NIULPE 2nd Class key terms

A spring-loaded valve that opens automatically at a set pressure to release steam and prevent a boiler from exceeding its maximum allowable working pressure. It is the boiler's final overpressure protection.

The highest pressure at which a boiler or pressure vessel may safely operate, established by design calculations, material thickness, and code rules, and stamped on the vessel nameplate.

A manual check in which the safety valve lever is lifted to confirm the valve opens freely and reseats without sticking. Performed while the boiler is at pressure, following plant procedure.

A vertical fitting connected to the boiler shell that dampens water-level surges so the gauge glass shows a steady level and provides mounting points for level controls and try cocks.

A transparent tube or flat glass assembly connected to the boiler at two points to visually indicate water level. Its connections must be blown down and checked regularly for blockage.

A safety control that senses low water level and shuts off the burner (and may trip the feed pump) before the boiler overheats. It must be tested periodically to prove it functions.

A brass plug with a low-melting alloy core installed in the crown sheet or shell of some boilers. If water level drops and the metal overheats, the alloy melts and releases steam to warn of danger.

A pressure test using water, typically at 1.5 times the MAWP, to verify the strength and tightness of a boiler or pressure vessel before service or after repairs.

A control scheme that regulates feedwater using drum level, steam flow, and feedwater flow signals together, so level is held steady despite sudden load changes and shrink or swell effects.

Apparent changes in boiler drum level caused by pressure changes: raising firing pressure collapses steam bubbles and level appears to shrink, while dropping pressure expands bubbles and level appears to swell.

The fraction of fuel heat released that is usefully absorbed, commonly assessed by measuring flue gas oxygen, carbon monoxide, and stack temperature to set excess air and reduce losses.

Air supplied above the theoretical amount needed for complete combustion. Enough is required to burn all fuel, but too much raises stack losses and lowers efficiency.

Measurement of oxygen, carbon dioxide, carbon monoxide, and temperature in the exhaust gas to evaluate combustion quality, set burner air-fuel ratio, and detect incomplete combustion.

A forced-air period before ignition that clears unburned fuel and combustible gases from the furnace and flue, preventing a furnace explosion at light-off.

A sensor (ultraviolet, infrared, or flame-rod type) that confirms a stable flame is present. The burner management system shuts off fuel if the scanner loses the flame signal.

The control system that sequences and supervises burner startup, purge, ignition, firing, and shutdown, and interlocks fuel supply with proven flame, airflow, and safe water level.

A heat exchanger that recovers heat from boiler flue gas to preheat incoming feedwater, lowering stack temperature and improving overall boiler efficiency.

Equipment that transfers heat from flue gas to combustion air before it enters the furnace, improving combustion and reducing stack losses. It may be recuperative or regenerative.

A vessel that heats feedwater and strips dissolved oxygen and other gases by steam scrubbing and venting, reducing corrosion in the boiler and condensate system.

Water supplied to the boiler to replace steam generated. It is a mix of returned condensate and treated makeup, and must meet chemistry limits for hardness, oxygen, and dissolved solids.

Controlled removal of boiler water to limit dissolved solids, sludge, and scale. Bottom blowdown removes sludge; surface or continuous blowdown controls dissolved solids concentration.

Scale is hard deposit formed on heating surfaces by precipitated minerals, reducing heat transfer; carryover is boiler water or foam entrained with steam, which can foul superheaters and turbines.

Localized corrosion caused by dissolved oxygen attacking steel, producing small deep pits. It is controlled by deaeration, chemical oxygen scavengers, and proper pH.

A turbine in which steam expands in stationary nozzles and strikes moving blades at high velocity, with pressure constant across the moving blades. It converts kinetic energy of the jet into shaft work.

A turbine in which steam expands in both stationary and moving blades, so pressure drops across the moving rows and the rotor is driven by both impulse and reaction forces.

A staging arrangement using one set of nozzles and several rows of moving blades separated by fixed guide vanes, absorbing the steam jet's energy in stages and reducing rotor speed.

The sub-atmospheric pressure maintained in a steam turbine condenser. Lower absolute pressure increases the heat drop available across the turbine and improves cycle efficiency.

A shell-and-tube heat exchanger that condenses turbine exhaust steam on the outside of tubes cooled by circulating water, recovering condensate and maintaining turbine vacuum.

The ideal thermodynamic cycle for steam power plants: pump compression, constant-pressure heat addition in the boiler, expansion through the turbine, and constant-pressure condensation.

A rotodynamic pump that uses a rotating impeller to impart velocity to liquid, converting it to pressure in the volute or diffuser. It is suited to high flow at moderate head.

The margin of absolute pressure above a liquid's vapor pressure at the pump suction. Available NPSH must exceed required NPSH to prevent cavitation.

Formation and collapse of vapor bubbles in a pump or valve where local pressure falls below vapor pressure. It causes noise, vibration, and erosion damage to impellers and casings.

Relationships for a given impeller diameter stating that flow varies with speed, head with speed squared, and power with speed cubed. They guide speed control and energy savings.

A thermodynamic property combining internal energy with flow work, useful for tracking heat added or removed in boilers, turbines, and condensers. Steam tables tabulate it for each state.

The amount by which steam temperature exceeds its saturation temperature at a given pressure. Superheated steam contains no moisture and improves turbine efficiency and blade life.

The three modes by which heat moves: conduction through solids by molecular contact, convection by fluid motion, and radiation by electromagnetic waves. Boilers use all three.

An electronic controller that varies the speed of an AC motor by changing supply frequency and voltage, commonly used on pumps and fans to match load and save energy.

An industrial digital computer that executes ladder-logic or function-block programs to control equipment, read sensors, and interlock plant processes reliably.

A fluid that absorbs heat by evaporating at low pressure and releases it by condensing at high pressure in a refrigeration cycle. Selection depends on safety, efficiency, and environmental rules.

The repeating sequence of compression, condensation, expansion through a metering device, and evaporation that moves heat from a cold space to a warmer environment.

A procedure that isolates energy sources and applies locks and tags so equipment cannot be started while personnel service it. It prevents unexpected energization and stored-energy release.

Entry into a space with limited access and poor ventilation that may contain hazardous atmosphere. It requires testing, permits, attendants, and rescue provisions before entry.

Scheduled inspections, lubrication, cleaning, and parts replacement performed on a time or usage basis to prevent failures and keep equipment reliable and efficient.

A written authorization and record for maintenance work that identifies the task, equipment, required parts, labor, and safety permits, and tracks completion and cost.