilers, including materials, design, fabrication, inspection, and testing requirements.

Confined space oxygen limit

Acceptable oxygen for entry is 19.5 to 23.5 percent by volume. Below 19.5 percent is oxygen deficient; above 23.5 percent increases fire and combustion risk.

Lockout tagout principle

Energy isolation procedure where each worker applies a personal lock and tag, verifies zero energy, and only the person who applied a lock removes it after work is complete.

Natural draft stack effect

Pressure difference produced by the density difference between hot flue gas in the stack and cooler outside air. Draft increases with stack height and gas temperature.