Basics of Flow Meters

Measuring the flow rate of solids, liquids, and gases is referred to as flow measurement, and is a very important and widely used control variable. Many industries such as power, chemical, water, waste-water treatment, energy, mining and petroleum have many requirements for flow measurement and control.

A flow meter is a device that measures the rate of flow or quantity of a moving fluid in an open or closed conduit. There are two basic ways of measuring flow: volumetric basis and weight basis.

Flow measuring devices are generally classified into four groups:

1. Mechanical type flow meters: Fixed restriction, variable head type flow meters using different sensors such as orifice plates, venturi tubes, flow nozzles, pitot tubes, quantity meters such as positive displacement meters, mass flow meters, etc.
2. Inferential type flow meters: Variable area flow meters (Rotameters), turbine flow meters, target flow meters, etc.
3. Electrical type flow meters: Electromagnetic flow meters, Ultrasonic flow meters, laser doppler anemometers etc.
4. Other flow meters: Purge flow regulators, flow meters for solids flow measurement, cross-correlation flow meter, vortex shedding flow meters, flow switches, etc.

Flowmeters need to be integrated into existing piping or new installation. There are two methods for flowmeter installation: inline and insertion. With the inline method, connectors are provided for upstream and downstream pipes. For the insertion method, a sensor probe is inserted into the pipe.

Most flowmeters are installed with straight sections of pipe on either side for flow to normalize. For the inline method, the diameter of pipes should be same as the flowmeter size. Insertion design is easier to install and more economical in large diameter pipes.

To select the suitable flowmeters many factors should be taken in mind. The most important is fluid phase (solid, liquid, gas, steam) and the other is flow condition (clean, dirty, viscous, open channel etc.). The next important factor is line size and flow rate. Other properties that will affect the selection of flowmeter are density, pressure, temperature, viscosity etc. You should consult an application engineer before specifying a flow meter to assure proper installation, lowest installed cost and safety.

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