— A Practical Guide for Engineers, EPCs, and Project Teams

Vortex flowmeters are among the most versatile flow measurement technologies in the process industries. They measure steam, gas, and low-viscosity liquids with a single device, have no moving parts, and offer wide turndown and low pressure loss. Yet the same versatility that makes them attractive also makes them easy to misapply. A vortex flowmeter sized for one medium can be completely unsuitable for another. A meter that works perfectly on compressed air can fail on saturated steam. A meter installed without proper straight pipe runs can produce errors that no amount of calibration will correct.

This guide explains how to select vortex flowmeters for steam, gas, and liquid applications—covering the principles of operation, the medium-specific considerations that drive selection, sizing calculations, installation requirements, and the common mistakes that lead to poor performance.

1. How Vortex Flowmeters Work

The vortex flowmeter operates on the Kármán vortex street principle. A bluff body (also called a vortex shedder) is placed in the flow stream. As fluid passes around the bluff body, alternating vortices are shed from each side. The frequency of vortex shedding is directly proportional to the flow velocity:

$$f=\frac{St\times v}{d}$$

Where:

• $f$ = vortex shedding frequency

• $St$ = Strouhal number (a dimensionless constant for a given bluff body geometry)

• $v$ = fluid velocity

• $d$ = bluff body width

A piezoelectric or capacitive sensor detects the pressure pulses generated by the vortices. The flowmeter converts this frequency into a flow rate.

The critical insight: The Strouhal number is constant only within a specific Reynolds number range—typically Re > 10,000 (some sources state Re > 20,000 for optimal linearity)-. Below this threshold, the relationship between shedding frequency and velocity becomes non-linear, and measurement accuracy degrades significantly.

2. The Medium Matters: Steam, Gas, and Liquid Are Not Interchangeable

The three media categories—steam, gas, and liquid—impose fundamentally different requirements on vortex flowmeter selection. Understanding these differences is the foundation of correct selection.

2.1 Steam: The Most Demanding Application

Steam is the most challenging medium for vortex flowmeters because its density varies dramatically with both temperature and pressure.

Saturated steam: Density depends on either temperature or pressure (they are directly related). A temperature-compensated vortex flowmeter with a built-in Pt100 sensor and a saturated steam density table can automatically correct for density changes-.

Superheated steam: Density depends on both temperature and pressure. This requires both temperature and pressure compensation—either through an integrated multivariable transmitter or through external pressure and temperature inputs.

Why compensation is mandatory: Without compensation, a vortex flowmeter measures volumetric flow at actual conditions. Steam volumetric flow changes dramatically with pressure and temperature, even when the mass flow rate is constant. A 10% pressure variation can cause a 10% error in mass flow calculation if not compensated-2.

Key accuracy figures:

• Volumetric flow (steam, gas): ±1.0% of reading-

• Mass flow (saturated steam, temperature-compensated): ±1.7% of reading

• Mass flow (saturated steam, temperature/pressure compensated): ±1.5% of reading-

Wet steam consideration: Wet steam containing less than 2% water droplets by mass is considered dry saturated steam. Beyond this threshold, the liquid droplets can cause signal noise and measurement errors. Vortex flowmeters are generally not suitable for wet steam with significant liquid content-52.

2.2 Gas: Density Compensation and Standard Condition Conversion

Gas flow measurement with vortex flowmeters requires careful attention to density and standard condition conversion.

Key considerations:

• Pressure effect: Gas density is directly proportional to absolute pressure. A change in line pressure changes the volumetric flow rate even when mass flow remains constant.

• Temperature effect: Gas density is inversely proportional to absolute temperature.

• Standard condition conversion: Most gas flow measurements are reported at standard conditions (e.g., 0°C, 1 atm or 15°C, 1 atm). This requires converting actual volumetric flow to standard volumetric flow using the ideal gas law or a compressibility factor.

Temperature and pressure compensation is mandatory for gas flow measurement. The vortex flowmeter must either integrate pressure and temperature sensors (multivariable) or accept external inputs from separate transmitters.

Accuracy: ±1.0% to ±1.5% of reading for volumetric flow, depending on whether compensation is applied-.

Minimum velocity requirement: Gas applications require a minimum flow velocity to generate measurable vortices—typically 5 m/s or higher-2.

2.3 Liquid: The Simplest Application (with Caveats)

Liquid flow measurement is the most straightforward vortex application because liquid density varies little with temperature and pressure (unless the temperature is near the boiling point).

Key considerations:

• Viscosity: Vortex flowmeters are suitable for low-viscosity liquids (typically < 20 mPa·s). High-viscosity liquids dampen vortex formation and reduce measurement accuracy-.

• Minimum velocity: Liquid applications typically require a minimum velocity of 0.5 m/s to generate stable vortices-2.

• Vapour pressure: For liquids near their boiling point, flashing or cavitation can occur across the bluff body. This must be evaluated during selection.

Accuracy: ±0.7% to ±0.75% of reading for liquids-.

Reynolds number: As with gas and steam, the Reynolds number must exceed 10,000 for linear operation. For high-viscosity liquids, this may require a smaller meter size to maintain the required Reynolds number at low flows.

3. Step-by-Step Selection Process

Step 1: Define the Medium and Its Properties

Step 2: Determine the Flow Range

The single most common selection error is sizing based on pipe diameter rather than flow range.

Turndown expectations:

• Gas and steam: Better than 20:1 with proper sizing-

• Low-viscosity liquids: Better than 10:1-

• Theoretical maximum: 1:100, but practical limits are lower due to low-flow signal-to-noise issues-

Why sizing matters: If the normal flow rate is only 10% of the meter's range, the vortex signal may be too weak to detect reliably. If the flow rate exceeds 80% of range, the pressure loss becomes excessive and the meter may be damaged by erosion. The solution is often to select a meter smaller than the pipe size—typically one or two sizes smaller—to increase velocity through the meter-.

Step 3: Calculate the Required Meter Size

The meter size is determined by the flow range, not the pipe size. The following steps apply:

1. Calculate the expected velocity for the minimum and maximum flow rates using the pipe inside diameter.

2. Compare with the meter's velocity range:

• Liquid: Typically 0.5–6 m/s (some sources allow up to 10 m/s)-37

• Gas: Typically 5–60 m/s-37

• Steam: Typically 7–50 m/s (saturated) and 7–70 m/s (superheated)-37

3. If the velocity is below the minimum, select a smaller meter size.

4. If the velocity is above the maximum, select a larger meter size.

5. Verify the Reynolds number exceeds 10,000 at the minimum flow rate.

Sizing example: A gas line with a DN100 pipe and a flow range of 100–800 Nm³/h. The normal operating pressure is 5 bar(g) and the temperature is 30°C. Calculate the actual volumetric flow at operating conditions, then calculate the velocity in DN100 pipe. If the velocity at minimum flow is below 5 m/s, consider a DN80 or DN50 meter to increase velocity.

Step 4: Select Compensation Requirements

Implementation options:

• Integrated multivariable vortex flowmeter: The meter includes temperature and pressure sensors and calculates compensated mass flow internally.

• External compensation: Separate temperature and pressure transmitters feed a flow computer or DCS, which performs the compensation calculation.

Best practice: For steam and gas applications, integrated multivariable vortex flowmeters are preferred because they eliminate wiring complexity, reduce installation cost, and ensure that compensation is always correctly applied.

Step 5: Select Construction and Materials

Step 6: Verify Installation Requirements

Vortex flowmeters are sensitive to flow disturbances. The installation location must provide sufficient straight pipe runs to ensure a fully developed velocity profile.

Minimum straight pipe requirements (per ISO 12764 and manufacturer recommendations):

Key principles:

• Install upstream of valves and fittings wherever possible-.

• Install in horizontal pipe with the sensor on top for liquid service to prevent gas pockets.

• Avoid vertical downward flow for gas and steam; liquid service can use vertical upward flow.

• Ensure concentric alignment between the meter and the pipe; eccentric installation causes uneven flow distribution-26.

• Support the pipe upstream and downstream to prevent vibration transmission-26.

4. Common Applications and Selection Examples

Example 1: Saturated Steam Header

Application: Measure saturated steam flow in a DN80 header. Pressure: 8 bar(g), temperature: 175°C. Flow range: 200–1,500 kg/h.

Selection:

• Medium: Saturated steam → temperature compensation sufficient

• Meter: Integrated multivariable vortex flowmeter with Pt100 sensor

• Size: Calculate velocity at minimum and maximum flow. If velocity at 200 kg/h is below 7 m/s in DN80, consider DN50 meter

• Compensation: Temperature compensation with saturated steam density table

• Output: 4–20 mA + HART for mass flow

Example 2: Compressed Air

Application: Measure compressed air in a DN50 pipe. Pressure: 7 bar(g), temperature: 35°C. Flow range: 50–400 Nm³/h.

Selection:

• Medium: Gas → temperature and pressure compensation required

• Meter: Multivariable vortex flowmeter with integrated pressure and temperature sensors

• Size: Convert Nm³/h to actual m³/h at operating conditions. Calculate velocity. Ensure minimum velocity exceeds 5 m/s

• Compensation: Both temperature and pressure

• Output: 4–20 mA + HART for standard volumetric flow

Example 3: Cooling Water

Application: Measure cooling water flow in a DN100 pipe. Temperature: 30°C. Flow range: 20–120 m³/h.

Selection:

• Medium: Liquid → no compensation required (temperature is stable and well below boiling point)

• Meter: Standard vortex flowmeter with piezoelectric sensor

• Size: Calculate velocity. At 20 m³/h in DN100, velocity is approximately 0.7 m/s (above the 0.5 m/s minimum)

• Output: 4–20 mA + pulse for totalisation

5. Common Mistakes to Avoid

6. Applicable Standards

7. Why Choose Anhui Tiankang for Vortex Flowmeters?

Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial instruments for nearly five decades. Our vortex flowmeters are designed for reliable performance across steam, gas, and liquid applications.

Vortex flowmeter product portfolio:

Key specifications:

• Sizes: DN15 to DN300 (standard); larger sizes available with insertion type

• Accuracy: ±0.75% of reading (liquid); ±1.0% of reading (gas/steam)-

• Turndown: Up to 20:1 for gas and steam; 10:1 for liquids

• Temperature range: -40°C to +400°C (model-dependent)

• Pressure rating: Up to PN100 (higher on request)

• Body materials: 304 SS, 316L SS, Hastelloy C-276

• Output: 4–20 mA, pulse, RS485/Modbus, HART

• Protection: IP65/IP67; Ex d IIC T6, Ex ia IIC T6

Core advantages:

• Complete certifications: CCC Ex, ATEX, IECEx, SIL

• CNAS-accredited laboratory: Full performance testing including flow calibration

• Engineering support: Sizing calculations, compensation design, installation guidance

• Proven track record: Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects

• One-stop supply: From flowmeters to temperature/pressure transmitters to cables—one supplier, one interface

8. Conclusion

Selecting the right vortex flowmeter is a systematic process that starts with the medium, not the pipe size.

Key takeaways:

The most important rule: A vortex flowmeter is a velocity device. It needs sufficient velocity to generate a measurable signal and sufficient Reynolds number to maintain linearity. If the application cannot provide these conditions at the minimum flow rate, the vortex flowmeter is the wrong choice—no amount of compensation or calibration will fix it.

Remember: The cost of proper selection is a few hours of engineering. The cost of poor selection is a meter that never works correctly, a process that cannot be controlled, and a replacement project that could have been avoided.

Contact Us

For vortex flowmeter selection advice, sizing calculations, or project quotations, please contact:

Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/

Anhui Tiankang – Your partner for reliable vortex flow measurement solutions.