Steam Flow Metering
You cannot manage what you do not measure, and steam is the hardest utility to measure well. Steam meters read velocity or pressure difference, and turning that into kilograms needs the steam’s density, which changes with pressure. A meter without density compensation is wrong whenever pressure moves.
You'll learn
- Describe the common steam flow meter types
- Explain why density compensation is essential
- Estimate the error of an uncompensated meter
- Use steam metering for efficiency and costing
Before this: Reading steam tables, Pressure and temperature measurement
Why meter steam
- Boiler efficiency by the direct method needs steam flow.
- Steam cost per department or product needs flow at each user.
- Leaks, failed traps and waste show up as unexplained consumption.
- Load profiles show whether a boiler is oversized or a new user can be added.
Steam meters are installed in the main from the boiler house and, increasingly, at major users, so that each department sees what it uses.
Meter types
| Type | How it measures | Notes |
|---|---|---|
| Orifice plate with DP transmitter | Pressure drop across a restriction | Simple, well understood; limited turndown; permanent pressure loss |
| Vortex | Frequency of vortices shed by a bluff body | Wide turndown, no moving parts; needs straight pipe runs |
| Variable-area and spring-loaded meters | Displacement of a cone or plate against a spring | Compact; good turndown on smaller lines |
Every meter needs the straight pipe lengths its maker specifies before and after it, dry steam, and a strainer or separator upstream. Water droplets and disturbed flow are the commonest sources of error after density.
Density compensation
A meter senses something that depends on density as well as on mass flow. A DP meter’s mass flow goes with the square root of density times pressure drop; a vortex meter measures volume, so mass flow is volume times density. Steam density changes a lot with pressure, so the meter needs a pressure (and for superheated steam, temperature) input to compute density continuously.
- Density at 10 kg/cm²(g): 5.548 kg/m³; at 8: 4.584 kg/m³.
- Uncompensated DP meter: reads high by √(5.548 ÷ 4.584) − 1 = 10.0%.
- Uncompensated vortex meter scaled at fixed density: reads high by 21.0%.
Key takeaways
- Steam metering underpins efficiency, costing and leak detection.
- Orifice/DP, vortex and spring-loaded variable-area meters are common; all need straight runs and dry steam.
- Every steam meter needs pressure (and for superheat, temperature) compensation.
- Uncompensated, a 10→8 kg/cm²(g) pressure change causes 10–21% error.