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Selecting the Right Flow Meter for the Job

A flow meter is simply instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. When selecting the correct flow meters for any particular need you first need to evaluate the nature of the process fluid and of the overall installation. A few other things to consider are existing knowledge of plant personnel and their experience with calibration and maintenance, the availability of spare parts and average time between failure history. After considering these issues, then consider installation cost, though this issue is not as important as the first four, as you will see.

For flow sensor selection, will you need continuous or totalized flowrate information? Will you need this information locally or remotely? If it’s done remotely, you need to know if the transmission is analog, digital, or shared. If it happens to be shared, what will the required (data-update frequency be?

Next, look at the piping that will accommodate the flowmeter. Also, check the properties and flow characteristics of the process fluid. 

Take a look at the fluid and flow characteristics. For the given fluid, check its pressure, temperature, allowable pressure drop, density, conductivity, viscosity and vapor pressure at maximum operating temperature. Understand how these properties may influence each other. Be sure to look at safety or toxicity information too. Finally, analyze data on the fluid's composition, presence of bubbles, solids, tendency to coat, and light transmission qualities.

In terms of piping, you’ll want to avoid downward flow in liquid applications. Things that can effect this are direction size, material, schedule, flange-pressure rating, accessibility, up or downstream turns, valves and regulators, and straight-pipe run lengths. Check to see if vibration or magnetic fields are present or are potentially present in the area. Be sure to verify if the area is classified for explosion hazards and whether electric or pneumatic power is available.

You’ll want to determine whether the flow information will be better understood in mass or volumetric units before specifying a flow meter. Volumetric flow is not very meaningful when measuring the flow of compressible materials. That is, unless density is constant. When the volumetric flow of incompressible liquids is measured, the presence of suspended bubbles will cause an error. If this occurs, air and gas must be removed before the fluid reaches the meter. Water flow meters are insensitive to density, pressure and viscosity variations and are not affected by changes in the Reynolds number.

Before selecting a flow meter be sure to talk to a professional and ask some great questions. Along with your research and talking to flowmeter vendors, you’ll be well prepared for purchasing the right ones for your use.

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