Reference

Glossary

The working vocabulary of steam, hot water and thermic-fluid engineering — each term defined the way it is used on a plant. Lessons link here; this page is the canonical definition home. It grows with the academy.

Absolute pressure: Pressure measured from a true vacuum: gauge reading plus the atmosphere (≈1.013 bar). Steam tables — and steam — work on this scale. See the lesson on pressure & units.
Gauge pressure: Pressure measured above atmospheric — what plant gauges display. Written kg/cm²g, bar(g) or psig.
Saturation temperature: The one boiling/condensing temperature that belongs to each pressure. At 1 atm it is 100 °C; at 10.5 kg/cm²g about 185 °C.
Saturated steam: Steam at its saturation temperature — vapour sitting exactly on the curve. The normal working state of process steam.
Wet steam: Steam carrying entrained water droplets. Described by dryness fraction; a well-run shell boiler delivers roughly 95–98% dry steam.
Dryness fraction: The mass fraction of a wet-steam sample that is actually vapour. 0.97 means 97% vapour, 3% water mist.
Superheated steam: Steam heated beyond its saturation temperature. Good for turbines and transport; poor at surrendering heat to a process surface.
Sensible heat (hf): The heat in the liquid: energy that raises water's temperature to the boiling point. Roughly 1 kcal per kg per °C.
Latent heat (hfg): The heat of vaporisation — the large slab of energy absorbed when water becomes steam and released when steam condenses, all at constant temperature. The workhorse of steam heating.
Total heat (hg): Sensible plus latent: the full energy content of saturated steam, per kilogram, counted from 0 °C.
LMTD: Log-mean temperature difference — the honest average driving force between two streams whose temperature difference changes along the surface: (ΔT₁−ΔT₂)÷ln(ΔT₁/ΔT₂). With isothermal steam it is one line of arithmetic. See heat transfer with steam.
U-value (overall coefficient): The overall heat-transfer coefficient in Q = U·A·ΔT — the sum of every resistance between steam and product: films, wall, fouling, and any air. The largest resistance governs. Units here: kcal/h·m²·°C.
Fouling: Scale or product residue accumulating on heat-transfer surface, adding thermal resistance and stealing capacity batch by batch. Tracked honestly as a fouling factor in exchanger design; removed by cleaning schedules, not hope.
Enthalpy: The formal name for the heat content the h-columns of a steam table measure, in kJ/kg or kcal/kg.
Flash steam: Steam that forms spontaneously when hot condensate drops to a lower pressure and its surplus sensible heat re-evaporates part of it. A resource, not a leak.
Condensate: The hot, treated water left when steam has condensed. Carries roughly a quarter of the steam's original energy plus its full water-treatment value.
Steam trap: An automatic valve that discharges condensate and air while holding back live steam. The most numerous — and most neglected — device in a steam system.
Stall: The part-load condition where a temperature-controlled heater’s space pressure falls to (or below) the condensate back pressure — the trap’s differential vanishes and condensate backs up though the trap is healthy. Cured by gravity drainage or a pumping trap, never by a bigger trap. See heating with steam.
Pumping trap: A steam trap combined with a small steam-powered pump in one body: drains normally while differential pressure exists, pumps when it does not. The engineered exit from stall for temperature-controlled equipment.
Separator (steam): A baffled vessel that spins entrained water droplets out of flowing steam and drains them through its own trap — the point-of-use cure for wet steam, fitted before PRVs, meters and quality-critical processes.
Strainer: A Y-shaped body holding a fine screen that catches pipe dirt before it reaches traps, control valves and meters. On steam lines the screen leg is mounted sideways so it cannot fill with condensate.
Vacuum breaker: A small check valve that admits air when cooling equipment pulls a vacuum — protecting vessels from collapse and letting condensate drain from stalled coils. The deliberate opposite of an air vent.
Water hammer: The violent impact of a slug of condensate accelerated by steam flow — a leading cause of steam-system damage and injury. Preventable by correct drainage.
Thermal shock (condensation hammer): Water hammer’s second mechanism: a pocket of steam surrounded by cooler water condenses suddenly and the void collapses, slamming water fronts together. Needs no long pipe — one fitting, or one cold condensate tank, is enough.
Air blanketing: The insulating film air forms on a heat-transfer surface when it rides in with steam. Small air fractions cause outsized losses in heat transfer.
Partial pressure (Dalton’s law): In a gas mixture each gas contributes its own share of the total pressure. Steam mixed with air condenses at the temperature of its partial pressure — lower than the gauge suggests. The reason air-bound heaters run cold. See the lesson on wet steam, air and clean steam.
Carryover: Boiler water spat into the steam main with the steam — droplets, foam and their dissolved solids — caused by violent firing rates, high water level or high TDS. Makes steam wet and dirties superheaters and valves. See blowdown & TDS control.
Priming: Violent carryover: the boiler water surface foams or surges into the steam space and the boiler throws water into the main. Brought on by high TDS, oils, high level or sudden load spikes. See blowdown & TDS control.
Swell & shrink: The drum level’s false moves: on a sudden load pickup, falling pressure puffs up the steam bubbles and the level momentarily rises (swell); on a load drop it dips (shrink). The reason fast boilers need two-element level control.
PRS / PRDS: Pressure-reducing station / pressure-reducing and desuperheating station: the assembly that steps distribution-pressure steam down (and, in a PRDS, cools it) for a user.
Desuperheater (DSH): A device that injects a controlled quantity of feed-grade water into superheated steam to bring it back near saturation, where it condenses properly. Controlled to 5–10 °C above saturation; paired with a PRV it forms the PRDS.
Safety valve: The self-acting valve that lifts at set pressure to protect a vessel or line from overpressure. The last line of defence — never an operating control.
Fusible plug: A bronze plug cored with tin in the furnace crown, kept cool by water cover. On low water the tin melts, steam douses the fire and sounds the alarm — a last-line safety device with nothing to switch, power or bypass.
Control valve: The modulating valve, with actuator, that a control loop uses to regulate flow, pressure or temperature.
Kv: A valve's flow coefficient: the water flow in m³/h that passes at 1 bar pressure drop. The sizing currency of control valves (imperial cousin: Cv).
Equal percentage: The control-valve characteristic in which each increment of travel multiplies flow by the same factor — gentle at small openings, with rising gain that offsets a heater’s falling gain. The default trim for steam temperature control.
Rangeability: A control valve's own controllable ratio — maximum to minimum controllable Kv, a property of the trim alone (typical catalogue bands: regulators ~20:1, globe ~30–50:1, rotary higher). See control-valve selection.
Turndown: The flow ratio the installed system actually achieves — always less than the valve's rangeability, because real pressure conditions eat into it. The specification that quietly disqualifies more hardware than any other; check it against the real load range, not the nameplate duty.
Deaerator: The feedwater vessel that strips dissolved oxygen and CO₂ — mechanically, by heating and scrubbing — before water enters the boiler.
Calorifier: A hot-water vessel heated by a steam coil or bundle — the storage route from steam to hot water. Rides demand peaks at the price of standing losses and strict temperature discipline against legionella.
TDS: Total dissolved solids in boiler water. Rising TDS causes carryover and priming; controlled by blowdown.
Hardness: Dissolved calcium and magnesium salts, reported as ppm CaCO₃. Their solubility falls with temperature, so they scale exactly where the metal is hottest. Removed by softening before the water ever reaches the boiler.
Softening (ion exchange): Passing water through a resin bed that swaps calcium and magnesium for sodium, removing hardness. The resin is regenerated with brine. Watch outlet hardness and salt use — a slipping softener surfaces in the boiler weeks later.
Steam accumulator: A pressure vessel mostly full of saturated water, charged by boiler steam in the lulls and discharged by flash as pressure falls during demand peaks — a battery for steam that lets the boiler run at the average load. See the accumulator lesson.
Blowdown: The deliberate discharge of concentrated boiler water to hold TDS in range — a necessary loss whose heat can be recovered.
F&A rating: "From and at 100 °C": a boiler's nameplate evaporation under idealised conditions. Actual output at working pressure and feed temperature is always lower.
Boiler efficiency: Heat into steam ÷ heat in fuel. Measured directly (steam vs fuel) or indirectly (100 minus the counted losses — stack, blowdown, radiation).
Economiser: A heat exchanger in the boiler flue that pre-heats feedwater with stack heat — usually the first waste-heat recovery a boiler house gets.
Acid dewpoint: The flue-gas temperature below which sulphur-bearing condensate forms on heat-recovery surfaces and corrodes them. Sets the floor for economisers on coal, oil and husk; condensing recovery belongs to clean gas.
Economic thickness: The insulation thickness at which the total of heat-loss cost and insulation cost is minimum. Published in standards tables by pipe size and temperature — around 50 mm mineral wool for typical Indian process mains.
IBR: The Indian Boiler Regulations — the statutory regime governing boilers and steam pipelines above threshold conditions in India.
Thermic fluid: Heat-transfer oil (mineral or synthetic) circulated in a closed loop — process heat up to ~300 °C at near-atmospheric pressure.
Film temperature: The temperature of the thin boundary layer of thermic fluid at a heated tube wall — bulk temperature plus 20–40 °C. Every fluid’s real ceiling; exceeded by low flow or hot spots while the bulk gauge reads innocent. See thermic fluid 101.
Acid number (TAN): Total acid number of a thermic fluid sample — the oxidation tell-tale. Rising TAN means hot oil is meeting air, usually in a hot or badly breathing expansion tank.
Dry-out: The patient first heat-up of a thermic fluid system: holding and creeping through the 100–120 °C band until residual water has boiled off through the expansion tank. Rushing it cavitates pumps and hammers coils.
Pressurised hot water (PHW): Water held above 100 °C by system pressure and circulated as the heating medium — steam's quieter cousin for 90–130 °C duties.
Reverse return: Piping a hot-water network so the first user fed is the last returned — every user sees the same total pipe length, and the network balances itself by geometry instead of throttled valves.
Stratification: Hot water floating stably on cold in a storage tank — preserved by tall geometry and gentle, diffused connections. A stratified tank delivers true-hot water and returns true-cold to the heat source; one crude inlet turns it into a lukewarm barrel.
Thermocompressor: A steam-jet device that uses high-pressure motive steam to entrain and recompress low-pressure steam to a useful intermediate pressure — recovery without moving parts.
MVR (mechanical vapour recompression): A compressor doing the thermocompressor’s job on electricity — lifting low-pressure vapour to a useful pressure with high effective ratios, at the price of rotating machinery and kW. Standard in evaporators; contending more widely as power economics shift.
Specific volume (vg): The volume one kilogram of dry saturated steam occupies, in m³/kg. Falls steeply as pressure rises — the column that sizes every steam pipe.
Choked (critical) flow: Compressible flow that has reached sonic velocity in a restriction: the flow rate then depends only on the restriction's area and the upstream pressure. Governs leaks, safety valves and thermocompressor nozzles alike.
Entrainment ratio: In an ejector or thermocompressor, the kilograms of suction steam carried per kilogram of motive steam — the machine's working efficiency figure.
Excess air: Combustion air supplied beyond the chemical minimum. Necessary in moderation, expensive in excess — read from the flue-gas O₂: air factor ≈ 21 ÷ (21 − O₂%).
GCV / NCV: Gross vs net calorific value of a fuel: GCV counts the latent heat of the water vapour formed in combustion, NCV excludes it. Always ask which one an efficiency figure was computed on.
CCTS: India’s Carbon Credit Trading Scheme: emission-intensity targets for obligated sectors with tradeable credit certificates — PAT’s mechanism, re-currencied from energy to CO₂. Coverage and factors are notified in stages; verify current rules. See steam and carbon.
Condensate return fraction: The share of steam raised that comes home as condensate — the single most valuable percentage in a steam system.
COP: Coefficient of performance of a heat pump: useful heat out per unit of electrical work in. The number that decides a heat pump's economics.
Specific steam consumption (SSC): Steam used per unit of product — the benchmark figure that lets one plant be compared with another, and with itself last year.
M&V (measurement & verification): The discipline of proving a saving: a baseline on the judging meters, a declared boundary, normalisation for production and weather, and repeated verification. A saving is a counterfactual — M&V constructs it honestly. See proving the savings.
Baseline: The before-picture a saving is judged against: a long-enough period, on the same meters that will judge the after, fixed in writing before the project. Reconstructed baselines are folklore with decimals.
Steam locking: A plug of steam trapped between forming condensate and its trap — classically in a syphon-drained rotating dryer — holding the trap shut while the equipment floods. Cured by a steam-lock release path, not a different trap. See why steam traps fail.
Air binding: An air pocket seating a trap shut — the trap sits closed and cool while its equipment starves. Thermodynamic discs and inverted buckets are the vulnerable families; the cure is a parallel air vent.
Double trapping: Two steam traps piped in series. The downstream trap shuts on the upstream trap's flash, floods the pipe between them and strangles both. One drain point, one trap — redundancy goes in parallel.
Back pressure (at a trap): The true pressure at a trap's outlet: return-line pressure plus ~0.1 kg/cm² per metre of lift plus whatever failed-open neighbours add. It is subtracted from the trap's working differential — and often was not, at sizing time.
Pressure-powered pump: A displacement pump for condensate: a vessel that fills by gravity, then empties by motive steam pressure through check valves. No motor, no seal, no NPSH question — the machine for water at the boil. Folded into one body with a trap it becomes the pumping trap.
NPSH: Net positive suction head — the margin, in metres, between a pump's suction pressure and the liquid's vapour pressure. Hot condensate leaves a vented receiver only ~2 m of it; standard centrifugal pumps want more, and the shortfall is heard as cavitation rattle.
Cavitation: Vapour bubbles forming where local pressure dips to vapour pressure — in a pump eye or downstream of a throttling valve — then collapsing against metal. Sounds like pumped gravel; eats impellers in months.
Carbonic-acid corrosion: CO₂ released from feedwater alkalinity rides the steam, dissolves in fresh condensate and drops it to pH 5–6. Signature: a smooth groove dissolved along the bottom of return lines, threads failing first. Cures live in the water plant.
Oxygen pitting: Localised attack by dissolved O₂ — deep isolated pits under rusty tubercles in feed lines, receivers and economisers. Driven by shutdown air in-leakage and weak deaeration; a pitted line can pinhole within a few seasons.
Wiredrawing (wire-drawing): Seat erosion by a throttled jet — high-velocity steam through a half-open gate valve, or flashing condensate through a valve asked to throttle it — cutting fine parallel score-lines across the seating faces. The seat never shuts tight again. Gates isolate, globes regulate; on condensate duty only the trap throttles.
Flow-accelerated corrosion (FAC): Inside-out thinning of hot feedwater and economiser piping where flowing low-oxygen water dissolves the protective magnetite faster than it reforms. Fails suddenly and has killed; found by thickness survey programmes — specialist and inspector territory.
Vacuum steam: Saturated steam below atmospheric pressure, condensing below 100 °C (0.58 bar(a) ≈ 85 °C). Keeps steam's isothermal, high-coefficient heating for gentle duties — at the price of continuous vacuum plant and pumped condensate drainage. See vacuum steam heating.
Culinary (filtered) steam: Plant steam upgraded for food contact: separator, fine filtration to a few microns, and a boiler-chemistry programme with food-acceptable additives. The grade for direct injection into product.
Clean steam: Steam generated afresh in a dedicated stainless generator (CSG) from RO/DM water with no chemical dosing, distributed in stainless and tested at the point of use for dryness, superheat and non-condensables. Pure steam is the same idea validated until the condensate meets injection-grade water rules.
Steam tracing: A small steam line run in contact along a product pipe under shared insulation, replacing the pipe's heat loss so the product stays hot enough to flow — bitumen, edible oils, sulphur, caustic. Each tracer run drains through its own trap. See the steam tracing lesson.
Vent condenser: A small heat exchanger mounted on a vent — flash vessel, receiver or deaerator — that condenses the escaping steam into incoming make-up water instead of the sky. The forgiving recovery route where no steam consumer matches the flash supply.
Pressurised condensate return: A closed condensate system held above atmospheric pressure all the way home, so flash never forms at receivers and its heat stays in the loop. The premium recovery architecture — bought with tighter trap differentials and real management discipline. See the pressurised condensate loop.
Throttling calorimeter: The classic dryness test: throttle a steam sample to atmosphere and read its temperature. Throttling preserves enthalpy, so if the sample lands superheated its heat content — and therefore the line's dryness fraction — falls out of one line of arithmetic.
BSP / NPT threads: The two incompatible pipe-thread families: BSP (55°, the Indian standard) and NPT (60°, the American one). They screw together convincingly and never seal — the classic leak on imported equipment. Match the family or fit a proper adaptor.
PN / Class rating: The two flange pressure-rating families (IS/EN "PN" and ASME "Class"). A rating is read from the pressure–temperature table at working temperature, not the number on the rim — and the two families never mix on one joint.
Pipe schedule: The standard wall-thickness series for pipe (Sch 40, Sch 80…). Schedule 40 is the steam default; schedule 80 buys back the wall a cut thread removes and gives small-bore condensate lines a corrosion allowance.
Spec break: The marked point where a pipe specification changes — classically just downstream of a pressure-reducing station, where a lighter class becomes legitimate only because a safety valve guarantees the pressure can never climb back.
Warm-up (equalising) valve: The small bypass around a large isolation valve, opened first so downstream pipework warms and pressurises gently before the main disc moves against no differential. Protects seats, prevents start-up hammer, and retires the cheater bar.
Gauge siphon (pigtail): The water-filled loop seal a steam pressure gauge sits behind, with an isolation cock — the element reads pressure but never sees live steam temperature. A gauge mounted bare reads optimistically for a while, then lies forever.
Steam balance: The one-page accounting of every kilogram per hour — generation against every consumer, loss and vent, arranged by pressure level, each figure labelled measured or estimated. The step-zero drawing every audit and recovery project stands on; the closing gap is itself a finding. See the steam balance lesson.
Back-pressure turbine: A steam turbine that performs a pressure letdown the plant needed anyway — extracting shaft power while the exhaust continues to process at the lower pressure. Cogeneration at its simplest, governed by one rule: power follows heat, never the reverse.
Stoichiometric (theoretical) air: The chemistry-minimum air a fuel is owed for complete combustion — 2.67 kg of O₂ per kg of carbon, 8 per kg of hydrogen less what the fuel's own oxygen pays, all divided by air's 23.2% oxygen content. Roughly 1.3–1.5 kg of air per 1,000 kcal released, whatever the fuel; real firing adds excess air on top. See the combustion arithmetic lesson.
Evaporation ratio: Kilograms of steam raised per kilogram of fuel burnt — the every-shift efficiency proxy that needs no test boundary. Husk plants run around 4–4.5, Indian coal 5–6, furnace oil 13–14, each at its own GCV and feed temperature. A falling ratio is a question; an efficiency test is the answer.
Heat rate: Fuel energy spent per unit of electricity generated, in kcal per kWh — the scorecard of any cogeneration scheme, fairly counted only when the heat genuinely delivered to process is credited. The lower, the better; a pure condensing station's heat rate is what cogeneration exists to beat.