NIULPE Class 5 key terms

A spring-loaded valve that opens automatically at a set pressure to release steam or water and protect the boiler from overpressure; it closes again once pressure falls below the set point.

The pressure at which a safety valve begins to open, stamped on the valve and matched to the boiler's maximum allowable working pressure so the boiler is protected before any component is overstressed.

The highest pressure at which a boiler or pressure vessel may legally operate, determined from its design, materials, and thickness, and recorded on the manufacturer's stamping and the operating certificate.

A float- or probe-type control that senses water level and shuts off the burner if the water falls below a safe level, preventing overheating and possible furnace explosion from a dry boiler.

A vertical transparent tube connected to the boiler shell that shows the water level directly; its upper and lower connections and blowdown valves allow the operator to confirm the level is true.

Small hand valves mounted at high, middle, and low levels on the boiler shell, used to verify water level independently of the gauge glass by observing whether steam or water escapes.

A device that automatically adjusts feedwater flow to hold the boiler water level at a chosen point, usually by sensing level or drum level and modulating a feedwater control valve.

A heat exchanger that raises the temperature of incoming boiler feedwater using extracted steam or hot process fluid, improving cycle efficiency and reducing thermal shock to the boiler.

A vessel that heats feedwater with steam to drive off dissolved oxygen and other gases, reducing corrosion in the boiler, economizer, and condensate piping; it also serves as a storage tank.

A heat exchanger in the flue gas path that warms feedwater before it enters the boiler, recovering heat that would otherwise leave through the stack and lowering fuel consumption.

A heat exchanger that uses exhaust flue gas to warm combustion air entering the furnace, improving combustion and boiler efficiency while reducing stack losses.

A bank of tubes heated by flue gas that raises steam above its saturation temperature, increasing cycle efficiency and reducing condensation losses in turbines and distribution piping.

A device that sprays feedwater or condensate into superheated steam, or passes it through a cooler, to hold final steam temperature within limits and protect superheater and turbine metals.

Steam in equilibrium with liquid water at a given pressure, so its temperature equals the boiling point for that pressure and any heat removal causes immediate condensation.

The energy absorbed or released when a substance changes phase at constant temperature, such as the heat needed to turn boiling water into steam without raising its temperature.

A thermodynamic property combining internal energy with the flow work of a fluid, expressed as heat content per unit mass; it is used to calculate heat transfer and boiler efficiency.

The air supplied to a burner beyond the theoretical amount needed for complete combustion, expressed as a percentage; enough ensures complete burning, while too much wastes heat up the stack.

The exact quantity of air required to burn a given fuel completely, with no oxygen left over and no unburned fuel remaining, calculated from the fuel's chemical composition.

Measurement of oxygen, carbon dioxide, and carbon monoxide in exhaust gas to judge combustion quality; the readings guide adjustment of air flow and reveal incomplete burning or excess air.

The small pressure difference that moves combustion air into the furnace and flue gases out the stack; it may be produced naturally by a tall chimney or mechanically by fans.

A fan located before the furnace that pushes combustion air into the burner and windbox, overcoming air-side and burner resistance to supply the required airflow.

A fan placed after the boiler that pulls flue gas through the unit and discharges it to the stack, maintaining a slight negative pressure in the furnace and preventing gas leakage outward.

The temperature of flue gas leaving the last heat recovery surface; a high value signals heat loss and fouled or dirty surfaces, and it is a key indicator of boiler efficiency.

A pressure test in which the boiler or vessel is filled with water and pressurized above its working pressure to prove strength and tightness before being placed in service or after major repairs.

A nondestructive method that uses high-frequency sound waves to measure remaining wall thickness of tubes and shells, detecting metal loss from corrosion or erosion without cutting the material.

Localized corrosion that forms small deep pits where dissolved oxygen attacks boiler metal, often under deposits; prevention relies on deaeration and chemical oxygen scavengers.

A hard deposit of calcium and magnesium salts that forms on heated boiler surfaces when water is concentrated, insulating the metal and causing overheating, lost efficiency, and tube failure.

Treatment that removes hardness minerals such as calcium and magnesium from feedwater, usually by ion exchange, so they cannot form scale on boiler heating surfaces.

Deliberate removal of water from the boiler to control the concentration of dissolved solids; bottom blowdown clears sludge, while surface blowdown controls dissolved chemicals and conductivity.

The combined concentration of dissolved salts and other substances in boiler water, monitored by conductivity and limited by blowdown to prevent carryover, foaming, and deposits.

The escape of boiler water droplets or foam with the steam, which contaminates piping and equipment with dissolved solids and can cause erosion and deposit buildup downstream.

An automatic valve that discharges condensate, air, and noncondensable gases from steam lines while preventing the escape of live steam, keeping equipment operating efficiently.

A valve that allows flow in only one direction and closes automatically when flow reverses, protecting pumps and boilers from backflow and preventing sudden loss of water.

A valve with a sliding gate that opens fully to give a straight, low-resistance flow path; it is used for isolation rather than throttling because partial opening erodes the seat.

A valve whose plug moves against a seat in a curved body, creating a tortuous path; it provides good throttling control at the cost of higher pressure drop than a gate valve.

Formation and collapse of vapor bubbles in a pump or valve where local pressure drops below vapor pressure; the collapse produces noise, vibration, and erosion of internal parts.

The absolute pressure margin above a liquid's vapor pressure at the pump suction; adequate margin prevents cavitation and must be provided by the system and pump design.

An industrial digital controller that executes a stored program to read sensors and drive outputs, commonly used for burner sequencing, interlocks, and boiler safety shutdowns.

A sensor, such as an ultraviolet or infrared detector, that confirms a burner flame is present and signals the control system to shut off fuel if the flame is lost.

A safety logic function that prevents or stops an action unless specified conditions are met, such as blocking fuel flow when airflow or water level is inadequate.