Power Eng 4th Class 4B flashcards

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

MAWP

Maximum allowable working pressure: the highest pressure at which a boiler or vessel may be operated, established by design calculations and stamped on the nameplate. Safety valves are set at or below this value.

Three-element feedwater control

A drum level control strategy using steam flow, feedwater flow, and drum level signals. It corrects level without instability from swell or shrinkage during sudden load changes.

Low-water fuel cutoff

A safety device that shuts off the burner when boiler water falls below the safe minimum level, preventing overheating and catastrophic failure of the heating surface.

Excess air

Air supplied above the theoretical amount needed for complete combustion. Measured as flue gas oxygen; too little causes CO and smoke, too much wastes heat up the stack.

Balanced draft

A furnace draft arrangement with both forced draft and induced draft fans, keeping the furnace slightly negative (about −0.05 to −0.1 kPa) so no hot gas escapes.

Furnace purge

A pre-ignition procedure passing air through the furnace and flue for at least five air changes to remove combustible gases before fuel is admitted.

Blowdown rate

The percentage of feedwater discharged to control dissolved solids, calculated as feedwater TDS divided by (boiler water TDS limit minus feedwater TDS), times 100.

Deaerator

A vessel that heats feedwater to saturation and strips dissolved oxygen to very low levels, typically below 0.005 mg/L, using steam and trays or spray nozzles.

Scale

Hard mineral deposit on boiler heating surfaces. It has very low thermal conductivity, so even 1 mm can raise fuel consumption by 5 to 10 percent.

NPSH available

The absolute suction head above vapour pressure available at the pump inlet. If it falls below NPSH required, cavitation occurs, causing noise, erosion, and loss of head.

Affinity laws

Pump and fan rules: flow varies with speed, head with speed squared, and power with speed cubed. Reducing speed to 80 percent cuts power to about 51 percent.

Surge

An unstable compressor condition at low flow where discharge pressure exceeds what the machine can sustain, causing flow reversal and violent vibration. Anti-surge control recycles flow.

Gate valve

A valve with a gate that moves perpendicular to flow. It gives low pressure drop for isolation service but should not be used for throttling because the gate erodes.

Thermodynamic trap

A steam trap using a disc that opens on high-velocity condensate and closes on flash steam. Common on high-pressure drip legs and tracing lines.

Safety relief valve blowdown

The pressure drop below set pressure at which a safety valve reseats, typically 4 to 7 percent of set pressure. Too little blowdown causes simmering and leakage.

Water hammer

A pressure surge caused by rapid velocity change in a pipe, such as sudden valve closure or slug flow of condensate. It can rupture fittings and supports.

Relative humidity

The ratio of actual water vapour pressure in air to the saturation vapour pressure at the same dry-bulb temperature, expressed as a percentage.

Air changes per hour

The number of times the total room air volume is replaced in one hour, calculated as airflow in cubic metres per hour divided by room volume in cubic metres.

Bypass factor

The fraction of air that passes a cooling coil without contacting the coil surface, calculated as (leaving dry-bulb minus apparatus dew point) divided by (entering dry-bulb minus apparatus dew point).

Tonne of refrigeration

A cooling capacity equal to 3.517 kilowatts, or 12,000 Btu per hour, the heat absorbed to melt one tonne of ice in 24 hours.

Coefficient of performance

Refrigeration effect divided by compressor work input. Higher COP means more cooling per unit of electrical energy. Carnot COP equals evaporator temperature divided by temperature lift, in kelvin.

Subcooling

Cooling liquid refrigerant below its condensing temperature before the expansion valve. It increases refrigerating effect and prevents flash gas at the valve inlet.

Three-phase power

Electrical power in a balanced three-phase circuit, calculated as the square root of three times line voltage times line current times power factor.

Star-delta starting

A motor starting method connecting windings in star for starting, then delta for run. It reduces starting current and torque to one third of direct-on-line values.

Locked-rotor current

The current a motor draws at standstill with rated voltage applied, typically six to eight times full-load current. It causes voltage dips during starting.

Proportional band

The percentage change in measured variable needed to drive the controller output from zero to full scale. It equals 100 divided by proportional gain; narrow bands cause cycling.

Live zero

The 4 mA minimum of a 4 to 20 mA current loop, representing zero percent of range. It allows a broken wire to be distinguished from a true zero reading.

RTD

A resistance temperature detector, usually platinum, whose resistance rises predictably with temperature. It is more accurate and stable than a thermocouple.

Compression ratio

The ratio of cylinder volume before compression to volume after compression, calculated as swept volume plus clearance volume divided by clearance volume.

Brake thermal efficiency

Brake power delivered at the shaft divided by the fuel energy input rate. Typical diesel engines achieve 35 to 45 percent.

Lockout/tagout

An energy isolation procedure in which each worker applies a personal lock and tag to the disconnect, then verifies zero energy before work begins.

Heat equation

The formula Q equals mass times specific heat times temperature change. For water the specific heat is 4.18 kilojoules per kilogram per degree Celsius.