What are Flow meters?

A flow meter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. Selecting a Flow Meter The basis of good flowmeter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation. Here are some key questions which need to answered before selecting a flow meter :

• What is the fluid being measured by the flowmeter or flowmeters (air,water,etc…)?

• Do you require rate measurement and/or totalization from the flow meter?

• If the liquid is not water, what viscosity is the liquid?

• Is the fluid clean?

• Do you require a local display on the flow meter or do you need an electronic signal output?

• What is the minimum and maximum flow rate for the flow meter?

• What is the minimum and maximum process pressure?

• What is the minimum and maximum process temperature?

• Is the fluid chemically compatible with the flowmeter wetted parts?

• If this is a process application, what is the size of the pipe?

Flow Measurement Orientation

When choosing flowmeters, one should consider such intangible factors as familiarity of plant personnel, their experience with calibration and maintenance, spare parts availability, and mean time between failure history, etc., at the particular plant site. It is also recommended that the cost of the installation be computed only after taking these steps. One of the most common flow measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly, an attempt is made to justify the use of a device because it is less expensive. Those “inexpensive” purchases can be the most costly installations.

The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation.

The first step in flow sensor selection is to determine if the flowrate information should be continuous or totalized, and whether this information is needed locally or remotely. If remotely, should the transmission be analog, digital, or shared? And, if shared, what is the required (minimum) data-update frequency? Once these questions are answered, an evaluation of the properties and flow characteristics of the process fluid, and of the piping that will accommodate the flow meter, should take place. In order to approach this task in a systematic manner, forms have been developed, requiring that the following types of data be filled in for each application : Click here to download the Flow meter Evaluation Form

Fluid and flow characteristics: In this section of the table, the name of the fluid is given and its pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?) and vapor pressure at maximum operating temperature are listed, together with an indication of how these properties might vary or interact. In addition, all safety or toxicity information should be provided, together with detailed data on the fluid’s composition, presence of bubbles, solids (abrasive or soft, size of particles, fibers), tendency to coat, and light transmission qualities (opaque, translucent or transparent?).

Expected minimum and maximum pressure and temperature values should be given in addition to the normal operating values when selecting flowmeters. Whether flow can reverse, whether it does not always fill the pipe, whether slug flow can develop (air-solids-liquid), whether aeration or pulsation is likely, whether sudden temperature changes can occur, or whether special precautions are needed during cleaning and maintenance, these facts, too, should be stated.

Concerning the piping and the area where the flowmeters are to be located, consider : For the piping, its direction (avoid downward flow in liquid applications), size, material, schedule, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available straight-pipe run lengths.

The specifying engineer must know if vibration or magnetic fields are present or possible in the area, if electric or pneumatic power is available, if the area is classified for explosion hazards, or if there are other special requirements such as compliance with sanitary or clean-in-place (CIP) regulations.

The next step is to determine the required meter range by identifying minimum and maximum flows (mass or volumetric) that will be measured. After that, the required flow measurementaccuracy is determined. Typically accuracy is specified in percentage of actual reading (AR), in percentage of calibrated span (CS), or in percentage of full scale (FS) units. The accuracy requirements should be separately stated at minimum, normal, and maximum flow rates. Unless you know these requirements, your flowmeter’s performance may not be acceptable over its full range.