High Risk Work Licence - Standard Boiler (BS) key terms

A brass fitting with a low-melting-point core screwed into the crown of a firebox or furnace, designed to melt and release steam into the firebox if the crown is uncovered by water.

The upper plate of a firebox or furnace that is directly exposed to radiant heat from the fire and must remain covered by water to avoid overheating and collapse.

A spring-loaded valve that opens automatically at a set pressure to discharge steam and prevent the boiler pressure from rising above its maximum allowable working pressure.

The highest gauge pressure at which a boiler may lawfully and safely operate, determined from its design, materials and thickness calculations, and stamped on the boiler nameplate.

A test in which the steam outlet is closed and the firing held at maximum to confirm the safety valves keep pressure from rising more than the permitted percentage above working pressure.

A transparent tube or flat glass assembly connected to the boiler shell that shows the water level in the boiler, fitted with isolating cocks and a blowdown valve for proving it clear.

Two or three small cocks fitted at the top, middle and bottom of the normal water level, used as an independent check on the water level if the gauge glass is suspect.

A dial instrument, usually of the Bourdon tube type, that indicates steam pressure in the boiler and is marked with the maximum allowable working pressure in red.

A situation where the water level falls below the safe limit, exposing heated surfaces; firing must be stopped immediately and the boiler must not be fed until it has cooled.

The carry-over of boiler water with the steam, caused by high water level, foaming, sudden steam demand or dissolved solids, which can damage turbines and superheaters.

Any contamination of steam by water droplets, dissolved salts or suspended solids leaving the boiler, leading to deposits and corrosion in downstream pipework and equipment.

The combined concentration of dissolved salts and other substances in boiler water, measured by conductivity or evaporation and controlled by blowdown to limit scale and carry-over.

Deliberate discharge of boiler water to remove dissolved solids and sludge; bottom blowdown clears sludge from low points, while surface or continuous blowdown controls dissolved solids concentration.

A valve on the lowest part of the boiler shell or mud drum used to discharge sludge and sediment, usually a slow-opening screw valve or a quick-opening cock in series with a second valve.

A cylindrical header at the bottom of a water-tube boiler that collects sludge and sediment and provides a low point for blowdown and for mounting the bottom blowdown valve.

A boiler in which hot combustion gases pass through tubes surrounded by water, such as the Lancashire or packaged shell boiler, giving large water and steam storage but lower pressure capability.

A boiler in which water flows inside tubes heated externally by combustion gases, allowing higher pressures, faster steam raising and smaller water content than a fire-tube design.

The large corrugated or plain cylindrical tube in a shell boiler in which fuel is burned and from which radiant heat is transferred to the surrounding water.

A bank of tubes that further heats steam above saturation temperature after it leaves the boiler, improving efficiency and reducing condensation in the steam mains.

Steam at the same temperature as the boiling water from which it was produced, so that any loss of heat causes immediate condensation; its temperature is fixed by pressure.

A heat exchanger in the flue gas path that preheats feedwater before it enters the boiler, recovering waste heat and lowering fuel consumption.

Equipment that transfers heat from flue gases to incoming combustion air, raising flame temperature and boiler efficiency, but that can suffer corrosion if gas temperatures fall below the acid dew point.

Water supplied to the boiler to replace steam drawn off, normally treated to remove hardness and dissolved gases and often heated before entry.

A vessel that heats feedwater with steam under pressure to drive off dissolved oxygen and carbon dioxide, reducing corrosion in the boiler and condensate system.

A device that uses a jet of steam to force feedwater into the boiler against its own pressure, commonly found as a standby feed device on small or older boilers.

A pump, usually centrifugal or reciprocating, that delivers treated water into the boiler at a pressure above steam pressure, often controlled automatically by the water level.

An automatic system that continuously adjusts the feedwater valve or pump speed in response to water level changes, keeping the level steady without the cycling of on-off control.

A hard deposit of calcium and magnesium salts formed on heated surfaces when hard water is evaporated, which reduces heat transfer and can cause overheating and tube failure.

Treatment, usually by ion exchange, that removes calcium and magnesium from feedwater to prevent scale formation in the boiler.

Localised corrosion producing deep pits on boiler metal, caused by dissolved oxygen in the feedwater, especially where water is stagnant or during shutdown periods.

Cracking of stressed boiler steel caused by concentrated alkaline boiler water entering seams and crevices; controlled by limiting alkalinity and avoiding high-stress riveted or rolled joints.

A feedwater control strategy using steam flow, feedwater flow and drum level signals together, so the feedwater is adjusted for demand before level changes become significant.

A draught system in which a fan supplies combustion air to the furnace at above atmospheric pressure, so the furnace and flue are under positive pressure.

A draught system in which a fan at the outlet of the gas path pulls combustion gases through the boiler, keeping the furnace slightly below atmospheric pressure.

A combination of forced and induced draught fans that holds the furnace at a slight negative pressure, controlling both air supply and gas removal for efficient, safe combustion.

Air supplied above the theoretical quantity needed for complete combustion, expressed as a percentage, ensuring all fuel burns but increasing heat losses if excessive.

The minimum mass of air theoretically required to burn a given quantity of fuel completely, with no oxygen or unburnt fuel left in the products.

Measurement of oxygen, carbon dioxide and carbon monoxide in the exhaust gases to check that excess air and combustion efficiency are within acceptable limits.

A poisonous, colourless gas formed by incomplete combustion; its presence in flue gases indicates poor mixing, insufficient air or a chilled flame, and it can also be a furnace explosion hazard.

A violent ignition of unburnt fuel that has accumulated in the furnace or flue, usually caused by repeated failed ignition attempts or loss of flame without fuel isolation.

A period of forced air flow through the furnace and flue before ignition to remove any accumulated combustible gases, typically lasting several air changes of the gas path.

Loss of the burner flame during operation, which must be detected promptly by a flame scanner so that fuel is shut off and a purge carried out before any restart attempt.

Acid attack on economiser and air preheater surfaces when flue gases cool below the temperature at which sulphuric acid vapour condenses, often seen as rusty deposits.

A pressure test in which the boiler is filled with water and pressurised above working pressure to prove its strength and tightness, usually after repair or at statutory intervals.

Examination of a boiler's internal and external condition, fittings and records by a competent person at required intervals, to confirm it is safe to continue in service.

A record kept for each boiler containing operating readings, blowdowns, tests, defects, repairs and inspection results, providing evidence of compliance and maintenance history.

The legal obligation of operators and employers to take reasonable steps to protect the health and safety of themselves and others affected by their work, including safe boiler operation.

Someone with the training, knowledge, experience and qualifications needed to carry out a specified task safely, such as operating a boiler or certifying an inspection.