NIULPE 4th Class key terms

A spring-loaded valve that opens automatically at a set pressure to release steam or vapor and prevent a boiler or pressure vessel from exceeding its maximum allowable working pressure.

A valve certified for both steam and liquid service that opens automatically to relieve excess pressure, used on pressure vessels and hot water systems where either vapor or liquid may be released.

The pressure at which a relief device is adjusted to begin opening, marked on the device and matched to the vessel's maximum allowable working pressure so protection occurs before failure.

The pressure rise above the maximum allowable working pressure that a vessel experiences while a relief device is discharging at full capacity, normally limited to a small percentage by code.

The highest pressure at which a boiler, vessel, or piping system may legally operate, determined from design, materials, and thickness calculations and stamped on the equipment nameplate.

A vertical fitting attached to a boiler shell that slows water movement so the low-water cutoff and gauge glass connections sense a steady level rather than a turbulent one.

A float-operated safety control that shuts off fuel to a boiler when the water level falls below a safe minimum, preventing overheating and possible shell failure.

A transparent tube or flat glass assembly connected to the boiler at the normal water level, letting the operator directly observe the water level inside the boiler.

Small manual valves mounted at three levels on a boiler to confirm water level by hand, used as a backup check when the gauge glass is cloudy or suspected of being blocked.

A control that automatically adjusts feedwater flow to hold boiler water level at a set point, often responding to level, steam flow, and drum level signals together.

A heat exchanger that recovers heat from boiler flue gas to preheat incoming feedwater, lowering fuel consumption and stack temperature.

Equipment that transfers heat from flue gas to combustion air before it enters the furnace, improving combustion efficiency and reducing heat lost up the stack.

The amount of combustion air supplied beyond the theoretical amount needed for complete burning, expressed as a percentage and measured by analyzing flue gas oxygen content.

Ideal burning in which exactly enough oxygen is supplied to convert all fuel carbon, hydrogen, and sulfur to their fully oxidized products, with no oxygen or unburned fuel left over.

Testing of exhaust gas for oxygen, carbon dioxide, and carbon monoxide content to judge combustion efficiency and confirm that excess air is neither too rich nor too lean.

A timed flow of air through the furnace before ignition that clears accumulated combustible gases, preventing a furnace explosion when the burner is lit.

A sensor that continuously verifies the presence of a stable flame and signals the burner management system to shut off fuel if flame is lost.

A rugged digital controller that executes burner management and process logic, reading sensors and operating valves and motors through a programmed sequence of steps.

A burner that varies fuel and air input continuously across a firing range to match steam or hot water demand, rather than simply switching on and off.

The ratio of a burner's maximum firing rate to its minimum stable firing rate, indicating how far it can throttle down without losing flame stability.

A device that automatically discharges condensate and noncondensable gases from steam lines while preventing live steam from escaping.

Steam produced when hot condensate at saturation pressure is released to a lower pressure, causing part of the liquid to vaporize instantly; recovered flash steam saves fuel.

The system that collects condensed steam from traps and returns it to the boiler feedwater supply, conserving heat and treated water.

A sharp pressure surge and banging noise caused by rapidly moving steam or water being stopped or meeting trapped condensate, capable of damaging piping and fittings.

A valve that allows flow in only one direction and closes automatically when flow reverses, protecting pumps and preventing backflow in feedwater and steam lines.

A valve with a sliding wedge or disc that opens fully to give a straight, low-resistance flow path, intended for on-off isolation rather than throttling.

A valve with a plug and seat that forces flow to change direction, giving good throttling control and tight shutoff but higher pressure drop than a gate valve.

A pump that uses a rotating impeller to throw liquid outward and create pressure, suited to high flow rates and commonly used for boiler feedwater service.

Formation and collapse of vapor bubbles in a pump when local pressure drops below vapor pressure, causing noise, vibration, and erosion of the impeller.

The absolute pressure margin above a liquid's vapor pressure at the pump suction, required to prevent cavitation and specified by the pump manufacturer.

A positive-displacement machine using a piston in a cylinder to compress gas, capable of high pressure ratios and often used for instrument air or refrigeration service.

A fluid that absorbs heat by evaporating at low pressure and releases it by condensing at high pressure, circulating through the refrigeration cycle.

The temperature of refrigerant vapor above its saturation temperature at a given pressure, used to confirm the evaporator is properly fed and to protect the compressor.

A metering device that regulates refrigerant flow into the evaporator, maintaining a set superheat by responding to temperature and pressure at the evaporator outlet.

The concentration of dissolved calcium and magnesium salts in water, which form scale in boilers and must be reduced by softening or internal treatment.

A hard deposit of mineral salts that forms on hot boiler surfaces, insulating the metal and causing overheating, lost efficiency, and tube failure.

A treatment vessel in which hard water passes through resin that swaps calcium and magnesium ions for sodium ions, preventing scale formation.

Deliberate draining of water from a boiler to control dissolved solids, sludge, and surface contamination; continuous and bottom blowdown serve different purposes.

A measure of a water sample's ability to carry electric current, used as an indicator of dissolved solids so blowdown can be controlled automatically.

Localized corrosion caused by dissolved oxygen attacking boiler metal, producing small deep pits and often treated with chemical oxygen scavengers and deaeration.

A vessel that heats feedwater and strips dissolved oxygen and other gases by spraying and steaming it under slight pressure before it enters the boiler.

The widely adopted construction standard covering design, materials, fabrication, inspection, and testing of boilers and pressure vessels, referenced by most jurisdictions in law.

A required examination of a boiler or pressure vessel by an authorized inspector acting under state or provincial law, with certificates issued for defined operating periods.