Flow Measurements


 Flow Measurements

Flow rate is the rate of change of Volume or Mass per time.

A flow meter is a device that measures the amount of flow of liquid, gas or vapor moving through a pipe.

Volumatic flow rate (m³/sec)

Mass flow rate (Kg/sec)

 

Volumatic Flowrate 

 Volumetric flow rate ,volume flow rate, rate of fluid flow, or volume velocity.

 is the volume of fluid which passes through pipe per unit time has a symbol Q (sometimes V̇).

  • The SI unit is  (m3/s).
  • In US unit and imperial units  expressed as cubic feet per second (ft3/s) or gallons per minutes (GPM)

The volume flow rate       Q = VA

Where V : flow velocity

           A: cross section area

 Mass Flowrate

Mass flow rate = volume flow rate x density

ṁ = Q . ρ

ṁ  mass flowrate  (in kg/s)

Q  volume flow rate (m3/s)

ρ   density  (in kg/m3).

 

Flowmeters types

Many types of flow meters available for use in industrial automation. Depending on the product to be measured

( Obstructive)

Flow meters will directly obstruct the fluid flow.

  • Mechanical (Turbine) flow meters.
  • Positive displacement (oval gear )flow meters
  • Vortex flow meter.
  • Differential pressure flow meters.

 

( Non obstructive)

Flow meters will not obstruct the fluid flow.

  •  Coriolis flow meters
  • Electromagnetic flow meters
  • Ultrasonic flow meters

 

( Obstructive)


  • Turbine flow meters.

When fluid enter the flowmeter passes through the inlet component causes the turbine rotates proportional to the fluid velocity

As each blade passes through a magmatic field created at ac voltage

 pulse is generated in the pick up coil generated an output frequency proportional to the volumetric flow through the flowmeter

Output frequency is used to represent the flow rate and totalization of fluid passing through the turbine flowmeter

Application : Turbine flowmeters are widely used in flow measurement

  • Organic liquids, inorganic liquids,
  • liquefied gas, Natural gas
  • Not suitable for dirty water that carries large particles that may cause damage or obstruct the rotor turbine.
  • Not suitable with too high viscosity fluids .
  • Not suitable with steam.
  • The maximum working temperature of the turbine flowmeter is 120°C



  • Oval gear flow meters

Positive Displacement (PD)


There are two oval gears inside the meter , the flow of the fluid causing the gears rotation every revolution called (pocket)

Counting the pocket frequency gives a measurement of the Volumatic flowrate

A sensor detects the rotation of the gears to determine the volume flow rate of liquid.

Rotational velocity is directly proportional to volume flow rate

Q=VA

Application : 

  • Designed for use with higher-viscosity fluids
  • It is suitable for flow measurement For different measuring media
  • Not suitable for steam
  • Not recommended for low-viscosity fluids, including water.

 

  • Vortex flowmeter


Operational theory:  according to fluid oscillation principle, flow will  generate vortices when passing by the shedding body (BLUFF BODY)  AS an obstruction body then mechanical sensor then sensing element  used to  convert mechanical motion to frequency

For liquid, Gases and steam

The frequency of the vortices is related to flow velocity and the area of shedding body

  • Suitable for steam

Differential pressure flowmeter


Primary element responsible for making differential in pressure 
  • Orifice plate
  • Flow nozzle
  • Venturi tube




To measure flow with DP transmitter we must measure three elements (P , DP, T)


or use multivariable transmitter has the 3 transmitter on it ( P, DP, T)


 (Non obstructive)

Coriolis flowmeter




It Contains one or two tubes which are energized by (drive coil )give a fixed vibration, When a fluid (gas or liquid) passes through this tube the mass flow will cause the tube to twist resulting in a phase shift in voltage  generated from (inlet and outlet coil).

Can detect Low Flow rates and  density of Gas/Liquid with High Accuracy.



 Magnetic Flowmeter




Mag meters are volumetric meters that have no moving parts.

 This is ideal for those areas where you wouldn’t want to be exposed to the measured liquid while working on the meter.

 




Ultrasonic Flowmeter




the principle behind these meters is that an ultrasonic signal is transmitted downstream and upstream

The differential time is used to calculate the velocity of the liquid. That velocity is then used to calculate the volumetric flow through the pipe.

 high accuracy flowmeter

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