Vortex Flowmeter Fails to Read Low Flow — Root Cause Found in Sizing, Not the Meter
A plant installed a DN80 vortex flowmeter on a compressed air branch line. Daytime readings were normal, but at night the DCS displayed 0 or flickered between 0–2 m³/h, and cumulative totals consistently fell short of compressor output. Replacing the meter did not help — the problem lay in oversized diameter and mismatched turndown ratio relative to low-flow conditions.
Why It Happens
Vortex meters rely on Kármán vortex shedding; shedding frequency is proportional to flow velocity (
f = St·v/d). At low velocities, vortices weaken and pulses are easily drowned out by pipe vibration and fluid noise, causing the meter to show zero or jump erratically. Three common causes:
Turndown ratio and lower limit mismatch. Typical vortex turndown is 10:1–15:1. Lower limit = upper limit ÷ turndown ratio. Sizing by upper-limit flow alone pushes the minimum detectable flow too high.
Oversized diameter. For a given flow rate, larger pipe diameter means lower velocity — and weaker vortex signals.
Insufficient Reynolds number. Stable shedding generally requires Re ≥ 2×10⁴. Gases have low density, so the minimum detectable velocity in the same diameter is higher than for liquids. Always verify against the manufacturer's specifications.
Takeaway: For low-flow gas applications, calculate the actual minimum flow first, then select diameter and turndown ratio accordingly. When retrofit space is limited, consider a smaller bypass meter or a different technology better suited to wide-range low-flow measurement.
