How accurate is a vortex flowmeter?

Our comparison table gives ±0.75 to ±2% of reading as a typical range. With liquids the figure is usually at the better end; with steam and gases, the accuracy of the mass flow also depends on the pressure and temperature measurements used for the compensation. In every case it depends on a long enough straight run. We confirm the expected accuracy for your application before you order.

Can it measure liquids that do not conduct electricity?

Yes. Unlike an electromagnetic flowmeter, a vortex meter does not need conductivity: it measures demineralised water, condensate, solvents and light fuels just as well. What it needs is a clean liquid of low viscosity that fills the pipe.

Which vortex flowmeters do you supply?

Vortex flowmeters are not yet in our online catalogue. We supply them from Krohne, which we represent in Greece across its whole range, including the OPTISWIRL vortex series. Send us the fluid, the flow range, the pressure, the temperature and the pipe size, and we will propose the right meter, with price and delivery time.

FLOW MEASUREMENT TECHNOLOGIES

Vortex flowmeters: steam, gases and liquids, with no moving parts

A vortex flowmeter has a bluff body across the pipe. As the fluid flows past it, vortices form behind it, alternately on one side and the other, and the rate at which they form is proportional to the flow velocity. A sensor counts them, and the meter turns that frequency into a flow rate. With no moving parts and a robust design, it is one of the most common choices for steam, and it also measures gases and low-viscosity liquids. Vortex flowmeters are not yet in our online catalogue; we supply them from Krohne, whose range includes the OPTISWIRL vortex series.

The manufacturer we represent

How a vortex flowmeter works

Put a bluff body in a flow and the fluid cannot follow its shape: it separates at the edges of the body and rolls up into vortices, which break away alternately from one side and the other and travel downstream in two staggered rows. It is the same effect that makes a flag flutter in the wind. In the meter, the bluff body spans the pipe from wall to wall, and its edges are sharp, so that the vortices always break away at the same points.

Over a wide range of flows, the number of vortices per second — the shedding frequency — is proportional to the flow velocity: twice the velocity, twice the frequency. Each vortex causes a small pressure pulse, which a sensor just behind the bluff body, or built into it, picks up; the transmitter counts the pulses and, from the known cross-section of the pipe, works out the volume flow. With no flow there are no vortices and no pulses, so there is no zero point to drift.

The meter measures the actual volume flow at operating conditions, which for a liquid is usually all you need. With steam and gases the density changes with pressure and temperature, so to give mass flow — or gas volume at standard conditions — the meter also needs the pressure and the temperature: from separate transmitters, or, on some models, from sensors built into the meter. For saturated steam one of the two is enough, because one determines the other.

The method has a lower limit: below a minimum velocity the vortices no longer form regularly, and the meter cuts its reading to zero. That limit is higher for gases and steam than for liquids, and higher for viscous liquids. It sets the usable range of the meter — typically from 10:1 to 20:1, depending on the fluid and the size — and it is the reason a vortex meter is often one size smaller than the pipe: to keep the velocity high enough at low flows.

The flow: liquid, gas or steam
The bluff body across the pipe, with sharp edges
The vortices that break away alternately from the two sides of the body (orange)
The sensor just behind the body, which picks up the pressure pulse of each vortex
The sensor signal: one pulse per vortex
The vortex frequency rises in proportion to the velocity: twice the velocity, twice the frequency

Where a vortex flowmeter fits — and where it does not

A strong choice for

Saturated and superheated steam, from the boiler house to the steam network and the users: one of the most common technologies for steam, with pressure and temperature compensation for mass flow and energy
Gases such as compressed air, nitrogen, natural gas and other process gases, in medium and large pipe sizes
Clean, low-viscosity liquids — water, demineralised and boiler feed water, condensate, solvents, light fuels — including liquids that do not conduct electricity, where an electromagnetic flowmeter cannot measure
High temperatures and pressures, where a robust body with no moving parts holds up well — the exact limits depend on the model
Little maintenance and a stable reading: nothing wears, and since there is no flow signal without vortices, there is no zero point to drift
A moderate pressure loss: in most cases lower than that of an orifice plate, though higher than an electromagnetic or ultrasonic flowmeter

Think twice when

The flow is very low, or varies over a very wide range: below the minimum velocity the meter reads zero — so at start-up, at night, or in a line sized for future growth, the low flows may simply go unmeasured
The liquid is viscous, such as heavy oil: the vortices form only at higher velocities and the usable range shrinks — a positive displacement or Coriolis flowmeter is usually the better fit
The pipe vibrates strongly, for example next to pumps, compressors or control valves: the sensor can take vibration for vortices, especially at low flows — modern sensors filter much of it out, but a calm location still matters
The flow pulsates, as after a piston pump or compressor, or is two-phase — wet steam with water droplets, liquid with gas bubbles, gas carrying liquid: the vortices become irregular and the reading unreliable
The fluid is dirty or sticky, or carries fibres or solids: deposits on the bluff body change its shape and the measurement — for slurries, an electromagnetic flowmeter is the usual choice
There is no room for a straight run: a vortex meter needs one of the longest — typically 15–20 D before it and 5 D after it — unless a flow conditioner is used

How it compares with the other technologies

Typical accuracy of each flow measurement technology, ± % of the reading, on a logarithmic scale. The further left, the more accurate. The technology of this article is highlighted.

Coriolis±0.1–0.2%
Electromagnetic±0.2–0.5%
Ultrasonic±0.5–2%
Differential pressure±0.5–2%
Variable area±1.6–4% *
Vortex (the technology of this article)±0.75–2%
Thermal mass±1–3%
Positive displacement±0.2–0.5%
Turbine±0.5–1%

Typical ranges for liquids (thermal mass: gases), ± % of the reading; for variable area, the accuracy class (*). The accuracy of a specific instrument depends on the model, the size and the installation — we confirm it for your application.

All technologies

Installing a vortex flowmeter

A vortex flowmeter needs a fully developed, undisturbed flow at the bluff body — more than most technologies. Typically that means 15–20 D of straight pipe before the meter and 5 D after it (D = nominal diameter); after two bends in different planes, or after a control valve, the run before the meter has to be longer still, and a flow conditioner can shorten it. Control valves belong after the meter. The gaskets must not protrude into the pipe, and the meter must be centred on it, because a step at the inlet disturbs the vortices.

Before choosing the size, check that the velocity stays within the range of the meter at both the lowest and the highest flow: a meter one size smaller than the pipe, with reducers before and after it, often keeps the low flows measurable. Then:

A long straight runTypically 15–20 D of straight pipe before the meter and 5 D after it. The control valve goes after the meter, beyond the straight run that follows it.

Liquids: a full pipeWith liquids the pipe must always be full at the meter: in a vertical line with the flow going up, or low down in a horizontal line. Not at the highest point of the line, where air collects, and not in a line where the liquid flows downwards.

Steam: the transmitter beside the pipeWith steam and other hot fluids, the transmitter usually goes beside the pipe rather than directly above it, where the heat rises. Insulate the pipe, but leave the neck of the meter and the transmitter free. Drain the condensate before the meter, so that slugs of water do not reach the bluff body.

Steam and gases: pressure and temperatureFor mass flow, or gas volume at standard conditions, a pressure transmitter and a temperature sensor go after the meter, at the distances the manufacturer gives, so that they do not disturb the flow reaching the bluff body. The transmitter of the meter, or a flow computer, combines the three signals.

Flush the line before fitting the meter: welding beads and scale can damage the sensor or build up on the bluff body.
Choose a calm spot, away from pumps, compressors and other sources of vibration, and support the pipe on both sides of the meter.
Mount the meter with the arrow on its body pointing in the direction of flow, and size it on the lowest and highest flow, not on the pipe size alone.
With steam, start up slowly: drain the condensate first and open the valves gradually, so that water hammer does not hit the meter.

Vortex flowmeters by industry

Power & EnergySteam from the boilers to the turbines and the users, boiler feed water and condensate, and fuel gas to the burners — with pressure and temperature compensation for mass flow and the energy balance.

ChemicalSteam to reactors, heat exchangers and tracing, nitrogen and other process gases, and clean solvents — including liquids that do not conduct electricity.

Oil & GasSteam networks in refineries, fuel gas and nitrogen to the units, and light hydrocarbons in clean service; the robust body suits high pressures and temperatures.

Food & BeverageSteam for cooking, sterilisation, pasteurisation and cleaning in place, and compressed air and nitrogen in the utilities — measured at each user, to see where the energy goes. The lines carrying the product itself call for hygienic design — usually an electromagnetic or Coriolis flowmeter.

Water & WastewaterAir from the blowers to the aeration tanks, and clean process water in the plant. For the main water flow, an electromagnetic flowmeter is the usual choice.

Vortex flowmeters from Krohne

Vortex flowmeters are not yet in our online catalogue. We supply them from Krohne, which SEMAC Automation represents in Greece across its whole range — including the OPTISWIRL vortex series. Tell us the application: the fluid, the flow range, the pressure and temperature, and the pipe size — and our engineers will suggest the right meter, with price and delivery time.

KrohneProcess instrumentation — flow, level, temperature, pressure and analysis Learn more

Vortex flowmeters: frequently asked questions

It has no moving parts, it copes with high temperatures and pressures, its pressure loss is moderate, and it covers a wider range than an orifice plate. With pressure and temperature compensation — or one of the two for saturated steam — it gives mass flow and energy. The usual alternative for steam is differential pressure; Coriolis and ultrasonic flowmeters measure steam only in specific cases.

Every vortex meter has a minimum velocity, below which the vortices do not form regularly and the reading drops to zero. It depends on the fluid — it is higher for gases and steam than for liquids, and higher for viscous liquids — and on the size of the meter. If the low flows matter, a meter one size smaller than the pipe, with reducers, raises the velocity; we check the range against your lowest and highest flow before we propose a size.

Ask about vortex flow measurement

Tell us the fluid (steam, gas or liquid), the flow range, the pressure and temperature, the pipe size, and whether you need volume, mass flow or energy — and our engineers will suggest the right meter, with price and delivery time.