DC Boiler Operator flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Latent heat of vaporisation

The heat absorbed when water changes to steam at constant saturation temperature, with no rise in temperature; released again on condensation. It is far larger than sensible heat for the same mass.

Three T's of combustion

Time, temperature, and turbulence. Complete combustion needs enough residence time, high enough furnace temperature, and thorough mixing of fuel and air. Shortfall in any one causes smoke, CO, or unburned fuel.

Excess air

Air supplied above the theoretical amount needed for complete combustion, expressed as a percentage of theoretical air. Too little produces CO and smoke; too much raises stack loss and lowers efficiency.

Stack loss

Heat carried out of the boiler in the flue gas. It is usually the largest loss and increases with higher flue gas temperature and higher excess air, so both are trimmed to practical minimums.

Boiler horsepower

A measure of evaporation rate equal to 9 810 W, or 33 475 Btu/h, roughly the energy to evaporate 15.6 kg of water per hour from and at 100 °C.

Saturation temperature versus pressure

The temperature at which water boils at a given pressure. It rises with pressure: about 100 °C at atmospheric and roughly 184 °C at 1 000 kPa, which is why high-pressure boilers need thicker drums.

Safety valve accumulation

The allowed pressure rise above set pressure while the valve passes full boiler output, limited to 6 % of set pressure. The valve must be sized so this limit is never exceeded.

Try cocks

Small valves on the water column used to verify water level independently of the gauge glass. They confirm whether the glass is reading true when a connection may be plugged.

Fusible plug

A brass fitting with a low-melting core, installed at the crown sheet or upper firebox. It melts and releases steam to warn of low water and overheating, but does not replace the low-water cutoff.

Feed check valve

A valve in the feedwater line that allows flow into the boiler but prevents backflow when feed pressure drops. It protects the feed pump and piping from hot boiler water.

Blowdown valve arrangement

A slow-opening valve on the boiler shell plus a quick-opening valve downstream. The shell valve is opened first and closed last, so the quick-opening valve never isolates live pressure alone.

Water column

A vertical fitting connected to the boiler at steam and water space, carrying the gauge glass and try cocks. Its lower connection must be blown down regularly to prevent plugging and false level readings.

Furnace prepurge

Forced draft airflow through the furnace and gas passages before lightoff, at least five air changes or typically 30–60 s at full airflow. It clears any accumulated combustible gas before ignition.

Warm-up rate limit

A cold boiler is heated at about 55 °C (100 °F) per hour on thick-walled drums. Faster heating causes thermal stress and uneven expansion that can crack tube seats and drum ligaments.

Low-water response

Secure the burner immediately and verify level with try cocks. Do not add feedwater to a boiler whose crown sheet or tubes may be uncovered until the metal has cooled, to avoid a violent steam flash.

Paralleling boilers

Bringing a second boiler onto a common steam header. Its pressure is raised slightly above header pressure, then its stop valve is opened slowly to avoid a pressure surge or water carryover.

Normal boiler shutdown

Reduce firing rate, blow down the water column and gauge glass, secure the burner, purge, close the main steam stop, then hold water level while the boiler cools slowly.

Caustic embrittlement

Stress corrosion cracking of steel at riveted seams and tube ends caused by concentrated caustic soda. It is avoided by limiting caustic alkalinity and using phosphate rather than hydroxide treatment.

Oxygen pitting

Localised corrosion from dissolved oxygen, forming deep pits that perforate tubes. It is prevented by mechanical deaeration plus a chemical scavenger such as sodium sulfite or hydrazine.

Carryover

Boiler water carried out with the steam, contaminating the steam system and superheater. It is controlled by keeping dissolved solids below the TDS limit, avoiding load swings, and not running high water level.

Bottom blowdown

Blowdown from the lowest point of the boiler to remove sludge and sediment. It is done in short, repeated openings with the boiler under load, not one long opening, to limit water level disturbance.

Blowdown percentage

Feedwater dissolved solids divided by the maximum allowable boiler water solids, times 100. Higher blowdown wastes fuel and water, so treatment is optimised to keep it low.

Scale thermal penalty

Scale has low thermal conductivity, so it raises tube metal temperature and fuel use for the same steam output. Even a thin layer can cause overheating and tube failure.

Flame safeguard control

A programming control that supervises prepurge, pilot proving, main flame proving, and lockout. It must lock out on flame failure and require manual reset before another lightoff attempt.

Low-water fuel cutoff

A safety device that trips the burner when water falls below the lowest safe level. It must be tested while the boiler is firing, because a stuck float may only show up under real conditions.

Ionisation flame rod

A flame detector that senses the current carried by flame ionisation between the rod and burner. A fouled, wet, or grounded rod causes nuisance lockouts and must be cleaned and checked.

High gas pressure switch

A safety switch in the burner gas train that trips and locks out the burner if gas pressure exceeds its setpoint. It protects against overfiring and flame instability.

Manual-reset safety limit

A high steam pressure or high water temperature limit set above the operating controller's setpoint. It trips the burner and requires a deliberate manual reset, so the cause is investigated first.

Hydrostatic test

A pressure test with the boiler filled with water, normally at 1.5 times maximum allowable working pressure for a new boiler. Safety valves are removed or isolated, and the test detects leaks and weakness.

Internal boiler inspection

An examination of the waterside and fireside with the boiler cooled, drained, isolated, and locked out, valves tagged, and the space verified safe. It finds scale, corrosion, cracks, and tube defects.

Combustion analyser trending

Recording O2, CO, CO2, and stack temperature over time. Trends reveal fouling and burner drift that a single reading misses, and they document efficiency between formal inspections.

Safety valve pop test

A test confirming the valve opens at its set pressure and reseats cleanly. Set pressure is verified by the valve tag and the test, not by the boiler pressure gauge alone.