Basics: volumetric vs mass flow
Volumetric flow (m³/h, US gpm) is the volume passing per unit time. Mass flow (kg/h, t/h) is what you usually care about for custody transfer, combustion and material balance. The two are linked by density:
ṁ = ρ · Q
Because gas density changes with pressure and temperature, gas volumes are normally referred to standard or normal conditions (Nm³/h, SCFH). A volumetric meter on gas service needs pressure and temperature compensation to give an accurate standard volume or mass.
Two more ideas matter for almost every meter: Reynolds number (whether the flow is laminar or turbulent) and the velocity profile, which is distorted by bends, valves and pumps upstream. Most meters assume a fully developed turbulent profile, which is why straight pipe runs are specified.
Differential pressure (orifice, venturi, nozzle)
A restriction in the pipe makes the fluid speed up, and its pressure drops (Bernoulli). The pressure difference across the restriction is measured by a DP transmitter. Flow is proportional to the square root of the DP:
Q = K · √ΔP → Q% = 100 · √(ΔP% / 100)
- Orifice plate: cheap, standardised (ISO 5167), no moving parts. Turndown is limited (about 3:1 to 4:1) because at 10 % flow you only have 1 % DP. Permanent pressure loss is high.
- Venturi: much lower permanent pressure loss; used on large water and air lines.
- Flow nozzle: robust at high velocity; common on steam and boiler feedwater.
Electromagnetic (mag) meters
Faraday's law: a conductive liquid moving through a magnetic field generates a voltage proportional to its velocity. There is no obstruction and no pressure loss, and accuracy is typically ±0.2–0.5 % of reading.
- The liquid must be conductive (typically above 5 µS/cm). Hydrocarbons, demineralised water and gases cannot be measured.
- The pipe must run full. Mount on a vertical upward line, or at a low point on horizontal lines.
- Grounding (earthing rings or electrodes) is critical, especially on lined or plastic pipe.
- Choose the liner and electrode material for the chemistry and abrasion of the fluid.
Vortex meters
A bluff body sheds vortices alternately from each side; the shedding frequency is proportional to velocity (the Strouhal principle). Vortex meters are popular for steam, gases and clean liquids, with no moving parts.
- They need sufficiently turbulent flow (high Reynolds number). Below a minimum velocity the meter reads zero, so check the low-flow cut-off against your minimum process flow.
- Pipe vibration can create false signals. Modern meters filter this, but avoid mounting near pumps or compressors.
Coriolis meters
The fluid flows through vibrating tubes; the Coriolis force twists the tubes in proportion to mass flow. The tube frequency also gives density directly. Accuracy of ±0.1 % or better makes Coriolis the reference choice for custody transfer, dosing and blending.
- They are expensive, heavy and large in big line sizes, and have a significant pressure drop in small sizes.
- Entrained gas in liquid (two-phase flow) degrades accuracy. Install so that gas can't collect in the tubes.
- Zero the meter at process conditions with the flow blocked in and the tubes full.
Ultrasonic & turbine meters
Transit-time ultrasonic meters measure the difference in sound travel time with and against the flow. Multi-path spool meters are used for gas custody transfer; clamp-on meters are excellent for temporary checks without cutting the pipe. Turbine meters count rotor revolutions: very accurate on clean, low-viscosity fluids, but the bearings wear and they need filtration.
Selection guide
| Technology | Liquids | Gas | Steam | Typical accuracy | Pressure loss |
|---|---|---|---|---|---|
| Orifice / DP | ✓ | ✓ | ✓ | ±1–2 % of span | High |
| Magnetic | ✓ conductive only | ✗ | ✗ | ±0.2–0.5 % rdg | None |
| Vortex | ✓ clean | ✓ | ✓ | ±0.75–1 % rdg | Low–medium |
| Coriolis | ✓ | ✓ | limited | ±0.1 % rdg | Medium–high |
| Ultrasonic | ✓ | ✓ | limited | ±0.5–1 % rdg | None |
| Turbine | ✓ clean | ✓ | ✗ | ±0.25–0.5 % rdg | Low |
Figures are typical; always check the vendor datasheet for your size and conditions.
Installation tips that save commissioning time
- Straight runs: respect the upstream/downstream diameters in the standard or vendor manual. Typical values are 10–40 D upstream of an orifice (depending on the fittings) and about 5 D upstream / 2 D downstream for a magmeter.
- Impulse lines (DP): for liquids, mount the transmitter below the taps so gas bubbles vent back to the line; for gas, mount above so condensate drains back. For steam, use condensate pots or fill the legs so both legs hold equal water columns.
- Flow direction: check the arrow on the meter body before the pipe is bolted up. It is an embarrassingly common error.
- Configuration: confirm the range, units, damping and square-root setting against the instrument index before the loop check.