How to Size a Vortex Flow Meter

A vortex meter measures steam, gas and clean low-viscosity liquids with a simple, rugged body — no impulse lines, no three-valve manifolds, no moving parts and no zero drift. The most common failure is a size that is wrong for the actual operating flow: too large and the meter stalls at low flow; too small and it over-ranges. This guide walks through the check in five steps.

Step 1 - Gather the operating conditions

You need four inputs:

1. Media (liquid, gas or steam) and its composition
2. Operating temperature and pressure — these fix the density
3. Min / normal / max flow in the line
4. Your pipe size and orientation

For steam and gas the working density drives everything. A meter sized at normal density can fall out of its velocity window during startup or maximum-demand periods, so check all three flow points.

Step 2 - Match velocity at operating conditions, not just line size

Vortices only form cleanly inside a designed velocity window. Typical guidance — always confirm against the selection sheet for your exact size:

If your normal flow is too slow for the line size, select a reduced bore mounted with concentric reducers upstream and downstream — do not force a full-bore meter onto a low flow.

Step 3 - Check turndown and Reynolds number

A standard vortex covers roughly 10:1, up to 25:1 on special ranges. The K-factor (pulses per unit volume) depends only on the shedder geometry — not on density or viscosity — so accuracy stays flat across the range as long as the flow is fully turbulent, roughly Re above 20,000 at the shedder. Confirm your minimum flow sits above that Reynolds cutoff.

Step 4 - Verify pressure loss

Permanent pressure loss is roughly:

ΔP ≈ 1.1 x ρ x v²

where ρ is the operating density (kg/m³) and v is the velocity (m/s). For saturated steam at 27 m/s the loss is only a few kPa, but for a dense liquid at high velocity it can matter — compare it against your available line pressure.

Step 5 - Apply the installation rules

• Straight run: 15D upstream, 5D downstream
• Keep the meter away from pumps, control valves and other vibration sources (allowable < 2g)
• Fill the pipe fully — no entrapped gas in liquid lines, no condensate pockets in gas or steam lines
• For liquids, vertical mounting with upward flow is preferred

Worked example - steam on a DN80 line

Saturated steam at 6 bar(g), 160 °C, density ≈ 3.7 kg/m³. Max flow 2,800 kg/h ≈ 0.21 m³/s at operating conditions:

• Through DN80 (cross-section ≈ 0.005 m²) that is ≈ 40 m/s — above the comfortable steam window
• Moving to DN100 gives ≈ 27 m/s at max flow, with roughly 3 kPa pressure loss
• The 400 kg/h low-end sits near 4 m/s — confirm it stays above your turndown low limit

Rough figures for illustration — send us your media, pipe size and flow range, and we will return a confirmed bore and model in one working day.