Basics of Flow Measurement

Flow measurement is the process of knowing the amount of liquid or gas or any other material transferred from one point to another. The flow measurement is one of the most important parameters in the field of instrumentation.

The main purpose of flow measurement in industries is to determine the amount of material in and out of a process or transfer of water in any area or transfer of gas in the home through pipelines and almost everywhere material transfer is there in liquid or gas state.

Without flow measurement, it is not possible to do cost accounting, plant material balancing, operation on the continuous or batch processes, or even quality control.

Factors affecting the flow Rate

The following factors affect the flow rate.

  • The density of the moving fluid
    • Density is inversely proportional to the flow rate of the flowing fluid.
    • More head is required to move a fluid with the same flow rate whose density increase or with the same head, the flow rate decreases if the density increases.
  • The viscosity of the moving fluid
    • The viscosity of the fluid is also inversely proportional to the flow rate of the moving fluid.
    • The more viscous the fluid is, the lesser the flow rate.
    • For example, honey is more viscous than water, and hence considering the same head provided by the system, honey’s flow rate will be less than that of the water.
  • Temperature and state (liquid or gas or liquid with gas molecules or gas with condensate)
    • The fluid temperature has a great effect on the density and viscosity of the moving fluid.
    • Temperature change in gas molecules also changes their flow rate because the increase in temperature imparts some energy to the molecules.
  • Pipe size
    • Pipe size also plays an important role in the flow rate of the moving fluid.
    • For the same fluid passing through a pipe, if pipe size is doubled, then the flow rate will become half and if pipe size is decreased to half, then the flow rate will increase to double.
  • The friction created between fluid and pipe’s surface
    • The friction between the fluid and the pipe’s surface also plays an important role in the flow rate of the moving fluid.
    • The greater the friction, the lesser the flow rate will become because of the friction losses.
    • Generally, the fluids with more viscosity have more friction with the pipe’s surface.

Types of Flow Measurement

There are three types of flow measurement.

  • Inferential flow measurement
  • Volumetric flow measurement
  • Mass flow measurement

Inferential Flow Measurement

  • Inferential flow measurement is the most basic type of flow measurement in which flow is inferred from a characteristic effect of a related phenomenon. The best example of inferential measurement is the amount of our body temperature increase indicates an increase in fever or any other disease and hence fevers or disease can be inferred from rising in our body temperature.
    • Following are the different flow measuring methods using the inferential technique:
  • Variable head or differential meters
  • Variable area flow meters
  • Magnetic flow meters
  • Turbine flow meters
  • Target flow meters
  • Thermal flow meters
  • Vortex flow meters
  • Ultrasonic flow meters

Volumetric Flow

  • Volumetric flow measurement tells us the amount of volume of the fluid that has been transferred.
    • The best example of a volumetric flow meter is the flow meters we see in petrol pumps.
    • Volumetric flow meters are also termed quantity flow meters.
    • Different types of volumetric flow meters are :
  • Reciprocating piston
  • Oscillating piston
  • Nutating disc
  • Sliding vane
  • Rotating vane
  • Lobbed impeller
  • Peristaltic pump
  • Diaphragm pump

Mass Flow

  • The Mass flow meter tells us the actual amount of flow that has passed in a given unit of time through a given area where we have installed a mass flow meter.
    • The mass flow measurement is the best choice for those applications where the density and temperature of material keep on changing. With the change in density and temperature, the fluid volume also keeps on changing. Thus, in this case, the mass flow meter measures the exact amount of mass passing through the pipeline. Here in changing density and temperature conditions volume keeps on changing and hence if we use volumetric flow measurement then there will be an error in flow measurement.
    • Generally, wherever mass flow meters are used, we call them metering skid or custody transfer meters.
    • Following are the mass flow meters we use in skids known as metering skids:
  • Coriolis mass flow meter
  • Thermal mass flow meter

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