Power Eng 2nd Class flashcards

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

Sensible heat

Heat that changes the temperature of a substance without changing its phase. Calculated as Q = m × c_p × ΔT, with mass in kg, specific heat in kJ/kg·K, and temperature change in K.

Steam quality

The mass fraction of vapor in a wet steam mixture, from 0 at saturated liquid to 1 at saturated vapor. It is found from enthalpy using x = (h − h_f)/h_fg.

Log mean temperature difference

The driving temperature difference for a heat exchanger, calculated as the difference of the terminal differences divided by the natural log of their ratio. Used with the overall coefficient to size area.

Carnot efficiency

The maximum possible efficiency of a heat engine operating between two temperatures, equal to 1 minus the ratio of absolute sink temperature to absolute source temperature.

NPSH available

The absolute pressure head at the pump suction above the liquid vapor pressure. If it falls below the pump's required value, cavitation damages the impeller and reduces flow.

Pump affinity laws

For a centrifugal pump, flow varies with speed, head with speed squared, and power with speed cubed. Impeller diameter follows the same pattern for modest trims.

Compressor clearance volume

The volume remaining in the cylinder at the end of discharge. It traps gas that re-expands on the suction stroke, reducing volumetric efficiency as compression ratio rises.

Turbine isentropic efficiency

The ratio of actual enthalpy drop across the turbine to the enthalpy drop that would occur in an ideal isentropic expansion between the same inlet and exhaust pressures.

Governor droop

The percentage speed decrease from no load to full load for a governor. Zero droop is called isochronous and allows parallel machines to share load poorly without a droop setting.

Excess air

The air supplied beyond the theoretical stoichiometric requirement, expressed as a percentage. Too little causes carbon monoxide; too much raises stack heat loss.

Drum swell and shrink

Apparent drum level rises on load increase because steam bubbles expand, and falls on load decrease. It misleads single-element level control, so three-element control is used on large boilers.

Indirect boiler efficiency

Boiler efficiency found by subtracting all losses from 100 percent. Losses include dry flue gas, moisture in fuel and air, unburned carbon, radiation, and blowdown.

Furnace purge

A forced airflow through the furnace and gas passages before ignition to remove accumulated combustible gases. The manufacturer's specified purge time must be proven before fuel is admitted.

Live zero signal

A 4–20 mA transmitter output where 4 mA represents the zero value of the measured span. The offset allows detection of a broken wire or failed transmitter.

Integral control action

A control mode whose output changes in proportion to the accumulated error over time. It eliminates steady-state offset but can cause overshoot if the integral time is too short.

Fail-safe valve position

The valve position on loss of air or power, set by the actuator spring. Selection depends on which position leaves the process in the safest condition.

Flame scanner

A sensor, often ultraviolet or infrared, that proves the burner flame is present. Its signal must distinguish flame from hot refractory and trip fuel on loss of flame.

Synchronous speed

The rotating speed of a motor's magnetic field, equal to 120 times frequency divided by the number of poles. Induction motors run slightly slower by the amount of slip.

Motor slip

The difference between synchronous speed and actual rotor speed, expressed as a fraction of synchronous speed. Slip increases as the mechanical load on the motor increases.

Star-delta starting

A starting method that connects the motor windings in star for starting and delta for running. Starting current and torque are reduced to one third of direct-on-line values.

Variable frequency drive

A device that controls motor speed by varying supply frequency and voltage. Below base speed it holds a constant voltage-to-frequency ratio to maintain magnetic flux.

Conductivity blowdown control

Boiler blowdown is adjusted to keep dissolved solids, estimated by conductivity, within the manufacturer's limit. Excessive solids cause carryover, foaming, and scale.

Oxygen pitting

Localized corrosion caused by dissolved oxygen attacking steel, forming small deep pits. Controlled by deaeration and a chemical oxygen scavenger held at the specified residual.

Caustic embrittlement

Stress corrosion cracking of boiler steel caused by concentrated caustic in crevices and at stressed joints. Controlled by water chemistry limits and by avoiding caustic concentration.

Coefficient of performance

Refrigeration effect divided by work input for a refrigeration cycle. For an ideal Carnot refrigerator it equals the cold absolute temperature divided by the temperature difference.

Refrigeration ton

A cooling capacity equal to 12,000 Btu per hour, or about 3.517 kilowatts. It is the rate of heat removal needed to melt one ton of ice in 24 hours.

Evaporator superheat

The temperature rise of refrigerant vapor above its saturation temperature at the evaporator outlet. Adequate superheat protects the compressor from liquid slugging.

Pressure dew point

The temperature at which moisture condenses from compressed air at operating pressure. It must be below the lowest piping temperature to prevent liquid water in the lines.

Predictive maintenance

Maintenance scheduled from measured equipment condition, such as vibration, thermography, oil analysis, or ultrasound. It replaces fixed intervals with condition-based intervention.

Vibration signature of imbalance

A dominant vibration at one times running speed, usually radial, that grows with speed. Misalignment instead shows elevated two times running speed and axial one-times vibration.

Management of change

A formal procedure requiring review and approval before any change to equipment, process, or setpoints. It ensures hazards and code impacts are assessed before implementation.

Safety valve set pressure

The inlet pressure at which a safety valve begins to open. It must not exceed the maximum allowable working pressure of the protected equipment, and capacity must match the boiler rating.

Lower explosive limit

The lowest fuel concentration in air that will support combustion. Gas detection alarms typically trip at a fraction of the LEL to keep the atmosphere safely below the flammable range.

Confined space oxygen limit

Acceptable entry oxygen is between 19.5 and 23.5 percent. Below that range asphyxiation risk rises; above it, clothing and materials burn far more readily.

Shift handover

A documented and signed transfer of equipment status, open permits, abnormal conditions, and outstanding work between outgoing and incoming operators. It prevents lost information at shift change.

Plant heat rate

The thermal energy input required per unit of electrical output, usually Btu per kWh. A lower heat rate means better efficiency and is a key plant performance indicator.