What a vortex flow meter is
A vortex flow meter measures flow by counting the vortices shed behind a bluff body placed in the stream. Each vortex arrives as a pressure pulse, and the frequency at which the pulses arrive is proportional to velocity over a wide band, so the instrument turns a mechanical rhythm into a flow rate without any moving part. Because the body sits in the line and the electronics count a frequency rather than read a displacement, the type tolerates temperature and pressure that would shorten the life of a mechanical meter.
That is why it is the common answer for steam. Saturated steam, superheated steam, compressed air, nitrogen and most process gases are measured with a vortex meter wherever the line is a clean single-phase stream, and the same instrument will measure low-viscosity liquids such as water and condensate. Where a plant meters a boiler header, a dyehouse or a compressor house, it is usually this type doing the metering.
This page covers how one is configured: what the body, the bore and the process connection are being asked to survive, when temperature and pressure compensation has to be added before the figure means anything, and which of the neighbouring flow technologies takes over when the medium is not a clean single-phase stream.
How a vortex flow meter measures
The bluff body is a deliberately unstreamlined obstruction. As the stream passes it, vortices are shed alternately from the two sides, and each shed vortex removes a small packet of pressure from the edge behind it; a sensor in the body detects the alternation as a frequency. Across the band of Reynolds numbers the type is designed for, that frequency is proportional to velocity, so the meter converts it into a volumetric rate using the bore area. The proportionality belongs to the geometry of the body rather than to the particular gas, which is why one body can be offered for several media.
Three consequences follow. The meter needs a single-phase stream, because a vortex shed by a liquid is disturbed by droplets and a vortex shed by a gas is disturbed by condensate, so wet steam and two-phase flow are outside the type. It needs a velocity inside its band, because at the low end the shedding stops being regular and at the high end erosion of the body sets the ceiling. And it needs a settled, swirl-free profile at the body, because the frequency is a velocity measurement in disguise and any disturbance upstream is read as a change in flow.
The signal being a frequency is the practical reason the type survives conditions that defeat others: a frequency does not drift the way an analogue span does. What it is not immune to is the medium changing state inside the meter, which is why a steam duty is normally paired with compensation rather than measured raw.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Bore and body style | Flanged, wafer and insertion variants across a range of nominal sizes | Whether the line has to be opened for a full-bore body or can take an insertion type, and whether the velocity at the duty falls inside the band the body works in. |
| Body and wetted material | Carbon steel and stainless steel as the common starting points | Whether the body survives the medium and the temperature, which is settled before accuracy is discussed at all. |
| Medium family | Steam, gas or liquid, which the meter is ordered for rather than changed afterwards | The calibration and the compensation arrangement. The same body measured raw is a different instrument from the same body compensated for steam. |
| Temperature and pressure compensation | Standard, temperature only, pressure only, or both, in an integral or a separate arrangement | Whether the output is a true mass or standard-volume figure or only a volumetric one. A gas or steam figure without compensation is a volume at line conditions, which is not what most accounting needs. |
| Output and communication | Pulse, current, serial and HART options | Whether the signal is counted as a total, shown as a rate, or read by a control system. |
| Power supply | Loop-powered from low-voltage DC, or from an internal battery | Whether a supply exists at the point of installation, and how long the instrument can be left unattended. |
| Process connection and flange standard | The flange standard, pressure class and facing of the existing pipework | Whether the meter bolts on directly or needs adapters and a second sealing interface. |
| Ambient duty and mounting | Vibration level, insulation, and whether the head has to be reached for reading | Whether a heavier body is warranted, and how the enclosure is oriented, since a head sitting above a hot line ages faster than one set to the side. |
How to choose a vortex flow meter
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| Whether the stream is single phase | Wet steam, entrained droplets and two-phase mixtures disturb the vortex and the reading with it | State whether the steam is dry and whether a separator exists upstream, and whether the gas can condense in the line. |
| Minimum, normal and maximum flow | The low end decides the bore, not the maximum, because a bore sized on the peak leaves the normal duty at the bottom of the band | Give all three figures, and the hours a day each applies. |
| Medium temperature and pressure | These decide the body rating and, for steam and gas, whether compensation is required before the figure means anything | State the working figures as well as the design figures, and whether the condition varies. |
| Pipe size and the velocity it implies | The body is chosen so the duty sits inside the working band, which may mean a bore different from the pipe | Give the pipe outside diameter and wall thickness, not the nominal size alone. |
| Straight pipe run available | Swirl from an elbow, a partly open valve or a reducer is read as a change in flow | Send a sketch of what is immediately upstream and downstream of the proposed point. |
| Vibration at the point | Pipe vibration at the shedding frequency is indistinguishable from flow | Describe the line and what it is attached to, and whether the meter can be moved. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a vortex flow meter usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a vortex meter |
| Electromagnetic flow meters | Sits wetted in the line and measures only conductive liquids. It cannot measure steam or gas at all, which is usually the duty a vortex meter is bought for; conversely it does not care whether the liquid is clean, so slurry and dirty water go to it rather than to a body that would erode. |
| Ultrasonic flow meters | Mounted outside the pipe or inserted through it, so the line need not be opened and the medium need not be conductive. Clamp-on types suit retrofits and large existing mains, but their reading depends on the pipe wall and the acoustic coupling, and they are not the usual answer for steam. |
| Thermal mass flow meters | Reads gas flow from heat transfer at low velocities, which is exactly where a vortex body stops shedding reliably. It is a gas-only instrument and it reads mass rather than volume. |
| Turbine flow meters | Counts a rotating element in the stream, accurate on clean low-viscosity liquids and widely used for batch dispensing. Bearings and blades are what make it intolerant of steam, dirt and dry gas. |
| Variable area flow meters | Gives a local visual indication without power, simple and inexpensive on small purge and utility lines. Ill-suited to opaque media, to totals, and to anything read remotely. |
| Coriolis mass flow meters | Measures mass flow and density directly, which is what custody transfer and high-value dosing actually need. Pressure drop and cost both rise steeply with bore, so it is not the general answer for a large steam header. |
The vortex meter and the thermal mass meter are the two types that measure gas without a moving part; the boundary between them is velocity rather than medium.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The medium, and whether it is steam, gas or liquid, and whether the steam is dry
- Minimum, normal and maximum flow, with the hours a day each applies
- Medium temperature and pressure in service, and whether they vary
- Pipe outside diameter, wall thickness and the flange standard it must match
- Whether the figure required is a volume at line conditions or a compensated mass or standard volume
- Output required, and what will read it
- Power available at the point of installation
- Straight pipe run available upstream and downstream
- Ambient conditions, including vibration and whether the line is insulated
- Quantity, destination port and required delivery date
