Vortex Flow Meters

What Is a Vortex Flow Meter?

A vortex flow meter is a volumetric flow meter that determines flow velocity by counting the vortices that detach at regular intervals from a bluff body placed in the stream. In sufficiently turbulent flow, the shedding frequency is directly proportional to velocity, so the instrument works on the same basic principle whether the medium is a liquid, a gas or steam.

Each meter body is calibrated with water at the factory and given its own calibration coefficient, known as the K-factor. The K-factor is determined by the geometry of the body and the shedder bar; since it does not depend on fluid density or viscosity, a calibration carried out with water also holds in steam and gas service. The electronics automatically compensate the K-factor for the effect of process temperature on the body material.

In process instrumentation, vortex flow meters are used above all in steam distribution lines, boiler houses, utility systems such as compressed air and nitrogen, and hydrocarbon processes. Unlike differential pressure flow measurement, they require no impulse lines, no manifold and no separate differential pressure transmitter. For accurate measurement it is sufficient that the fluid is homogeneous and single-phase and that the flow stays above the minimum Reynolds number.

The Kármán Vortex Street: How Vortex Flow Measurement Works

Vortex measurement is based on a well-known phenomenon of fluid mechanics: behind a bluff body in the flow path, vortices break away alternately from one side and then the other, forming a regular vortex pattern. The measuring chain works in these steps:

Flow Separation at the Shedder Bar

Flow meeting the shedder bar mounted across the pipe bore separates at both edges of the bar. The separated flow generates vortices that break away alternately downstream of the bar; fluid mechanics calls this regular sequence the Kármán vortex street.

Relationship Between Shedding Frequency and Flow Velocity

The shedding frequency (f) is directly proportional to the flow velocity (v) and inversely proportional to the bar width (d): f = St · v / d. Because the Strouhal number (St) remains practically constant over the measuring range, the frequency is a direct measure of velocity; multiplying velocity by the pipe cross-section gives the volumetric flow rate.

Detecting the Oscillations with the Sensor

The shed vortices exert alternating forces on the bar. These oscillations are picked up by a sensor that has no contact with the process and sits outside the process seal boundary. A mass-balanced sensor design, combined with adaptive digital signal processing, helps to distinguish pipe vibration from the genuine flow signal.

Calculating Volumetric and Mass Flow

The electronics convert the frequency into volumetric flow by means of the K-factor. In multivariable versions, a temperature sensor fitted in the shedder bar — together with pressure data from an external pressure transmitter where required — is used to calculate density and hence mass flow. Results are transmitted via 4–20 mA/HART, pulse, FOUNDATION Fieldbus or Modbus.

Reynolds Number and Measurable Flow Range

The linear relationship between frequency and velocity depends on the flow being sufficiently turbulent. The stated volumetric accuracy applies above a Reynolds number of 20,000 for liquids and 15,000 for gas and steam. As the Reynolds number drops towards 10,000, the error limit increases linearly to ±2%, and between 10,000 and 5,000 it widens to a band of ±2–6%; for sizes from 15 to 100 mm the minimum measurable Reynolds number is 5,000.

In most field applications the practical lower limit is a flow velocity of about 0.2 m/s in liquids and about 2 m/s in gas or steam. This is why vortex meters are frequently selected one or more sizes below the line size. The exact minimum and maximum flow, pressure loss and expected accuracy should be established by a sizing calculation that takes the density and viscosity of the fluid into account.

Mounting orientation has no effect on accuracy: the meter keeps its specified accuracy in horizontal, vertical and inclined lines. In horizontal piping, the shedder bar should be positioned horizontally so that solids carried by liquids, or droplets entrained in gas and steam lines, cannot disturb the shedding frequency.

Design and Functional Features

·         Gasket-free, fully welded body: The body contains no ports, gaskets or cavities that could plug, which reduces potential leak points. In the weld-end version the flange gaskets disappear as well, because the body is welded straight into the process pipe.

·         Isolated, replaceable sensor: The sensor is not wetted by the process and can be exchanged while the process seal remains intact. The same sensor design is used in every line size, so one spare sensor can support meters of different diameters.

·         Sensor replacement with the process running: An isolation valve option offered for critical applications lets the user confirm that there is no process fluid in the sensor cavity and replace the sensor online, without the need for a bypass line.

·         Multivariable output: A Type N thermocouple that uses the shedder bar as its thermowell allows temperature-compensated mass flow to be calculated for saturated steam and liquids. With an external pressure transmitter operating in HART burst mode, pressure compensation is available for saturated steam and combined temperature and pressure compensation for superheated steam.

·         Low-superheat alerting: In superheated steam service, an alert or alarm can be raised when the degree of superheat approaches saturation, so that deteriorating steam quality is recognized early.

·         Reducer body: Reducing flanges are built into the body, making the measuring section one nominal size smaller than the line. As it shares the face-to-face dimension of the standard body, it can be installed without altering the pipe layout, and no reducers have to be fitted on site.

·         Dual-sensor and quad configurations: The dual-sensor body serves safety applications with redundant electronics and sensors. In the quad configuration, two shedder bars and four transmitters provide 2oo3 voting together with an independent output for process control.

·         Remote electronics option: At measuring points with high temperatures or difficult access, the transmitter can be mounted at a separate location with a standard or armored signal cable; standard lengths are 3, 6, 10, 15 and 23 m.

·         Phase-change detection diagnostic: An optional process diagnostic recognizes when the fluid changes from liquid to gas and issues an alert via the digital, analog or pulse output — useful on separator water dump lines and in blowdown cycles.

·         In-situ verification and operating-hours counter: Device diagnostics use an internal signal generator to verify the electronics and sensor in the field without stopping the process. An optional operating-hours counter records the total running time.

·         Wide range of bodies and connections: Wafer, flanged, reducer, weld-end, threaded-end, dual-sensor, quad and high-pressure body options make it possible to match different piping standards.

Vortex Flow Meter Specifications

The values below apply to the vortex flow meter family described on this page. Accuracy, velocity limits and temperature ranges vary with body style, line size, wetted material and selected options; final values should be confirmed by a sizing calculation and at project stage.

The maximum measurable velocity and the permanent pressure loss depend on fluid density and are calculated for each application with sizing software. Hazardous-area approvals may narrow the process and ambient temperature limits.

Key Advantages

·         Lower risk of leaks and plugging: A welded design without ports, gaskets or impulse lines reduces potential leak points and plugging-related failures in steam and hazardous-fluid lines.

·         One calibration for three fluid phases: Thanks to a K-factor that is independent of density and viscosity, the same meter can be used in liquid, gas and steam service; no periodic zero adjustment is needed.

·         A simpler measuring point: The impulse lines, manifold and differential pressure transmitter of orifice metering systems are not required; the reducer body can eliminate on-site reducer fitting, and the multivariable output can remove the need for a separate temperature measurement.

·         Process continuity: Because the sensor can be exchanged without opening the process seal, maintenance and verification work rarely requires draining the line or shutting down a plant section.

·         Suitability for safety systems: The IEC 61508 certified 4–20 mA output and the dual-sensor and quad configurations make it possible to build architectures that meet SIL 2 and SIL 3 requirements in safety instrumented functions.

·         High pressure and wide temperature capability: Connections up to ASME Class 1500 and PN 160, with temperature options reaching down to –200 °C and up to +450 °C, suit the meter to demanding conditions in energy and hydrocarbon processes.

·         Stability in vibrating installations: The mass-balanced sensor and adaptive signal processing limit the influence of pipe vibration from pumps and compressors on the measurement.

·         Smaller spare-parts inventory: With a common sensor design across all line sizes, a single type of spare sensor may be enough for many measuring points of different diameters.

Steam, Gas and Liquid Applications

Vortex flow meters cover a wide field — from general-purpose lines with homogeneous, single-phase flow at an adequate Reynolds number to critical applications with high temperature, high pressure and safety requirements.

Steam Generation and Distribution

Mass flow is calculated with temperature compensation for saturated steam and with temperature plus external pressure measurement for superheated steam; low-superheat alerting gives early warning of deteriorating steam quality. ASME B31.1 compliance options are available for boiler external piping. Where a body has no integrated temperature sensor, a separate measurement can be made with temperature transmitters.

Utilities and Industrial Gases

The lightweight, economical wafer body is a practical choice for utility lines carrying compressed air, nitrogen and water. For large-diameter or very low-pressure gas lines, ultrasonic gas flow meters can be considered as an alternative.

Oil and Gas Production

Gas passing through a separator water dump line can be detected remotely with the phase-change diagnostic. Materials meet the NACE MR0175/ISO 15156 recommendations for oilfield production environments containing H₂S; a certificate of compliance is available as an option.

Refinery Processes

Materials compliant with NACE MR0103 for corrosive refinery environments, nickel alloy and super duplex wetted-material options, and connections up to ASME Class 1500 allow use in high-temperature, high-pressure hydrocarbon lines.

Safety Instrumented Systems

Where redundancy is critical, the dual-sensor body is preferred; for functions requiring 2oo3 voting, the quad configuration is used. The IEC 61508 certified 4–20 mA output can be used in safety functions up to SIL 3.

Blowdown and Clean-in-Place Cycles

In blowdown and clean-in-place (CIP) cycles using steam, nitrogen or air, a single in-line vortex flow meter measures the flow of the main process fluid and also detects the change from liquid to gas, providing a signal for cycle control.

Oxygen Service and Low-Temperature Applications

A special cleaning option is offered for oxygen service. With suitable wetted materials, the extended temperature range option allows measurement of low-temperature fluids approaching –200 °C.

Vortex Flow Meter Sizing and Selection Criteria

With vortex flow meters, a good result starts with correct sizing: by choosing a smaller body or a reducer body for the same line size, the measuring range is matched to the actual flow band of the process. For a technical assessment we recommend preparing the following information:

Fluid and Process Conditions

·         Fluid and phase: liquid, gas, saturated or superheated steam (the fluid must be homogeneous and single-phase)

·         Density and viscosity at operating conditions (for the Reynolds number calculation)

·         Minimum / normal / maximum flow, temperature and pressure

·         Cavitation risk in liquids: keeping line pressure above a minimum based on the pressure loss (ΔP) and vapor pressure (pv), e.g. 2.9 × ΔP + 1.3 × pv; a back-pressure valve if necessary

·         Corrosive components (H₂S, chlorides etc.) and wetted-material requirements

Sizing and Measuring Range

·         Minimum flow remaining above the practical lower limit (approx. 0.2 m/s for liquid, approx. 2 m/s for gas/steam) and the minimum Reynolds number

·         Need for a body smaller than the line size or a reducer body

·         Upper limit for accurate measurement at maximum flow and acceptable permanent pressure loss

·         If mass flow is required: temperature, pressure or combined compensation

Installation and Site Conditions

·         Upstream / downstream straight pipe runs (35D / 5D ideal; at least 10D upstream)

·         Mounting orientation: shedder bar positioned horizontally in horizontal lines

·         Level and source of pipe vibration (pump, compressor, control valve)

·         Remote rather than integral electronics at high process temperatures, and cable length

·         Hazardous-area classification and the approval type required

Function, Redundancy and Safety

·         SIL requirement: the IEC 61508 certified output is available only with 4–20 mA/HART and cannot be combined with the multivariable options or the phase-change diagnostic

·         Redundancy need: dual-sensor body or quad configuration with 2oo3 voting

·         Need for sensor replacement with the process running (not available for wafer bodies and some size/material combinations)

·         Compatibility of the multivariable output with body style and size (40–300 mm for flanged bodies, 50–300 mm for reducer bodies)

Integration and Documentation

·         Control system interface: 4–20 mA/HART, FOUNDATION Fieldbus or Modbus RS-485

·         Pressure transmitter supporting HART burst mode for pressure compensation

·         Need for NAMUR-compliant alarm and saturation levels

·         Pressure equipment, NACE, ASME B31.1 and material / calibration certificate requirements

For two-phase, pulsating or very low-Reynolds-number flows, keep in mind that a technology other than vortex may be more suitable; in these cases Coriolis mass flow meters or differential-pressure-based solutions can be evaluated as well.

Functional Safety, Hazardous-Area Approvals and Standards

The main certification, approval and compliance options available for the vortex flow meter family are listed below. The scope of each approval should be confirmed together with the output type and selected options.

The volumetric accuracy values do not imply legal metrology approval for fiscal or trade metering; such requirements must be assessed separately for each project. Limits depending on temperature class and protection type should be confirmed against the selected approval code.

TLY Enerji Engineering Support for Your Vortex Measuring Points

How well a vortex flow meter performs in the field depends largely on sizing: a body chosen at the same size as the line may drop out of its measuring range at low flow. TLY Enerji considers fluid properties and actual operating flows together, and assesses body size, reducer versus standard body, Reynolds number, pressure loss and cavitation risk.

We configure multivariable measurement and external pressure transmitter integration for steam systems, redundant architectures and SIL documentation for safety applications, and approval selection for hazardous areas according to project requirements. We support site teams with installation details, DCS/PLC integration, verification of configuration and low-flow cutoff settings during commissioning, and with sensor replacement and diagnostic evaluation during operation.

Vortex Flow Meter FAQ

How does a vortex flow meter work?

When the flow meets the shedder bar mounted across the pipe bore, vortices break away alternately behind the bar. The shedding frequency is directly proportional to flow velocity. A sensor that has no contact with the process detects the oscillations these vortices cause on the bar; the electronics convert the frequency into volumetric flow using the factory-determined K-factor and transmit it via a 4–20 mA/HART, pulse, Fieldbus or Modbus output.

How is steam mass flow measured with a vortex flow meter?

In multivariable versions, a Type N thermocouple fitted in the shedder bar measures the process temperature, the density of saturated steam is calculated and mass flow is derived; typical accuracy with this method is ±2.0% of reading. When pressure is received from an external pressure transmitter via HART burst mode, pressure compensation for saturated steam and combined temperature and pressure compensation for superheated steam become possible; with pressure compensation the accuracy improves to ±1.3%.

Can a vortex flow meter measure low flow rates?

Vortex measurement has a lower limit: in practice it needs a flow velocity of about 0.2 m/s for liquids and about 2 m/s in gas and steam service, and a minimum Reynolds number of 5,000 for 15–100 mm sizes. For low flows, the body is selected smaller than the line. For example, a 50 mm flanged body measures water over approximately 1.81–59.4 m³/h, whereas a reducer body in the same line lowers the minimum to about 1.10 m³/h.

What are the benefits of a vortex meter with a reducer body?

In a reducer body, the reducing flanges are integral and the measuring section is one nominal size smaller than the line. Flow velocity therefore increases at low flow rates, extending the measurable range, and no reducers or extra pipe spools have to be installed on site. As it has the same face-to-face dimension as the standard body, the meter can be changed when process conditions change without affecting the pipe layout or drawings.

How much straight pipe does a vortex flow meter need?

Rated accuracy depends on the length of straight pipe from the nearest flow disturbance. With 35 pipe diameters upstream and 5 pipe diameters downstream, no K-factor correction is needed. If the upstream run is shortened to the recommended minimum of 10 pipe diameters, the K-factor can shift by up to 0.5%; this effect can be compensated by applying a correction for installation effects.

Can a vortex flow meter be used in SIL applications?

Yes. The 4–20 mA output is certified to IEC 61508 by an accredited third-party body and is suitable for safety functions up to SIL 2 in single (1oo1) use and up to SIL 3 in redundant (1oo2) use. The dual-sensor body provides a redundant arrangement, while the quad configuration provides 2oo3 voting. The safety-certified output is not offered in combination with Fieldbus or Modbus outputs or with the multivariable options.

Does the process have to be shut down if the sensor fails?

No. The sensor has no contact with the process and sits outside the process seal boundary, so it can be replaced without draining the line or opening the process seal. The isolation valve option for critical applications allows the sensor cavity to be checked for fluid and the sensor to be exchanged online without a bypass line. This option is not available for wafer bodies or for some size and material combinations.

Does pipe vibration affect vortex flow measurement?

Strong vibration can create a false flow signal when there is no flow. A mass-balanced sensor design and adaptive digital signal processing keep this effect to a minimum, and the factory settings are adequate for most applications. If a signal appears at zero flow, the low-flow cutoff, the trigger level or the low-pass filter setting can be adjusted. Once flow starts, the vibration effect generally remains well below the flow signal.

Can a vortex flow meter be used in two-phase flow?

Accurate volumetric measurement requires a homogeneous, single-phase fluid; accuracy is not guaranteed for wet steam, liquids containing gas bubbles or gases carrying liquid droplets. The optional phase-change diagnostic can, however, detect a transition to gas in a liquid line and raise an alarm. Where two-phase flow is continuous, a different measuring technology should be considered.

What is the difference between a vortex flow meter and an orifice meter?

Orifice metering measures the differential pressure across a restriction element with impulse lines and a differential pressure transmitter. A vortex flow meter measures the shedding frequency directly; it needs no impulse lines, manifold or separate transmitter, requires no zero adjustment and usually offers a wider measuring range. On the other hand, it has minimum Reynolds number and velocity limits, so the choice should take process conditions and applicable standards into account.