Power Eng 4th Class 4A key terms

Heat added to or removed from a substance that changes its temperature without changing its phase. In boiler work it warms feedwater up to the boiling point before any steam is formed.

The heat energy absorbed when a liquid changes to vapour at constant temperature and pressure, or released on condensation. For water at atmospheric pressure it is roughly 2257 kJ per kilogram.

A property combining internal energy with the flow work of a fluid, written h = u + Pv. Steam tables list it so heat balances across boilers, turbines and heat exchangers can be calculated directly.

The temperature rise of steam above its saturation temperature at the same pressure. Superheated steam carries more energy per kilogram and condenses less readily in piping and turbine stages.

The mass fraction of a wet steam mixture that is vapour, expressed from 0 for saturated liquid to 1 for dry saturated steam. High quality protects turbine blades from water droplet erosion.

The ratio of useful energy output to energy input for a cycle or plant. It shows how much of the fuel heat actually becomes shaft work or steam, with the remainder lost in stack gas, radiation and condenser heat.

The ideal thermodynamic cycle for steam power plants, made up of pumping, constant-pressure boiling, expansion through a turbine and condensation. Practical plants add superheat and feedwater heating to raise efficiency.

An accounting of all heat entering and leaving a boiler or plant, usually expressed as percentages of fuel energy input. It identifies stack, radiation and blowdown losses so efficiency improvements can be targeted.

The pressure at which a boiler or vessel is normally operated, as distinct from its design or maximum allowable pressure. Instruments, safety valves and controls are set relative to this value.

Maximum allowable working pressure: the highest pressure at which a boiler or pressure vessel may legally operate, set by its design, material and thickness. Safety valves are set at or below this figure.

A vertical fitting on a boiler that dampens water level movement so the gauge glass shows a steady level. Its connections must be kept clear to avoid false readings that could cause a low-water trip.

A transparent tube or flat glass assembly that shows the actual water level inside a boiler. It must be blown down and checked regularly because a blocked connection can display a dangerously false level.

The deliberate discharge of boiler water to remove dissolved solids and sludge. Bottom blowdown clears sludge from the mud drum, while continuous surface blowdown controls dissolved solids concentration.

A hard deposit of calcium and magnesium salts that forms on hot boiler surfaces when feedwater is not properly treated. It insulates the metal, causing overheating, tube failure and wasted fuel.

A form of stress-corrosion cracking in riveted or stressed boiler steel, promoted by concentrated caustic soda in crevices. Modern water treatment limits alkalinity and avoids high-stress riveted seams.

A vessel that heats feedwater with steam under slight pressure to drive off dissolved oxygen and carbon dioxide. Removing these gases before the feed pump prevents pitting and corrosion in the boiler.

Localised corrosion of boiler metal caused by dissolved oxygen attacking small areas of the surface. It produces deep pits that can perforate tubes, and is controlled by deaeration and chemical oxygen scavengers.

Sodium sulfite, an oxygen scavenger added to boiler water where it reacts with residual oxygen to form sulfate. It is not used at higher pressures because it breaks down into corrosive gases.

An ion exchange unit that swaps calcium and magnesium ions in raw water for sodium ions, preventing hard scale. Softened water still contains the original dissolved solids, so blowdown control is still required.

A measure of how well boiler water conducts electricity, used as an indicator of dissolved solids concentration. It guides continuous blowdown so that total dissolved solids stay within limits.

The air supplied to a burner beyond the theoretical amount needed for complete combustion, usually expressed as a percentage. Too little causes smoke and carbon monoxide; too much wastes heat up the stack.

The exact quantity of air theoretically required to burn a given fuel completely, based on its chemical composition. Practical combustion always needs some excess air above this figure.

Measurement of oxygen, carbon dioxide and carbon monoxide in exhaust gas to judge combustion quality. The oxygen reading is used to tune air-fuel ratio and minimise stack losses.

A sensor that detects the presence of a burner flame using ultraviolet or infrared light. It must prove flame before fuel is admitted and trip the fuel supply if flame is lost.

A forced flow of air through the furnace and gas passages before ignition to clear any accumulated combustible gas. A purge is also required after a trip before attempting a restart.

A safety control that shuts off fuel to a boiler when the water level falls below a safe limit. It must be tested regularly because a failed cutoff can lead to overheating and catastrophic failure.

A spring-loaded valve that opens automatically at a set pressure to release steam and protect the boiler from overpressure. It must be set at or below the maximum allowable working pressure and tested periodically.

A pump that uses a rotating impeller to impart velocity to a liquid, converting it to pressure in the casing. It is the standard choice for boiler feed and circulating duties, and needs priming.

The formation and collapse of vapour bubbles in a pump when local pressure drops below the liquid's vapour pressure. It causes noise, vibration and pitting damage to the impeller.

Net positive suction head: the margin of absolute pressure above vapour pressure available at the pump suction. Insufficient NPSH is the usual cause of cavitation and reduced pump performance.

A valve with a sliding wedge that fully opens or closes a flow path with little resistance when open. It is used for isolation rather than throttling, because a partly open gate erodes quickly.

A valve in which flow passes through a seat at right angles to the stem, giving good throttling control. Its higher pressure drop makes it unsuitable for simple isolation duties.

A valve that allows flow in one direction only and closes automatically when flow reverses. It protects pumps and boilers from backflow and water hammer in shared headers.

A pressure surge caused by sudden stoppage or reversal of liquid flow in a pipe, producing a loud banging noise. It can burst pipes and fittings and is prevented by proper valve closure and air chambers.

A device that automatically discharges condensate and non-condensable gases while retaining live steam. Correct trapping keeps heat transfer efficient and prevents water hammer in steam mains.

A controller that acts on proportional, integral and derivative terms to keep a measured variable at setpoint. Integral action removes offset, while derivative action reduces overshoot on fast processes.

An instrument that converts a measured variable such as pressure, level or temperature into a standard electrical or pneumatic signal for the control system. It allows readings to be sent long distances reliably.

The target value that a control loop maintains, such as boiler drum level or steam pressure. The controller compares the measured value with the setpoint and adjusts its output accordingly.

A safety procedure in which energy sources are isolated, locked and tagged before maintenance work begins. It prevents accidental start-up or release of stored energy that could injure workers.

Entry into an enclosed area such as a boiler drum or header with limited access and poor ventilation. It requires a permit, gas testing, an attendant and rescue arrangements before anyone enters.