Tutorial: Vortex shedding flowmeters

Cost-effective and reliable flowmeter technology retains a high position in process plant toolboxes.

09/18/2008


One technology that scored well in our recent Product Research on flowmeters that we have yet to cover here is vortex shedding. This type of device calculates liquid, gas, or steam flow by observing the vortices formed when a fluid stream passes an obstruction with known characteristics.

When a bar (called a bluff body) is inserted across the interior of a pipe, fluid flowing through the pipe has to pass on one side or the other. As it passes, vortices form on both sides of the body. The frequency of these vortices is determined by the velocity of the fluid stream. Instrumentation devices use a detector to count the frequency, such as strain gages, ultrasonic, capacitance, magnetic, or other types of sensors to generate electrical pulses from the liquid vibrations. The transmitter counts these pulses and converts the reading to a stream velocity from which it calculates fluid volume.

This approach has many advantages:

  • Accuracy and turn-down ratio are sufficient for most general process applications;

  • Relatively inexpensive;

  • Wide range of sizes available;

  • Durable and not prone to drift;

  • Stable, so they need little calibration or maintenance;

  • Good for gas, steam, and liquid;

  • Most can mount in any position so long as the pipe is full;

  • No moving parts;

  • Works with conductive or non-conductive liquids; and

  • Configuration options include spool or wafer designs, and insertion styles for larger sizes.

However, there are some downsides you should keep in mind:

  • Any design that introduces an obstruction causes a pressure drop and creates clogging potential. The extent varies between manufacturers;

  • Solids depositing on the bluff bar can disrupt accurate readings;

  • Two-phase flows and slurries are not well-suited for this kind of flowmeter;

  • Over time, the bluff bar’s shape may change from wear which can cause drift;

  • Liquid viscosity is generally limited to 30 cp;

  • Specific designs can be dedicated either to gas or liquid, but not both; and

  • Ensure sufficient up- and down-stream straight pipe to control turbulence.

Some of the more sophisticated designs add sensors for variables such as fluid temperature and pressure. These readings are often very useful, and can be used to calculate a mass flow reading when known fluid characteristics are added.

These discussions are general, so review specifics with your supplier.

Vortex shedding flowmeters are manufactured by many companies, including:

Aalborg
Cancoppas
GE Sensing
Honeywell PS
Racine Federated
Rosemount (Emerson PM)
Universal Flow Monitors

You can also search online at the Control Engineering Supplier Search .

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
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