Metering steam

Every lesson in this Academy prices steam by the tonne — but a plant that never weighs its steam is pricing folklore. Metering turns the steam system into an accountable utility: which department uses what, whether the project saved what it promised, and where tonight's steam went. This lesson covers the two meters that matter in Indian practice, the installation rules that decide whether they tell the truth, and the step from measuring kilograms to accounting energy.

The two workhorses

The orifice meter — a precision hole in a plate, measured by the pressure drop across it. Rugged, cheap, repairable with a spanner, understood everywhere — and honest about its limits: flow follows the square root of the differential, so a 4:1 flow range spans a 16:1 differential range, and below about a quarter of full flow the reading fades into noise. Fine for steady loads (a boiler main at constant duty); frustrating on swinging ones. The vortex meter — a bluff bar in the flow sheds vortices whose frequency counts the velocity. Turndown of 10–15:1, no square root, no impulse lines to choke — the modern default for consumer branches with real load swings. Its honest limit sits at the bottom: below a minimum velocity the shedding stops and the meter reads a confident zero, so size the meter body to the flow range, not the pipe (a DN80 meter in a DN100 line, swaged in, is normal and correct — lesson C3's "right looks small" yet again).

Mass needs pressure: the compensation rule

Both meters sense velocity or volume — but steam is sold and burned by the kilogram, and the kilograms in a cubic metre swing with pressure (vg, lesson A5). A meter calibrated for 7 kg/cm²g reads ~10% wrong at 6 — pressure sag alone fakes a "saving". Real steam metering therefore includes pressure (and for superheat, temperature) compensation: a transmitter feeding the flow computer so every reading is converted at actual density. The audit question for any existing meter is one line: where is its pressure signal? No compensation, no kilograms — just optimistic volume. And upstream of everything: dry steam. A meter fed wet steam counts water as steam at steam prices; a separator ahead of the metering run (lesson C10) is part of the meter.

Installation: where truth is won

Meters read swirl, disturbance and water as flow. The rules that keep them honest: straight lengths — typically around ten diameters clear upstream and five downstream (more after double bends; the vendor table is law); drainage — a drain pocket (lesson C4) just upstream, because a slug of condensate through any meter is both a false reading and a hammer blow (lesson C5); orientation and tappings per the book — steam orifice impulse lines have a right and a wrong geometry, and a plugged or flooded impulse line is the classic slow lie; and location with intent — one meter on the boiler main gives the plant total (and, against fuel, a live efficiency check on lesson B4); one per major department turns steam from overhead into an accountable cost, which changes behaviour faster than any poster campaign.

From kilograms to energy

Steam accounting's final step prices what the kilogram carries: flow × enthalpy, netted against returned condensate — the true kcal delivered per department (lesson F1's stack, measured instead of estimated). Modern flow computers do this arithmetic natively; the discipline is keeping their pressure, temperature and condensate inputs honest — the meter's arithmetic is only as good as lesson B4's instrumentation hygiene. Savings claims stand on this: measure before, measure after, same meters, stated period — the M&V habit lesson F8 formalises. A project "verified" by fuel bills alone in a plant with swinging production has proven only the weather.