Boiler Glossary

ASME CSD-1

The ASME standard for controls and safety devices on automatically fired boilers with fuel input under 12,500,000 Btu/hr, adopted by many jurisdictions.

ASME Section I

The ASME code for power boilers: steam above 15 psig, and hot water above 160 psig or above 250°F.

ASME Section IV

The ASME code for heating boilers: steam at 15 psig or less, and hot water at 160 psig or less and 250°F or less.

ASME Section VIII

The ASME code for unfired pressure vessels, such as tanks, separators, and receivers.

ASME stamps (S, H, M, U)

Certificates of authorization that let a shop build to the ASME code: S for power boilers, H for heating boilers, M for miniature boilers, and U for pressure vessels.

Auxiliary low water cutoff

A second, independent low water cutoff, usually set lower than the first and often manual reset.

Blowdown

Draining boiler water to remove dissolved and suspended solids. Surface (continuous) blowdown controls dissolved solids; bottom blowdown removes sludge.

Blowdown separator

A vessel that takes blowdown and lets the flash steam vent. Cold water tempering or an aftercooler brings the water down to what the drain allows; the California Plumbing Code limits water discharged under pressure into a drain to 140°F.

Boiler horsepower (BHP)

A unit of boiler output: 33,475 Btu/hr, or 34.5 lb/hr of steam from and at 212°F. It describes output, not fuel input.

Combustion efficiency

The share of the fuel's heat not lost up the stack, found from flue gas O2 and stack temperature.

Condensate

Steam that has given up its heat and turned back to water. Returning it saves water, chemicals, and fuel.

Dissolved oxygen

Oxygen carried in the water. It causes pitting corrosion in boilers and piping; hotter feedwater holds less of it.

Economizer

A heat exchanger in the stack that uses flue gas heat to warm feedwater or process water.

Excess air

Air supplied beyond what's needed to burn the fuel. Some is needed for safe, complete combustion; too much carries heat up the stack.

Feedwater

The water pumped into the boiler: returned condensate plus makeup water.

Feedwater tank

An atmospheric tank that collects condensate and makeup, holds a few minutes of storage, and feeds the boiler feed pumps.

Firebox boiler

A firetube boiler with a rectangular, water-jacketed firebox as its furnace instead of a round furnace tube.

Firetube boiler

A boiler where hot combustion gases pass through tubes surrounded by water. Common in commercial and industrial plants.

Flame safeguard

The burner management control that sequences the purge, ignition, and flame proving, and shuts the fuel off on flame failure.

Flash steam

Steam that forms when hot condensate or blowdown drops to a lower pressure.

Flue gas O2

The oxygen left in the flue gas, measured with a combustion analyzer on a dry basis. On natural gas, 3% O2 is about 15% excess air.

Flue gas recirculation (FGR)

Returning some flue gas to the burner to cool the flame and lower NOx.

From and at 212°F

A rating basis: feedwater at 212°F turned into steam at 212°F and atmospheric pressure. Real boilers make somewhat less steam per horsepower because feedwater comes in colder and steam leaves at pressure.

Fuel-to-steam efficiency

Steam (or hot water) output divided by fuel input. It includes stack losses and shell losses, so it is lower than combustion efficiency.

Hydrostatic test

Filling a boiler or vessel with water and pressurizing it to prove it is tight: well above MAWP on new construction, and at or near MAWP after most repairs.

Jackshaft linkage

A control where one motor turns a shaft linked to both the fuel valve and the air damper, also called single-point positioning.

Lead-lag sequencing

Controls that start and stop multiple boilers to follow the load, rotating which boiler leads.

Low water cutoff

A control that shuts off the burner when the water drops to a set level, protecting the boiler from overheating.

Makeup water

Fresh, usually softened, water added to replace steam and condensate that don't come back, plus blowdown.

MAWP

Maximum allowable working pressure: the highest pressure the boiler or vessel is built for, stamped on its nameplate. At least one safety valve is set at or below it.

MMBtu/hr

Millions of Btu per hour. Burner input and boiler output are both stated this way; input is output divided by efficiency.

National Board Inspection Code (NBIC)

The code that covers installation, inspection, repair, and alteration of boilers and pressure vessels after they are built.

National Board R certificate

Authorization from the National Board of Boiler and Pressure Vessel Inspectors to repair and alter boilers and pressure vessels under the National Board Inspection Code.

NFPA 85

The National Fire Protection Association code for boiler and combustion system hazards, applied to boilers with fuel input of 12.5 MMBtu/hr or more.

NOx

Nitrogen oxides formed during combustion. Air districts limit boiler NOx in ppm corrected to 3% O2.

NPSH

Net positive suction head: the pressure at a pump inlet above the water's vapor pressure, stated in feet of head. Hot feedwater needs enough tank height above the pump to keep it from cavitating.

Oxygen trim

A control that reads flue gas O2 and trims the combustion air in real time to hold a low setpoint.

Parallel positioning

Linkageless control: fuel and air each have their own servomotor, set point by point across the firing range.

Pass

One trip of the combustion gases through the length of the boiler. A three-pass boiler sends the gases through the furnace and two banks of tubes.

Purge

Blowing air through the furnace before ignition and after shutdown to clear unburned fuel. It is required for safety and carries heat out of the boiler.

Refractory

Heat-resistant lining in the furnace, doors, or burner throat.

Retube

Replacing a firetube boiler's tubes. In most cases, replacing rolled and beaded tubes is not an R stamp repair; tube work that involves welding is done as a code repair under a National Board R certificate. Your Authorized Inspector confirms what applies.

Safety valve

A spring-loaded valve that opens to relieve steam pressure before it exceeds the boiler's limit. Hot water boilers use relief valves.

Scotch marine boiler

A horizontal firetube boiler with an internal furnace and two to four gas passes, built as a self-contained package.

Short cycling

A burner turning on and off often because the load is below its minimum firing rate. Each start costs a purge and adds wear.

South Coast AQMD Rules 1146, 1146.1, 1146.2

The South Coast air district's NOx rules for boilers, split by nameplate heat input: 1146.2 up to 2,000,000 Btu/hr, 1146.1 above 2 and below 5 million, and 1146 at 5 million Btu/hr and above.

Stack temperature

Flue gas temperature leaving the boiler. Net stack temperature subtracts the combustion air temperature.

Steam sparge

A perforated pipe below the water line that injects steam to heat a feedwater tank.

Steam trap

A valve that lets condensate and air out of steam equipment and lines while holding the steam in.

Therm

100,000 Btu of fuel. Natural gas bills are usually in therms.

Tubesheet

The thick plate at each end of a firetube boiler that the tubes are rolled or welded into.

Turndown ratio

A burner's highest firing rate divided by its lowest. A 10:1 burner can fire down to 10% of its rating.

Ultra-low NOx burner

A burner designed to reach single-digit ppm NOx, often with premix, staged combustion, or flue gas recirculation.

Variable speed drive (VSD)

A drive that changes motor speed. On a burner fan, it cuts electricity at low fire compared with a damper.

Vertical boiler

A compact firetube boiler standing on end, used where floor space is tight.

Water column and gauge glass

The fitting and glass tube that show the operator the boiler's water level.

Watertube boiler

A boiler where water flows inside the tubes and the flame and hot gases are outside them. Common for high pressures and very large capacities.

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