BD First Class Boiler Attendant key terms

The highest pressure at which a boiler or pressure part may legally and safely operate, set by design calculations and stamped on the boiler. Safety valves and controls are set with reference to this limit.

A spring-loaded automatic valve that opens at a preset pressure to discharge steam and protect the boiler from overpressure. It must reset automatically once pressure falls below the set point.

A test confirming that all safety valves together can discharge enough steam to keep boiler pressure from rising more than the permitted percentage above the set pressure when firing is at maximum.

A pressure test in which the boiler is filled with water and pressurised above its working pressure to prove strength and tightness of seams, tubes and fittings before being returned to service.

A vertical fitting attached to the boiler shell that dampens water movement so the gauge glass shows a steady level; its connections must be kept clear to give a true reading.

A transparent tube or flat glass assembly connected to the boiler at top and bottom that shows the actual water level inside the shell or drum, allowing operators to verify level directly.

A control that senses an unsafe drop in boiler water level and shuts off the burner before the crown sheet or tubes are overheated, preventing a dangerous meltdown or explosion.

Water supplied to a boiler to replace steam that has been drawn off. Its temperature, purity and dissolved oxygen content strongly affect boiler life and efficiency.

A vessel that heats feedwater with steam to drive off dissolved oxygen and carbon dioxide, reducing corrosion of boiler metal and preheating the water before it enters the boiler.

A chemical such as sodium sulfite or hydrazine dosed into feedwater to remove any remaining dissolved oxygen that the deaerator did not eliminate, protecting metal from pitting.

Deliberate discharge of water from the boiler to control dissolved solids and sludge. Bottom blowdown removes sediment; surface or continuous blowdown controls chemical concentration in the water.

The combined concentration of dissolved salts and minerals in boiler water. Excessive levels cause foaming, carryover and scale, so blowdown rates are set to keep it within limits.

Contamination of steam by boiler water droplets or foam, which deposits solids on superheaters, turbines and valves. It is usually caused by high dissolved solids, organic matter or sudden load changes.

Formation of stable bubbles on the boiler water surface, often from high dissolved solids, oils or organics, which leads to priming and wet steam leaving the boiler.

Hard mineral deposit, mainly calcium and magnesium salts, baked onto heating surfaces. It insulates tubes and shells, raises metal temperature and can cause overheating, cracking or tube failure.

Soft, suspended solids in boiler water that settle in low areas rather than adhering to heating surfaces. It is removed by bottom blowdown before it bakes into scale.

An ion-exchange unit that replaces hardness-causing calcium and magnesium ions in makeup water with sodium ions, preventing scale formation in the boiler.

Fresh water added to the system to replace steam losses, condensate loss and blowdown. Its quality determines the amount of treatment and blowdown needed.

A boiler in which hot combustion gases pass through tubes surrounded by water, so the shell contains the water and the tubes contain the fire; limited in pressure by shell diameter and thickness.

A boiler in which water circulates inside tubes that are heated externally by combustion gases, allowing higher pressures, faster steaming and smaller drum sizes than fire-tube designs.

The top plate of the furnace in many fire-tube boilers, directly exposed to flame. It must remain covered by water or it will overheat and collapse.

A plug with a low-melting alloy core fitted in the crown sheet or tube plate; if water level falls and the metal overheats, the alloy melts and releases steam to warn the operator.

The upper pressure vessel of a water-tube boiler where steam separates from water, feedwater is introduced and steam is drawn off to the superheater or plant.

A bank of tubes that heats saturated steam above its saturation temperature, raising thermal efficiency and reducing condensation losses in the plant and turbine.

A heat exchanger that uses flue gas leaving the boiler to preheat feedwater, lowering stack temperature and fuel consumption while reducing thermal shock to the boiler.

Equipment that transfers heat from flue gas to incoming combustion air, raising furnace temperature and improving combustion efficiency, often fitted after the economiser.

Steam at the same temperature as the boiling water from which it formed, containing no superheat. Its temperature is fixed by its pressure and it condenses readily on cooling.

The heat absorbed or released when a substance changes state without changing temperature, such as the heat needed to turn boiling water into saturated steam.

The total heat content of a unit mass of steam or water, combining sensible heat and latent heat, used in calculations of boiler output and heat balance.

Air supplied to the furnace beyond the theoretical amount needed for complete combustion, ensuring all fuel burns but raising stack losses if excessive.

The ideal burning of fuel with exactly the theoretical oxygen required so that no oxygen remains and no unburned fuel is left; real firing always needs some excess air.

Measurement of oxygen, carbon dioxide and carbon monoxide in exhaust gas to judge combustion quality and tune the air-to-fuel ratio for efficiency and safe operation.

The ratio of useful heat delivered to the steam, expressed as a percentage of the heat energy released by the fuel burned, accounting for stack, radiation and blowdown losses.

A feedwater control strategy using drum level, steam flow and feedwater flow signals together so that level stays stable during sudden load changes.

A programmed control that supervises burner light-off and shutdown, proving flame presence and locking out fuel on failure to prevent furnace explosions.

A sensor, often ultraviolet or infrared, that confirms a stable flame is present in the burner and signals the combustion safeguard to keep fuel valves open or shut them.

A forced flow of air through the furnace and passes before light-off or after shutdown to clear combustible gases, preventing explosion on ignition.

A non-destructive method using sound waves to measure remaining wall thickness of tubes and shells, detecting thinning from corrosion or erosion during inspection.

The cylindrical outer steel plate of a fire-tube boiler that contains the water and pressure; its thickness and condition set the boiler's pressure limit.