MD Boiler Operator key terms

A boiler in which hot combustion gases pass through tubes surrounded by water, so the fire is inside the tubes and the water is outside. Common in low-pressure heating and small steam plants.

A boiler in which water flows inside tubes that are heated by fire or hot gases on the outside. It can produce high pressure and large steam quantities and is common in power plants.

A cylindrical pressure vessel at the top of a water-tube boiler that separates steam from water, holds a water reserve, and receives feedwater. It connects to tubes and carries internal separating equipment.

A lower cylindrical drum on a water-tube boiler that collects sludge and sediment and distributes water to the generating tubes. It is blown down to remove accumulated solids.

A rod threaded through and riveted or welded to two boiler plates to tie them together and resist pressure that tries to separate them. Used mainly on flat surfaces such as firebox sheets.

The area of boiler metal exposed to hot gases on one side and water on the other, measured in square feet or square metres. It indicates the boiler's ability to absorb heat.

A bank of tubes that receives saturated steam from the boiler and heats it further with hot flue gas, raising its temperature above saturation. Superheated steam improves efficiency and reduces condensation in turbines.

A heat exchanger that uses hot flue gas leaving the boiler to preheat incoming feedwater. It lowers stack temperature and fuel use but must be protected against corrosion and freezing.

A spring-loaded valve that opens automatically at a set pressure to release steam and prevent the boiler from being overpressurized. It is the final overpressure protection and must be tested regularly.

A transparent tube or glass assembly connected to the boiler at two points so the water level inside the boiler can be seen directly. It is blown down and checked each shift.

A safety control that senses a low water level and automatically stops the burner before the boiler overheats. It usually works with a float chamber or electrode probe and requires regular testing.

Small valves mounted at different heights on the boiler shell or water column that are opened to confirm the water level by observing whether water or steam comes out. Used as a backup to the gauge glass.

A valve at the lowest point of the boiler used to discharge water and settled sludge to the drain. Blowing down controls dissolved solids and removes sediment from the mud drum.

A dial instrument that shows the steam pressure inside the boiler in psi or kPa. It must be accurate, marked with the maximum allowable working pressure, and checked against a test gauge.

A control system that monitors the burner flame with a scanner and shuts off fuel if the flame is not proven at light-off or is lost during operation. It prevents explosions from unburned fuel.

The ideal burning of fuel with exactly the amount of oxygen needed to convert all carbon to carbon dioxide and all hydrogen to water, leaving no excess oxygen or unburned fuel.

Air supplied to a burner beyond the theoretical amount needed for complete combustion. A small amount ensures all fuel burns, but too much wastes heat by sending it up the stack.

The pressure difference that moves air into the furnace and flue gases out through the stack. It may be natural, forced by a fan, induced by a fan, or balanced between both.

Breaking liquid fuel into a fine mist of small droplets so it mixes readily with air and burns cleanly. Pressure, steam, or air atomizing nozzles accomplish this.

The range of firing rates over which a burner can operate smoothly and safely, expressed as the highest firing rate divided by the lowest. A higher ratio gives more flexibility.

The lowest temperature at which a fuel gives off enough vapor to form an ignitable mixture near its surface. It indicates fire hazard during storage and handling.

A period of forced air flow through the furnace and flue passages before ignition to remove any accumulated combustible gases. It is required before every light-off attempt.

Water supplied to the boiler to replace steam that has been generated and used. It is usually a mixture of returned condensate and treated makeup water.

Fresh water added to the boiler system to replace steam and water lost through leaks, blowdown, and process use. It must be treated to prevent scale and corrosion.

A vessel that heats feedwater with steam and removes dissolved oxygen and other gases, reducing corrosion in the boiler and piping. It also serves as a feedwater storage tank.

A hard crust of mineral deposits, mainly calcium and magnesium compounds, that forms on boiler heating surfaces when water is not properly softened. It reduces heat transfer and can cause overheating.

A condition in which bubbles build up on the boiler water surface, often from high dissolved solids or organic matter, causing erratic water level indications and possible carryover.

The movement of boiler water droplets or foam out of the boiler with the steam. It can contaminate piping, turbines, and processes and is usually caused by high solids or priming.

A chemical such as sodium sulfite or hydrazine added to feedwater to remove the last traces of dissolved oxygen. It protects boiler metal from pitting corrosion.

The total amount of dissolved minerals and salts in boiler water, measured in parts per million. It is controlled by blowdown because high levels cause foaming and carryover.

A sharp banging or shock in piping caused by the sudden stopping or reversing of steam or water flow, often from trapped condensate. It can damage pipes, valves, and fittings.

A pressure test in which the boiler is filled with water and pressurized above its normal operating pressure to check for leaks and structural strength. It is done during inspection and after repairs.

A method of storing an idle boiler by draining, cleaning, and drying it and keeping the interior dry with desiccant. It prevents corrosion during long shutdowns.

A safety procedure in which energy sources are isolated, locked, and tagged before maintenance so equipment cannot be started unexpectedly. Only the person who applied the lock removes it.

A serious condition in which the boiler water level falls below the safe limit. The burner must be shut down immediately, and water must not be added until the boiler has cooled.

A widely adopted set of rules for designing, building, inspecting, and stamping boilers and pressure vessels. Section I covers power boilers and Section IV covers heating boilers.

The government agency or official responsible for enforcing boiler codes, issuing permits and operating certificates, and conducting or requiring inspections in a given area.

A code published by the National Board of Boiler and Pressure Vessel Inspectors that gives rules for installing, inspecting, repairing, and altering boilers and pressure vessels after they are built.

A document issued by the jurisdiction or an authorized inspector confirming that a boiler has been inspected and found safe to operate, usually valid for a stated period.

The highest pressure at which a boiler or pressure vessel may legally be operated, based on its design, materials, and thickness. It is marked on the nameplate and on the pressure gauge.