NIULPE 2nd Class flashcards

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

Maximum allowable working pressure

The highest pressure at which a boiler or pressure vessel may be operated, set by the weakest pressure-containing part and stamped on the nameplate. Safety valve settings cannot exceed it.

Fusible plug

A brass fitting with a low-melting alloy core installed in the crown sheet or upper firebox. It melts near 445 °F when uncovered by water, releasing steam into the furnace as a low-water warning.

Try lever test

A manual check that a safety valve opens freely, performed with the boiler at no less than 75 percent of set pressure. It proves freedom of movement, not the popping pressure.

Gauge glass lowest reading

The bottom of the visible water glass must align with the lowest permissible water level. Operating below that line risks uncovering heating surface and overheating the metal.

Stoichiometric air

The exact mass of air needed to burn a fuel completely with no oxygen left over. Natural gas requires about 17.2 pounds of air per pound of fuel.

Excess air

Air supplied beyond the stoichiometric amount, expressed as a percentage. It ensures complete combustion but raises stack loss; typical targets are 5 to 15 percent depending on fuel.

Pre-purge

A forced flow of air through the furnace and gas passages before ignition, measured in air changes, that clears any accumulated combustible gases before the burner is lit.

Trial for ignition

The limited time after the fuel valve opens during which the flame scanner must prove main flame. If no signal appears, the burner management system closes the fuel valve.

Deaerator

A vessel that heats feedwater to saturation to drive off dissolved oxygen and other gases. It typically holds oxygen below 7 parts per billion with a chemical scavenger.

Economizer

A heat exchanger that transfers heat from flue gas to incoming feedwater, raising boiler efficiency. Inlet temperature must stay above the acid dew point to prevent corrosion.

Blowdown percentage

The fraction of feedwater deliberately discharged to control dissolved solids, calculated as blowdown flow divided by feedwater flow times 100. It equals steam flow plus blowdown in balance.

Steam trap

An automatic valve that discharges condensate and non-condensable gases while retaining steam. Types include thermostatic, float-and-thermostatic, inverted bucket, and thermodynamic designs.

Rankine cycle efficiency

Net work output divided by heat added in the boiler, computed from steam table enthalpies. The largest loss is heat rejected in the condenser.

Critical speed

A rotor speed at which operating speed matches a natural frequency of the shaft assembly, producing high vibration. Turbines accelerate quickly through criticals and never dwell there.

Overspeed trip

A protective device that stops steam admission when rotor speed reaches about 110 percent of rated speed. It is tested before startup and after governor maintenance.

Governor droop

The speed decrease from no load to full load divided by full-load speed, expressed as a percentage. Isochronous governors have zero droop and hold constant speed on isolated loads.

NPSH available

The absolute suction head above the liquid vapor pressure at the pump inlet, after subtracting suction friction losses. It must exceed the pump's required NPSH to prevent cavitation.

Affinity laws

Relations for centrifugal pumps: flow varies with speed, head with speed squared, and power with speed cubed. The same relations apply when impeller diameter is changed.

Best efficiency point

The flow at which a pump's hydraulic efficiency peaks. Running far from it causes recirculation, vibration, or cavitation and shortens bearing and seal life.

Compressor surge

A flow reversal condition in a centrifugal compressor when flow drops below the surge line, causing loud pulsation and possible damage. Anti-surge valves recycle flow to protect the machine.

Boiler horsepower

A measure of boiler capacity equal to 33,475 Btu per hour, or about 34.5 pounds of steam per hour evaporated from and at 212 °F.

Log mean temperature difference

The driving temperature difference for a heat exchanger, computed as the difference of the two terminal differences divided by the natural log of their ratio.

Latent heat of vaporization

The heat required to change water at saturation temperature to saturated steam without a temperature change. At atmospheric pressure it is about 970 Btu per pound.

Steady-flow energy equation

For a control volume, heat in minus work out equals the change in enthalpy plus kinetic and potential energy. In a turbine with no heat loss, work equals the enthalpy drop.

Motor slip

The difference between synchronous speed and actual rotor speed, divided by synchronous speed. Slip increases with load and is a few percent for induction motors.

Live zero signal

A 4 to 20 milliamp instrument loop uses 4 milliamps to represent zero. A reading below 4 milliamps therefore indicates a broken wire or failed transmitter, not a true zero.

Overload relay

A starter device that protects a motor from sustained overcurrent by tripping on heat or current. It does not provide short-circuit protection, which the circuit breaker or fuse supplies.

Fail-safe valve position

The position a control valve moves to when air or power is lost. Air-to-close valves open on air failure and are chosen when an open path is the safe condition.

Ton of refrigeration

A cooling capacity of 12,000 Btu per hour, equivalent to melting one ton of ice per day. It is the standard unit for rating refrigeration and air conditioning equipment.

Superheat

The temperature of refrigerant vapor above its saturation temperature at the same pressure. Measured at the evaporator outlet, it protects the compressor from liquid slugging.

Condenser approach

The difference between condensing temperature and the entering cooling medium temperature. A rising approach indicates fouling, scale, or non-condensable gases in the condenser.

Chilled water capacity

For water, cooling capacity in Btu per hour equals 500 times flow in gallons per minute times the temperature rise in degrees Fahrenheit across the coil.

Lockout/tagout

A procedure that isolates every energy source, applies a personal lock for each worker, and verifies zero energy before work. Only the person who applied a lock removes it.

Confined space atmospheric test

Testing for oxygen, flammable gas, and toxic gas before entry and continuously during occupancy. Acceptable oxygen ranges from 19.5 to 23.5 percent by volume.

Hot work permit

A written authorization for welding, cutting, or grinding that requires removal of combustibles, gas testing, and a fire watch during and after the work.

OEE

Overall equipment effectiveness, the product of availability, performance, and quality, each measured against planned production time. It shows where losses occur in a plant.

MTBF

Mean time between failures, calculated as total operating time divided by the number of failures. It is a reliability measure used to set maintenance intervals and spares levels.

Predictive maintenance

Maintenance triggered by measured condition such as vibration, oil analysis, or thermography, rather than by calendar time. It catches faults before failure and avoids unnecessary work.

Critical spare part

A stocked item whose failure would cause long downtime or a safety hazard and whose lead time is long. Consequence and lead time, not price, set the stocking decision.