What are Venturi Flow Meters, and How Do They Work?

Wherever you find a pipeline today, you’ll typically find a Venturi flow meter within it. Venturi flow meters accurately measure flow rates in a broad range of pipe sizes, whether that pipeline carries gasoline, water, or a slurry mixture.

On the extreme end are Venturi flow meters that measure sewage discharge at sprawling wastewater treatment plants. For example, Primary Flow Signal (PFS) installed a rectangular Halmi Venturi flow meter at the Blue Plains Advanced Wastewater Treatment Plant in Washington, DC., purported to be one of the world’s largest treatment plants. Its capacity is 370 million gallons per day.

The Italian physicist Giovanni Battista Venturi first observed a change in fluid pressure as it flowed through a pipe with a constricted section or chokepoint. This phenomenon became known as the Venturi effect. According to Instrumentation Tools, “A fluid’s velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy. Any gain in kinetic energy a fluid may accrue due to its increased velocity through a constriction is balanced by a drop in pressure.”

The Venturi’s flow meter’s operation is connected to the famous Bernoulli equation. “Bernoulli’s Equation (or Bernoulli’s principle) determines fluid velocities through pressure measurements. According to Fluid Handling Pro, “It starts with qualifications of non-viscous, steady, incompressible flow at a constant temperature.” Engineers can measure the change in pressure using Venturi meters and nozzles, which tell you the flow rate. The meter can measure the fluid flow and reduce its flow when necessary. Fluid flow is calculated when the meter checks the pressure difference between the two readings and the temperature.

In modern applications, the principal advantage of the Venturi flow meter is its long life expectancy of the meter body. Some global water and wastewater utilities still rely on 100-plus-year-old meters, particularly in line sizes above 48 inches.

While there’s a clear advantage to a century-long life expectancy, many engineers wonder how accurate these meters are after 30, 40 or more years of service. The answer to the question: “It depends.” It depends on specific conditions (piping), minimum to maximum flow rate range, water chemistry, secondary instrumentation (type of differential pressure transmitter), the design of the Venturi meter and the selection of materials it’s manufactured from.

Selecting the Right Venturi Flow Meter for Your Application

Typically, engineers begin their selection process around the liquid type. What’s the material, viscosity, fluid temperature and expected pressure drop? Additionally, what are the expected minimum and maximum flow rates?

Here’s just a sample of the many Venturi flow meters PFS designs and builds:

HVT-FV Fabricated Venturi
The HVT Fabricated Venturi Flow Meter offers substantial design and engineering flexibility with end arrangements. (flanged, mechanical joint, weld end, etc.) combined with various body materials and coatings to achieve pressure, temperature, flow range, line fluid, line size, laying length and cost objectives dictated by the specific applications

HVT-CI Cast Iron/ HVT-DI Ductile Venturi
The HVT-Halmi Venturi Flow Element in Cast Iron and Ductile Iron form is offered exclusively by Primary Flow Signal, Inc. for the highest accuracy and reliability for flow measurement of pressurized line fluids, including liquids and gases, as well as high-viscosity line fluids, solids-bearing line fluids, and harsh or contaminated line fluids.

HVT-SMS Sealed Meter System
The PFS Sealed Meter System (SMS) provides accurate and reliable measurements for applications that contain contamination, such as sewage or sludge, without the concern for plugged impulse lines. One of the most significant advantages this system offers is performing maintenance and calibration services without removing any system parts or stopping the flow. You can also extend the minimum to maximum flow rate range by using dual or triple DP transmitters while maintaining accurate and repeatable flow rate documentation.

PFS can assist you with an analysis of your existing venturi flow meter accuracy, no matter who the manufacturer was or how old the meter is – contact PFS for assistance.