Power Eng 5th Class key terms

The maximum pressure at which a boiler or pressure vessel is designed and permitted to operate under normal service, stamped on the nameplate and used as the basis for safety valve settings and inspection intervals.

A spring-loaded automatic valve that opens at a set pressure to release steam or vapour and prevent a boiler from exceeding its maximum allowable working pressure. It reseats automatically as pressure falls.

A pressure test in which the vessel is filled with water and pressurised above its working pressure to prove strength and tightness. Water is used because it stores little energy if the vessel ruptures.

Extra metal thickness built into a boiler or vessel wall beyond what is needed for strength, so that predictable metal loss from corrosion or erosion over the service life does not reduce the wall below the safe minimum.

A vertical fitting on a boiler connected to the steam and water spaces that dampens surges so the gauge glass and low-water cutoff see a steady level rather than the churning surface inside the drum.

A transparent tube or flat glass assembly connected to a boiler's water and steam spaces so the operator can directly observe the water level. It must be blown down and checked regularly to stay reliable.

A safety control that senses boiler water level and shuts off the fuel supply if the level falls below a safe minimum, preventing overheating and catastrophic failure of the pressure vessel.

A manual check of a safety valve in which the lever is lifted briefly while the boiler is under pressure to confirm the valve opens freely and reseats without sticking. Done with care to avoid damage.

The controlled release of water from a boiler to remove dissolved solids and sludge that accumulate as steam is generated. Bottom blowdown removes sludge; surface blowdown controls dissolved solids concentration.

Priming is carryover of boiler water droplets into the steam outlet; foaming is a stable froth on the water surface that promotes it. Both are worsened by high dissolved solids, oil, or sudden load changes.

Combustion in which exactly the theoretically correct quantity of air is supplied to burn all the fuel, leaving no excess oxygen and no unburned fuel. In practice some excess air is always needed.

Air supplied to a burner beyond the theoretical amount needed for complete combustion, expressed as a percentage. Enough ensures complete burning; too much wastes heat up the stack and lowers efficiency.

Measurement of oxygen, carbon dioxide, carbon monoxide and temperature in the exhaust gas to judge combustion quality. It reveals whether the burner runs lean, rich, or with excessive air and heat loss.

A control system that proves a flame is present before fuel is admitted and shuts the fuel off if the flame is lost, preventing explosive accumulation of unburned fuel in the furnace.

A timed flow of air through the furnace and gas passages before ignition to sweep out any accumulated combustible gases, typically several air changes, so light-off cannot cause an explosion.

A violent ignition of unburned fuel that has collected in the furnace or flue, usually caused by failed purging, loss of flame without fuel shutoff, or repeated failed light-off attempts.

Feedwater control that uses drum level, steam flow and feedwater flow signals together. Steam flow anticipates demand and feedwater flow corrects the response, avoiding the shrink and swell errors of level-only control.

Apparent changes in boiler drum level caused by pressure changes, not by real changes in water quantity. A sudden steam demand drops pressure, makes steam bubbles expand, and the level falsely rises.

The valve and actuator assembly that adjusts feedwater flow into a boiler in response to level or flow signals, maintaining drum level within limits across the load range.

The concentration of dissolved salts and minerals in boiler water, measured in parts per million. It is controlled by blowdown because excessive levels cause carryover, scale, and corrosion.

A hard crust of mineral deposits, mainly calcium and magnesium salts, that forms on hot boiler surfaces. It insulates the metal, causing overheating, tube failure, and wasted fuel.

An ion exchange unit that swaps hardness ions such as calcium and magnesium for sodium, preventing scale formation in the boiler. The resin is restored by regenerating with brine.

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

Localised corrosion in the form of small deep cavities on boiler and condensate piping metal, caused by dissolved oxygen. It is prevented by deaeration and chemical oxygen scavengers.

Cracking of stressed boiler steel caused by concentrated caustic soda in crevices and seams. It is controlled by limiting alkalinity, avoiding over-concentration, and good water treatment practice.

Formation and collapse of vapour bubbles in a pump when local pressure drops below the liquid's vapour pressure. It causes noise, vibration, and erosion of the impeller and reduces flow.

The absolute pressure margin above a liquid's vapour pressure available at a pump suction. Adequate NPSH available over NPSH required prevents cavitation and loss of pumping.

A pump that adds energy to liquid by spinning it outward from an impeller inside a casing. It produces high flow at moderate head and is the most common boiler feed and circulating pump type.

A positive displacement pump using a piston or plunger moving in a cylinder to push liquid. It delivers a nearly constant flow against high pressure regardless of head, suiting small high-pressure feed duties.

A control method that lets a compressor run with its intake valve closed or held open so it produces no air while the receiver is at pressure, reducing starting load and cycling wear.

A pressure vessel that stores compressed air, smoothing pulsations from the compressor and allowing it to run in efficient cycles rather than constantly matching demand.

A refrigerant metering device that controls flow into an evaporator to maintain a set superheat at the outlet, using a bulb sensing suction line temperature to move the valve.

The temperature of refrigerant vapour above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the refrigerant has fully boiled and no liquid reaches the compressor.

The heat exchanger in a refrigeration system where low-pressure liquid refrigerant absorbs heat from the space or process and boils into vapour, producing the cooling effect.

A graph relating dry-bulb temperature, wet-bulb temperature, humidity ratio, relative humidity and enthalpy of moist air, used to analyse heating, cooling, humidifying and dehumidifying processes in buildings.

The amount of water vapour in air expressed as a percentage of the maximum the air can hold at that temperature. It affects comfort, condensation risk, and the load on cooling coils.

A safety procedure in which energy sources are isolated, locked, and tagged before maintenance, ensuring equipment cannot be started or energised while someone is working on it.

Entering an enclosed area such as a boiler drum or vessel with restricted access, requiring atmosphere testing, isolation, ventilation, an attendant, and a permit before anyone goes in.

A unit equal to one million ohms, used to express very high resistance. Insulation resistance of motors and cables is measured in megaohms to confirm the insulation has not broken down.

The relationship stating that voltage equals current multiplied by resistance in a circuit. It is the basis for calculating load current, voltage drop, and protective device ratings in plant electrical work.