Two pressure gauges can read exactly the same number while the steam behind them is not the same steam. One line carries steam that is 4% water by mass; the other carries steam 30 °C hotter than its boiling point. Same pressure, different fluid, different behaviour in your process. To see the difference you need one picture — the temperature–enthalpy diagram, the map every steam engineer carries in their head.

Walk one kilogram of water across the map

Take 1 kg of water at 30 °C and heat it at constant atmospheric pressure. Its story has three chapters:

  • Sensible heating. Temperature climbs as heat goes in — roughly 1 kcal per °C. From 30 °C to 100 °C costs about 70 kcal. The water stays water.
  • The plateau. At 100 °C the temperature stops. Heat keeps pouring in — 539 kcal of it — and every kcal goes into tearing water molecules free of the liquid, not into warming anything. This is the latent heat chapter: the flat shelf on the diagram where water becomes steam at constant temperature.
  • Superheating. Only when the last droplet has evaporated does the temperature move again. Now you are heating a gas — about 0.5 kcal per °C — and the steam climbs off the shelf into the superheated region.

Repeat that walk at a higher pressure and the shelf sits higher (saturation temperature rises — the lesson What is steam?) and is shorter (latent heat falls). Stack up the shelves for every pressure and their ends trace two curves that meet at the top — the famous bell. Inside the bell, water and steam coexist. Left of it, water. Right of it, superheated steam.

Dryness fraction — the honesty rating of steam

Real steam leaving a working boiler is never perfectly dry: the violence of boiling throws a fine mist of droplets into the outlet. Steam that is 97% vapour and 3% droplets by mass has a dryness fraction of 0.97. On the diagram it sits on the shelf, but not at its end — 97% of the way across.

The heat it carries follows directly: h = hf + x·hfg — the full sensible heat, but only the fraction x of the latent heat. The missing latent heat never got invested, and the droplets that carry the shortfall must be drained out of your pipes before they cause damage.

The numbers — what 3% wetness costs the practice plant
Boiler steam: 10.5 kg/cm²g, latent heat hfg ≈ 476 kcal/kg At dryness x = 0.97: latent carried = 0.97 × 476 ≈ 462 kcal/kg Shortfall ≈ 14 kcal/kg — 3% of the boiler's working payload At 6,000 kg/h, that is ≈ 84,000 kcal/h of capacity not delivered — and ≈ 180 kg/h of hot water travelling down the steam main instead.

The wetness also grows along the pipe: every kcal the main loses through its insulation condenses more steam into droplets. That is why long mains need drain points along their length — the lesson Draining steam mains — and why the driest steam in the plant is at the boiler, not at the user.

The three states, and who wants which

  • Wet steam (inside the bell) — what boilers actually make. Acceptable at x ≈ 0.95–0.98; separators and good offtake design keep it there.
  • Dry saturated steam (the right-hand edge of the bell) — the ideal for heating: maximum latent heat, condenses at one temperature, transfers heat superbly. All process-heating design assumes you are close to this line.
  • Superheated steam (right of the bell) — the choice for turbines and long transmission lines, but a poor heater: it must first cool down to saturation before it can condense, and while doing so it behaves like a thin, lazy gas. It gets its own lesson.
At site
  • Take steam from the top of the boiler offtake and headers — droplets travel along the bottom of the pipe.
  • A separator plus trap set close to the boiler is the cheapest dryness improvement money buys.
  • Sudden pressure drop (a reducing valve) partially dries — even superheats — steam: downstream of a PRS your steam is drier than upstream. Expect it in your measurements.
  • Persistent water hammer at start-up is the phase diagram talking: condensate is pooling where wet steam was left undrained.
Pin this
  • Constant-pressure heating: sensible climb → latent shelf → superheat climb.
  • The bell is the two-phase region; its right edge — dry saturated — is where heating processes want to live.
  • Dryness fraction x: h = hf + x·hfg. Wet steam carries less latent heat and drops water in your pipes.
  • Higher pressure: hotter shelf, shorter shelf — more temperature, less latent heat.
  • Wetness grows along the main; drain accordingly.
Steam stories

Follow the bell upward and the shelves keep shrinking until, at 221 bar(a) and 374 °C, the shelf vanishes entirely — the critical point, where water and steam become indistinguishable and latent heat falls to zero. Modern supercritical power boilers work above this point: inside them water never boils at all. It simply slides from liquid-like to gas-like with no bubbles, no drum, and no surface — a kettle with nothing to watch.