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Flow Measurement

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Flow measurement can be defined as the quantification of movement of a fluid. The flow measurement is assumed as the oldest recorded work in the instrumentation field. In industrial field, flow measurement is of great importance as from physiological processes to rocket science, the characteristic of flow is required. Its applications also extends to measurements in day to day processes like gas stations, water service, and so on

Generally flow is measured in two ways , volumetric basis and on the basis of weight .The flow of solids are usually measured in terms of mass per unit time or weight per unit time. Liquid flow is measured volumetrically or in the basis of weight. Gaseous flow is normally measured volumetrically.

When we are dealing with flow meters there are two terms called ‘turn down’ and ‘rangeability’. Turn down is defined as the ratio of full-scale flow to the minimum flow, which can be measured within a stated accuracy. If the turn down is in the ratio of 20:1, it means that the flow meter can measure from 20 per cent to 100 percent of the scale. This gives the accuracy of the meter. Rangeablity is the ratio of maximum to minimum range to which the meter can be calibrated (Refer: Calibration of flow meters ) 

There are many techniques used for flow measurement (Refer: How to Select a Flow meter).  Let us now look at some of the flow meters used in the industry.

1) Mechanical type flow meters

  • Piston Meters

Piston meters  or the rotary piston is semi positive displacement meter  consists of a rotating piston in a chamber whose volume is known. They are used for domestic water measurement

  • Variable area meter

The variable area meter or the rotameter  is available for a wide range of liquids but are commonly used for measurement of air and water

  • Turbine flow meter

The turbine flow meter converts the rotating of the turbine into a human readable scale and to the display

  • Single jet meter

It consists of an impeller with radial vanes which is impinged with a single jet

  • Woltmann meter

A rotor and helical blades are inserted axially into the flow   in a Woltmann meter . it is considered as a turbine flow meter

  • Paddle wheel meter

Similar to single jet meter except the fact that the impeller will be small .

  • Current meter

It is used to determine the flow though a large structure like a penstock in a hydro electric plant . the measurement is done by averaging the flow velocity over a large area

  • Nutating disc meter

Here a nutating disc which is ergonomically mounted is used to determine the fluid flow. Nutating disc meter is usually used in the measurement of water supply

  • Pelton wheel

A pelton wheel turbine which is also known as the radial turbine converts the mechanical action of the pelton wheel rotating in a liquid to user readable form

  • Multiple jet meter

Multiple jet meter is a velocity type meter which works similar to a single jet meter except the fact that here the flow is directed equally to the impeller by the ports

  • Oval gear meter

it is a positive displacement meter which is used to measure the flow

  • Inferential meter

Inferential meter reduces the volumetric flow by measuring some properties of the  liquid

2) Pressure based flow meters

  • Orifice plate
  • Venturi meter
  • Dall tube
  • Pitot tube
  • Multi hole pressure probe
  • Differential pressure transmitters

3) Optical flow meters

4) Open channel flow measurement

  • Level to flow measurement
  • Area/velocity measurement
  • Dye testing
  • Acoustic  Doppler velocimetry

5) Thermal flow meters

6) Vortex flow meters
7) Electromagnetic flow meters
8) Ultrasonic flow meters
9) Mass flow meters

  • Angular momentum mass flow meter –
  • Corioles mass flow meters
  • Thermal mass flow

10) Laser Doppler flow measurement

Calibration of flow meters

Flow meters are considered to be pretty accurate and in ideal case they are not affected by its environmental conditions.  But in practical case we have to consider various environmental factors too. We can see that due to improper installation and other factors industrial flow measurements are often prone to errors for avoiding these errors we have to calibrate the flow meters. Usually in situ methods are employed for calibrating flow meters.

How to Select a Flow meter

You know a flow meter is an instrument used by measure the linear or non linear mass flow rate in gases or liquids. Flow meters are classified into different basis according to the method used to measure the rate of flow.  All these types of flow meters have their own merits and demerits. For selecting the  best flow meter we must  consider many aspects like the process conditions, turndown requirements, accuracy, installation requirement and so on.  We are giving some guide lines which will help you to select the best flow meter for your requirement

  1. First enquire about the type of flowmeter  which has been  used in similar application before. This is the simple and most popular method. There are many manufactures around the planet making flow meters are they will be having websites. In the product specifications they will be mentioning its applications.
  2.  Another option is using the most familiar type of flow meter such as the differential pressure flow meter.  We will not recommend this as it’s not a much accurate option.  You should do further check before finalizing the flow meter.
  3.  Some other factors according to which flow meters are chosen are based on service, rangeability, pressure loss, accuracy, installation requirements, cost.
  4. Special attention should be taken for flowmeter intended to be used on sour service applications. NACE requirements, testing requirements etc should be  carefully considered.
  5. When choosing flow meters employed in hazardous area requirements, winterization requirements etc all requirements should be full filled .
  6. Special service applications such as custody transfer metering , multiphase flow metering etc. needs specific types of flowmeters  in order to  suit their requirements and hence the same shall be carefully considered.
  7. The installation requirements of the corresponding flow meter should be satisfied, as the piping layouts can be affected by straight run requirements of flowmeters.