Can a turbine flowmeter measure gas?

Yes, with a meter built for gas: its rotor is lighter and its blades are shaped for the low density of a gas. Gas turbine meters are widely used for natural gas, with the volume converted to standard conditions from the pressure and temperature, often by a volume corrector. A meter built for liquid must not be used on gas: the rotor would overspeed and the bearings would fail.

FLOW MEASUREMENT TECHNOLOGIES

Turbine flowmeters: the flow measured by the speed of a rotor

A turbine flowmeter has a bladed rotor in the middle of the pipe, turned by the flow itself: the faster the fluid moves, the faster the rotor spins. A sensor on the meter body counts the blades as they pass, and every pulse stands for a known volume — so the pulse rate gives the flow rate and the pulse count the total. It is accurate, compact and quick to follow changes in the flow, and it suits clean, low-viscosity liquids — water, solvents, light fuels — and clean gases. It does not suit viscous or dirty fluids, its bearings wear over time, and it needs a straight run or a flow straightener before it. Turbine flowmeters are not yet in our online catalogue; for liquids, we supply them from Flomec.

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How a turbine flowmeter works

On the axis of the pipe sits a rotor with angled blades, much like a small propeller, held by supports before and after it and turning on bearings. The fluid flowing past pushes on the angled blades and turns the rotor, and within the meter’s range the rotor spins at a speed proportional to the average flow velocity. The bore of the meter is fixed, so velocity times cross-section is the volume flow: the rotor speed measures the volume flow rate. Because the rotor is light, it follows changes in the flow quickly.

The turns are counted from outside the flow. A magnetic pickup on the meter body senses each blade as it passes: the blade changes the magnetic field and induces a voltage pulse. The pulses per unit volume — per litre or per cubic metre — are the meter’s K-factor, which the manufacturer determines for each meter by passing a known volume through it. The pulse rate divided by the K-factor gives the flow rate, and the pulse count divided by the K-factor the total volume. An electronic register shows both, or passes them on to a PLC as pulses or as a 4–20 mA signal.

The K-factor stays constant only within the meter’s linear range, typically around 10:1 between the maximum and the minimum flow for liquids. At the bottom of the range, friction in the bearings and the drag of the fluid hold the rotor back, so the meter reads low; that is why every turbine meter has a minimum flow. Viscosity matters too: as the liquid gets thicker, the K-factor shifts and the linear range shrinks — so turbine meters are made for low-viscosity liquids, and the K-factor the manufacturer gives holds for a stated viscosity.

Gases are measured the same way, with meters built for gas: a lighter rotor, with blades shaped for the much lower density. Gas turbine meters are widely used for natural gas. Like any volumetric meter on a gas, they measure volume at line pressure and temperature, so the reading is converted to standard conditions from the pressure and temperature — often by a volume corrector next to the meter.

The flow of the fluid
The rotor, with angled blades, turned by the flow — its speed is proportional to the flow velocity
The magnetic pickup on the meter body, which senses each blade as it passes
One pulse for each blade that passes — the pulse rate gives the flow rate
The K-factor, pulses per litre, against the flow rate: constant across the linear range, falling at the lowest flows

Where a turbine flowmeter fits — and where it does not

A strong choice for

Clean, low-viscosity liquids — water, demineralised water, solvents, and light fuels such as diesel and kerosene — where it is accurate across its linear range
Non-conductive liquids such as fuels and solvents, which an electromagnetic flowmeter cannot measure
Clean gases — natural gas, compressed air, nitrogen — with a meter built for gas
Loading, dosing and short batches: the light rotor follows the flow quickly, and with a batch controller the transfer stops at the quantity you set
Compact installations: the meter is a short section of pipe, and its pulse output goes straight to a PLC, a flow computer or a batch controller
Duties where repeatability matters, such as fuel loading and test benches: a well-maintained turbine meter repeats its reading very closely

Think twice when

The liquid is viscous — heavy oils, resins, syrups: the K-factor shifts with viscosity and the low end of the range is lost — a positive displacement or Coriolis flowmeter suits them better
The fluid carries solids, fibres or abrasives: they damage the blades and bearings or jam the rotor — a strainer is essential, and slurries need another meter, such as an electromagnetic one
The fluid is steam: turbine flowmeters are not used on steam — vortex or differential pressure flowmeters are the usual choice
There is no room for a straight run or a flow straightener: swirl after a bend or a valve speeds the rotor up or holds it back, and the reading shifts
The flow often drops below the meter’s minimum: at the low end the bearings hold the rotor back and the meter reads low
The flow pulsates, as after a reciprocating pump, or the liquid can carry air, as when a tank runs empty: the rotor speeds up faster than it slows down, and air spins it as if it were liquid — in both cases the meter reads high

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±0.75–2%
Thermal mass±1–3%
Positive displacement±0.2–0.5%
Turbine (the technology of this article)±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 turbine flowmeter

A turbine flowmeter measures the velocity of the flow, so the flow reaching it must be straight and free of swirl. Our table gives 10 to 20 pipe diameters of straight pipe before the meter and 5 after it as a typical range; where there is no room, a flow straightener before the meter removes the swirl and allows a shorter run. It also needs clean fluid: a strainer before it, with the mesh the manufacturer specifies. Most turbine meters are installed in a horizontal line — the manufacturer’s instructions decide whether another position is allowed.

For liquids, the meter must stay full: put it where it cannot drain — not at a high point of the line — and fit control valves after it, not before, so that it works under back pressure, which also keeps the liquid from boiling or cavitating at the rotor. Wherever air can get into the line, an air eliminator goes before the meter. Isolation valves before and after it, and a bypass where the flow must not stop, let it be taken out for service. Then:

Straight run before and afterA bend, a valve or a pump before the meter leaves swirl in the flow, which turns the rotor faster or slower than the flow alone would. Typically 10 to 20 diameters of straight pipe before the meter and 5 after it, with the control valve further downstream — the manufacturer’s instructions give the exact lengths.

Or a flow straightenerWhere there is no room for the full straight run, a flow straightener — a bundle of tubes or vanes along the pipe — goes between the bend and the meter. It takes the swirl out of the flow, so the straight run before the meter can be shorter; how much shorter is given by the manufacturer’s instructions or the applicable standard.

A strainer before the meterSolid particles damage the blades and the bearings or jam the rotor, so a strainer — here a Y-strainer — goes before the meter, with the mesh the manufacturer gives. The particles collect in its leg, and it has to be cleaned at intervals.

Liquids: a full pipeAir spins the rotor as if it were liquid, so the meter reads high, and a pocket of air rushing through can overspeed the rotor. Install the meter on a low section that stays full, with the control valve after it — not at a high point of the line, where air collects.

Flush new pipework before the meter goes in — with a spool piece in its place — so that welding slag and other debris do not hit the rotor.
At start-up, fill a liquid line slowly and vent the air, and pressurise a gas line slowly: a rush of fluid into an empty line can spin the rotor far beyond its maximum speed and damage the bearings.
Size the meter for the flow range, not for the pipe size, and keep the flow within its linear range: below the minimum flow the meter reads low, and running above the maximum wears the bearings quickly.
Centre the gaskets so that they do not stick out into the bore, install the meter in the flow direction marked on its body, and — where the volume is billed — check it at regular intervals against a known volume, as the bearings wear over time.

Turbine flowmeters by industry

Oil & GasLight fuels at loading racks, depots and refuelling points, transfers between tanks, and natural gas measured with gas turbine meters — where the volume is often what is billed.

Water & WastewaterClean water: bulk supply to networks and large consumers, irrigation and treated water — not raw wastewater, whose solids would damage the rotor.

ChemicalSolvents and other clean, low-viscosity chemicals, dosed into batches and mixers — with the materials of the rotor and bearings chosen for the liquid.

Power & EnergyDiesel to generators, fuel gas to burners, and clean cooling and process water — with pulse outputs that go straight into the plant’s control system.

MarineMarine gas oil and other light fuels to main and auxiliary engines and generators, and fresh water on board — not heavy fuel oil, which is too viscous.

Turbine flowmeters from Flomec

Turbine flowmeters are not yet in our online catalogue. For liquids, we supply them from Flomec, a Great Plains Industries brand, which makes turbine flow meters — as well as oval-gear and electromagnetic meters — for fuels, oils, chemicals and water, from portable transfer to fixed installations. Tell us the application: the fluid and its viscosity, the flow range, the pressure and temperature, the pipe size, and whether you need a local display, pulses or a 4–20 mA signal — and our engineers will suggest the right meter, with price and delivery time.

FlomecFlow meters for fuels, chemicals and water Learn more

Turbine flowmeters: frequently asked questions

Our comparison table gives ±0.5 to ±1% of reading as a typical range — deliberately conservative: well-installed meters in clean, low-viscosity liquids often do better, and their repeatability is better still. The accuracy holds within the linear range; it falls below the minimum flow, with a more viscous liquid, with swirl in the flow and as the bearings wear. We confirm the expected accuracy for your fluid and flow range before you order.

The number of pulses the meter gives per unit volume — for example, pulses per litre. The manufacturer determines it for each meter by passing a known volume through it, and states it with the meter. The register, or the PLC, divides the pulse rate by the K-factor to get the flow rate and the pulse count to get the total. A large meter turns more slowly than a small one at the same velocity, so it gives fewer pulses per litre.

Because the rotor responds to both the speed and the direction of the flow. A bend, a valve or a pump before the meter leaves swirl in the flow, which turns the rotor faster or slower than the flow alone would — so the reading shifts. Our table gives 10 to 20 diameters before the meter and 5 after it as a typical range; a flow straightener before the meter takes out the swirl and allows a shorter run. The manufacturer’s instructions give the exact lengths.

Turbine flowmeters are not yet in our online catalogue. For liquids, we supply them from Flomec, which makes turbine, oval-gear and electromagnetic meters for fuels, oils, chemicals and water, from portable transfer to fixed installations. Send us the fluid and its viscosity, the flow range, the pressure and temperature, the pipe size and the output you need, and we will propose the right meter, with price and delivery time.

Ask about turbine flow measurement

Tell us the fluid — liquid or gas — and its viscosity at the operating temperature, the flow range, the pressure and temperature, the pipe size, and whether you need a local display, pulses or a 4–20 mA signal — and our engineers will suggest the right meter, with price and delivery time.