Precession Vortex flowmeter

A precession vortex flowmeter measures gas volume by forcing the flow into a controlled swirl and reading how fast that swirl precesses around a diffuser. Because the swirl is generated inside the meter rather than shed from a bluff body, the instrument needs only three diameters of straight run, which is why it is the usual answer on packaged gas skids and retrofit stations where the pipe route is already fixed.

The range below covers gas service only: natural gas, LPG vapour, town gas, compressed air and nitrogen. Sizes run from DN25 to DN300, accuracy is plus or minus 1.5 percent of reading, and the compensated version carries pressure and temperature sensors in the same head so the total is stored in standard cubic metres rather than at line conditions.

What to check before you choose one

Start with the minimum flow. A precession vortex meter needs a minimum Reynolds number before the precession is stable, and on a distribution point with a strong day-night swing the night hours can fall below that floor and read as zero. If the quiet-hour flow is under ten percent of maximum, look at a roots meter or a turbine instead.

Then check the pressure loss. The meter costs 0.3 to 0.8 bar at full scale, which is fine on a 4 bar main and impossible on a 30 mbar network. Below about 0.5 bar line pressure, a rotary displacement meter is the only sensible choice.

Finally check the gas. Wet gas disturbs the vortex core and the reading goes noisy before it goes wrong, so a knock-out drum upstream is not optional on saturated service. Dirty gas fouls the guide blades over months and shifts the K-factor, so a strainer is worth fitting even though the meter tolerates dirt better than a turbine.

Models in this range

The flange type body is the standard unit for district metering and factory gas inlets, with a stainless or coated aluminium body and an IP65 head. The battery version runs three to five years on a duty cycle of a few seconds and stores daily totals for later download, which makes it the usual choice where no power is available at the metering point.

For custody transfer where the reading is invoiced, check the local metrology rule first. Many gas codes still specify a turbine or a displacement meter for billing, and a precession vortex meter may not qualify even where it is more practical to install.