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Flow Meters

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What is a flow meter?

This is a device for measuring the flow rate of a liquid or a gas quantity passing through a pipe; be it mass, Volumetric, linear or nonlinear.

Factors to consider when choosing a flow meter

  • Familiarity of plant personnel
  • Experience of plant personnel with maintenance and calibration
  • Availability of spare parts
  • Failure history at the plant site.2

Types of meters in the Market

Calorimetric flow meters

Calorimetric flow meters use the principle of fluid flow measurement; two temperature insulated sensors are placed in contact with the liquid. One sensor is continuously heated to monitor the fluid flow rate, using the cooling effect of the fluid. An increase in fluid flow allow heat energy to be drawn from the hot sensor leading to a reduction in the temperature difference between sensors. The reduction in temperature is proportional to the rate fluid flow. The variation of response times is due to the fluid thermal conductivity. For proper measurement, lower thermal conductivity needs higher velocity. For high accuracy, the rate of flow should be kept low.

Velocity flow meters

There is a direct relationship between the output signal and the velocity going through the meter. This includes electromagnetic flow meters and others.

Electromagnetic flow meters

The fluid works as the conductor, allowing the energised coils to create the magnetic field. The voltage is directly proportional to the rate flow. The two electrodes in the pipe will detect the voltage produced. These flow meters are always used to accurately measure corrosive liquids. They can only be used for electrical conductive fluids such as water since they have high power consumption.

Mass Flow Meters

The development of mass flow meters was as a result of the need for accuracy in flow measurements. They include; thermal and Coriolis, with the later commonly as the liquid flow applications. This instrument measures the mass flow rate directly. There is a direct relationship between the output signal and the mass going through the meter. a

Coriolis Flow Meters

The measurement of mass is insensitive to temperature, pressure, viscosity and density changes. Coriolis flow meters are known as universal meters due to their ability to measure gases, slurries and liquids. They adopt the concept of the Coriolis effect for the measurement of the mass moving along them. The U-shaped tube through which the fluid flows vibrates in an angular harmonic oscillation, causing the tubes to deform. The additional vibration component is added to the oscillation, resulting in a measurable phase shift on a few points of the tubes. The accuracy level of these meters is less than +/-0.1% with a 100:1 turn-down rate.

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