Saturday, August 2, 2014

Measuring Flow - The Basics

(courtesy of Azbil)
Flow measurement, the measuring of a flow rate for gasses, liquids and solids,  is used throughout process applications in power generation, chemical manufacturing, petro-chemicals, pulp and paper, water and wastewater, bio-science, semiconductor and many other manufacturing processes. There two primary ways to measure the flow: volumetrically and by weight.

Flowmeters are used to measure the rate of flow (or quantity) of material in an open or closed system. They normally are installed on piping systems with the flow sensor immersed in the process medium.

Flow measuring devices are categorized in (4) ways:

  1. Inferential Types: Such as variable area flowmeters (rotameters), target flow meters and turbine flow meters.
  2. Electrical Flow Meters: Such as electromagnetic flow meters, ultrasonic flowmeters and laser doppler anemometers.
  3. Mechanical Type: Such as orifice plates, venturi tubes, flow nozzles, pitot tubes, positive displacement meters and mass flow meters.
  4. Other: Such as vortex shedding flow meters, cross-correlation flowmeters, purge flow regulators, flow meters for solids flow measurement and flow switches.
Flowmeters can be integrated into existing piping systems or installed on new lines either inline, or insertion. Inline flowmeters mount in the piping system using an downstream and upstream connection. Immersion flowmeters use a probe, or sensor, placed in the flow stream.

For best sensing, flowmeters should be installed on piping systems with straight runs. Different manufacturers recommend different lengths of straight runs  upstream and downstream of the flowmeter for an optimum flow profile.

For proper selection criteria, you should always know the physical state of the process media ((solid, liquid, gas, steam), the condition of the media (clean, dirty, viscous, corrosive, flammable), piping size and flow rates, and finally process pressures and temperatures.

For best performance and maximum safety, an application expert should always be consulted before specifying a flowmeter.