How to Select Flow Meters

Image Credit: Aalborg Instruments, Clark Solutions

Flow meters are devices for measuring the flow rate or quantity of a moving liquid or gas. Industrial processes often require these meters for accurate flow measurements. They allow operators to properly monitor and control process flow and to more easily detect system failures. Flow meters can also be used to assist in the design of various types of water systems and facilities.

The basis of good flow meter selection is a clear understanding of the parameters surrounding a particular application. These may include fluid type, fluid viscosity, possible contaminants or corrosive elements, pipe or system size, etc. Flow meters can be classified based on type, performance specifications, and features.

Types of Flow Meters

There are four basic categories of devices: differential pressure (DP), positive displacement (PD), velocity, and true mass. 

• Differential pressure flow meters obtain the flow rate by measuring the pressure differential and extracting the square root. Choices for DP meters include cone-type devices, elbow tap meters, flow nozzles, laminar flow elements, orifice plates, Pitot tubes, rotameters, target meters, variable area flow meters, and Venturi tubes.
• Positive displacement flow meters divide the media into specific increments which can be counted by mechanical or electronic techniques. Examples of PD meters include nutating disc devices, oval gear meters, and piston-based designs.
• Velocity flow meters measure velocity (v), which operates linearly with respect to the flow rate (Q) based on the relationship with vessel area (A):

Q = v x A

Because there is no square-root relationship, their range is greater than DP devices. Choices for velocity meters include electromagnetic meters, paddlewheels, sonar-based devices, turbine meters, ultrasonic meters and vortex or shedding meters.

• True mass flow meters are used to directly measure the mass rate of flow. These flow meters include both thermal meters and Coriolis meters.

Performance Specifications

The GlobalSpec SpecSearch Database provides industrial buyers the ability to select flow meters based on performance specifications. These include the flow rate range, operating temperature, and operating pressure.

• Flow rate range is the range of fluid flow that the device is designed to measure. Mass, velocity, or volumetric flow rates above or below the specified values may not be measurable by the device and could damage the instrument or its calibration.
• Operating temperature is the accepted range of temperatures in which the device can properly operate. Handling media outside this range could damage the instrument.
• Operating pressure is the accepted range of pressures in which the device can properly operate. Pressures below this range may not be measurable and those exceeding this range could cause device failure.

Features

Flow meters can include a number of additions to make the device compatible with the system. These include mounting types, various end fittings, interface options, and electrical outputs. They may also include a number of different features to offer additional measuring capabilities such as temperature or density.

References

Engineering Toolbox - Types of Fluid Flow Meters

OMEGA Engineering - Introduction to Flow Meters

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