NIULPE Class 1 key terms

The ratio of a material's ultimate or yield strength to the maximum stress it is expected to carry in service. Codes set minimum values so boilers and pressure parts withstand normal loads plus surges without failure.

The highest gauge pressure at which a boiler or pressure vessel may legally operate, established by design calculations using material strength, thickness, and joint efficiency. It is stamped on the nameplate and governs safety valve settings.

A decimal factor applied in pressure-part thickness calculations that accounts for the strength lost at welded or riveted seams. Full radiography of a weld permits a higher value, allowing thinner plate for the same design pressure.

Controlled heating of a completed weldment, usually to a temperature below the lower critical point, then slow cooling. It relieves residual stresses, softens hard zones, and reduces susceptibility to cracking in thick boiler sections.

Time-dependent permanent deformation of metal held at elevated temperature under sustained stress. In high-pressure steam components it limits useful life, so designers select alloys and thicknesses based on creep-rupture data at operating temperature.

A drum level control scheme using steam flow, feedwater flow, and drum level signals together. Feedwater flow is matched to steam flow while level trims the balance, giving stable control during large load swings.

Apparent, temporary changes in indicated boiler drum level caused by pressure shifts that alter the volume of steam bubbles in the water. A sudden load increase swells level; a sudden load drop shrinks it, misleading single-element controls.

Air supplied to a burner beyond the theoretical amount needed for complete combustion, usually expressed as a percentage. Enough ensures full burnout of fuel and carbon monoxide control; too much wastes heat up the stack.

The exact mass of air required to burn a given fuel completely with no oxygen left over, also called theoretical air. It is the reference point from which excess air and burner air-fuel ratios are calculated.

The small negative pressure maintained in the combustion chamber of a balanced-draft unit, pulling gases toward the boiler outlet while keeping the setting under suction. It prevents hot gas and flame from leaking into the plant.

A fan located after the boiler that pulls combustion gases through the gas path and discharges them to the stack. It maintains furnace draft and must handle hot, dirty, corrosive gas, so it is sized for volume at temperature.

A fan upstream of the burners or air heater that pushes combustion air into the furnace under positive pressure. It supplies the air quantity demanded by the fuel firing rate and is the primary air source on balanced-draft units.

A heat exchanger in the gas path that recovers heat from leaving flue gas to warm boiler feedwater before it enters the drum. It lowers stack temperature and raises overall plant efficiency.

Equipment that transfers heat from flue gas to incoming combustion air, improving furnace efficiency and flame stability. Regenerative types use rotating baskets; tubular types use fixed tubes, and both are subject to cold-end corrosion.

Net positive suction head is the margin of absolute pressure above a liquid's vapor pressure at the pump suction, available from the system and required by the pump. Insufficient margin causes cavitation, noise, and impeller damage.

The high-pressure pump that delivers treated feedwater to the boiler at a pressure above drum pressure. It is usually a multistage centrifugal machine, often driven by a steam turbine in large plants.

A vessel that heats feedwater with steam to near saturation and strips dissolved oxygen and other gases, protecting downstream metal from corrosion. It also provides a storage tank that buffers feedwater supply.

A turbine stage in which steam expands in stationary nozzles and strikes moving blades at essentially constant pressure. The pressure drop occurs in the nozzles, and the rotor extracts energy from the steam's velocity.

A turbine design in which steam expands gradually through both stationary and moving blades, so pressure falls across the rotating row as well. The rotor is driven by pressure reaction as well as velocity change.

The ratio of actual work produced by a turbine to the work that would result from an ideal, frictionless expansion between the same inlet and exhaust states. It quantifies losses from blade friction, leakage, and moisture.

A thermodynamic property combining internal energy with the flow work of pressure and volume, useful for steady-flow devices. Steam tables list it, and the drop across a turbine gives the energy available per unit mass.

The amount by which steam temperature exceeds its saturation temperature at the same pressure. Superheating dries the steam, raises cycle efficiency, and protects turbine blades from moisture erosion.

A bank of tubes in the high-temperature gas path that adds heat to saturated steam from the drum, raising its temperature above saturation. Its outlet temperature is controlled by attemperation or gas bypass.

The upper pressure vessel of a watertube boiler where steam separates from water and leaves for the superheater. It holds a water level, houses internal separators, and provides a point for chemical addition and blowdown.

A sensor, typically ultraviolet or infrared, that confirms a burner flame is present. Its signal permits fuel admission and triggers a trip if flame is lost, preventing explosive fuel accumulation.

The interlocked control package that sequences purge, pilot, main flame, and shutdown for each burner and enforces safety permissives. It trips fuel on loss of flame, low air flow, or other unsafe conditions.

A required period of forced air flow through the furnace before ignition, typically five minutes at high flow, to sweep out combustible gases. The purge must be proven by air flow and damper position before fuel can be admitted.

A measure of a water sample's ability to carry electrical current, reflecting dissolved ion concentration. It is used to monitor feedwater purity, control blowdown rate, and detect condenser tube leakage.

Localized corrosion that forms small, deep pits on boiler metal where dissolved oxygen concentrates under deposits. It is prevented by mechanical deaeration and chemical oxygen scavengers such as sulfite or hydrazine.

Cracking of stressed boiler steel caused by concentrated sodium hydroxide in crevices and at riveted seams. Modern water treatment and welded construction have largely eliminated it, but phosphate control and avoiding caustic carryover remain important.

A controlled, continuous release of water from the boiler drum surface to limit dissolved solids and suspended material. The rate is set from conductivity or silica readings to protect steam purity and prevent carryover.

The entrainment of boiler water droplets or foam into the steam leaving the drum. It deposits dissolved solids on superheater tubes and turbine blades, causing overheating and imbalance, and is controlled by drum internals and blowdown.

A spring-loaded automatic valve that opens at a set pressure to release steam and protect the boiler from overpressure. It must be set at or below the maximum allowable working pressure and tested regularly.

A pressure test in which the boiler is filled with water and pressurized, usually to 1.5 times the maximum allowable working pressure, to prove structural integrity. It is performed on new construction and after major repairs.

A procedure that isolates energy sources and applies locks and tags so equipment cannot be started or energized during maintenance. It protects personnel from unexpected release of steam, electricity, or stored mechanical energy.

The controlled process of entering a boiler, drum, or tank that has limited access and possible hazardous atmosphere. It requires an entry permit, atmospheric testing, ventilation, an attendant, and standby rescue provisions.

A post-combustion system that injects ammonia or urea into flue gas over a catalyst to convert nitrogen oxides into nitrogen and water. It typically achieves high NOx removal but requires careful reagent control.

A particulate control device that charges ash particles and collects them on oppositely charged plates, then removes them by rapping. It handles large gas volumes at low pressure drop and is sensitive to flue gas temperature and moisture.

The measurement of oxygen, carbon monoxide, carbon dioxide, and unburned combustibles in exhaust gas to judge combustion quality. Results guide air-fuel trim and reveal excess air, air leakage, or incomplete burning.